Monte Carlo Simulation of Interlamellar Isotactic Polypropylene

Monte Carlo molecular simulations are reported to investigate the amorphous region between crystalline lamellae of isotactic polypropylene ( iPP). It is the first such study in which consideration is given to the nature of pendant side groups on the polymer chain. The crystalline lamellae are modeled after the R2 crystal of iPP, while the interphase and amorphous domains are equilibrated with respect to both spatial and topological degrees of freedom. A united atom force field is used to describe the interactions between the methyl, methylene, and methyne moieties that constitute a polypropylene molecule. Mass density and bond orientational order profiles, distributions of chain populations, interphase thicknesses, interfacial energies, local conformations as characterized by torsional angle sequences, and chain reentry statistics are studied for three different temperatures. Our results match well with the available experimental data and provide a molecular level understanding of the semicrystalline interphase for isotactic polypropylene. A comparison with linear polyethylene (PE) reveals the effects of chemical architecture on the crystal -amorphous interphase in semicrystalline polymers. Semicrystalline polymers have found widespread use due to their excellent mechanical and barrier properties. The crystal- lization of polymers corresponds to a first-order phase transi- tion, 1 similar to other substances. However, a significant difference between long chain molecules and their low molecular weight counterparts lies in the kinetics of the crystallization process, because of which the complete incorporation of polymer molecules into the ordered crystalline state is rarely achieved. Thus, "semicrystalline" polymers are structurally nonhomoge- neous materials and invariably consist of an intimate mixture of crystalline and amorphous phases. These polymers exhibit properties that are a consequence of the combined attributes of ordered (crystalline) and disordered (amorphous) constituent regions, with the unique feature that both crystalline and noncrystalline elements are topologically connected by chains that meander their way from region to region. A comprehensive understanding of the behavior of semicrystalline polymers thus demands extensive knowledge not only of the individual phases but also the interplay between them. The existence and lamellar nature of semicrystalline polymers have been known for approximately half a century. 2-8 Various facets that describe the state of the semicrystalline system can be identified, including the gross morphological structure, degree