Molecular mechanisms of the steady-state growth of the chain folded lamella and the cold crystallization across the glass transition temperature Tg are investigated by molecular dynamics simulation for a system of long polyethylene (PE)-like polymers made of 512 united atoms C512. The present paper aims to reconsider results of our previous simulations for short PE-like polymers C100 by carrying out very long simulations up to 1 μs for more realistic systems of much longer chains, thereby to establish the firm molecular image of chain-folded crystallization and clarify the specific molecular process of cold crystallization. We observe that the chain-folded lamella shows fast thickening-growth keeping marked tapered growth front. Despite the fast growth in much longer chains, the fold-surface is found to be predominantly of adjacent-reentry. Detailed inspections of the molecular pathway give an insightful image that can explain the apparently contradicting results. In addition, the fold-structure with specific spatial heterogeneity is found to give rise to heterogeneous mobility within the crystalline region. On the other hand, investigations of the cold crystallization during slow heating of the glassy film across Tg is found to give a granular texture made of small crystallites. The crystallites are found to nucleate preferentially near the free surfaces having lower Tg, and to be dominantly edge-on showing a definite tendency to orient their chain axes parallel to the free surface.

Molecular mechanisms of crystal growth and homogeneous nucleation from the melt of polyethylene-like linear polymer are investigated by molecular dynamics simulations. The present paper is aimed at extending our previous work with respect to the system size and the boundary condition, thereby enabling detailed studies on the structures of sufficiently large lamellae and fully equilibrated melt. Lamellae of uniform thickness but with marked tapered edges are found to grow at constant velocity from the substrate. Three-dimensional shape of the growing lamellae exhibits peculiar undulation at the growth front, the origin of which is suggested to be the inhomogeneous thickness distribution within the lamellae. Trajectories of chains crystallizing onto the growth front reveal an unexpected pathway for chain folding, where a partially attached chain stem forms a new fold by plunging its head back into a neighboring stem position through slithering snake motions of the chain. Detailed statistics of folds and cilia show that the folds are rather neat and mostly make re-entries into the nearest or the second or third nearest neighboring stem positions, whereas the cilia are generally short but with a small number of longer cilia forming thick amorphous layers. Structure of supercooled melt investigated versus temperature reveals that, at moderate degree of supercooling, the overall chain conformation remains Gaussian random coil but the persistent length of chains increases monotonically with increasing supercooling. Exceptions are at the largest supercooling where homogeneous nucleation takes place; usual melt structure becomes rapidly unstable and emerges many crystallites of random orientations. During early 10–20ns after the quench, density of melt, radius of gyration of chains, and fraction of kinked bonds show marked alterations. These structural changes are highly cooperative and are considered simply due to the emergence of many embryonic crystals in the melt. Conformations of the chains forming nuclei are also traced to reveal that the homogeneous nuclei are fringed micelle like aggregates of chains, but the chains as a whole have folded conformations, which are similar to those reported in previous simulations on a single polyethylene in a vacuum.

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 Lp and the radius of gyration Rg of the molecules, together with the overall crystallinity χc. In quasiequilibrium melt of moderate supercooling, the chains have random-coil conformations. However, the temperature dependence of the averaged Lp 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 Rg, 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 Rg. Rapid cooling of the melt is found to give poorly crystallized states having fringed-micellar organization. The effective segment length Lp shows considerably faster increase than Rg, 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.

Herein, we propose a large-scale simulation approach to perform the stretch-induced crystallization of entangled polyethylene (PE) melts. Sufficiently long (1000 ns) united-atom molecular dynamics (UAMD) simulations for 16000 chains of 1000 consecutive CH₂ united-atom particles under periodic boundary conditions were performed to achieve the crystallinity observed in experiments. Before the isothermal crystallization process, we applied uniaxial stretching as pre-elongation to the embedded strain memory on the entangled PE melts. We confirmed significant differences in the morphologies of crystal domains and scattering patterns for pre-elongation ratios of 400% and 800%. The obtained scattering patterns were consistent with the experimental results. Uniaxial stretching MD simulations revealed that the elastic modulus at 800% pre-elongation was stronger than that at 400% pre-elongation. From this observation, we can derive the structure-property relationship, wherein the magnitude of the pre-elongation governs the crystal domain structures and mechanical properties.