Friday, September 28, 2012

The Largest Molecule

This is a bit of a puzzler that I don't have the answer to. I used to have the answer (or I thought I did), but not anymore. The question is this: what is the largest molecule? (By molecule, I am referring to something that has covalent bonds between the atoms. Metals and salts don't qualify.)

If you've never thought about this before, you might be thinking about biochemicals, such as various proteins or DNA. As far as the molecules that most chemists work with go, they certainly are very large, but they are positively microscopic to what follows.

As a polymer person, I always like to shock people and suggest some kind of everyday object like a bowling ball. These types of objects are made from large polymers chains that have been crosslinked together with covalent bonds. If crosslinked enough, all the polymer chains become part of the same network. Maxwell's Demon would be able to start at one atom and hop along the bonds to reach any other atom without having to jump to another molecule.

I used to think the largest molecule was a bowling ball, but given the core/shell construction used nowadays, I'm not so sure. Are outer and inner parts covalently bonded? I kinda doubt it. Another common suggestion are tires. The problem with that suggestion however, is that a tire is made up of more than just rubber. In car tires, there are large quantities of steel or other cords in the belts and also the rim bead that are not chemically bonded to the rubber. Also the carbon black isn't even "bonded" to the rubber, only adsorbed.

So what is the largest molecule? You could argue that just the vulcanized rubber in a tire is one large molecule (I can't disagree) and that therefore the largest tires would have the largest molecules as part of their construction. That's a good start, but I can think of crosslinked rubber materials that are still bigger.

What about roll goods, like EPDM or other rubbers that are used as moisture liners or commercial and industrial flat roofs? You can buy rolls that are 30 feet wide and a 100 feet long. Ignoring the carbon black again, these rolls might suggest to you that least in one dimension, they could qualify as the largest molecule. But I can think of a bigger molecule yet.

How about crosslinked HDPE (PEX) tubing? A quick search came up with 1 1/2" x 500 ft tubes - all a single molecule (except for the colorant). But since the EPDM and the PEX tubing are made in a continuous extrusion process, there really is no reason that you couldn't make even larger molecules. You're really only limited by your ability to roll up the finished goods.

Or am I wrong? Is there still something out there that would be bigger yet? If you have a suggestion, please leave it in the comments. (Yes you can post anonymously.)

Thursday, September 27, 2012

Biofuels might not be so green after all

As we saw yesterday, it's very easy to be excited about certain aspects of bio-based materials (They're GREEN! They're sustainable! And biodegradable! And...) while overlooking other aspects (waste water, waste biomass, waste gas emissions...). While yesterday I discussed polyydroxybutyrate (PHB), today's topic is biofuels.

A new study from EMPA (the Eidgenössische Materialprüfungs- und Forschungsanstalt, which in English is the Swiss/Federal Laboratories for Materials Science and Technology) shows that when a complete life cycle analysis is undertaken, most biofuels are actually worse for the environment, human health and resource use than petroleum fuels. To put is simple, these green fuels are not green at all.

You can read the entire 113 page report or look at a summary such as this graph:(click on the graph if it is too hard to read the fine print)
The only two aspects where universally biofuels meet or beat petroleum are in a reduced global warming potential and ozone depletion. Looking at the other extreme, land and water eutrophication is the biggest downfalls across the widest range of biofuel alternatives.
"Biofuels can allow the reduction of fossil fuel use and of greenhouse gas emissions but with the risk of shifting impacts and creating new environmental problems; indeed, only very few biofuel pathways show lower or at least no higher impacts than the fossil fuels for all indicators. The most promising pathways are those based on methanisation of residues or on reforestation activities."
While some may see this a justification to keep going with petroleum ("Drill Baby Drill!"), this report shows the strengths and weaknesses of the various options, but also cautions
"The study confirms the high diversity in the impact patterns of biofuel pathways and therefore the necessity of assessing biofuel projects with specific data. The uncertainty of the results is high due to lack of data and modelling uncertainties. There is for example a need for more specific modelling of agricultural N2O. This uncertainty should lead to general caution when promoting biofuels."
Like most of the large questions facing civilization, we don't know the answer ahead of time. We just have to keep an open mind to options as we go along. Running open-ended experiments is a discomforting thought to scientists and engineers, but we really have no choice, do we?

