#Anti enthalpy

[aroaceharlot]

I'm going to turn the concept of enthalpy into a person just to kill them.

[precisionantibody]

Understanding Antibody Affinity Measurement: Key Principles and Techniques

The Importance of Antibody Affinity Measurement

Antibodies are proteins produced by the immune system to identify and neutralize foreign substances like pathogens. The binding strength between an antibody and its antigen is referred to as affinity. High-affinity antibodies bind more tightly to their antigens, leading to more effective neutralization. In therapeutic applications, such as monoclonal antibody development, selecting antibodies with high affinity ensures better efficacy at lower doses. For diagnostics, high-affinity antibodies improve the sensitivity and specificity of assays, leading to more accurate detection of disease markers.

Key Concepts in Antibody Affinity

Antibody affinity is governed by several factors, including the molecular interactions between the antibody's variable region and the antigen's epitope. These interactions involve hydrogen bonds, hydrophobic interactions, Van der Waals forces, and electrostatic attractions. Antibody Affinity Measurement The binding strength is typically expressed as a dissociation constant (Kd), which is the concentration at which half of the antigen-binding sites are occupied. A lower Kd value indicates a higher affinity.

Techniques for Measuring Antibody Affinity

Several methods are employed to measure antibody affinity, each with its own advantages and limitations. The most widely used techniques include surface plasmon resonance (SPR), enzyme-linked immunosorbent assay (ELISA), isothermal titration calorimetry (ITC), and biolayer interferometry (BLI).

Surface Plasmon Resonance (SPR):

SPR is a label-free, real-time technique that measures the interaction between an antibody and an antigen immobilized on a sensor chip. As the antibody binds to the antigen, changes in the refractive index near the sensor surface are detected, allowing determination of the association and dissociation rates. SPR provides a detailed kinetic analysis, making it one of the most accurate methods for affinity measurement.

Enzyme-Linked Immunosorbent Assay (ELISA):

ELISA is a widely used method in which an antigen is immobilized on a solid surface, and the antibody is added to detect binding. The strength of the interaction is determined by the amount of antibody bound, which is then quantified using an enzyme-substrate reaction that produces a color change. Although ELISA is less precise in determining kinetic rates compared to SPR, it is highly versatile and widely used in research and clinical settings.

Isothermal Titration Calorimetry (ITC):

ITC measures the heat change associated with the binding of an antibody to its antigen. By analyzing the thermodynamics of the interaction, ITC provides information on the binding constant, stoichiometry, and enthalpy. This method is particularly valuable for characterizing the energetics of binding, though it requires relatively large amounts of purified antibody and antigen.

Biolayer Interferometry (BLI):

BLI is another label-free technique that uses an optical sensor to measure changes in the thickness of a biological layer as an antibody binds to an antigen. Like SPR, BLI allows for real-time kinetic analysis, making it suitable for determining affinity with high accuracy.

Applications and Future Directions

Accurate antibody affinity measurement is critical in both research and industry. Anti Idiotype Antibodies In therapeutic antibody development, high-affinity antibodies are preferred for targeting diseases like cancer and autoimmune disorders. In diagnostics, highly specific and sensitive antibodies are essential for early disease detection. With advancements in affinity measurement techniques, researchers can better understand antibody-antigen interactions, leading to the development of more effective treatments and diagnostics.

[iamjjmmma]

“Number All My Bones: There and Back and There Again” Part 1, Chapter 4

Beginning: https://bit.ly/2NtGPgu

Previous: https://bit.ly/2H5dDej

Next: https://bit.ly/2tD9Q03

It’s only a taser; I know. I know the basics about these types of guns, although violence isn’t my main research preference. Still, I duck inside, my heartbeat still somewhat yelling at me, my head definitely yelling at me to get back to my work, that it’s probably just some sort of census. But the doorbell rang, and Papyrus immediately sprang out of his seat, with that golly-gee smile impressioned all over his face, and sprinted towards the door. Sans sprinted after him, and I after Sans, all of us except Papyrus seeming to remember the rule that no one was supposed to answer the door except for me. But the door opened before I could say anything, and there stood the one woman I would cry over just a few weeks later. Her name tag read “Ica Grey, Head of the Anti-Monster Department”, the “Jess” part obscured by a shadow for a little while, but I knew who she was. The streak of grey hair, the crossed arms, the badges on her blue dress told me everything. She was the one who had started the “MF” tag, the one who had started the monsters coming home without any sort of occupation, the one who had started the monster children not allowed to take the same classes as humans, the monsters being denied from the hospitals. The dehumanization process didn’t need to be done; it simply was, and it was since when we were born. My smile stretches until it turns taut. “Hello, Miss.” Her hand settles on her taser for a moment, but it stutters just before it settles by her side. “Hello, Doctor. I’ve heard a lot about you.” I nod. “I can say the same. Especially with your ‘MF’ endeavors. What does it stand for, though? I’ll take a wild guess. ‘Monsters Forbidden.’” She nods back, although I can practically see her teeth gritting. Her hand moves closer to the gun. Betty whimpers a little, and Sans and Papyrus hush the other children away before they get embroiled in the grown-up soup of politics and science. In another world, maybe I would have gone with them. But that world is faraway, much too far from now to even think of existing. Miss Grey put her hand by her hip. “Are we conducting the meeting or not?” I nodded, although I didn’t even think about giving her any more than that. I was prepared to send all of the children upstairs, thinking they went into the living room, but it was only Betty, reading a history book for her tutoring program, no doubt. I was about to say something, but one look at the scary lady behind me all in blue sent her tiptoeing away and making her way up the stairs. As we sat on the couches, the coffee in the pot cold by now after my morning cup, I made my move, even though I knew it wouldn’t work by a long shot. “Do you mind putting the gun away? I have four little kids here, and I don’t want them getting-” She laughed, ran her long fingernails through her hair once or twice. “Of course not. You’re the scientist, aren’t you? You should know by now that it’s only a safety precaution. Not that I’d willy-nilly fire at one of your kiddos, right?” I sighed, went into a conversation about geothermics I wouldn’t give to my students until it was May and the graduation caps were being shipped. I counted myself using the words “entropy”, “enthalpy,” “quasistatic”, “Carnot cycle”, and “calorimetry” at least twice each before she started to nod off before nearly bumping her nose on the edge of the couch. Science that would have gone over her head even if she had a fifty-foot mitt to catch it. She jerked herself up so quickly that she started falling forwards, and I almost stretched out my hands to catch her before she could regain her composure.“Well, Dr. Gaster, this was all very, very informative, but can you please focus on the effectiveness of your project?” I went into a slight smile. Finally. “Alright, Miss. The expansion of the Core will help to power our city by-” She put a hand over her mouth in mock shock, but I knew she was yawning underneath. A professor tends to notice these things easier. “So it basically makes our gas bills cheaper?” I laughed, and I almost put a hand over my own mouth. I shifted into a different language, one that politicians love to speak. “What-?! No. No, not at all. If the expansion is complete, you won’t even have to pay for electricity at all. Ever. And thanks to it, we’re starting to see a big change. Not only in the bills-” I stopped. I was getting a little preachy. I laughed again. Even if I was preachy, it wouldn’t ever stop me from loving the feeling. So I gave in when she asked how the Core worked. Just this once. “Well, it converts geothermal energy from the mountain to-” I couldn’t say “magical”, but there was another word for that. A word I could use. “-idiopathic energy by using the underground chambers. These chambers have magnets with turbines that allow the electricity to be transformed from idiopathic to-” She put her hand over her eyes, although I know they were closed underneath. “It converts electricity to heat.” “Oh, I see.” Huh. So she wasn’t asleep after all. “A non-polluting, unlimited, self-sustaining power source. Of course…” I stand up, and she puts her weight on her toes as if she’ll follow, but she stays right there where she is. People say I’m a good judge, even though I’m a better scientist, but in cases such as this, I can’t always pull out a clear verdict about someone. “...none of this would happen if you don’t sign the agreement tomorrow.” She nods, but puts her hand closer to her taser just in case. “Yes, but that doesn’t mean anything.” “What do you mean that doesn’t mean anything? I’ve just explained an energy agenda that I doubt you’ll find anywhere else, and-” “That still doesn’t explain the rest of your kind.” “Are you-?!” “Yes, Doctor. I am. You think that just because you’ve made energy out of the dirt means that you haven’t come from it. You come up here and steal our jobs, steal our money, all because you think you’re better than the rest of us. You-” I stretched out my hand, reach for anything looking vaguely like a door handle to push. “Miss Grey, I didn’t say any of that-” “Oh, just because you didn’t say it doesn’t mean it isn’t-” I saw her in the corner before I heard her. Betty had come back from upstairs, probably because of all the fuss we made down here, and was looking at me with some of the most terrified two eyes I’ve ever seen. “Excuse me, ma’am.” She didn’t bother me as I went over and patted Betty’s shoulder. Poor girl. Only a few minutes here, and already we’ve escalated beyond what I would ever think of doing if Jessica wasn’t… Jessica. “Betty, it’s alright. The both of us were just having a discussion, alright? It’s very important. So what I need for you to do is to go back upstairs and-” “Doctor.” “Just a minute. What I need for you to do is go back upstairs and tell the others that everything is fine. And even if it does escalate, I’m stronger than I look, huh?” I patted her shoulder again for good measure. “Doctor, please. You’re not talking to anyone.” “Miss, what do you mean I’m not talking to anyone? Betty’s right here, isn’t she?” Chara and Asriel have come back down, too. I suppose the conversation died just enough. “Isn’t she?” Chara shakes his head, while Asriel shrugs his shoulders. “She’s still upstairs playing puzzles with Papyrus. An’ I think she’s learning how to play chess, too.” I look to my right, and Betty’s gone. Anxiety can do more than you could ever imagine, I suppose. If it can keep me staying awake at night after a dream that only mildly alarmed me, it can do what it just did. Anxiety also kept me heading towards my room after Jessica left, after calling down the kids and getting Papyrus to help me fix a pizza and some chicken, telling them that dinner was probably right around the corner. And just as anxiety foretold, something’s wrong. One of my books on human-monster history has fallen on the floor, but even without any sort of education in physics, I can tell it doesn’t fall like that. It’s at least halfway across the room, my bookshelf still in place right next to the door, and when I picked it up, another eerie fact sent a chill down my spine, and I almost felt my coat shaking along with it. It was open only a few pages in towards the end. Experience has taught me otherwise. If books don’t fall flat on the covers, front or backs, it normally falls with the middle pages open and spread out. Meaning if it didn’t fall, someone had to have taken it. Was it Sans or Asriel or Betty or anyone being tutored by him, forgetting to pick it up after they’d left? Or was it Papyrus, who was trying to get his own little revenge for me not getting him the book at the library? Alright. Focus. It’s probably one of them. I put back the book, and I sighed, going out to fix myself another cup of coffee. Anxiety can do everything, I suppose.

