Thursday, May 06, 2010

Mistaken Identity

I thought I might have run into an old buddy the other day, flow-induced crystallization, but I was in error. [1] I was running a constant temperature DMA on some new plastic materials under development and in the plot of viscosity over time saw this:
Many materials will show an increase in viscosity over time – any system that cures (epoxies, reactive polyesters, styrene-ester systems…), as well as materials that crosslink upon excessive heat. But this was a simple thermoplastic that wasn’t expected to have such a reaction running around. So what was causing this massive increase at a temperature in which the polymer was molten?

One possibility was flow-induced crystallization (FIC), a term that describes crystallization that results from flow. [2] Without the flow, the crystallization would not have occurred as the temperature of the polymer would be too high. The flow field is able to partially orient the polymer, (extensional fields work so much better for this than shear fields) and in doing so, reduce the entropy change in the transition from melt to crystal. Now recall with me from your wonderful days in chemical thermodynamics that at equilibrium,

DS = DH/T

Since DS has decreased while DH hasn’t, T must increase. i.e., the temperature at which the melt crystallizes is higher than normal. And so the flow field can cause crystallization at temperatures higher than those at which it would normally occur.

The sample was being sheared continuously during the testing, so it is possible that this might have induced crystallization. [3] However, a few more tests allowed us to rule this out. First, the viscosity increases that we saw occurred faster as the temperature increased - even in the somewhat odd world of flow-induced crystallization, this doesn't happen anymore than it happens in the quiescent world. The real killer however, was what we saw when the plastic sample was kept at the constant temperature without the shear for most of the induction period noted above, and then the shear was started up: the initial viscosity was already at the elevated level, forcing us to conclude that the oscillatory shear was in fact inhibiting the viscosity climb.

So what I thought was an old buddy was actually a case of mistaken identity.

[1] I say old buddy as my dissertation was in the area of flow-induced crystallization. I have run into it more than a few times in my professional career, so this buddy does call from time to time, but it’s not like I’ve friended him on Facebook or anything.

[2] “Flow” in this case is a broad term meaning any applied mechanical stress or strain. Some researchers will even use the specific terms “strain-induced” or “stress-induced”. It’s still the same phenomenon underneath.

[3] The shear was oscillatory and I was working in the linear region. Both of these constraints that would have made it more difficult to create the orientation needed.

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