Wednesday, September 26, 2012

"From Methane to Plastic to Methane - Without Waste" ?!?!?!?!?!?

Did anyone else notice that the New York Times yesterday attempted to overcome the Second Law of Thermodynamics?

That's right, they claimed that a new process could repeat the cycle illustrated here on the right WITHOUT WASTE!. Starting with methane, microbes would ferment it and produce the biodegradable polymer polyhydroxybutyrate (PHB), which could then be used as desired. After being used, the product would then be thrown into a digester which would then generate methane, that methane then being used to start the cycle all over again.

No waste, huh? Oh please!There is waste in every single step of that cycle. That it's not shown in the diagram doesn't mean it's not there. At the very least, the steps involved in forming the PHB into a usable product will require lots of energy to melt and pump it. If you've stood over a hot extruder that is melting and processing plastics, then you know there is a lot a waste heat given off.

But let's ignore all that and just focus on the mass balance in this cycle. Taking methane as the initial feed, it has the chemical formula CH4. The repeat unit in PHB (shown below) is C4H6O2. Right away, this doesn't look good. Not only is the C/H ratio off (it went from 1:4 to 2:3), but there is suddenly oxygen that wants to be in the mix. That means for every molecule of methane, 10 atoms of hydrogen have to be disposed of (hint: WASTE!) and 2 atoms of oxygen need to be called in from somewhere. I've limited knowledge of the biochemistry here, so it is possible that some of this hydrogen will be taken up by the microbes and used elsewhere, but the microbes also produce waste products of their own. I'll talk more about this again in a minute.
PHB exists in microbes as a source of stored energy. People store their excess calories as fat, microbes store it as PHB. But regardless of how it is stored, it is kept inside the cells which means you have to get it out of the little critters. And being a polymer, you can't just get it to diffuse out. Instead, you have to go in there and rip it out. Line up the bugler to play Taps, because it's the end of the line for the bugs. They lived a short happy life serving us well to make plastic. And while we are happy to get the plastic, you have all these dead microbe parts that you need to dispose of, and that is waste. Material waste. Material waste that isn't shown on the chart above, but still exists. Depending on the exact situation, the amount of PHB in a microbe can be as high as 75% (on a dry basis - meaning the water is recovered and reused), so that there is at least a 25% mass loss in this step.

Once you have the PHB, there is still more waste to come. While polymer processes that run continuously can be very efficient with material throughput, startups and shutdowns will lead to loss of material, although this is usually just a few percent or less. Consumer use of the products will also lead to some further waste as not all of it will end up being collected for the final digestion. Despite the ubiquity of recycling bins for aluminum cans, the national rate in the US is only 65% - we can't expect PHB to do any better, so that means are least a 35% loss of material.

Finally, in the digestion stage, the mass balance that was run above now needs to be reversed. To convert the PHB back to methane, the two atoms of oxygen need to disappear (WASTE) and 10 atoms of hydrogen need to be called in from somewhere.

So here is the bottom line on the waste:
  • Fermentation - 10 atoms of hydrogen per molecule of methane
  • Recovery of raw PHB - 25% mass loss from the dead microbes
  • Processing - minimal losses
  • Consumer use and recovery - At least a 35% loss
  • Digestion - 2 atoms of oxygen per molecule of methane

As you can see, the chart above has a number of missing inputs and output and therefor does not represent a mass balance. But more importantly, the question I have is this: why would you want to create a processing loop such as this? Once the PHB is made, keep it as a plastic and reprocess it. A tremendous amount of energy and matter has been used to make it and that should be respected. Degradation of polymers, particularly if it only to then serve as a feedstock for recreating that polymer anew is terribly inefficient, even if the concept flows well as a visual.

"Without waste"? Hardly. The New York Times should be better than this. Could you talk to a engineer or a chemist next time before you hype something, please?

It must be noted that it was the New York Times that proposed the waste-free title. Mango Materials makes no such claim on their website. Good for them.