[mogood]

Low melting point alloy manufacturers introduce the advantages of tin alloys

Tin alloy is a kind of fusible compound and bonding metal of refractory metal, which is then made into alloy material through powder metallurgy process. The introduction of low melting point alloys into future high-tech weapons and equipment manufacturing, cutting-edge technological progress and rapid development of nuclear energy will greatly increase the demand for high-tech, high-quality and stable tin-bismuth alloy products.

1. The alloy has good damping properties.

Compared with aluminum alloy, steel and iron, the modulus of elasticity of tin-bismuth alloy is lower. Under the same stress conditions, tin-bismuth alloys can consume more deformation work, have noise and vibration reduction functions, and can withstand greater shocks and vibrations. Load, suitable for preparing anti-vibration parts.

Moreover, tin-bismuth alloy has a small specific gravity and is currently the lightest structural material. The specific strength of tin-bismuth alloy is generally higher than that of aluminum alloy and steel, and only slightly lower than the highest strength fiber-reinforced material; it is also the same as aluminum alloy and steel. , But much higher than fiber reinforced materials, so it has a good advantage.

2. The alloy has good thermal conductivity, thermal stability, anti-electromagnetic interference and shielding performance.

Tin-bismuth alloy has good castability and low reactivity of magnesium and iron. Iron crucibles can generally be used for smelting. The enthalpy per unit volume of the fusible substance is low, resulting in a higher die casting speed than aluminum. Therefore, the casting processing accuracy of the casting is high. The bismuth alloy die casting has the smallest wall thickness. Tin-bismuth alloy can be processed at high speed, with high production efficiency and higher production efficiency than die-cast aluminum parts, which is suitable for mass production in the automotive industry.

3. The alloy has good dimensional stability.

The shrinkage rate is stable, and the dimensional accuracy of castings and finished products is high. Except for magnesium-aluminum-zinc alloys, most tin-bismuth alloys have basically zero dimensional changes due to phase transformation during heat treatment and long-term use.

Excellent cutting performance, its cutting speed is much higher than other metals, low tool consumption when cutting tin-bismuth alloy, low cutting power, tin-bismuth alloy, aluminum alloy, cast iron, low-alloy steel parts with the same cutting power consumption ratio is a fixed value. After the tin-bismuth alloy is cut, there is no need for grinding or polishing, and no cutting fluid is required to obtain a machined surface with extremely low roughness.

In addition to the above points, tin-bismuth alloys also have the advantages of low density but high strength and good rigidity. The rigidity of high-quality tin-bismuth alloys increases with the increase in cubic ratio thickness. Therefore, the rigidity of tin-bismuth alloy is very conducive to design

[mtf-unit-epsilon11]

Item #: SCP-009

Object Class: Euclid

Special Containment Procedures: Object is to be contained within a sealed storage tank of heat-resistant alloy with dimensions not less than 2m x 2m x 2m.

Under no circumstances should SCP-009 be exposed to temperatures in excess of 0°C when not undergoing testing, and no water-based solutions shall be allowed within 30 meters of the object's containment area. Object's chamber is to be fitted with temperature sensors which must be monitored at all times, and is to be kept refrigerated by no fewer than three (3) redundant cooling units. Any malfunction of sensors, or of coolant systems, is to be reported and repaired immediately. If at any time the temperature in the containment area climbs above -5°C, the chamber is to be locked down and flooded with coolant until temperatures return to safe levels (-30°C to -25°C).

Containment area is to be kept in total vacuum during testing, and personnel interacting with SCP-009 must wear full environmental protection gear. Following testing, all equipment, personnel, and other materials must undergo dehydration procedures and be quarantined for no less than 12 hours. Any moisture found displaying properties of SCP-009 is to be quarantined and added to the containment area as soon as possible. Living organisms found to be contaminated by SCP-009 are to be terminated by chemical dessication and extracted molecules of SCP-009 added to containment.

Description: SCP-009 is approximately ███ liters of a substance which superficially resembles distilled water (H2O), except with a distinct bright red hue. This red hue is discernible in all phases, and serves as the most expedient method of identifying contaminated matter before its anomalous properties manifest. In contrast to mundane water, SCP-009 assumes a liquid phase at temperatures between -100°C and 0°C, and a solid state above those temperatures. At temperatures below -100°C, SCP-009 vaporizes into a gaseous phase similar to steam.

Examinations of the atomic structure of SCP-009 have proved inconclusive. The substance appears to be identical to normal water molecules, with the exception of [REDACTED] in contrast to standard laws of enthalpy. Dr. █████, Site ██'s resident expert on Xenospatial Physics suggests that SCP-009 may originate in a universe with alternate physical laws.

The most hazardous property of SCP-009, however, is its ability to contaminate normal H2O. When in contact with any aqueous solution, SCP-009 will, through unknown mechanisms, transfer its anomalous properties to other objects and creatures. Testing has shown it capable of assimilating ice, steam, tea, fruit juice, seawater, blood, and [DATA EXPUNGED]. The time it takes for this process to occur varies depending on temperature and the exact chemical composition of affected matter, and had been observed as taking between 3 minutes and ██ hours.

Experiments on D-Class personnel have illustrated the process of conversion by the substance, which has been found to follow a consistent pattern:

1. Initial Exposure: Subject is exposed to SCP-009, and it begins assimilating any moisture present on the exposed surface. Creatures in this stage do not commonly notice any unusual symptoms except for a slight warming sensation.

2. Surface Conversion: Frost begins to form on the exposed area as the heat produced by the subject and SCP-009 itself raises its temperature above 0°C. This stage can take anywhere from one (1) minute to █ hours, during which time subjects begin to feel [REDACTED] crystals from the epidermis.

3. Deep Tissue Conversion: Exponential increase in temperature of SCP-009 causes runaway reaction throughout subject's body, resulting in [REDACTED]. Actual blood loss is minimal due to ice crystals [REDACTED], allowing subjects to remain alive and conscious for up to ██ hours.

4. [DATA EXPUNGED]

Testing on D-Class personnel was discontinued as of 4/23/20██.

Addendum: Circumstances of Retrieval: Subject was found in ████, Alaska, on 11/05/19██. The Foundation became involved after reports were obtained from the native ████ Tribe, who came across the mangled bodies of a team of seal hunters which had apparently been ship-wrecked ██ kilometers from the village.

All victims were found encased in red ice. Cause of death recorded as internal bleeding, though closer examination found [REDACTED]. It is surmised that the low ambient temperatures in the area retarded the freezing process. This prolonged the time to total conversion by ██ hours, and allowed the victims to remain conscious until [DATA EXPUNGED].

Origin of SCP-009 is currently unknown. Investigation into similar events or materials in the area is ongoing. Evidence at the scene suggests [REDACTED], possibly involving SCP-███.

See Exploration Log A009-1 for details.

Addendum: 11/09/19██

After initial report and retrieval of specimens, it was confirmed that the arachnoid entity found by MTF-B7 (see attached file) was indeed a previously unknown instance of SCP-3023. Investigation has revealed the instance originated in [REDACTED] as a result of [DATA EXPUNGED].

Addendum: 12/06/19██

After repeated inquiries, it should be noted that the portion of coastline upon which the initial victims were found was barren rock approximately █ meters from the seashore, and was sufficiently dry and cold to prevent significant contamination of the surrounding area. Had the site been closer to the water, there is little doubt an extinction-level event would have ensued.

Consideration of upgrading SCP-009 to Keter class under review.

Addendum: 12/16/20██

Super-cooling of SCP-009 for the purposes of experimentation is disallowed until further notice. Personnel are advised that liquid nitrogen is only to be used on the subject in controlled amounts, and only until temperatures have reached acceptable levels.

Related note: Possible application of SCP-009 in cold fusion research pending evaluation.

Memo from O-5 Command: 1/09/20██

We've decided to keep this thing Euclid for now. We understand the concerns raised, but as long as you keep the power on and nobody goes near its containment area, there shouldn't be a problem. That's why we're keeping it in Site ██, after all.

As for the cold fusion research, we're putting a pin in that for now. Frankly, we don't have it in the budget for another SNAFU like Site ██. The salvage team still hasn't found Dr. █████'s [REDACTED]

 Hide Report

The following experiment record was recovered via a chance occurrence of SCP-507 shifting into a universe in which the described test was carried out using SCP-107. The applicability of the reported findings to our own universe is pending review.

Input: 10mL of SCP-009

Result: "Red snow" fell in test area for 27 minutes with moderate intensity. Grass growing in test area began runaway reaction which ended with entire area being "frozen" within minutes. Notably, anti-enthalpathic reaction of SCP-009 did not extend past the effective radius of SCP-107, for reasons still under investigation. Non-grass plants in area turned bright red in color, greatly expanded, and mutated to display cyan-colored "tentacles" similar to those of species Drosera capensis. Mucilage produced by these tentacles later found to be tiny beads of SCP-009. How the plant is able to survive with SCP-009 integrated into its cell structure is currently under investigation, with preliminary hypothesis being the plant is a reflection of flora from the substance's native universe.

[arxt1]

Geometric and Thermodynamic Volume of Hairy Black Branes. (arXiv:2006.10880v1 [gr-qc])

With the objective to generalize previous results found for a handful of explicit solutions, we study the extended thermodynamics of a black brane with minimally coupled scalar hair in D-dimensional asymptotically anti-de Sitter spacetimes. Using Komar integration and the Hamiltonian formalism to calculate the conserved charges, we obtain a Smarr relation that is applicable to a wide variety of solutions and suggests a more general definition of the thermodynamic volume. This volume is found to be proportional to the geometric volume, and a simple prescription is given to calculate the constant of proportionality. Moreover, the method of Hamiltonian perturbations yields an extended first law of thermodynamics for hairy black branes, thus giving a definition for their enthalpy. These results are then verified by applying them to some of the explicit solutions that exist in the literature.

from gr-qc updates on arXiv.org https://ift.tt/2YjzEhV

[zanesoutter371-blog]

Papa Child Wedding celebration Tracks.

