Friday, January 30, 2015

Molecular weight, SEC and the US Supreme Court

So a dozen people walk up to the bar and start discussing molecular weight distributions and size-exclusion chromatography (SEC). Let's listen in to what some of them had to say:
"MJ:And as the district court found...the presumed meaning of that term "average molecular weight" when you when you're using this technology is peak average molecular weight. And the there are other technologies such as osmometry and light scattering that give rise to a different presumed meaning of what average molecular weight is, because they produce different measurements. But the only kind of peak average that you can read from the chromatogram is peak average molecular weight. And the Federal Circuit went right by the finding that the presumed meaning would be peak average molecular weight and gave essentially treated the three possibilities, peak, number and weight average, as though they were equal."

MA:"...if you use SEC, then peak molecular weight is produced and that you'd need further calculations to do other things."

JK: "You know, is molecular weight usually measured in kilodaltons or something else."
And who are these twelve people? Why the nine US Supreme Court Justices and the three attorney arguing in front of them.

It's plenty surprising that molecular weight distributions and SEC would be argued about in the Supreme Court, and what is even more surprising, is that they never misspoke about the subject (compare this to previous technical discussions such as the crack smoking poodle, or 2 + 2 is somewhere between 3 and 5, right?). So how did this come to be?

Teva Pharmaceuticals makes the drug Copaxone, used to treat multiple sclerosis. The patent claims from Teva all refer to a copolymer made from amino acids having a molecular weight between X and Y. Sandoz thought the patent claims for this drug were too vague (did they refer to peak molecular weight, number average molecular weight or weight-average molecular weight?) and therefore invalid. Sandoz started making their own version of the drug and of course were sued. Teva won in the district court after expert witnesses convinced the jury that the claims were not too vague. Sandoz appealed to the Court of Appeals for the Federal Circuit (CAFC) and won. But that was in part because the CAFC made a de novo review of the patent claims - they ignored what the jury found and reviewed the matter as if the jury had never decided the issue at all. Appeals courts generally do NOT function like that, but the CAFC does it very frequently. And so Teva appealed to the Supreme Court, arguing that the de novo review should not have happened.

The Supreme Court agreed. No more de novo reviews for the CAFC. The case has now been returned to them to be retried, but without the option of a de novo review. While Sandoz may still prevail, the odds have suddenly gotten a lot tougher. This was a big win for Teva, even though the fight is still not over. It's just that the rules have been changed mid-game in their favor.

This was all argued back in October, and since it appeared that it would be mostly about a legal issue (de novo reviews), I never bothered to follow it. Who knew that it would over its course get into a discussion about molecular weight measurements? Maybe I can get to see them argue of glass transition temperatures someday. Now that would be worth making the trip to DC just to watch.

Previous Years

January 30, 2014 - Does "Literally" Really Mean What You Think It Does?

January 30, 2013 - Clamshell Packaging Takes a Knock

January 30, 2012 - 3-D Printing, Medical Organs and Piracy

January 30, 2009 - Boron Boride

January 30, 2009 - Buckytubes for All!!

Monday, January 26, 2015

King Tut and BPA

King Tut was in the news last week, or more correctly, his burial mask was (although I bet most people don't really know the difference). The breaking news is over a polymeric material and the incompetence of some conservators to do the right thing. Specifically, it is reported that his iconic beard broke off, and was hastily repaired with an epoxy.
"Three of the museum's conservators reached by telephone gave differing accounts of when the incident occurred last year, and whether the beard was knocked off by accident while the mask's case was being cleaned, or was removed because it was loose. They agree however that orders came from above to fix it quickly and that an inappropriate adhesive was used. All spoke on condition of anonymity for fear of professional reprisals. 'Unfortunately he used a very irreversible material — epoxy has a very high property for attaching and is used on metal or stone but I think it wasn't suitable for an outstanding object like Tutankhamun's golden mask,' one conservator said."
But it gets worse. The three conservators really made a mess of things, so much so that you have to wonder if they were possibly named Larry, Moe and Curly.
"The conservator said that the mask now shows a gap between the face and the beard, whereas before it was directly attached: 'Now you can see a layer of transparent yellow.' Another museum conservator, who was present at the time of the repair, said that epoxy had dried on the face of the boy king's mask and that a colleague used a spatula to remove it, leaving scratches."
Take a look at this picture and see the damage yourself:
That is one bad glue job on ol' King Tut
That is an awful patch job - it looks like something I would do puttering around in the garage on the weekend. And that would include getting excess adhesive all over the place. Look closely and you can also see freshly scratched gold in numerous areas of the chin where the excess was removed. King Tut's burial mask, damaged for all eternity.