Finally, many companies over the years have attempted to make PHB and other polyhydroxy alkanoates using a variety of microbes. The efforts have been only mildly successful. It is not a trivial task and I wish Mango Materials the best of luck in their efforts.

Monday, September 24, 2012

Bird-Brained Rheologists

I ran across a couple of reports 1 and 2 (see pages 74 and 75) that both state that mud daubers and swallows are able to build their mud-based nest by taking advantage of thixotropy. From reference 1:

"Mud dauber wasps build little cells of mud, stuck to walls. When they add new pellets to the cells, they add a bit of water from their crop, and they buzz. The vibrations liquefy the mud, letting it spread into the earlier, drier pellets. Then old and new pellets vibrate together, achieve the same consistency, and are stable when vibration stops.

That brings us back to barn swallows. They collect mud pellets from puddles and gradually add several rows of pellets to form the nest cup. But they apparently don’t just whack each new pellet into place. Instead, when they add pellets to the growing base, they use a dabbing or dabbling motion. This jiggles the old (drier) and new (wetter) pellets until the water content is similar and their consistency is equalized. As soon as the dabbling stops, the junction of new and old pellets becomes stable. Wouldn’t it be fun to find out if young adult barn swallows know to do this automatically or if they have to learn the hard way (if their first nest-building attempts collapse)!"

I'm having a difficult time finding a primary literature reference documenting this behavior and more importantly to my interests, the rheology of the mud. (Without having rheology curves, we don't know if this is thixotropy or shear-thinning or both, but thixotropy is such a cool term, let's just go with it.) While I don't doubt that something like this is occurring, I have a few questions:
  • It has been my observation that swallow nests are not built in a single day. This would then mean that the uppermost layer of the mud in the partially built nest would be quite dry the next morning. How is this large disparity in the moisture content addressed?
  • What is the nature of the mud at the very base of the nest - the part that is in contact with the house/cliff/barn...? This mud must not only adhere to the external structure, but also to the rest of the nest. (It is the original "tie-layer" or compatibilizer, isn't it?) How is meeting this disparity in requirements accomplished?

I would love to hear from any serious ornithologist or entomologist about this, particularly if they can provide any primary research about the nature of the mud(s) used in nest building.

Friday, September 21, 2012

UV Exposure Meter in a Wiper Blade. Not a Great Idea

A few years ago, the replacement wiper blades that I bought for a car had a feature that I had never seen before: small UV-sensitive pieces that were suppose to change color to indicate that the wiper blades needed to be replaced. They didn't work very well as the blades needed replacing long before the color change occurred, but I kept the blades around (outside) to see what would happen. The dot that was originally black [1] has in the course of a few weeks become yellow [2].

Making an appropriate UV-sensitive indicator would be extremely challenging. The amount 0f UV light that the wiper blades receive is all over the map - literally. It is well known and documented that UV exposure varies with latitude, but also with longitude. For instance, Georgia and Arizona share many of the same degrees of latitude, but the weather in Georgia is much cloudier and rainier than in Arizona. Other factors that influence UV exposure would be how long the car was parked inside or outside during the day and the direction the car faced when parked outdoors.
Parking the car in the shade of say a tree for example, is not as effective at reducing UV exposure as you might think. Rayleigh scattering intensity goes with the inverse 4th power of the wavelength, so UV light is the most highly scattered light from the sun. If the wiper blades can see blue sky at all, they are getting some UV exposure.

But all these factors could be swept aside and ignored if you make the assumption that UV exposure leads to degradation of the rubber and that the degradation of the rubber is the reason that wiper blades need to be replaced.

That unfortunately is false. Wiper blades are made with 2 well-defined 90 angles. I learned from past clients that this sharp angle is essential for wiper performance, but friction and erosion from dirt lead to this angle becoming rounded off over time. This is why wipers need to be replaced. This means that you cannot correlate UV exposure to wiper wear. As an extreme example consider a very rainy climate where the amount of UV exposure will be lower, but the demand and wear on the wiper blades will be very high.

[1] It could also have been clear too and just looked black because of the black background.