Quickly that will definitely be Papa's Time; the third Sunday in June, proclaimed a recognized day from national regard in 1972 through Head of state Richard Nixon. She is actually interchangeably called Ceridwen, Kerdwin, Cyrddven, Kerridwyn, Cerridwyn or even Kerridwen; or Black Mom, Queen from organized crime, Female from Awen, Inspiration of the Bards; or Jagged Treasured, Fair Chorus, Bent Purity, Sacred Songstress, Crooked Female. After Joshua Alameda Franklin, a gay papa from pair of, determined that Dietsportal.info his 9-year-old son had been actually intimidated, strangled, and slammed to the ground in the course of an after university course, he made a decision enough sufficed. The unfamiliar opens in a Confederate military medical center near Raleigh, North Carolina, where Inman is bouncing back from fight cuts in the course of the American Civil War The soldier is wheelsed of defending a cause he never believed in. After looking at the advice coming from a blind man as well as moved by fatality of the guy in the bedroom alongside him, he chooses one nightfall to unclothe the hospital as well as come back house to Cold weather Mountain, North Carolina. And Jean Val Jean, as a result, ends up being Hugo's the majority of ethical as well as excellent character: a straightforward man and a father brown therefore devoted to Cosette that he agrees to lay down his lifestyle to ensure that she might find contentment along with Marius. Community Cats: This is actually an additional non-profit web site along with a checklist from home ideas - most get intensely off package inside a plastic bathtub approach, but one fascinating possibility suggested is using Mylar coverings when the temperature level is actually very cool - to reflect the pet's body heat back to the creature. Visualize if there have been as several issue fragments as anti-matter, there will have been no matter excluded after demolishing obliteration and also deep space would possess been actually barged up with energies which our company often get in touch with dark energy or online photon enthalpy, on below -nuclear ranges. Decision was actually regularly the exact same ... Tony would certainly decided not to sign yet another lending, Michael would tied in to a lecture, asking for appreciation, to which Tony responded to that his daddy certainly never actually cared about him - had left him early in lifestyle - and also while Tony was attempting to pacify the male just recently, he wasn't visiting lend him more funds, nor ruin his personal debt as he had in recent. Eventually I was actually revealed a photograph of Dr. Josef Mengele as well as I recognized him pointing out, 'That is actually Father Josef.'" Father brown Josef did work in the health center together with director Camille Laurin which would certainly later become Quebec's Minister of Health and wellness. I am the initial one to accept that I despise possessing cold feet, I love using unclear pantofles throughout the wintertime as well as that is actually definitely an incentive with working at property as this implies I do not have to take all of them off!!! You produced me sob like a little one, remembering myself when my daddy died Three Decade earlier when I was 16, and also just how that modified my lifestyle ever since, picturing currently exactly how my other brothers & sibling felt after that particularly the one who was just sex.

[alexanderbowmanme-blog]

POP and DSC

CALORIMETRIC STUDIES OF POLYOXYPROPYLENE

  by

  ALEXANDER CHARLES DAVID BOWMAN [BSc. (Hons.)]

      A thesis submitted in partial fulfilment of the requirements of the University of Greenwich for the degree of MSc Open (Science) in Biochemistry

     SCHOOL OF SCIENCE

UNIVERSITY OF GREENWICH –MEDWAY CAMPUS,

CHATHAM MARITIME

KENT ME4 4AW, UK.

   May, 2005

      ABSTRACT    

CALORIMETRIC STUDIES OF POLYOXYPROPYLENE

The calorimetric phase transitions of polyoxypropylene 1000 (POP) have been examined by high sensitivity differential scanning calorimetry (HSDSC). HSDSC has been employed to acquire thermodynamic parameters associated with thermal aggregation in aqueous solutions of POP as a function of: repeatability (reversibility), concentration and scan rate. The effects of D2O, metal ions (NaCl and MgCl2), urea, guanidinium hydrochloride and methanol on the aggregation transition of POP have also been examined. POP transitions are reversible i.e. repeatable in that the first and second heating scans are superimposable. The aggregation transition temperature of decreases with increasing concentration of POP. However, the calorimetric parameters are independent of scan rate (10, 30, 60 0C/min) at a fixed concentration of POP. This allows the use of equilibrium thermodynamics for the interrogation of calorimetric parameters. The parametric values governing the quadratic relationship between changes in enthalpy resulting from aggregation as a function of temperature are similar to those descriptive of the linear relationship between heat capacity change and temperature.

The phase transition of POP has a sharp leading edge indicating aggregation, the calorimetric signal then tails off gradually. The difference between the initial and final heat capacities for POP is negative, indicating a loss of structured water from the hydration sphere around the POP.

In the presence of D20 the aggregation transition temperature is decreased, but the excess heat capacity (DCp,ex) is increased slightly yet the difference in heat capacity between the pre- and post-transitional baseline (DCp,d) remains negative and of the same value as in water. Similarly the metal ions Na+ and Mg2+ decrease the aggregation transition temperature of the POP compared to that in water. Increasing concentrations of urea and guanidinium hydrochloride, at a fixed concentration of POP, increase the aggregation transition temperature, decrease DCp,ex and result in DCp,d values which become increasingly less negative compared to POP in water. The effects of urea and guanidinium hydrochloride on POP, measured by HSDSC, are opposite to the calorimetric effects observed for by these agents on proteins.

ALEXANDER CHARLES DAVID BOWMAN BSc. (Hons.)

  DECLARATION

 We, the undersigned, declare that the work presented in this thesis is the candidate’s own work unless otherwise stated.

      Alexander C. D. Bowman                 3/05/2005

       Prof. B. Z. Chowdhry                        3/05/2005

       ACKNOWLEDGEMENTS

 I remain indebted to Professor Babur Z.Chowdhry for his time and supervision throughout the course of this study. I also thank Professor S. Leharne for setting up the mathematical model for analysis of the POP1000 HSDSC data.

        ABBREVIATIONS

 Symbol            Meaning

a                                    Extent of reaction

CP                   Cloud point

EtOH               Ethanol

GdnHCl          Guanidinium hydrochloride

H2O                 Water

HSDSC           High sensitivity differential scanning calorimetry

kDa                 Kilo Dalton

LCST              Lower critical solution temperature

Ln                   Natural logarithm

M                    Mole

MeOH             Methanol

Mg                  Milligram

mL                  Millilitre

MW                 Molecular weight

n                      Aggregation number

NaCl                Sodium chloride

POE                Polyoxyethylene

POP                 Polyoxypropylene

RSD                Relative standard deviation

T1/2                             Temperature at which a equals 0.5

ΔCp,d               Difference in heat capacity between initial and final states of a system

ΔHCal                        Calorimetric enthalpy  

ΔHVH                        Van’t Hoff enthalpy

ΦCp,xs            Apparent excess heat capacity

 CONTENTS                                                                          PAGE NUMBER

 TITLE PAGE

 ABSTRACT                                                                                                              

 DECLARATION AND ACKNOWLEDGEMENTS

 ABBREVIATIONS                        

  CHAPTER 1:            INTRODUCTION                                                                      

 1.1.                              POLYOXYPROPYLENE (POP)

 1.1.1.                           POP SYNTHESIS

 1.1.2.                           PHYSICO-CHEMICAL PROPERTIES OF POPs

 1.1.3.                           USES OF POP

 1.1.4.                           POLYPROPYLENE GLYCOL (PPG)

 1.1.5.                           POP BLOCK CO-POLYMERS                                                    

  CHAPTER 2:            HIGH SENSITIVITY DIFFERENTIAL SCANNING CALORIMETRY

 2.1.                              INTRODUCTION

 2.1.2.                           HSDSC

 2.1.3.                           INSTRUMENTATION

 2.1.4.                           THEORETICAL CONSIDERATIONS

 2.1.5.                           MATERIALS

 2.1.6.                           HSDSC EXPERIMENTAL PROTOCOLS

  CHAPTER 3:            POP RESULTS AND DISCUSSION

 3.1                               HSDSC PROFILE FOR POP

 3.2                               HSDSC OF POP 1000 IN H2O

 3.3                               HSDSC OF POP 1000 IN D2O

 3.4                               HSDSC OF POP IN THE PRESENCE OF METAL IONS

 3.5                               HSDSC OF POP IN THE PRESENCE OF UREA

 3.6                               HSDSC OF POP IN THE PRESENCE OF GUANIDINIUM HYDROCHLORIDE

  CHAPTER 4:            SUMMARY AND FUTURE STUDIES

 REFERENCES

                                                                                                                                                CHAPTER 1

             INTRODUCTION

 1.1.      POLYOXYPROPYLENE (POP)

 Polyoxyproylene

 Polyoxyproylene (POP) is a polymer used by itself or in conjunction with other polymers to form co-polymers commercially known as poloxamers, superonics, or pluronics [1]. Wurz discovered POP in 1860.  Its structure consists of three carbon atoms bonded to six hydrogen atoms and a double bond to an oxygen atom (figure 1.1):

          The structure of POP is composed of n number of repeating units of oxypropylene.  POP can be synthesised from propylene via an oxypropylene intermediate.  POPs are hydrophobic, can exist in the solid or liquid state, and are obtainable over a wide range of molecular weights.  POPs have many uses in e.g. cosmetics, inks and drug delivery systems some of which will be discussed later.

 1.1.1    POP synthesis

 POPs are synthesised from the organic compound propylene (figure 1.2). Propylene is converted to the halohydrin, propylene chlorohydrin, by the addition of a halogen, such as chlorine and water to the double bond in propylene.  To propylene chlorohydrin concentrated aqueous –OH is added to obtain the epoxide, oxypropylene (figure 1.3) [2].  Oxypropylene is the starting block for the synthesis of POP. Addition polymerisation under controlled temperature and pressure conditions yields POP. POP has a zigzag chain structure (figure 1.4) [3].

                          Figure 1.4   The schematic zigzag chain structure of POP [3].

 1.1.2    Physico-chemical properties of POPs

 POP can form an extensive network of hydrogen bonds in aqueous solution.  When POPs are added to water the water molecules form large clusters around the POP and the structural order of water is enhanced.  Therefore the unassimilated water molecules will further interact to give stronger hydrogen bonds between the water molecules [4].

Triblock co-polymers of POE (polyoxyethylene)-POP-POE can form micelles at high temperatures whereby the POP blocks increase in hydrophobicity and undergo phase separations when the temperature is raised [2].  These micelles are formed when the POP molecules reduce their exposure to water by self-association and because the POE, which is hydrophilic, occupies the space between the water and POP [5].  At temperatures above 20°C water is a poor solvent for the POP and in a block co-polymer this will induce micellization into spherical, rod-like and maybe layered aggregates depending on the concentration and temperature of the co-polymer [6].  Self-association of the POP in water and the POE block behaving as an ice-like structure in water induces an enthalpy change but can result in an entropy penalty due to enhanced structuring of water [1].  The critical micelle concentration (CMC) is an important value in that if it is lower than the polymer concentration it may affect its function e.g. partitioning of a drug added to a co-polymer may occur [7].  High temperature or the introduction of aqueous salt solutions may influence the CMC value of a particular polymer.

