Entanglement

How entangled is a polymer melt?

A fundamental property of melts of long flexible polymers is that they are entangled, like cooked spaghetti in a bowl.  It is relatively easy to pull one molecule (or strand of pasta) along its own path, but difficult to move it “sideways”, because of other molecules running in other directions that cannot be passed through.

A melt or solution of long entangled polymer chains flows like honey — very viscous, and even “elastic”, for example snapping back when you cut a flowing stream with scissors.  This behavior comes from the stretchy, springy nature of the long molecules, and the fact that they are entangled together.  Different kinds of polymers — made from different chemical “beads” strung together in a long necklace — are more or less elastic in this way, depending on how entangled they are…

Glassy systems

“What makes dense, nearly-glassy liquids so sluggish?”

Glasses are both practically important materials and theoretically fascinating systems.  They are fascinating because 1) they are rigid solids without being crystalline, and 2) they become solid not abruptly but gradually as the temperature is lowered, by becoming progressively more viscous until we can no longer say they are liquids.  They are practically important, because window glass, optical fibers, amorphous silicon for electronics, and many polymer materials including polystyrene and “vinyl” polymers (PVC) are glasses.

We have performed simulations both of idealized “hard-sphere” models for fundamental studies of the glass transition, and atomistic simulations of real polymers to understand how free surfaces and plasticizers shift the glass transition in these materials…

Crystallization

How can we predict, and control, the rate at which polymer melts crystallize?

A melt of long polymer molecules is hard to crystallize, even if cooled below the freezing temperature.  Polymer crystallization proceeds by nucleation.  Small ordered regions appear by thermal fluctuations.  If they are too small, they quickly melt away.  If they are large enough, they grow by adding chain segments on the boundary.  How large is large enough, depends on a competition between the interior of the nucleus, which has a lower free energy than the melt, and the surface of the nucleus, which has an interfacial tension — a free energy cost.

So to predict how readily a given polymer will nucleate, we need good values for the interfacial tension between a crystal and its melt, and for the heat of fusion of the crystal (which tells us how much lower is the free energy of the crystal than the supercooled melt).  And to control nucleation, an important strategy is to introduce nanoparticles that have a favorable interfacial energy with the crystal versus the melt…

Conducting polymers

“How does disorder along a polymer semiconductor impede charge motion?”

Polymeric organic semiconductors are being developed as possible materials for organic electronics and photocells, to take advantage of modern polymer synthesis and processing. These materials are very different from traditional “hard” semiconductors like silicon.  They are made of flexible long-chain molecules with conductivity primarily along chains, arranged in a nanoscale mixture of ordered and disordered domains.

In the ordered domains, straight chains are regularly packed like pencils in a box, while in disordered domains the chains are randomly packed, like cooked spaghetti.  This disorder can impede the motion of charges along the chains.  So predicting electronic properties of these materials is a big theoretical challenge, because we must treat the effects of disorder, which means averaging over many large disordered conformations…

Polymer phase behavior

How can we predict when polymers will mix, or chains will align?

Polymers are notoriously difficult to blend.  A tiny repulsive interaction between unlike monomers, when summed along an entire long chain, can be enough to overcome the entropic tendency of chains to mix uniformly.  So predicting the repulsive interaction per monomer (the “chi parameter” χ) is very challenging, because of the sensitive required…

Semiconducting polymers for photocell materials are rather stiff, and can potentially form nematic phases, in which chains tend to align in a common direction without forming a crystal. Nematic order can be helpful for making thin-film devices with superior properties.  We have developed a new simulation method to measure the nematic coupling, which quantifies the tendency for nearby chain segments to align. With this, we can predict the nematic transition for real polymer structures…