Multiscale Theory For Semicrystalline Polymers

TECHNICAL SUMMARY

This award supports theoretical research and education on polymer crystallization.

The long-chain nature of polymer molecules dictates that polymeric crystals adopt a chain-folded lamellar form, but the basic question of how the crystals nucleate is ill understood. There is experimental evidence that polyethylene, the most common semicrystalline polymer, actually nucleates via an intermediate ?rotator' mesophase. However, no theoretical basis yet exists to assess this intriguing hypothesis, or to ascertain how widespread this phenomenon might be in other polymers. Progress requires a theory of the mesophases, able to compute not only their free energy relative to the melt and crystal phases, but also the free energy of the interface between the melt and the mesophase or crystal.

An effective theory of the crystal-melt interface in semicrystalline polymers would also predict the concentration of ?tie chains', which link together adjacent crystalline lamellae and lead to the toughness and ductility of plastics. Also, there is much current interest in the effect of flow on crystallization kinetics and morphology. This is crucial in commercial use of semicrystalline polymers, which are very sensitive to flow effects. Without a good theory of quiescent crystallization, any effort to understand how flow speeds up nucleation is severely handicapped.

The PI aims to develop the theoretical basis to understand the mesophases in polyethylene. It will lead to predictions for bulk and surface free energies, and it will determine whether nucleation in polyethylene can indeed occur via a mesophase. Given the inherently multiscale nature of the problem, a transformative synthesis of techniques will be employed: atomistic simulation of ordered phases, novel use of ?solid-state simulations' to characterize mesophase domain walls, mesoscopic discrete-spin simulation of partially ordered mesophases, and a new adaptation of grafted chain ?brush' theory of the interface between ordered polymer phases and adjacent melt. This unique combination of strategies will lead to a better understanding of how polymer crystals nucleate, and will bring us closer to achieving optimal properties of these truly modern materials.

This intellectually rich problem of practical importance provides an excellent opportunity for education, offering students and postdocs broad exposure to theory, analytical methods, atomistic and mesoscale simulation. The PI is developing an undergraduate course for Fall 2009 in Polymers and Complex Fluids, which is well aligned with the multiscale approach of this proposal. The Chemical Engineering Department has a strong record of minority and gender representation among its graduate students, with about 30 percent women. The College of Engineering has an active Women In Engineering Program and a Multicultural Engineering Program. All simulations will be performed with open source software, to remove any barrier to use by others.

NON-TECHNICAL SUMMARY

This award supports theoretical and computational research and education on how polymers crystallize.

Semicrystalline polymers, although relatively young, are the most ubiquitous materials of the modern age. The mass of such materials now produced worldwide each year exceeds the production of steel. Even so, their ultimate potential for desirable mechanical and physical properties is as yet unfulfilled. This is because, in contrast to the centuries-old field of metallurgy, the science base for semicrystalline polymers is still very much a work in progress, with many key results obtained only in the past few decades. Likewise, improved control of polymer molecular structure through advances in catalysis has emerged only relatively recently.

The PI will use theoretical techniques to explore a possible microscopic mechanism for crystallization through an intermediate polymer phase.

This intellectually rich problem of practical importance provides an excellent opportunity for education, offering students and postdocs broad exposure to theory, analytical methods, atomistic and mesoscale simulation. The PI is developing an undergraduate course for Fall 2009 in Polymers and Complex Fluids, which is well aligned with the multiscale approach of this proposal. The Chemical Engineering Department has a strong record of minority and gender representation among its graduate students, with about 30 percent women. The College of Engineering has an active Women In Engineering Program and a Multicultural Engineering Program. All simulations will be performed with open source software, to remove any barrier to use by others.