 The presence of alcohols and salts in a polymer solution substantially change the physical and chemical properties of the polymer, for example, alcohol solutions are used in water-based printing inks in order to reduce the surface tension of the polymer [8]. Small-angle neutron scattering experiments carried out on pluronics F-88 and P-84, triblock co-polymers of POP, have shown that the addition of salts to the polymers changes the micelle orientation and also that the micellization depends on the salt concentration and temperature whereby micellization increases for the pluronics with temperature [9].  Knowledge of the phase transition properties of polymers will enable determination of how the physico-chemical properties of polymers may be altered by an addition of a co-solute or co-solvent.  Understanding the mechanisms behind these processes i.e. how a co-solvent/ co-solute affects the self-assembly of co-polymers is important in terms of their potential clinical and industrial applications [10-12].

 Many analytical techniques have been used, for example mass spectrometry, chromatography, and ultracentrifugation to determine the molecular weight of POPs. The molecular weight of commercially available POP is in the range of 1-20 kDa.  Furthermore, POPs can exist in the solid or liquid state with a range of melting points from –193 to 87°C.

      1.1.3    Uses of POP

 POPs are used in diblock co-polymers and triblock co-polymers and have many industrial applications.  The most common monomer that is used with POP is POE, and by changing the block lengths during synthesis, the co-polymer will change in size and enable the co-polymer to have many uses [12].  For instance POE-POP-POE, a triblock co-polymer, is used in the synthesis of detergents, dispersion stabilisers and control release agents in the pharmaceutical industry, and water-based printing inks [5].  Also, thin liquid films of the diblock co-polymer POE-POP on the surface of fused quartz has been investigated [13]. Crodamol PMP (a polyoxypropylene (2) myristyl ether propionate block co-polymer) has been employed in human cosmetics for decades and has potential uses in veterinary pharmaceuticals for food-producing animals [14].

 The POP monomer can be used in a variety of pharmaceutical products where it acts as an emulsifying, wetting, thickening and/or dispersing agent [15].  Pharmaceutical co-polymers are used as anti-infectives and antibiotic drug delivery systems for the treatment of bacterial and viral infections. High molecular weight co-polymers can also act as vaccines [16]. In Toxoplasma gondii infection in mice, POPs have been tested for acute anti-infection activity [17].

 CRL1005 (consists of 95% of POP and 5% of POE) can stabilise emulsions and the binding of antigens between the interface of oil and water and in turn can be used in oil-free vaccines due to its carrier effects [6].  Pluronic F-68, POE-POP-POE block co-polymer, can prevent aggregation occurring in the human growth hormone. However, pluronic F-68 cannot stabilise the hormone when thermal stress is induced [18].  It can also inhibit thrombosis, improve perfusion of damaged tissue, and decrease the viscosity of the whole blood [19].

 When pluronics are absorbed across cell membranes, they have been found to increase the translocation of the anti-cancer drug doxorubicin, into the cytoplasm.  This leads to doxorubicin accumulation, in turn allowing the drug to bypass P-glycoprotein found in resistant cells.  This finding is important in investigating the mechanism of co-polymer transportation across a membrane and may provide an explanation of pluronic drug formulation behaviour [20].

   1.1.4    Polypropylene glycol (PPG)

 Polypropylene glycol (PPG) is a liquid form of POP, its structure is [H{O-CH(CH3)CH2}n-OH] and it has the same properties and uses as POP. PPG is important in separating and purifying enzymes, proteins, nucleic acids, and other substances that are involved in biological processes.  For example, a study has shown that PPG did not affect the release of glutamate (which is converted into glutamic acid) from the bacteria Corynebacterium gluytamicum. This is due to its hydrophobic structure not inducing the bacterial cell membrane to release glutamic acid unlike polyethylene glycol, which is less hydrophobic, incorporating and not interacting with the lipid membrane [21]. PPG forms intra- and intermolecular hydrogen bonds.  PPGs of a low molecular weight are water soluble while those of higher molecular weight are only partially soluble [22].

 1.1.5    POP block co-polymers

 Many investigations [23, 24] have been conducted on block co-polymers containing the POP block, however there is limited information on POP itself. Poloxamers and poloxamines commonly known as pluronic and tetronic or superonics, are two common nonionic block polymers containing blocks of POP and POE.  These two block co-polymers exhibit characteristics of amphiphiles due to the hydrophobic and hydrophilic components, respectively. These amphiphilic co-polymers have the ability to self-organise into supramolecular structures at interfaces [11]. Poloxamines have a different structure from poloxamers (figure 1.5), they are tetrafunctional block co-polymers containing four POE-POP blocks joined together at the centre by an ethylene diamine moiety.

 Poloxamers posses a symmetrical structure (figure 1.5b) whereby (a) and (b) denote the number of oxyethylene and oxypropylene monomers per block, respectively [25].

                   Poloxamines have a range of molecular weights from 1650 to 30000 Da.  Low molecular weight poloxamines are viscous oils or pastes, and high molecular weight poloxamines exist as solids [26].  Nonionic block co-polymers that are synthesised from oxypropylene are synthesised in such a way that each co-polymer displays a unique physico-chemical property with different levels of adjuvant activities in drugs [27].  Both poloxamers and poloxamines have a variety of uses as the size (chain length) of the  oxyethylene (OE) and oxypropylene (OP) can be varied (Table 1.1) .

  Table 1.1 Uses of some poloxamers and poloxamines.

 Polymer

Average Molecular Weight (kDa)

OE

Blocks

OP

Blocks

Uses

     Poloxamers      

188

8.4

52 x 2

30

Antithrombotic, haemorheological activities,  phagocyte activation, and neutrophil degranulation

401

2.0

5 x 2

67

Inhibition of multidrug resistance and adjuvant  activities

407

12.6

98 x 2

67

Slow release gels and stimulating the production of  endothelial growth factor

     Poloxamines      

904

6.7

15 x 4

17 x 4

Nanoparticle engineering and macrophage stimulation

908

25.0

119 x 4

17 x 4

Long circulating particles and macrophage  stimulation

   Health and safety aspects include: classification as a primary irritant, a mild protoplasmic poison,  a central nervous system depressant and is thought to be carcinogenic in animals (lethal doses in rats by inhalation 4000mg/kg , ingestion 930 mg/kg) [4].

                    CHAPTER 2

 2.         DIFFERENTIAL SCANNING CALORIMETRY

 2.1.      INTRODUCTION

 Over the last three decades calorimetry has gained popularity as an instrumental technique, facilitating the understanding of energetic and thermodynamic properties of synthetic and natural (bio-) polymers.

 Heating of aqueous solutions of POP results in phase separation, however and interestingly POE does not phase separate below the boiling point of water, except in the presence of salts. Explanations presented to account for the molecular processes occurring during phase separation in aqueous polymer solutions have been reviewed by Almgren et al, [28]. One reason suggests that the temperature dependence of aqueous solubility of POP is on account of the molecules ability to be housed within an ice-like conformation [29]. It is worth noting that poly (oxyethylene) [POE] has been shown to be readily incorporated into such an ice-like structure [29]. In the case of POE, this structural conformation results in a favorable (exothermic) enthalpy change while at the same time resulting in an entropy penalty due to the increased structuring of water. At lower temperatures this enthalpy contribution along with the combined entropy contribution of the polymer chains and the free energy of mixing, outweighs the entropy penalty. However an increase in temperature reverses this equilibrium, resulting in phase separation.

 The pendant methyl group in POP causes a tension in the ice-like structure of water in the hydration sphere, consequently phase separation occurs at lower temperatures in relation to that observed in aqueous solutions of POE [29]. Finney and Soper [30] reported that by using neutron scattering, “there was no evidence for the ordering/ice-like structure of water molecules around non-polar methyl groups”, but this hypothesis is controversial. Furthermore, other research groups have inferred that the increasingly hydrophobic POE as a function of temperature results from the changing orientation of OE segments [31]. With reference to the backbone segments –O-C-C-O-, a trans-gauch-trans orientation about the bonds is preferred [31,32]. Polar conformational states such as these favor interaction with water, on average, there being two water molecules to each OE unit [32]. This is a state of low energy as well as of low probability, there being only two of these conformations [31]. At higher temperatures there is a preference for less polar orientations. These are of higher energy and higher probability, there being some 23 non-polar conformations.  

 Evidence has shown that less polar structures interact less favorably with water. Water loss due to higher temperatures allows OE chains to be in closer proximity to each other. This model allows successful explanation for phase separation of OE in aqueous solutions [31] and non-aqueous solvents [33]. Alterations in -C-C- bonds from gauche to trans leading to polarity changes, have been confirmed by C-NMR [33, 34]. POP phase separation seems to occur for the same reasons.

 Crowther and Eagland [35] have examined the rheology and density of aqueous POP solutions (MW 400) and concluded that POP solution behavior is dictated by hydrophobic interactions. Privalov [36] has postulated that protein denaturation in aqueous solutions is accompanied by an exposure of hydrophobic amino acid residues to the solvent, which results in an increase in ordered water structure. This effect is observed in HSDSC, as an increase in the heat capacity between the onset native state and the final unfolded protein conformation. The inference drawn is thus that in systems where the hydrophobic interactions become reduced a negative change in heat capacity should result.                

 2.1.2   High Sensitivity Differential Scanning Calorimetry

 In recent years several macroscopic analysis techniques have been introduced or modified to meet contemporary demands for the quantitative and qualitative analysis of chemical and physical phenomena in biological systems. Differential scanning calorimetry (DSC) is one such technique. It is used to measure the excess apparent specific heat of a system (DCp,ex) in a continuous manner as a function of temperature at a fixed scan rate of  e.g. 0.1 to 1 0C min-1. Biologically relevant macromolecular and polymolecular structures in their native state are stabilised by numerous weak forces and usually undergo conformational and/or phase transitional changes on heating and/or cooling in the easily accessible and pertinent temperature range of -10 to 105 0C (263.15 to 378.15 K).

Figure 2.1

An idealised DSC scan (C

p

vs.

T plot) indicating the parameters used to describe the data. The shaded area represents the enthalpy H (T).