Fortunately, the chemophobic missed out on a big story here regarding the chemistry of epoxy adhesives. They are usually two-part adhesives, with part A having molecules with multiple, terminal epoxide rings, and part B being a hydrogen-donating molecule, such an amine. Mixed together, the two parts react to form a crosslinked material - "irreversible" to quote one of the conservators. For part A, the most commonly used compound is DGEBA, an abbreviation for diglycidyl ester of bisphenol A. Yes, bisphenol A, that chemical that the chemophobic so love to fear for causing everything awful to our bodies and our children's bodies. While we cannot be certain at this point what the chemistry of the epoxy was, the most likely scenario is that it did contain DGEBA so that now King Tut is being exposed to BPA for his eternal afterlife.

To which I can only say:
King Tut  - exposed to BPA

Previous Years

January 26, 2012 - Viewing History through an Oil Refinery

January 26, 2011 - My Most Embarrassing Moment at Work

January 26, 2010 - Phosgene Death

January 26, 2009 - More on Dow and Rohm & Haas

January 26, 2009 - Biodegradation of Polymers

Friday, January 23, 2015

The Dastardly Death Ray vs. The Snowman

Please forgive my absence in posting this week. My thoughts have been elsewhere - on
I've written in the past about the Death Ray that I possess. It is the natural result of sunlight reflecting off the thin, curved window glass on the south side of my house. The curvature is enough to concentrate the sunlight so that it can burn the lawn and melt snow. Egged on by a comment from Chemjobber, I decided to put the Death Ray to the test and use it to sever a Snowman.

Once the world sees the destructive force of my Death Ray, it will bow before me and I will be unstoppable!

Conditions were ideal this last weekend for the construction of my victim - warm and cloudy. I had planned for weeks the exact location. My plan was perfect! The snowman was in the ideal location, now all I needed was sunshine. And that came yesterday. Here is the setup from yesterday morning before the sun was high enough to cause havoc:
The house faces due south. In just a few hours, the sunlight would bounce off the windows, becoming concentrated and unleash it full photonic fury upon the snowman.

Just before noon, the first of the caustics crossed the snowman's body. Goodbye Mr. Frosty:
But alas! My plan was a complete failure. The Death Ray barely affected the snowman at all! In fact, the sunshine that fell on the back of the snowman was far more destructive, creating the extensive cratering that is all so common on sunstruck snow. Oh cruel world!. My hopes for world domination are dashed. And worse yet, I now have a snowman right outside my kitchen window that each day will mock me. Instead of having results worthy of publication in both Science and Nature [*], I now can only publish them in a third-rate blog. Oh, why have the Fates seen fit to punish me so?

(The explanation for my failure is fairly simple. The Death Ray is able to cut tracks in the snow field (see the photos in the 2nd link above) while the sun can't because the grade of the yard in that area is away from the sun and towards the house. For Frosty, the slope of his back was facing nearly directly at the sun. Additionally, the sun was able to shine on Frosty for a number of continuous hours, while the caustic crossed the snowman in just a few minutes.)

And so it is back to the drawing board, but that is how science and engineering often is. Now matter how appealing a hypothesis is ("I can sever a snowman with the Death Ray"), we need to run the experiment to prove it, and in this case, face the difficult fact that the Death Ray cannot sever the snowman's body (or even make a noticeable dent). I'm glad I tried, but I think this will be the last time I write about the Death Ray. I'll get back to more normal topics next week.

[*] With a fully functional Death Ray, who was going to stop me? Certainly not Reviewer #3

Previous Years

January 23, 2014 - Dow Chemical Under Attack From Its Own Investors

Friday, January 16, 2015


As much as I would like to brag, or say "I told you so", I really don't feel that I'm prescient in the least. Any scientist with a little appreciation for their field would instantly recognize that when somebody says "Chemical X is poisonous! We need to stop using it!", the immediate follow-up questions should be:
  • OK, so what should we replace it with?
  • Why do you think the replacement is any better?
And yet that is what we face with the chemical bisphenol A, also known as BPA. It is the condensation product of 2 moles of phenol (hence the bisphenol) and 1 mole of acetone (hence the A). It is used to make the polycarbonate that is used for making Blu-ray/DVD/CD discs (other polycarbonate do exist as well). It is also used for making epoxies, some of which are used as adhesives, but others of which are used for lining steel cans to prevent their rusting.

