Stiff chains in tight spots: confinement and semiflexibility in semicrystalline polymers and entangled melts

Nontechnical Abstract

The most common plastic polymers, polyethylene and polypropylene, are made from simple molecules obtained from refining and natural gas feed stocks. These polymers are inexpensive, lightweight, versatile, and ubiquitous - and yet, given their molecular structures, their material properties are still far from optimal. One reason there is room for improvement is that our understanding of the molecular origin of plastic properties is very incomplete. A key property of plastics is that when you pull on them, they 'give', rather than snapping in two. To the hand and eye, that is what makes a plastic plastic. But how does a solid material manage to 'give' rather than snap? To find out, the PI's group will use computer simulations to look inside the crystals that make up common plastics, in which the long polymer molecules pack together like pencils in a box. There can be defects in the crystal packing - trapped twists, called 'twist solitons', that can move along the length of the molecule, like a bump under a rug. Understanding how polymer crystals change shape in response to stress without snapping may give insight into how to tailor inexpensive, weight-saving polymer materials for a wider range of uses.

Designing new polymer molecules is expensive. Molecular simulations can be used to predict key material properties of molecular structures, to guide which new molecules to try. A key property of polymers is how they flow when molten, because polymers are often made into finished products by melt processing. Modern theory does a good job of predicting polymer flow behavior, if one knows two key parameters: how 'entangled' a polymer is, and how 'slippery' it is. To predict these parameters for a given molecular structure, the PI's group will model a dense melt consisting of a single long self-entangled ring molecule. With these parameters, flow behavior of real polymers can be predicted, and used to guide design of new molecules.

Technical Abstract

Confinement of polymer chains by the repulsive potentials of neighboring molecules plays an essential role in the physics of semicrystalline polymers and entangled melts. The elementary excitations that remain active in the constrained geometry are a key common element in these disparate systems. In entangled melts, these excitations are motions within the 'tube'. In the crystalline lamellae of semicrystalline polymers, the excitations are twist solitons. Chain stiffness intensifies the effects of confinement, narrowing the tube and increasing the soliton energy. This proposal exploits molecular dynamics and Monte Carlo simulations combined with topological methods to characterize the most important motions in these highly constrained systems. These results will enable predictions for key material properties, including nucleation barriers, entanglement length and friction factor for melts, and the amorphous rubberlike modulus in semicrystalline polymers.

The first part of this proposal builds on success of using atomistic molecular dynamics simulations to characterize twist solitons in polyethylene. Twist solitons permit chain motion in lamellar crystallites, enabling the deformations that make plastics plastic. Solitons proliferate in disordered 'rotator' phases, which play a key role in nucleation of semicrystalline polyethylene. Polypropylene is the second most common polymer worldwide after polyethylene. It crystallizes in a packing of single-chain helices instead of all-trans ribbons. Are twist solitons and rotator phases in polyethylene a fluke, or a paradigm? Simulations will be used to discover if twist solitons and rotator phases also play a key role in polypropylene.

The second part exploits PI?s group expertise in entangled polymer melts. New methods will be applied to problems of fundamental importance: dynamical regimes for long entangled chains, structure of 'hairpins' formed as branched polymer arms relax, and the rubberlike modulus of the amorphous region in semicrystalline polymers.

Understanding crystallization in polyolefins could lead to advanced products in a competitive global marketplace. Trapped entanglements in the amorphous region are little understood, yet critical for large-strain plastic deformation without failure. This key material attributes enables environmentally friendly 'downgauging' of plastic films, doing the same job with much less material. Efficient methods to determine fundamental materials parameters from small scale simulations could unleash a new era in computer-aided materials design.