 HSDSC provides information on the temperature dependence of the excess apparent specific heat over a wide temperature range (Cp vs. T curves, figure 2.1). This allows the determination of the thermodynamic functions for well defined molecular systems.  These include: DHcal-calorimetric enthalpy; DHVH-Van’t Hoff enthalpy; and DT1/2-width of the transition in 0C at ½ Cex,.max. Attempts can then be made to relate these macroscopic thermodynamic parameters to data on microscopic structural/conformational changes occurring in the sample. Alternatively, changes in thermodynamic properties of a series of structurally related molecules can be examined.

  The use of HSDSC has become more widespread because of:

·       commercial availability, in the last twenty years, of suitable DSC instruments at reasonable cost ($ 50,000-80,000).

·       increasing availability of high purity, well characterized analytes in sufficient quantities for DSC, and the ability to synthesize a series of related analogues or homologous molecules.

·       the development of more sophisticated methods of data analysis.

·       increasing realisation of the need for thermodynamic parameters of biologically relevant phenomena.

·       better theoretical knowledge of systems under investigation.

·       increase in high-resolution structural analysis using methods such as X-ray crystallography, NMR and neutron diffraction.

·       availability of other techniques such as high-sensitivity differential scanning densitometry which yield complementary macroscopic data on the system being investigated.

 These factors allow a more quantitative physicochemical interpretation of the relationship between thermodynamic parameters obtained by DSC and molecular phenomena.

 2.1.3    Instrumentation

           A HSDSC consists of two cells: the reference cell and the sample cell (see figure 2.2; Privalov, 1980).

   Figure 2.2 Schematic of HSDSC cell assembly.

The cells contain solvent (reference) or solvent plus the sample (sample) and are heated to raise their temperature across a given range. When a temperature-induced process takes place in the sample cell, the calorimeter supplies more or less electrical power (depending on whether the process is endothermic or exothermic) to the sample cell through appropriate heaters to maintain it at the same temperature as the reference cell. The calorimeter thus records the difference in electric power, which is proportional to the difference in the heat capacity of the sample and the reference cells, as a function of temperature.

Table 2.1

  DSC instruments vary in the way this basic principle is put into practice (Table 2.1). In HSDSC instruments, such as the MC-2, both cells are enclosed (suspended) in adiabatic shields. Thermopile systems detect temperature differences between the cells, and activate heating mechanisms to ensure that very little, if any, temperature difference exists inside the adiabatic assembly. One of the cells is equipped with an additional electrical heater for calibration. Sample and reference cells are filled by volume. HSDSC instruments are highly accurate and sensitive in estimating heat capacity (errors are <0.1%). This means they can be used to study samples in the dilute solution range (<10-4M sample) needed to avoid intermolecular interactions. High sensitivity is also necessary to ensure that small changes in the thermodynamic parameters and the shape of the excess apparent specific heat vs. temperature curves can be detected.

 In HSDSC instruments (such as the MC-2 used in this project) there is near identity of cell geometry, cell volume, rate of heating, physical properties (e.g. thermal conductivity) and environment. This ensures that the calorimetric output reflects only the thermally induced events taking place in the sample and not other parameters such as non-identical heating of the two cells, enthalpy of ionisation of buffers, etc. This and the adiabatic cell block arrangement and fixed volume measurement, contribute to a near linear, highly reproducible and stable baseline for both cells filled with solvent (Chowdhry & Cole, 1989).

HSDSC cannot be used to study small molecules unless they form aggregates showing intermolecular co-operation, as in crystals. Since the enthalpies of chemical processes are rarely as large as 20 calg-1, it is evident that molecules having molecular weights, or molecular aggregates, in the thousands of Daltons are required to give sufficiently sharp transitions for useful HSDSC observation.

 In a scanning calorimeter, one measures the specific heat of a system as a function of the temperature. For a solution, the apparent specific heat of the solute, C2, is given by the expression:

C2=C1+1/W2(C-C1)

where C is the specific heat of the solution, C1 is that of the solvent, and W2 is the weight fraction of the solute. Since the quantity C-C1 is usually relatively small, for example, approximately 0.7% of C1 for a 1% aqueous solution of a protein, it is essential to employ a differential scheme of measurement in which C-C1 is directly measured. This is accomplished in a differential scanning calorimeter by using two closely matched cells filled with equal weights or, more usually, with equal volumes of solution. When one considers that in general a significant, or even major, fraction of the total change in apparent specific enthalpy is due to the simple heating or cooling of the solvent, it becomes evident that the highest possible sensitivity and accuracy should be realised.

 2.1.4        Theoretical considerations

Interpretations of DSC data are usually based on the equilibrium thermodynamic expression:

                       (dlnK/dT)p= DHVH/RT2     (1)

where K is the equilibrium constant for the process under study, T is the absolute temperature, and DHVH is the apparent or Van’t Hoff enthalpy. It is immediately evident that any equilibrium process observed during an increase in temperature is necessarily endothermic and that any exothermic process so observed must involve rate limitation.

 DIFFERENTIAL SCANNING CALORIMETRY (DSC)

 Biological samples are usually small and dilute, so conventional calorimetric measurements have until recently required too much material and have lacked sensitivity. However, recent technical advances have led to the development of microcalorimeters, which can detect the very small amounts of heat generated or consumed by polymer transitions. Here differential scanning calorimeters are considered, in which processes such as polymer transitions can be studied as a function of temperature.  A schematic diagram of a differential scanning calorimeter is shown below.

  Heat is supplied at the same rate to two matched cells. The solution cell will generally absorb more heat than the buffer cell, causing a slight difference in temperature DT between the two cells. A feedback loop monitoring T will supply a small amount of heat DQ to the solution cell, so as to equalize the temperatures. The change in heat capacity (DCp) = DQ/DT.

 Determination of DH and DS from DSC

A schematic illustration of how DSC traces are integrated to obtain enthalpy and entropy:

Note how the same data (CP as a function of T) can be used to obtain both DH and DS, and thereby DG as well.

 As an example of  how DSC can be used in a biologically relevant situation, consider the figure below which represents the typical thermal denaturation of a protein.

  RELATION BETWEEN CALORIMETRIC AND VAN'T HOFF ENTHALPIES

The two ways to determine the enthalpy of a transition, DHvH from the temperature dependence of the equilibrium according to the van’t Hoff equation, and DHcal from a calorimetric measurement, will not generally yield the same result. The relation between the two depends on the cooperativity of the transition.

• If DHvH = DHcal, this indicates a two-state transition. However, it has been noted that many reactions that are thought to be two-state do not obey this equality, for reasons currently unknown.

• If DHvH < DHcal, some intermediate states are involved. The number n of "cooperative units" can be defined from the relation:

 DHcal/DHvH = n

 As a simple example, if a polymer had n independent, identical domains, each unfolding

independently, since the equilibrium transition for each domain would proceed independently of the others.

• If DHvH > DHcal, this generally indicates intermolecular interactions between molecules

which must be overcome to get from one state to the other.

Both DHvH and DHcal can be obtained from DSC, by using the following equation:

                                DHvH = RT2m (2 + 2n)[ da/dT]T = Tm

a can be viewed as the reaction progress variable (the fraction of reactant converted to product), which can be evaluated from the heat capacity curve, such that:

    so that:

  [ da/dT]T = Tm    

=

 and

 DHvH = (2n +2)RT2m

 where Cp,ex (Tm ) is the excess heat capacity at the half-way point of the transition

(approximately the maximum in the Cp vs T curve

 2.1.5     Materials

Reagents

All chemicals (Analar grade) were obtained from Aldrich Ltd, Dorset, UK and double de-ionized water was used throughout. The POP 1000 sample gave a single elution peak by gel permeation chromatography analysis and was used without further purification. Polymer solutions of POP in the concentration ranges (2.5 – 51.46 g dm-3) were prepared using doubly distilled/deionised water. These were then left to chill at 277 K for an hour in order to enhance dissolution prior to examination by HSDSC.

 2.1.6      HSDSC EXPERIMENTAL PROTOCOLS

 Calorimetric measurements made on the dispersions were carried out using a Microcal, MC-2D ultrasensitive differential scanning calorimeter (MicroCal Inc., Northampton, MA, USA) interfaced to an IBM 486/SX computer for instrumental control and data acquisition. Samples were run against the appropriate reference sample (H2O/D2O/metal ions or co-solvents e.g. GdnHCl, urea or methanol). Reference samples in both cells were used to obtain the baselines, which were subsequently subtracted from the sample scans. During scanning all samples were kept under an atmosphere of nitrogen to suppress bubble formation. Calorimetric data was obtained at scan rates of 10, 30 and 60 Kh-1. Unless otherwise stated data acquisition and analysis was carried out using the DA2 software package, supplied by the instrument manufacturer. Uncertainties arising due to multiple scans have been estimated for the measured and calculated thermodynamic parameters: T1/2 (± 0.2%), DHcal (± 3%), DHVH (± 3%), n (± 6%) and DCp (± 6%).

A typical protocol for HSDSC experiments is given in Figure 2.3.

Figure 2.3

      CHAPTER 3: POP RESULTS AND DISCUSSION

 3.1.1. HSDSC PROFILE FOR POP

This study set out to examine the thermally induced aggregation transitions of POP by HSDSC.

The HSDSC output for POP 1000 shown in figure 3.1 clearly shows an asymmetric leading edge indicative of aggregation with a gently sloping tail end. Tm is 32 ºC and the critical micelle temperature (CMT) or “onset transition temperature” is approximately 28.5 ºC.

                            Figure 3.1  HSDSC output for POP 1000 showing the CMT and the negative change in  excess s heat capacity  between initial and final  states (ΔCp,d values).

    It is particularly of interest to note that the difference in heat capacity between initial and final states (ΔCp,d) is negative indicating an increase in structured water in the hydration sphere around POP. This in turn favors increased aggregation in POP as well as increased water ordering. This is indeed the case as shown by the thermogram and is explained by the hydrophobic portions of POP molecules coming together in order to maximize their distance from the bulk water in solution.

POP samples were also scanned as a function of scan rate (10, 30, 60 K/h). The experiments yielded very good quality data, as shown in Fig 3.2a.

                   Figure 2a HSDSC signal acquired for POP 1000 at a concentration of 5g/dm-3. The main profile shows the scan rate independence of the transition (10, 30 and 60 K/h) while the right hand corner insert portrays the captured transition at a scan rate of 57 K hr-1.