Once the BPA has reacted and formed the polymer, there is very little reason to be concerned about it. The polymer is good and stable. The concern over BPA arises from unreacted BPA in the polymer, not from the reacted BPA (although chemophobes will try and convince you otherwise). As much as the FDA and regulatory bodies around the world have looked at the available data and said that there is no reason for concern (see the link above), some people are concerned and want to avoid any contact with BPA.

One popular suggestion as an alternative is BPS, made by reacting two moles of phenol with one mole of sulfuric acid (hence the "S"). (If you are curious, there is a whole alphabet of bisphenols out there, even a BPZ.) But now we have a report that in zebrafish embryos, BPS appears to causing at least as much hormonal damage as BPA. While I don't ever want to create problems for any animal, let alone zebrafish, for us humans, well, we have to wait and see if the effects of BPS are the same in humans.

While the biological aspects of the research are beyond my understanding, the interpretation and conclusions are not. The researchers contradict themselves and don't even see it. Near the end of the article they state
"...manufacturers have turned to BPS with little proper toxicology testing to produce the “BPA-free” products demanded by society..."
and yet the authors have no problem with proposing their own open-ended experiment:
"These a societal push to remove all structurally similar bisphenol analogues..."
And the alternative compounds (that of course have been "properly" tested) are...?

That last statement quoted above is shockingly overreaching just by itself. So because BPS didn't work out in one (unreplicated) study on fish (and not humans), we need to toss out all bisphenol compounds, even the ones that are safe (such as BPA)? Wow. What an overstatement. Could the researchers take themselves and their results even more seriously? It's only January 16th, but I'm ready to predict that this quote will easily win the "Overstatement of the Year 2015 " Award.

Previous Years

January 16, 2014 - Recycling Plastics for the Do-It-Yourselfer

January 16, 2013 - Does Going Public Kill Innovation?

January 16, 2012 - Flying Cupcake Update

January 16, 2009 - The Pitch Drop Experiment

Thursday, January 15, 2015

Falling Oil Prices and ExxonMobil

I've been preaching for a while about how ExxonMobil is incorrectly blamed for any and all possible problems around the world that can be connected to our modern petroleum-consuming economies. While ExxonMobil may indeed be at fault for many of these accused problems either directly or indirectly, I again want to emphasize that they are a small fish in a large ocean.

Look at the recent price drop in petroleum around the world. ExxonMobil has nothing to do with this, and in fact, the low prices are hurting them. Revenues are dropping, investments are dropping and their stock price is dropping. Is anyone saying ExxonMobil is responsible for the low prices? Or si the blame being put on Saudi Arabia, whose National Oil Company, Saudi Aramco, has the largest reserves in the world, and who isn't cutting production?

People being people, ExxonMobil will again be vilified when prices do increase and their profits follow. No one will blame Saudi Aramco, and our profound, collective ignorance of the true dynamics of the petroleum industry will continue.

Previous Years

January 15, 2013 - Assault with a Deadly Hair Removal Gel

Wednesday, January 14, 2015

Monkey see, monkey do

Back in November, Dow Chemical was able to reach a truce with their "activist investor" Daniel Loeb of Third Point LLC., by letting Third Point put two of their people on the Dow Chemical Board of Directors. (In a rather intelligent move, Dow Chemical also added 2 additional directors so there are now 14 total. Had Loeb won his proxy fight, his 2 directors would have been 2/10 = 20% of the board. Now they are 2/14 = 14%. It may have been awhile since Dow's CEO was at the bench, but he certainly understands dilution to this day.)

The other large US-based chemical company whose name also starts with with the letter "D" is now in a similar fight with an activist investor, in this case Nelson Peltz and his company Trian Investors. No doubt emboldened by Loeb's (dubious) success, guess what Peltz wants? You got it - he wants to put directors on Dupont's board! Peltz obviously has a little more time on his hands than Loeb does, as Peltz wants to be one of the directors, while Loeb was quite content to let his cohorts have that role.

