Molecular Dynamics Simulation Studies Of Glass Dynamics Of Several Polymer Systems

Glass transition remains a major scientific challenge in condensed matter physics.Though researchers have done lots of experiments, theories, and simulations to reveal the issue of glass transition, many problems have not been well clarified due to constraint of various restricted conditions of expenriments and simulations. Since it does not easily crystallize and can be easily assembled, polymer is one of the most concerned systems for study of glass transition. In this letter, the glassy dynamics of colloidal polymers and molecular polymers is studied by molecular dynamic simulation.1. The study of glassy dynamics of colloidal polymers.(1). The effect of "monomer" size on glassy dynamics of colloidal polymers."Colloidal polymer" is the system which is composed of colloidal chains, and each colloid is termed the "monomer" of colloidal polymer. It is an important feather of colloidal polymer that the monomer size can vary in a certain range. Here,we propose a model to address the varying monomer size of colloidal polymer and quantitatively study their glassy dynamics in differnet monomer sizes. The mean square displacement of monomers exhibits Rouse-like sub-diffusion at intermediate time/length scale and the corresponding exponent depends on the volume fraction and the monomer size. We find that the threshold volume fraction at which the caging regime emerges can be used as a rescaling unit so that the data of localization length versus volume fraction for different monomer sizes can gather close to an exponential curve. The increase of monomer size effectively increases the hardness of monomers and thus makes the colloidal polymers vitrify at lower volume fraction. Static and dynamic equivalences between colloidal polymers of different monomer sizes have been discussed. In the scope that we investigate, we find that the curves of dyamic quantities (the mean square displacement and self-intermediate scattering function)can coincide well but the curves of staitc radial distribution function coincide well only at low volume fraction. The mode-coupling critical exponents for colloidal polymers are in agreement with that of flexible bead-spring chains.(2). The effect of controlled chain stiffness on glassy dynamics of colloidal polymers. In order to discrible the variability of chain stiffness of colloidal polymers,we introduce a beading potential to the model having been studied in chaper (1). We focus on the quantitative differences in the static structure and glassy dynamics of the colloidal polymers as flexible chains to stiff chains and the increase of chain stiffness.In the case of stiff chains, the intermediate peak of radial distribution function is induced by bending potential. It is different from the case of flexible chains that monomer directionality is presented in the stiff chains, and such monomer directionality is enhanced with the increase of chain stiffness. In the same monomer size and volume fraction, comparing to flexible chains, the mean square displacement of stiff chains exhibits larger exponent of the sub-diffusion. In the case of stiff chains,we find that the threshold volume fraction at which the caging regime emerges can be used as a rescaling unit so that the data of localization length versus volume fraction for different monomer sizes can gather close to an exponentially decayed curve. The values of localization length of stiff chains are whole larger than those of flexible chains and decay slower than those of flexible chains. In the same monomer size and volume fraction, the active energy of systems is linearly increased with the increase of chain stiffness. The mode-coupling critical exponents and critical volume fractions of stiff chains are separately larger and smaller than those of flexible chains. Static and dynamic equivalences between stiff colloidal polymers of different monomer sizes have also been checked. We find that the curves of mean square displacement can coincide well but the static radial distribution function does not coincide well because of the emergment of intermediate peak induced by beading potential. On the basis of establishment of dynamic equivalences, we find that, in the same chain stiffness, the mode-coupling critical volume fractions of colloidal polymers of different monomer sizes correspond to almost the same volume fraction of hard-sphere equivalences.2. The study of glassy dynamics of molecular polymers.(1). Metabasin dynamics of supercooled molecular polymer melt. The metabasin dynamics has been found for binary Lennard-Jones mixture model and colloidal suspension experiment. But, whether the metabasin dynamics exists in the molecular polymer system or whether there are qualitative or quantitative differences in metabasin dynamics between molecular polymers and simple molecules is not clear.Hence, the metabasin dynamics of supercooled molecular polymer melt is investigated by bead-spring model. We find that, in a small system, the a-relaxation process is composed of a few "crossing events"(metabasin-metabasin transitions) that monomer hops from one metabasin to another. Each crossing event is very rapid and involves a "democratic movement"(the large-displacement movement of major momomers) of many particles. Evaluation on the contributions of metabasin exploration and democratic movement shows that the structural relaxation is mostly governed by the latter. Through comparing the maximum non-Gaussian time with the sojourn time of metabasin, we find that, our calculated results show that the metabasin-metabasin transitions are not the main reason of spatially dynamical heterogeneity. However, the metabasin-metabasin transitions of binary Lennard-Jones mixture model are relevant for the spatially dynamical heterogeneity.(2). the effect of chain length on glassy dynamics of semi-flexible ring molecular polymers. We initially study glassy dynamics of semi-flexible ring molecular polymers by bead-spring model, focusing on the effect of chain length. It is confirmed that the increase of chain length leads to the emergence of dynamic frozen state/glass transition. In the case of long chains, the decoupling of self-collective intermediate scattering functions is found and such existence of decoupling suggest the emergence of inter-ring correlation in the region of characteristic size of ring, also the emergence of ring clusters which are composed of internal rings of fast and continuous motions.