  The insert seen on the right hand corner in Fig 3.2a denotes the scan rate normalized data for an aqueous solution of POP 1000 at a concentration of 5 mg/mL scanned at 57.9 K hr-1. The HSDSC output data is expressed in terms of power units (mcal s-1). Data normalization with regards to the scan rate leads to conversion of units to mcal K-1. By dividing the molar quantity of POP 1000 available in the sample cell we convert the data to molar heat capacity. Justification of the use of equilibrium thermodynamics in order to analyse data is only possible if it can be shown that the data are scan rate independent [49]. The main graph in Fig 3.2a illustrates that the acquired data for POP 1000 at a concentration of 5 mg ml-1 at the different scan rates used 12    are virtually super-imposable. At the higher temperature end a little fluctuation occurs in the superimposability which could be reflective of a real occurrence, or may be accounted for simply as a discrepancy in the method by which the calorimetric signal was manipulated in order to acquire the graph. A straight base line was placed by sight, using the software tools in the manufacturers DSC data analysis package (Microcal Software Inc. Northampton, MA, USA) to the pre-transition portion of the data and then subtracted from the signal as a whole. Secondly, of importance is the actual shape shown by the graph, characteristic of an aggregation transition whose thermodynamics may be explained using a mass action model of association [38].

HSDSC signals are dependent upon the concentration of POP (Fig. 3.2b). The figure clearly shows a direct correlation between increasing POP concentration and a corresponding decrease in the temperature values at which POP aggregation takes place. Previous studies have documented suitable methods for the quantitative analysis of calorimetric data resulting from such aggregation systems [38, 39]. It is important to note that the mechanism by which phase separation proceeds in aqueous solutions of POP is via nucleation and growth and furthermore that it is the nucleation phase which is observed by HSDSC.

        Figure 3.2b HSDSC signals acquired for POP solutions at various concentrations ranging from 2.5 to 51.5g/dm-3.

             MATHEMATICAL MODELLING

In brief, model fitting data encompasses the following: (1) based upon previously mentioned calorimetric data analysis techniques [39] (2) changes in enthalpy with respect to temperature for reactions stringently governed by thermodynamics is witnessed via HSDSC and given by:

   (1)

   dqp is the heat change at constant pressure; T is the temperature;  ΦCp,xs is the apparent excess heat capacity (i.e. the difference in heat capacity between reference and sample cells); α  is the extent of change in the system; ΔHcal (T1/2 ) denotes the experimentally determined enthalpy change (i.e. the integrated area of the HSDSC curve) at T1/2, the temperature at which α = 0.5 and ΔCp is the difference in heat capacity between the initial and final states of the system. Since we know that T1/2 is temperature independent and ΔHcal (T1/2) and assuming that ΔCp is independent in the system under investigation, then equation (1) above may be re-formulated as:

  (2)

 The degree of change to aggregates, α , for aqueous polymer systems investigated in this study is obtained from the temperature dependence of the equilibrium constant that describes the incorporation of single surfactant units in to micelles.

 (3)

  ΔHVH above represents the Van’t Hoff enthalpy. Ratios of the Van’t Hoff enthalpy to the calorimetric enthalpy give a measure of the magnitude of the co-operative unit involved in the process of micellization. Consequently the heat capacity change shown by equation {4} above, is scaled to show this co-operative effect in the system since the equilibrium constant mirrors those reactions involving the co-operative unit.

By integrating equation (3) we acquire K( T ) and this value can be used in the ensuing form of equation (4) to obtain a value for  α the fraction of surfactant in micellar form :

  (4)

  where C is total polymer concentration.

Assessing K at different temperatures allows an evaluation of the temperature dependence of ΦCp,ex in equation  (3). Sequential usage of the equations shown above allows a framework for model fitting of acquired HSDSC signals as well as for obtaining numerical values for the different thermodynamic parameters that occur in the above expressions.

Figure 3 gives an idea of the ability of the outlined model ability to adequately capture the main features occurring in the thermally induced transition. A worry in this study was delineating the temperature dependence of the calorimetric parameters. This is done by investigating solutions of differing POP (1000) concentration. HSDSC information shows unequivocally that the nucleation event is endothermic.

                            Figure 3.3

Results of fitting the acquired HSDSC data for a 5mg/dm

3

solution of POP 1000 to the thermodynamic model outlined in the text.

  Based upon the mass action description for nucleation given by equation (4), it is obvious that if the polymers aqueous solution concentration is increased then the degree of aggregation will itself increase. Furthermore since aggregation is endothermic then it suggests that the systems temperature will decrease. Consequently under equilibrium conditions it becomes a mandate for the temperature range over which aggregation transpires to drop as polymer concentration increases.

Appreciation of the assumptions made, allows us to be aware of possible limitations with the model fitting procedure. An important assumption is that n is constant, while it is clear that systems in which phase separation occurs by way of nucleation and growth, n must increase rapidly. Earlier mentioned was an argument for n representing the quantity of discrete reaction units that initially link forming the nucleation center and its subsequent growth occurring by the exchange of molecules between individual nucleation centers [40]. The concentration-dependent data acquired by fitting the calorimetric signals to the above equations are shown in Table3.1. There seem to be definite trends in the data with respect to changing concentration except for the parametric data acquired for the aggregation numbers n. The relative standard deviation (RSD) in n is approximately 6 %.The difference between the largest and smallest n values are about ± 6 % and thus it is reasonable to believe n is independent of polymer concentration. Conversely the variation in the other optimized parametric values relative to concentration is markedly greater than the RSD and must thus denote genuine functional relationships with concentration.

 Table 3.1 HSDSC parameters obtained for aqueous solutions of POP1000, as a function of POP concentration.

Concentration

(g dm-3)

DHcal

(kJ mol-1)

DHvH

(kJ mol-1)

n

T½

(K)

DCp

kJ mol-1 K-1

2.5

103

461

12.2

331.2

-13.3

5.0

111

556

12.0

324.6

-15.5

10.0

121

709

11.6

318.5

-15.4

30.6

138

953

11.9

310.5

-20.4

51.5

143

987

13.1

308.3

-25.9

 The graphical association between enthalpy values, T1/2 and concentration are represented in figures 3.4 and 3.5 respectively.  A log-linear association between T1/2 and concentration (R2 = 0.989) is clearly portrayed by the data. Excellent log-linear associations are also demonstrated between the Van’t Hoff enthalpy and concentration (R2 =0.988) and the calorimetric enthalpy and concentration (R2 =0.998).

         Figure 3.4. The concentration dependence of the optimized calorimetric enthalpy values obtained through the use of the theoretical model fitting procedure.

         Figure 3. 5.

The concentration dependence of the optimized Van’t Hoff enthalpy (ΔH

VH

) values using obtained using the theoretical model fitting procedure.

 It is a reasonable suggestion that these functional dependencies are based upon the relationship between the polymer concentration and temperature range over which aggregation occurs. This in turn leads to temperature dependence in the enthalpy parameters due to there being an observable heat capacity change between the initial and final states in the system. To test this hypothesis, Figures 3.6 and 3.7 reveal the functional relationships for ΔHcal (T1/2  ), ΔHVH (T1/2 ) and

ΔCp (T1/2 ) with T1/2 .The two groups of enthalpy data (figure 3.6) show excellent curvilinear association with temperature but the derived empirical expressions are not analogs of the physical processes involved and should not be extrapolated.

The following regression equations are derived from the graphs:

ΔHcal = 0.03 T2 – 22.8 T + 4041         R2 = 0.999

ΔHVH = 0.43 T2 – 297 T + 52030        R2 = 0.996

The heat capacity values portrayed in figure 3.7 fluctuate more; probably mirroring the higher level of uncertainty in determining this parameter. However the data can be reasonably fitted to the following linear relationship:

ΔCp = 0.48 T1/2 – 170                         R2 = 0.803

Heat capacity is the first derivative of enthalpy with respect to temperature. The ensuing relationship for heat capacity is derived if the Van’t Hoff enthalpy regression expression is differentiated:

  Since the slope and intercept values are of the same order of magnitude and are quite similar in value to the regression parameter this infers some agreement between the values captured for the heat capacities and Van’t Hoff enthalpies. However it is possible that a correlation such as this may just mirror some cross-correlation in the model fitting procedure. The observation that ΔCp seems to be dependent upon temperature poses a problem with respect to the model fitting procedure since the model assumes that ΔCp is temperature-independent. It should be appreciated that the model postulates that ΔHVH will vary with temperature.

The temperature dependency used in the model could cause large errors at the extremities of the temperature range over which the aggregation transition occurs, if it is accepted that ΔCp is temperature-dependent. The heat capacities temperature dependence should not however give

            Figure 3.6. The graphical relationship between the optimized T1/2 values and the Van’t Hoff (ΔHVH) and calorimetric enthalpy (ΔHcal) values acquired by using the model fitting procedure.

                   Figure 3.7. The relationship between the optimised values for T1/2 and the heat capacity change values acquired through the use of the linear model fitting procedure.

 large errors in the transitions mid range, where the apparent excess heat capacity values are large and consequently should exert a controlling effect on the optimization procedure. The important question to be asked is thus: can these regression parameters give a basis for predicting DSC output?

                  3.3 HSDSC STUDIES OF POP 1000 IN D2O

 The physico-chemical properties of D2O and H2O differ (see e.g. table 3.1).

 Table 3.1. The physical properties of H2O and D2O at 298.18 K.

     Property      

H2O

D2O

P/(g cm-3  )

0.99703

1.10436

Cp.m/ (J K-1 mol -1)

75.23

84.67

u/(m s-1)

1496.7

1399.2

ks/Tpa-1

447.7

462.5

kT / Tpa-1

452.5

464.4

Cv,m/(J K-1 mol-1)

74.44

84.42

α/Kk-1

0.25705

0.1722

 (Adapted from M.Nakamura and co-workers Thermochimica Acta: 253 (1995), p128)

 Calorimetric scans of POP in D2O and H2O are compared in Figure 3.1.

              Figure 3.1 The effects of D2O vs H20 on POP 1000 at a concentration of 5g/ml-1 and scan rate of 10K/h.    

    In fact D2O forms stronger hydrogen bond interactions than H2O. The differences in excess properties between H2O and D2O mixtures are reflective of the strengths of hydrogen bonds of H2O and D2O. Deuterium oxide is a heavier isotope of hydrogen and occurs naturally, accounting for 0.01 % of water. The Tm of POP in H2O occurs at 33.02 °C and that in D2O at 32.01 °C.

