Structural relaxation in binary hard spherical particles has been shown recently to exhibit a wealth of remarkable features when size disparity or mixture composition is varied. In this paper, we test whether or not similar dynamical phenomena occur in glassy systems composed of binary hard ellipses. We demonstrate via event-driven molecular dynamics simulation that a binary hard-ellipse mixture with an aspect ratio of two and moderate size disparity displays characteristic glassy dynamics upon increasing density in both the translational and the rotational degrees of freedom. The rotational glass transition density is found to be close to the translational one for the binary mixtures investigated. More importantly, we assess the influence of size disparity and mixture composition on the relaxation dynamics. We find that an increase of size disparity leads, both translationally and rotationally, to a speed up of the long-time dynamics in the supercooled regime so that both the translational and the rotational glass transition shift to higher densities. By increasing the number concentration of the small particles, the time evolution of both translational and rotational relaxation dynamics at high densities displays two qualitatively different scenarios, i.e., both the initial and the final part of the structural relaxation slow down for small size disparity, while the short-time dynamics still slows down but the final decay speeds up in the binary mixture with large size disparity. These findings are reminiscent of those observed in binary hard spherical particles. Therefore, our results suggest a universal mechanism for the influence of size disparity and mixture composition on the structural relaxation in both isotropic and anisotropic particle systems.

In this work, we have carried out Brownian dynamics simulation to describe the detailed characteristics of conformational and rheological responses to startup shear. In addition to the evaluation of the contour length of the primitive chain, the state of chain entanglement and the first normal stress difference as a function of strain, three methods are applied to determine the time-dependent shear stress. Our results show significant stretching of the primitive chain up to many Rouse times, followed by retraction, as the primary origin of stress overshoot for deformation rates lower than the reciprocal of the Rouse time but higher than the reciprocal of the reptation time. The analysis of such results reveals heterogeneous local chain stretching, demonstrating the coupling between stretching and orientation that extends to times considerably longer than the Rouse time. Explicit comparison between the simulation and the theoretical description from the GLaMM theory shows marked differences. For example, the simulation indicates a slower decline in the number of original entanglements than that at equilibrium up to many Rouse times whereas the GLaMM theory predicts a faster decrease. Moreover, contrary to the simulation that depicts a nearly constant slope in the stress–strain relationship during startup shear, the GLaMM theory shows an immediate and precipitous strain softening.

The crystalline morphology and structure of poly(L-lactide) (PLLA) in a PLLA film–chloroform system were investigated by means of wide angle X-ray diffraction (WAXD), polarized optical microscopy (POM) and atomic force microscopy (AFM). Birefringent and nonbirefringent ring banded supra-structures with radial periodic variation of thickness were obtained, which were induced by micro-evaporation of solvents and concentration gradient of PLLA. The ring banded morphologies consisted of multilayer lamellar crystals, which is a manifestation of alternating ridge and valley bands of periodic variation of thicknesses along the radial direction. The formation of the ring banded supra-structures is associated with diffusion and crystal growth induced periodic variation of concentration gradient, which is attributed to diffusion-related rhythmic growth and the competition between diffusion of polymer segments and growth of crystal lamellae. The mechanism of such banded-ring formation was explored on the basis of rhythmic growth resulting from non-linear diffusion.