Alexander J. Bourque
The goal of the project is to develop and test a new paradigm for the development and design of additives that significantly affect the nucleation and growth of polymer crystallites, the evolution of novel morphologies as a consequence of new crystallization behavior, and the engineering of polymer material properties through the modification of semicrystalline morphology. There is a persistent inability to provide direct observation of nucleation events, crystal growth mechanisms and the resulting molecular scale organization in semicrystalline polymers materials by experiment. Through this project, we have successfully employed molecular simulations to perform virtual experiments and reveal the molecular nature of crystal nucleation and growth in the presence of additive surfaces. Data for the propagation of a crystal growth front from the initial polyethylene/n-alkane melt interface into the bulk amorphous melt have been analyzed for growth rates and surface nucleation events. Regarding surface nucleation, mean first passage time methods have been employed to capture the surface nucleation of n-alkane crystal phase and to measure the subsequent lateral spreading rate following nucleation. The mechanism of growth has been identified in the crystallization of long n-alkanes and comparison is made to the classical theory proposed by Lauritzen and Hoffman. Most recently, we have begun systematically simulating melts of polyolefins with a number of foreign surfaces, to “scan” through the nucleating agent parameter space. The Stillinger-Weber (SW) potential is used to this end to represent the fundamental atomic structure and bonded interactions within a solid nucleating agent. The parameters of SW potential can be easily changed to represent a number of real materials (Si, Ge, C). The nucleation potential among candidate surfaces was evaluated using the techniques developed for nucleation on a polyethylene surface. Simulation results are used to guide the choice of additives for experimental study. Experimental efforts have been devoted to achieving dispersion of polar nano-particles in non-polar polyolefins, which is the main challenge in realizing thermo-mechanical property enhancement in polyolefin nanocomposites. Rheometry, differential scanning calorimetry, small angle light scattering, small and wide angle synchrotron X-ray scattering are employed to measure indirectly the early onset and ultimate morphologies of selected additives. Through an iterative approach of simulation and experiment, we hope to reduce the cycle time of material discovery for polyolefin materials.