 3.4 HSDSC OF POP IN THE PRESENCE OF METAL IONS

Figure 3.4.1 shows the HSDSC scans of POP in the presence of NaCl and MgCl2. There is a significant decrease in the Tm of POP in the presence of 1 mol dm-3 NaCl and MgCl2. The POP has a Tm of 32.9°C in comparison to 1M MgCl2 at 21.7°C and 1M NaCl at 20.8°C. It can be seen that by adding a neutral salt such as NaCl, that is half way along the lyotropic (Hofmeister) series, a change in heat capacity (Cp) results. This phenomenon can also be seen for the magnesium chloride ion.

   The Hofmeister series is shown below:

 Anions:  SO42- > H2PO4- > CH3COO- > Cl- > Br- >I- > ClO4- > SCN-

 Cations: NH4+> Cs+ > K+ = Na+ > Li+ > Mg2+ = Ca2+ > Ba2+  

 Salt addition affects water structure. It is observable that with an increase in cationic radius there is a drop in Cp corresponding to a decrease in water structure. The more positively charged the ion is, the more it repels water molecules due to their positive protons. These water molecules have to rearrange themselves in a different space and so water becomes less ordered. This reorientation accounts for weaker H-bonds due to disruption of bulk water structure layering over the ions and thus requiring less energy to break the structure.

 Table 3.4.1 POP thermodynamic parameters as a consequence of metal ions and D2O.

    DHcal

(Kcal/M)

DHvH

(Kcal/M)

n

DThalf

(oC)

DCp

(Kcal/M/K)_

H20

128

864

11.54

312

-18.9

 NaCl

143

954

13.09

299.7

-24.4

 MgCl2

110

920

12

300.3

-20.2

 D2O

140

848

10.8

310.8

-20.2

                   Figure 3.4.1 The effects of sodium chloride and magnesium chloride ions on HSDSC scans of POP 1000 solutions at a concentration of 5g ml-1.

                 3.5                HSDSC OF POP IN THE PRESENCE OF UREA

 The effects of different concentrations of urea (H2N-CO-NH2) on the calorimetric output for POP are shown in figure 3.5.1., and the effects on the calorimetric parameters are shown in figure 3.5.2. It is immediately obvious that urea:

1)     increases the T1/2, the temperature at which the Cp is a maximum.

2)     DHcal decreases with increasing concentration of urea but at high urea concentrations (above ~4M), DHcal appears to remain constant or even increase.

3)     DHvH decrease with increasing concentration of urea.

4)     the value of n also seems to decrease as a function of urea concentration. However it is arguable as to whether or not this decrease is real because n only decreases from about 11.5 (at 0M urea) to 10.5 (8M urea).

5)     the difference in heat capacity (DCp,d) between the initial and final states becomes more positive

                Figure 3.5.1. HSDSC scans of POP (20mg/mL) as a function of different concentrations of urea (scan rate: 1oC/min).

                  Figure 3.5.2. Plots of calorimetric parameters vs concentration of urea.

                     3.6                HSDSC OF POP IN THE PRESENCE OF GUANIDINIUM HYDROCHLORIDE

  The effects of increasing concentrations of guanidinium hydrochloride on the calorimetric output for POP are shown in figure 3.5.3 and the graphical plots for the effects of urea as a function of the different calorimetric parameters are shown in figure 3.5.4.

                       Figure 3.5.3. HSDSC scans of POP (20mg/mL) in the presence of the indicated concentrations of GndHCl (M).

                 Figure 3.5.4. Plots of calorimetric parameters vs concentration of GdHCl (M).

 The overall effects of GdHCl on the calorimetric phase transition of POP are very similar, in terms of general trends, to those found for the POP-urea system. However it is to be noted that there is an “outlier” in some of the plots shown in figure 3.5.4. (C and D). The reasons for this are not known or understood. Nevertheless, the “results” were reproducible indicating that either something unusual is happening or the “effects” are the results of the data analysis methods employed.

 Urea and GdHCl  both denature proteins. Less work has been conducted on the effects of GDHCl on proteins compared to urea. Nevertheless it is important to note that both these molecules cause the calorimetric denaturation temperature of proteins to decrease and this is the opposite of what happens with POP. In fact it is interesting to note that very few studies of the effects of urea and GdHCl on polymer systems have been undertaken using HSDSC. This therefore is a potentially rich area of research.

 Urea and GdHCl should be included in the Hofmeister series. There have been countless experimental and theoretical studies of the effects of both of these molecules (often referred to as chaotropes) on proteins where they act as denaturants. Generally speaking chaotropic agents have been hypothesized to increase the solubility of nonpolar substances in water. Consequently, their effectiveness as denaturing agents stems from their ability to disrupt hydrophobic interactions. But the manner in which they do so is not understood or agreed.

Urea (and GdHCl) is highly soluble in water and the corresponding solution has some interesting features:

(1)   it increases the solubility of hydrocarbons in water,

(2)   forms transition metal complexes,

(3)   has nonlinear optical properties, and

(4)   denatures proteins.

These properties have been rationalised with different mutually exclusive models of the characterisation of urea hydration. In a first model, first proposed by Frank and Franks, urea acts as a structure-breaker which indirectly alters the water structure in the vicinity of the solutes. It has been used to explain the increased solubility of hydrocarbons in aqueous urea solutions. This model is supported in several experiments such as NMR, X-ray, Raman and infrared experiments. The second model, known as the Shellman, Kreschek, Scheraga, Stokes model, attributes urea’s aqueous properties to its ability to form dimers and oligomers via hydrogen bonds (H-bonds). The authors of this model assume that urea–urea interactions play an important role in solvation phenomena in aqueous urea solutions with the formation of urea dimers and oligomers. The validity of these two models has been investigated mainly by a number of molecular dynamics studies, but the existence of urea dimmers in water strongly depend on potential functions used in the calculation. Urea forms dimers and higher aggregates as demonstrated by osmotic pressure measurements. In opposition, the experimental work of Keuleers and co-workers as well as that of Lee and co-workers show non-existing intermolecular urea–urea H-bonds.

           Chapter 4.

Summary

n  HSDSC has been employed to acquire thermodynamic parameters associated with thermal aggregation in aqueous solutions of POP as a function of repeatability (reversibility), concentration and scan rate.

n  Aggregation transition temperature decreases with increasing concentration of POP.

n  Calorimetric parameters are independent of scan rate at a fixed concentration of POP. This justifies use of equilibrium thermodynamics for interrogation of calorimetric parameters.

n  The phase transition of POP has a sharp leading edge indicating aggregation, the calorimetric signal then tails off gradually.

n  The calorimetric output for POP in water is a completely reversible process and differences in the initial and final heat capacities are negative, indicating a loss of structured water from POP’s hydration sphere.

n  In the presence of D2O the aggregation transition temperature is decreased but the excess heat capacity (ΔCp,ex) is increased slightly yet the difference in heat capacity between the pre- and post-transitional baseline (ΔCp,d)remains negative and of the same value as in water.

n  The metal ions Na+ and Mg 2+decrease the aggregation transition temperature compared to that in water.

.

n  Increasing concentration of urea and guanidine hydrochloride increase the aggregation transition temperature, decrease ΔCp,ex and result in ΔCp,d values which become increasingly less negative compared to the POP in water.

 Future studies

The studies reported in this work could be expanded to include the following:

1)     a more comprehensive examination of the effects of salts in the Hofmeister series on the calorimetric transitions of POP. This might allow an insight into the mechanism of action of salts on POP. Moreover it would be interesting to examine other metal ions, including a range of univalent, divalent and trivalent metal ions with the same counterion and also keep the identity of the metal ion the same and change the counter ion e.g. sulphate. It might then be possible to draw more detailed conclusions on their effects on POP. In addition the foregoing studies would allow correlations to be made with the effects of metal ions on water molecules and possible correlations between calorimetric parameters for POP in the presence of the metal ions with physical properties of the metal ions/salts e.g. ionic radii etc.

2)     it would also be very useful to study the effects of  a homologous series of alcohols (e.g.

methanol, ethanol and propanol etc.,) on the calorimetric properties of POP at a fixed concentration of the polymer.

     3)   the effects of GuSCN and Gu2SO4 as well as derivatives of urea (e.g. methylurea) on

           POP would also be valuable. GuSCN is a much more potent denaturant of proteins than

           GdHCl whereas Gu2SO4 stabilizes proteins.  

4)     the effects of carbohydrates (mono and disaccharides) at different concentrations on POP would also be worth investigating. In fact there is quite a lot of data in the literature relating to osmolytes and they could all be investigated.

5)     calorimetric studies of the effects of urea and GdHCl on different polymer systems have not been widely reported, especially using calorimetric techniques. It would therefore be interesting to compare and contrast the properties of different synthetic water soluble polymers in the presence and absence of the two chaotropic molecules.  

                 REFERENCES

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 2.                  Garbassi, F and Morra, M and Occhiello, E (1998), Polymer Surfaces From Physics to Technology, (2nd Edn), John Wiley and Sons Publishers, pp 3-43

 3 & 4.           Kirk-Othmer, Ibid  reference 1, p 737

 5 & 6.           Hicks, J (1982) Comprehensive Chemistry, (3rd Edn); Macmillan Press, pp 651-653

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 11.                Colin, P Ibid.  reference 7, p

 12.                Skoulios, A E, Ibid. reference 7, p 84

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 15.                Schmolka,I R (1995) The Versatile Surfactants for Medical Investigations. Conference Paper, 2nd International Conference on Clinical Haemorology, Big Sky, Montana.

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  19.                Personal communication with Dr Stuart, B University of Greenwich Laboratories.

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 21.                Ibid reference 1, p 722

 22.                Schild, G H and Tirrell, D A, J.Phys.Chem. 1990: 94, 4352

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 24.                Ibid, reference 23: p650

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 45.                Chowdhry B Z and Snowden M J and Leharne S A (1999) Eur. Polymer J. 35, 273

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 74, 75, 76.    Ibid reference  80, p 273.

 77 & 78.       Ibid reference 81

 79.                Ibid reference 80.

 80.                Nakamura M, Tamura K and Murakami S,. Thermochimica Acta, 253, 1995, 127

81.                Ibid reference 70.

 82.                Privalov P and Makhatadze G I, (1993) J.Mol.Biol : 232

 83.                Hey M J, Ilett S M and Davidson G, (1995) J.Chem.Soc. Faraday Trans. 91: 3897

 84 & 85.       Kjellander R and Florin E, (1981) J.Chem.Soc. Faraday Trans. 77: 2053

 86.                Karlstrom G, (1985) J. Phys.Chem.  89: 4962

 87.                Direct communication with the University of Greenwich, courtesy of Dr Stuart B.

 88.                Chowdhry B Z and Cole S C, (1989),7 TIBTECH; January

 89.                Leharne S A and Chowdhry B Z, (1998) Biocalorimetry: applications of calorimetry in the biological sciences, Edited by Ladbury J E and Chowdhry B Z, 158, John Wiley and Sons Ltd.