[2] I will have the lab here look into the chemistry of this color change and post what I find. My initial guess: it was a polyacetylene or polydiacetylene polymerization. As the polymerization proceeds, the conjugated backbone develops which then absorbs UV light. The color would first be blue but that wouldn't be visible on the black background....

Tuesday, September 18, 2012

The Switch to a Bio-based Economy

"(Biomass)Waste constitutes an enormous potential resource: hundreds of megatonnes across the world. Therefore a bio-based economy must be established on a corresponding scale. The starting point has to be large-volume chemicals- e.g., lubricants, surfactants, monomers for plastics and fibers, and industrial solvents - because they have the potential to make a substantial impact. Merely targeting fine chemicals, although economically attractive and important, would have negligible impact on sustainability of chemical production because the demand for such chemicals is small." (emphasis added)
Source: Valorization of Biomass: Deriving More Value from Waste C. O.Tuck, et al., Science 337 p. 695 2012 ($)

As you might suspect, I disagree with this little snippet of the article. (The rest of the article was very well written, but then, it did focus on the actual chemical reactions and other details, rather than making broad statements about engineering and economics.) As much as there is the rational desire outlined above for establishing a bio-based economy, there is no way that large volume chemicals will be the starting point. Simply put, the risk is too great.

Businesses succeed on the basis of two factors in the same way that gamblers, casinos and investors do - by making money and minimizing risks. In all situations, the higher the risk of the investment, the higher the potential payoff needs to be. A very large, if not the largest risk for anyone investing in a non-petroleum alternative is the price of petroleum itself, and as we all know too well, it is subject to big swings both up and down. The upswings are great for making the alternatives look good, but the downswings can be brutal. This then means that products with higher profit margins (i.e, fine chemicals) will be invested in first. Only when companies are truly convinced that downswings in petroleum are not going to occur will investments be made in lower margin items. This is especially true in commodity chemical production were large investments occur (and are recovered) over a number of years. Considering how fracking has made a huge impact on petroleum prices and markets both here and in Europe, downswings in prices are certainly not out of the picture.

I've always thought that if the government were serious [*] about developing non-petroleum alternatives, it would establish a base price for petroleum - say $3.00 a gallon for gasoline (and equivalent prices for other hydrocarbons). If the market price were to ever drop below that, the government would tax it until it met that base price. That would then allow the investment community to minimize their financials exposure to price drops and would increase long-term investments. But that will never happen. And so the investment schemes will follow what I've outline: higher margin, smaller volume investments will be made first, despite their minimal impact on establishing a bio-based economy.

[*] They're not. Jon Stewart has a great analysis of this if you think otherwise.

Monday, September 17, 2012

Toughening Up Hydrogels

Hydrogels are normally pretty soft materials that tear easily. Jello is the example that most people of familiar with. Ever had chewy jello? I don't think so. If you've ever made "Slime" from white glue and borax, you've seen another soft hydrogel. There are countless more examples. If you've ever had an EKG or EEG done in the last 20 years, the electrodes that were stuck to you have a hydrogel. What makes a hydrogel a hydrogel is that they have a crosslinked polymer network made from a water-soluble polymer and lots of water. Because of the crosslinking, the water is not able to dissolve the polymer but instead, the polymer absorbs it much like a sponge.

Researchers at Harvard have now created a hydrogel that can be stretched a very large amount, but maybe more importantly, if a hole is put into the hydrogel, the hole doesn't form a runaway tear, but instead stays constrained. If you have a subscription to Nature or feel like paying $32, can read the report at their website. If you don't, you're still in luck. Harvard has recently required that all their publications be made available to the public (HOOP YEAH!), so you can still read the article (4 pages) and the supplementary materials (17 pages!).

For this new material, the authors used two inter-crosslinked networks of a synthetic polymer - polyacrylamide - and an algae-based biopolymer - alginate. The nature of the inter-crosslinking is important here, and there are 3 linkages of concern. The polyacrylamide is covalently crosslinked with itself and the alginate polymer, while the alginate is ionically-crosslinked with itself with divalent calcium ions. These are weak crosslinks which are critical to the overall performance of the hydrogel. More on that in a minute.

