Molecular dynamics simulations of polymer crystallization in highly supercooled melt: primary nucleation and cold crystallization.

Molecular mechanisms of crystallization at large supercooling and structure of supercooled melt are investigated in our polyethylenelike polymer through molecular dynamics simulations. Three representative crystallization processes are here considered: (1) isothermal homogeneous nucleation in the melt, (2) crystallization by rapid cooling of the melt, and (3) cold crystallization during slow heating of an amorphous state. Molecular level structures of the melt and the emerging crystallites are characterized by the use of the specific parameters, the effective segment length L(p) and the radius of gyration R(g) of the molecules, together with the overall crystallinity chi(c). In quasiequilibrium melt of moderate supercooling, the chains have random-coil conformations. However, the temperature dependence of the averaged L(p) in the melt is found to show quite unexpected transition around the bulk melting temperature. At larger supercooling of 330 K, the homogeneous nucleation takes place after an induction period of about 4 ns. Characteristic conformational changes are here described by multimodal distributions of R(g), the main components of which correspond to relaxed random-coil chains in the melt and once-folded chains in the crystallites; the former chains transform continuously into the latter, having similar chain extension R(g). Rapid cooling of the melt is found to give poorly crystallized states having fringed-micellar organization. The effective segment length L(p) shows considerably faster increase than R(g), resulting in peculiar conformational frustration. Nearly amorphous samples obtained by very rapid cooling show pronounced cold crystallization by slow heating over the glass transition temperature, where crystallites of random orientations form a granular texture due to steric collisions of the growing lamellae. The generated crystal texture is only metastable and readily reorganizes by annealing at high temperatures, where the chains are found to make large conformational transformations, the refolding from the twice-folded conformation into the once-folded.