 90.                Hergeth W D and Alig I et al, (1991) Chem. Macromol. Symp. 52, 289

 91.                Courtesy of Dr Stuart B direct communication

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 93.                Harned H S and Owen B B, (1958) “The Physical Chemistry of Electrolytic Solutions”, Chapters 3 and 12, Reinhold publishers  New -York Edsal J T and Wyman J,  “Biophysical Chemistry”, Chapters 5 and 6 Academic Press, New- York

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 96.                Armstrong J and O’Brien R et al, (1995) J.Phys. Chem. 99, 4590

 97.                Schott H J, (1973) Coll. Int. Sci. 43, 150

Schick M J, (1962) Coll. Int. Sci. 17, 801

Armstrong J and O’Brien et al, (1995) J. Phys. Chem. 99 4590  

  98.                Schild H G and Tirrell D A, (1991) J. Phy. Chem. 94, 4352 and  Armstrong K J, Chowdhry B Z, Snowden M J and Leharne S A, (1998) Langmuir 14, 2004

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PTFE Products and attributes

PTFE's mechanical properties are low compared to other plastics, and can be used over a wide temperature range of -100°F to +400°F (-73°C to 204°C). ). It has excellent thermal and electrical insulation properties and a low coefficient of friction. PTFE is very dense and cannot be processed by melting. PTFE must be compressed and sintered to form useful shapes.

PTFE Sheet, Rod, & Tube - Thermal stability PTFE is one of the most thermally stable plastic materials. There are no appreciable decompositions at 260°C, so that PTFE, at this temperature, still possesses most of its properties. Appreciable decomposition begins at over 400°C.

PTFE Transition points -The geometry of the PTFE molecules (crystalline structure) varies with the temperature. There are different transition points, with the most important ones being the following: that at 19°C corresponding to a modification of some physical properties and that at 327°C which corresponds to the disappearance of the crystalline structure: the PTFE assumes an amorphous aspect conserving its own geometric form.

PTFE Expansion -The linear thermal expansion coefficient varies with the temperature. In addition, because of the orientation caused by the working process, the PTFE pieces are generally anisotropic; in other words, the coefficient of expansion varies also in relation to direction.

PTFE Thermal conductivity -The coefficient of the thermal conductivity of PTFE does not varies with the temperature. It is relatively high, so that PTFE can be considered to be a good insulating material. The mixing of suitable fillers improves the thermal conductivity (see filled PTFE).

PTFE Specific heat -The specific heat, as well as the heat content (enthalpy) increases with the temperature.

PTFE Behaviour in presence of foreign agents

PTFE Resistance to chemical agents -PTFE is practically inert against known elements and compounds. It is attacked only by the alkaline metals in the elementary state, by Chlorine trifluoride and by elementary Fluorine at high temperatures and pressures.

PTFE Solvent resistance -PTFE is insoluble in almost all solvents at temperatures up to about 300°C. Fluorinated hydrocarbons cause a certain swelling which is however reversible; some highly fluorinated oils, at temperatures over 300°C, exercise a certain dissolving effect upon PTFE.

PTFE Resistance to atmospheric agents and light -Test pieces of PTFE, exposed for over twenty years to the most disparate climatic conditions, have not shown any alteration of their characteristic properties.

PTFE Resistance to radiations -High energy radiations tend to cause the breaking of the PTFE molecule, so that the resistance of the product to radiations is rather poor.

PTFE Gas permeability -The permeability of PTFE is similar to other plastic materials. The permeability does not depend, obviously, only on the thickness and pressure, but also on the working techniques.

Physical - mechanical properties

Tensile and compressive properties These properties are to a large degree influenced by the working processes and the employed powder. PTFE, however, can be used continuously at temperatures up to 260°C, while possessing still a certain compressive plasticity at temperatures near to the absolute Zero.

PTFE Flexibility -PTFE is quite flexible and does not break when subjected to stresses of 0,7 N/mm2 according to ASTM D 790. Flexural modulus is about 350 to 650 N/mm2 at room temperature, about 2000 N/mm2 at -80°C, about 200 N/mm2 at 100°C and about 45 N/mm2 at 260°C.

Impact properties -PTFE possesses very high resilience characteristics also at low temperatures.

Plastic memory -If a piece of PTFE is subjected to tensile or compression stresses below the yield point, part of the resulting deformations remain (as permanent deformations) after the discontinuance of the stresses, with the result that certain strains are induced in the piece. If the piece is reheated, these strains tend to release themselves within the piece which resumes its original form. This property of the PTFE is commonly indicated as "plastic memory" and is made use of in different applications.

Also most of the semi-finished products, because of the transformation processes, possesses similar strains, to a certain degree. When it is desired to obtain semi-finished parts dimensionally stable at high temperatures, it is possible to subject the parts to a temperature of 280°C for one hour every 6 mm of thickness and then cool them slowly. The parts obtained in this manner are almost completely free from internal strains and are in general known as "conditioned" or "thermostabilised" material.

Hardness -The hardness Shore D, measured according to the method ASTM D 2240, has values comprised between D50 and D60. According to DIN 53456 (load 13,5 Kg for 30 sec) the hardness sways between 27 and 32 N/mm2.

Friction -PTFE possesses the lowest friction coefficients of all solid materials; between 0.05 and 0.09:

* the static and dynamic friction coefficients are almost equal, so that there is no seizure or stick-slip action

* when increasing the load, the friction coefficient decreases until reaching a stable value

* the friction coefficient increases with the speed

* the friction coefficient remains constant at temperature variations.

Wear -The wear depends upon the condition of the other sliding surface and obviously depends upon the speed and loads. The wear is considerably reduced when adding suitable fillers to the PTFE (see filled PTFE).

Electrical properties

PTFE Insulation -PTFE is an excellent insulator and precious dielectric as shown by the relative data reported in datasheet and maintains these characteristics throughout a large range of environmental conditions, temperatures and frequencies.

Dielectric strength -The dielectric strength of PTFE varies with the thickness and decreases with increasing frequency. It remains practically constant up to 300°C and does not vary even after a prolonged treatment at high temperatures (6 months at 300°C). It depends also upon the transformation processes.

Dielectric constant and dissipation factor -PTFE has very low dielectric constant and dissipation factors values; these remain unvaried until 300°C, in a frequency field of up to 109 Hz even after a prolonged thermal treatment (6 months at 300°C). The dielectric constant, dissipation factor as well as the volume resistivity and surface resistivity, considered as being independent from the transformation processes.

Arc-resistance -PTFE has a good resistance to the arc. The arc resistance time according to ASTM D 495 is 700 sec..

After a prolonged action there are no signs of surface charing.

Corona effect resistance -The discharges caused by the corona effect may result in erosions of the PTFE surface which, nevertheless, is indicated as a suitable insulator in case of high potential differences.

Surface properties

The molecular configuration of PTFE brings to its surfaces a high anti-adhesiveness. For the same reason these surfaces are hardly wettable, the contact angle with water is about 110° and it is possible to affirm that, beyond a surface tension of 20 dine/cm, the liquid no longer wets the PTFE. A special etching treatment renders the surfaces bondable and wettable.

Source:https://www.sukoptfe.com/ptfe-products-and-attributes

[arxt1]

No Violation of the Second Law in Extended Black Hole Thermodynamics. (arXiv:1906.05870v1 [gr-qc])

Recently a number of papers have claimed that the horizon area - and thus the entropy - of near extremal black holes in anti-de Sitter spacetimes can be reduced by dropping particles into them. In this note we point out that this is a consequence of an underlying assumption that the energy of an infalling particle changes only the internal energy of the black hole, whereas a more physical assumption would be that it changes the enthalpy (mass). In fact, under the latter choice, the second law of extended black hole thermodynamics is no longer violated.

from gr-qc updates on arXiv.org http://bit.ly/2KndTb4

[arxt1]

Joule-Thomson expansion of charged AdS black holes in Rainbow gravity. (arXiv:1905.03057v1 [hep-th])

In this letter we investigate the throttling process of the charged Anti-de Sitter (AdS) black holes in the rainbow gravity. In the extended phase space of these black holes, the cosmological constant plays the role of a varying thermodynamic pressure and the black hole mass is identified with the thermodynamic enthalpy. We derive exact expressions for the Joule-Thomson coefficient and the inversion temperature in terms of black hole parameters and constants of rainbow gravity analytically, and then perform a numerical analysis for the isenthalpic and inversion curves of charged AdS black holes. We will consider the isenthalpic curves for different values of the black hole mass.

from gr-qc updates on arXiv.org http://bit.ly/2VQB7fb

[arxt1]

Electron and proton heating in trans-relativistic magnetic reconnection. (arXiv:1708.04627v1 [astro-ph.HE])

The coronae of collisionless accretion flows, such as Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here, protons are non-relativistic while electrons can be ultra-relativistic. By means of 2D PIC simulations, we study electron and proton heating in the outflows of trans-relativistic ($\sigma_w$~0.1, where the magnetization $\sigma_w$ is the ratio of magnetic energy density to enthalpy density) anti-parallel reconnection. We explore the dependence of the heating efficiency on mass ratio (up to the realistic value), magnetization $\sigma_w$, proton plasma $\beta_i$ (the ratio of proton thermal pressure to magnetic pressure), and electron-to-proton temperature ratio $T_e/T_i$. For both electrons and protons, heating at high $\beta_i$ is dominated by adiabatic compression (adiabatic heating), while at low $\beta_i$ it is accompanied by a genuine increase in entropy (irreversible heating). For our fiducial $\sigma_w=0.1$, we find that at $\beta_i<1$ the irreversible heating efficiency is nearly independent of $T_e/T_i$ (which we vary from 0.1 up to 1). If $T_e/T_i=1$, the fraction of inflowing magnetic energy converted to electron irreversible heating decreases from ~0.016 down to ~0.002 as $\beta_i$ ranges from ~0.01 up to ~0.5, but then it increases up to ~0.03 as $\beta_i$ approaches ~2. Protons are heated more efficiently than electrons at low and moderate $\beta_i$ (by a factor of ~7), whereas the electron and proton heating efficiencies become comparable at $beta_i$~2 if $T_e/T_i=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection, when the magnetization exceeds unity. Our results have important implications for the two-temperature nature of collisionless accretion flows, and may provide the sub-grid physics needed in general relativistic MHD simulations.

from astro-ph.HE updates on arXiv.org http://ift.tt/2uKllWu