To me, the most eye-popping result is this stress-strain plot that shows the synergistic impact of using the two networks. Synergy is a word that is over-used in society, but clearly is the perfect descriptor here. The curve for the alginate isn't really visible, while the curve for the polyacrylamide is very low and short. But the curve for the combined networks is very steep (initially) and very long - the hydrogel is not only tough to stretch, but can be stretched a long ways.

I mentioned above the covalent crosslinks of the alginate and their mobility. This is key in preventing tears from running across the entire hydrogel. When there is a hole in the stressed hydrogel, the stress is concentrated right at the edges of the hole. For a relatively brittle hydrogel such as polyacrylamide, this higher stress on the network continues to tear the gel. But when the alginate is added, the weak crosslinks allow the alginate chains to move and absorb the energy, releasing it as they move to a lower energy state. This prevents the stress from being being applied to the polyacrylamide network.

It is well known that viscous dissipation works very well at reducing energy applied to a material, so the mechanism employed here is not revolutionary by any means. That it works this well however, is.

Friday, September 14, 2012

The PR for Medical Research is Overhyped??? You Don't Say

A new report (open access - and be sure to download the full pdf file) has reached a conclusion that should be pretty much obvious to anyone who has ever bothered to actually read the research that is covered by the mainstream media: it is overhyped. Not just in the PR releases, but even in the abstracts of the papers too.
"‘‘Spin’’ was identified in 17 (41%) abstracts, 19 (46%) press releases, and 21 (51%) news items."
They even give an explicit example of how a report on acupuncture in breast cancer patients that showed no statistical significance ended up being reported (in the abstract, the PR release and the mainstream media) as showing a reduction in hot flashes and improved sex drive for the patients!

This report focused on just medical research arena, but even in the fields of polymers and chemistry, the same insanity occurs, albeit probably to a smaller degree. I've discussed numerous examples of this over the years, including such overhyped claims of the invention of the perfect polymer, or cashiers having higher doses of BPA in their urine, both of which were not supported by their own research. In all these cases, you don't have to have inside knowledge of the field of research. You just have to read the articles, see what conclusions the researchers reach, and then compare them to the hype [*]. Sure, some of the articles do have tremendous amounts of jargon and technical details that can scare you away, but the analysis and conclusions are written with a great amount of non-jargon that anyone with a high school education can read.

I am never surprised by the overhyping that occurs in PR blurbs, as these blurbs are released by the PR department and not the researchers themselves, although I think that the researchers, en masse would have enough weight to have some serious influence over that department. But the fact that abstracts will also overexaggerate is what I found to be really bothersome. This indicates not only poor reviewing of the paper by the referees, something that is entirely unnecessary and avoidable, but also some complicity on the part of the authors. All of this leads to a further degradation of science in the public's eye, and we have only ourselves to blame.

[*] I always spend the extra time whenever I review research to ensure that my posts have the needed links so that everyone can read the articles and reach their own conclusions about what I am saying. It's a hassle, but it is necessary in my mind to maintain credibility (and why I try to focus as much as possible on free/open access articles).

Thursday, September 13, 2012

If the Zombies are after you...

...they really are looking for your brains, but it appears that a thick gel formation made from gelatin, sorbitol, glycerin and some food coloring can be a good stand in. That's what movies are using to simulate good, thick, fleshy, body parts. If you have some stored in the fridge, you could toss them at the invaders as a decoy that would delay them enough to give you a few extra steps to make it too your car [*]. Best of all, all the ingredients are edible so you could even sit down with the zombies for a picnic if you think they are just poor, misunderstood creatures that just need some understanding.

[*] But of course, you will then discover that you can't escape because either a) you forgot to grab the keys, b) the zombies already pulled the wires to the spark plugs, or c) you're one of the 25 stereotypical characters that has to die in order for this to be a proper horror movie.

Wednesday, September 12, 2012

Chain Folding in Alkanes Both Great and Small

One of the more significant discoveries in polymer science in the 1950's was that semi-crystalline polymers, such as polyethylene, do not crystallize as a fully extended chain ("spaghetti in a box") but rather as folded chains, a highly stylized example of which is illustrated [1] here on the right.

