Conformation-orientation coupling in helical polymers from atomistic simulations

Simulating crystallization is challenging for helical polymers such as isotactic polypropylene (iPP). In prior computational works, only polyethylene (PE) or other PE-like linear chains can successfully crystallize in molecular simulations. Crystallization of chains such as iPP involves the formation of helical structures, which may couple with orientational order to drive the overall crystallization. The thermodynamic driving force arising from conformation-orientation coupling, however, may be rather weak. Thus, direct observation of iPP crystallization in simulations is difficult. In this work, we apply all-atom molecular dynamics (MD) simulations and thermodynamic integration to quantify the conformation-orientational coupling for iPP. We quantitatively show that the coil-helix transition is entropically unfavorable even below the crystal melting temperature when iPP chains are immersed in an isotropic matrix. The coupling between iPP segments and orientational fields, however, can stabilize the helical conformation. We expect the conformation-orientation coupling to be critical for iPP crystallization.