Dupont's Board is already 14 members and Peltz is running a slate of four candidates. If they all win, they would make up almost 4/14 = 29% of the Board. Or maybe Dupont can copy what Dow did and let Peltz put two on the board while Dupont adds two more, in which case Peltz will only control 2/18 = 11% of the Board. That would be a significant swing.

I've not followed Dupont's business closely, so they may or may not be deserving of a kick in the pants to wake them up, but activist investors never sit well with me. They strike me as too much grandstanding and publicity seeking and not having enough focus on rational thought. Such behavior is typical for politicians. I don't approve of it from either group.

Previous Years

Tuesday, January 13, 2015

Kinetics, Thermodynamics and Polymer Phase Transitions

Polymers appear to break many rules of thermodynamics on a regular basis, when in actuality nothing of the sort occurs. I bring this up after reading an article in the Guardian yesterday about glass transitions in polymers that has too many errors to just let slide. I don't think that these errors are acceptable. While explaining the glass transition of polymers can run you afoul of numerous difficulties, The Wall Street Journal did an admirable job a few years back, so I don't think my standards are too high.

Consider this excerpt, which really is logically incoherent:
"Every polymer locks up its own particular temperature, known as the glass transition temperature, usually denoted Tg. This is the temperature at which the chains can no longer move fast enough to respond to any external forces to allow deformation to occur. If you hit the polymer fast with a hammer this means that a higher glass transition temperature is recorded than if you slowly pull on it."
If that last sentence came to you as out of nowhere, or maybe better, as a sudden blow to the head with a hammer, you're not alone. It's quite a non sequitur The discussion starts on Tg as a temperature, but then a hammer enters the picture and that changes everything about that temperature? And how is that change in temperature "recorded" when the hammer comes down? This is horribly unclear but I understand where the author wants to go.

A polymer can act glassy in two entirely different manners. First, it will behave as a glass when it is cooled below its glass transition temperature. But it can also act glassy (and the emphasis here in on "act") even if it is above its Tg when too great a stress is applied too quickly. In this latter case, the polymer strands do not have the time to relax and flow past each other and so they have to stay in place and take the stress without moving - just like a glass. I'm guessing that that is where the "hammer" comes from - a short, sharp blow from a hammer can shatter some polymers like glass. But to suggest that this glasslike behavior "changes" the Tg is entirely incorrect. And it contradicts the first sentence - "Every polymer locks its own particular temperature...."

Then consider this:
"The glass transition temperature is quite different: its value depends on how you measure it. This makes it something of a thermodynamic oddity; it isn’t a true phase transition at all. "
Oh boy, them's fighting words [*]. The glass transition temperature is a true phase transition, but it is a 2nd-order phase transition, not a first order phase transition such as the boiling of water that the author compares it to.

Measuring any phase transition (1st- or 2nd-order) in a polymer takes patience, at least if you want to measure a true equilibrium value. Ideally you need to cool down the sample very slowly so that the polymer chains can relax any internal stresses and reach a true equilibrium configuration. But the cooler you go, the longer it takes since the viscosity of the polymers increase rapidly. And so compromises between cooling speed and data accuracy need to be made. By looking at data from multiple tests, it is possible to extrapolate an equilibrium value which is why we can have tables of glass transition temperatures. Extrapolations like this a very common in science. Think about absolute zero. We've never been able to cool a sample to 0 K, but we still know that the temperature exists and we can extrapolate numerous thermodynamic data to it as well. And so it is with the glass transition temperature.

I stated that patience is need for measuring any phase transition in polymer. For even a 1st-order phase transition such as melting, the values you record will depend on the heating rate. Because polymers are such poor thermal conductors, measuring the melting transition at higher rates would increase the "thermal lag" between the transition and the applied temperature. Measurements are typically made at 10 oC per minute, a completely arbitrary value (since the rate has dimensions, it is arbitrary) that is a compromise between accuracy and measurement rate. If that rate is changed, the temperature of the transition would be reported differently. To requote the article, "...its value depends on how you measure it". No, the values don't vary in this case either. It is an experimental error, nothing more, and results from our impatience in waiting for equilibrium data.