The length between folds can be altered with the crystallization temperature - longer folds occurring at higher temperatures:
This particular plot was the result of crystallization for solution, but similar results can be found for crystallization from the melt. If I did my math correctly [2], a fold length of 120 nm corresponds to about 83 repeat units of the polyethylene.

83 repeat units is longer, a lot longer than what a new report expects for alkanes. Reading the paper requires a subscription/pay-per-view, but the Computation Organic Chemistry Blog has a nice review of it. In this case, the researchers found that 17 repeat units is the longest alkane that is still expected to be linear. Any longer than that, and the folded configuration is more stable.

Now this is not exactly an apple-to-apples comparison. Most obviously, the small-molecule alkane work is not about its crystallization. The research is computational and is supported by gas phase IR data, which means that there are minimal or no interactions with other molecules, only those interactions of the molecule with itself. But still, 18 vs. 83 is a very large difference. Obviously the neighboring segments in the 3-D structure of the crystal supports the longer fold length, regardless of whether the segments are from different molecules or a distant portion of the same molecule. The more serious question that I have is what is the longest alkane that doesn't show chain folding upon crystallization - i.e., how what is the longest alkane that crystallizes as "spaghetti in a box"?

[1] All illustrations are from "Principles of Polymer Morphology" by D.C. Bassett

[2] Update: A reader has pointed out that that a C-C bond is actually 1.54 Å long, not 1.54 nm long. However, since I misread the fold length off the chart as 120 nm and not 120 Å , 83 C-C bonds was still correctly calculated.

A C - C bond is 1.54 nm long. In the trans configuration, the bond is at a 19.5 angle from the backbone, so each bond adds 1.54 nm * cos(19.5o) = 1.45 nm. Then 120 nm / 1.45 nm is about 83 C-C bonds.

Thursday, September 06, 2012

Somewhere in a movie balcony...

Gene: Welcome to "At the Rheology Movies". I'm Gene Visko, film critic for the Chicago Tan Delta

Roger:...and I'm Roger Elast, film critic for the Chicago G Prime. Tonight we review the rheological thriller Rubber, a 2010 release directed by Quentin Dupieux. The movie is a very dark, and yet humorous film that follows a tire which goes on a bloody killing spree. If that doesn't make any sense to you, then this segment should help you feel more comfortable with the idea:

If you can accept that whole concept of "no reason", then you will enjoy the movie immensely.

Gene: Well Roger, I could accept that whole concept, but I really think it makes for tired film making, excuse the expression. Without that whole prologue, the movie would be just flat (excuse the expression). I do have to admit that the whole prologue made the movie roll along much better (again excuse the expression), and so I do have to give Quentin credit for that. More film makers should be using that trick. It would only inflate (groan) their bad movies by a couple of minutes. Maybe I'll just mentally insert this scene it in my mind whenever I am sitting clueless in a movie again, like say The Room. Now that I think about it, that introduction would help out "The Room" tremendously.

Roger: Gene, I loved this movie. It didn't take itself seriously and was almost a parody of a B-grade horror movie.

Gene: But Roger, look at the total disregard for rheology in it. The tire is able to internally generate a little set of vibrations which then cause the heads of people across the way to explode. Even accepting the whole "no reason" clause for every stupid aspect of the movie, look at how unrealistic the killer tire is. Internally generated vibrations like that violate the 2nd law of thermodynamics, and then being able to transmit that energy at a distance without any visible meanings of concentrating it so as to overcome the power drop with 1/r2 is just fantasy. Not "no reason": fantasy.

Roger: Gene, it's apparent to me that the vibrations are at the resonance frequency of the tire which allows them to become amplified without bound.

Gene: But there still would be viscous damping.

As an aside Roger, there was one point in the movie when the question arose of whether the tire would sink in water. You and I spoke of this before the show and I think we agree that the density of a tire would induce it to sink, what with the steel wires in it, the carbon black and more.

Roger: At least we agree on something. The movie is rated R and available on DVD. To summarize, I gave the movie a thumbs up and Gene went thumbs down. So until next time, the balcony is closed.