You may have noticed the two main points here both rely on the balance between kinetics and thermodynamics.
  • Because of the sometimes slow kinetics polymer have in relaxing internal stresses, they can be glasslike in appearance. This can appear at conditions well removed from the equilibrium glass transition temperature, but it does not mean that the glass transition temperature has changed.
  • Because of the slow rate at which polymers conduct heat, measuring any phase transition for a polymer is a compromise between getting the data at a reasonable rate and the accuracy of the data.

[*] Three times in my professional life I have witness arguments get so heated that I thought it would break out into a barehanded brawl. Two of those arguments were over the glass transition and statements just like this.

Previous Years

Monday, January 12, 2015

Much about Nothing; OR The FDA clears BPA - again

Back November while I was distracted by other matters, the FDA once again found that the current uses of BPA in food packaging are safe:
"FDA’s current perspective, based on its most recent safety assessment, is that BPA is safe at the current levels occurring in foods. Based on FDA’s ongoing safety review of scientific evidence, the available information continues to support the safety of BPA for the currently approved uses in food containers and packaging."
This will not end the debate, but my concerns are not so much on the safety of BPA, but rather on what the alternative will be.

And that is a point that BPA's critics are deafly silent on.

BPA has been used for over 50 years. There are still ongoing investigations about about it and an never ending debate about what risks are acceptable, there are even a greater number of unanswered questions about all of the alternatives, since none of them have been used for anywhere near that time period. Further, the regulatory reality would also be that if BPA is banned and the new alternative is found to be just as bad or even worse than BPA, there will be no going back to BPA, since afterall, it was banned. Imagine the outcry against the agency proposing to un-ban a previously banned substance. Senators Blather, Bluster and Bellyacher would immediately be on TV calling for hearings (the speakers at such hearings would of course largely be movie stars, sports figures and musicians since they have expert opinions on all matters of importance), and Representatives Rep, Publi and Can would be calling to impeach the President (unless it was a Republican president, in which case it would be Representatives Demi and Crat calling for impeachment).

Also keep in mind that this view on the safety of BPA is the same one taken by regulatory agencies in Canada, Europe and elsewhere. It's not just the FDA that is "in the pocket" of the canning manufacturers - that evil cabal has their fingers everywhere.

Previous Years

January 12, 2011 - My Polymer Chemistry Hero

Janauary 12, 2009 - Connections (with apologies to James Burke)

Thursday, January 08, 2015

Getting past the dreaded Pay-Per-View firewall

Now that I'm without great access to technical literature, I get to brush up on my skills for getting articles that are normally behind a pay-per-view firewall. I can't get that done on my good looks alone, so I have to be creative.

This morning, for instance, I had a request from someone looking for help with dissolving polyethylene oxide (PEO) in ethanol. I've dissolved it in water plenty of times, but that can actually be difficult because PEO is readily soluble in water at room temperature. Unless you are adding at an extremely slow rate, it will clump up and form "snots" that will never disperse. But if you adjust the temperature, either by adding ice to the water, or by boiling the water (actually 90 oC is hot enough), you can dump the PEO pretty quickly. Then let it warm/cool to room temp and away you go. The ice slows down the dissolution rate which prevents clumping, while the hot water is actually not capable of dissolving the PEO - it is merely getting dispersed quite well - so that upon cooling you have a clump free solution.

But back to the literature. I had no idea if the same tricks could work with ethanol, so I googled "PEO ethanol solubility" and this was at the top of the results. From the abstract, the paper looked spot on, but as expected, it was behind the dreaded pay-per-view firewall.

But I didn't let that stop me. I copied the article title into the Google Scholar search bar and - oh happy day - saw this:
A Google Scholar search result that make me happy
Hoop-yeah! A .pdf version is available. That was both quick and easy.

Why go through a firewall when you can quickly do an end run?

I passed all of this onto the requester in short order. And in case you are curious, it appears that the heat dissolution technique will work in ethanol, but when the solution cools the polymer starts to crystallize out creating a paste. That's all the more I'll say at this point, in order to respect the privacy of the requester.

(All of this reminds me that I need to update the "Free Access to Research in the Polymer Literature" page that you can access on the left hand side of the blog.)

Previous Years

January 8, 2010 - Time to Close Up Shop

Tuesday, January 06, 2015

Weeding-out Engineering Students

Over the recent holiday break, a discussion about "weed-out" courses was hashed out by fellow bloggers Chemjobber, "Just Like Cooking" and Thoreau. So while I may be late to the party, as long as there are incoming freshmen, there will always be "weed-out" classes and so these comments should be just as timeless.

The biggest difference in my discussion however, is that rather than focusing on educating future chemists, I'm going to speak to educating future chemical engineers, and there are some pretty big differences. Engineering classes typically don't start until the end of the sophomore year or early junior year. Junior year is pretty late in one's career to face a weed-out class and risk of changing one's major, but that is how it's still done. Keep in mind that at this point, the students have already survived the "weed-out" freshman chemistry class and now have to face their introductory "Mass and Energy Balances" class or something equivalent.

For those unfamiliar with the subject, it is mostly a matter of bookkeeping. You have a piece of equipment that has various mass and energy flows going in and out. The one and only equation you need to know is
"Accumulation = Inflow - Outflow + Generation - Destruction"
You set up some linear equations and solve for the unknowns. There probably are some unit conversions too, but this is all just a matter of keeping the details straight. If a student can't handle this, they are really going to lose it when it comes to doing the same in fluid mechanics. In that subject, they have run similar balances with 3-dimensional, multivariable calculus.

When I was a TA, I never once was told by the prof that we had to cut a certain number of people or reduce the size of the class or hold the students to some arbitrarily high and/or capricious standard(s). It always seemed to me that students self-selected changing their major. And blaming it on a bad prof isn't too likely, as this initial stuff was pretty straightforward and intuitive. It's no different than balancing a reaction. Having a rotten prof for fluid mechanics and heat transfer is an entirely different matter. [*]

This all reminds me of the one episode where the prof in fact actually bent over backwards to pass a student. He was a senior who never did any homework and only came to class for the exams. I personally don't have a problem with such an approach IF the student still does well. But that wasn't case here. The student always had the lowest grade on exams. So come the end of the year, I recommended an F for him. Imagine my surprise to see that shot down. He got a D. The prof justified it by saying the school would be done with the student. He would graduate and never be on this campus or any other again. A Gentleman's D.

[*] I know. Been there, done that. And am still irate at the profs all these years later. And it wasn't just me. The Class of 1984 from Minnesota is the "Lost Class", the one that never provides news items to the Alumni newsletter or donates money. 30 years later and the anger is still there. That's how much a bad prof or two can impact hundreds of students.

Previous Years

January 6, 2014 - How Cold Is It???

January 6, 2012 - Free Access to Articles in the Polymer Literature

January 6, 2010 - Polycarbonate (and BPA and Phosgene)

Monday, January 05, 2015

Time's "Person of the Year" - and Plastics

Plastics are critical to the fight against Ebola
Over the past couple of holiday weeks, Time Magazine came out with their "Person of the Year" issue. The cover, seen on the right, was all over the newsstands when I was shopping. What hit me the most was that the photo barely showed the person underneath - only their eyes and little bit of skin around the eyes were visible. The rest of the person - covered in plastic.

I've made the point before (1, 2) of how much worse this outbreak would have been without plastics, single-use plastics at that, the favorite whipping boy of anti-plastic environmentalists. I seriously question if many of the people reading this would be alive (including me [*]) and whether the world economy would have collapsed once every nation shut their borders if we didn't have such plastics.

Or perhaps the question is a non-starter, since a world without plastics would lack the ease of travel which leads to the ease of spreading the disease. But in the same vein, I wonder how many lives could have been saved during the Plague had plastics been available. It wouldn't have prevented them all, since germ theory wasn't well understood then, but still, there were enough basic public health practices that plastics could have improved the situation some.

None of this is to take away from the people that still have to don and doff the gear (doffing being the risky part) and they deserve out greatest respect. But there still needs to be some recognition that plastics play a critical role in the fight too. Plastics by themselves won't treat the ill or prevent the spread, the medical professionals need to attend to that, but without plastics, their jobs would go from being hazardous to suicidal.

[*] One of the largest Liberian communities in the US is here in the Twin Cities and many of them have traveled back there during the outbreak, so this city and state were following the outbreak with greater concern than other areas. Further, my father volunteers as an English teacher and many of his students are Liberian immigrants. There's your six degrees of separation.

Previous Years

January 5, 2011 - ANTEC Bound

January 5, 2010 - Amusing Names for Rheology Models