Influence Of Molecular-weight Polydispersity On The Glass Transition Of Polymers

Glass is a well-known state of soft matter. However, we still lack a deep and unified microscopic understanding of the glassy state despite of extensive studies. When the temperature approaches the glass transition temperature, microscopic structure varies little, but the dynamics slows down dramatically. The inconsistences of structure and dynamic variation increase the difficulty of understanding the nature of the glass transition. Polymers are good glass formers due to their inherent difficulties(the complex configurations) in forming crystal and are often used as model systems to study the glass transition. Most synthetic polymers have molecular-weight polydispersity, which is an important factor influencing polymer mechanical properties. In addition, the glass transition temperature is a key parameter for polymer applications, which also can be used as a single parameter to estimate the mechanical properties when defined consistently. Whether there is any relation between and molecular polydispersity is very important for polymer applications and for understanding the nature of the glass transition. However, the influence of molecular weight polydispersity on polymer has been paid to little attention in previous research. It is well known that increases with molecular mass and saturates at large mass. This can be described by the Fox-Flory equation: =∞-/. However, the origin of this dependence still remains elusive. In practice, “the mass” in the above equation has been controversially taken as the number-average or the mass-average molecular weight. Previous research about the mass dependence of polymer has not distinguished between them.Motivated by the above questions, by performing molecular dynamic simulations, we find that for polymers with the same number-average molecular weight, molecular weight distribution profile, either in Schulz-Zimm form or in bi-modal form, has little influence on the glass transition temperature and average segment dynamics(monomer motion, bond orientation relaxation and torsion transition). By contrast, average chain motion is affected by polydispersity. Non-Gaussian parameter shows that molecular-weight polydispersity dramatically enhances dynamic heterogeneity of monomer diffusive motions after breaking out of the cage, but it has weak influence on dynamic heterogeneity on the short time scale. These results imply that the irrelevance of molecular-weight polydispersity to the polymer is due to the local nature of the glass transition. The string-like cooperative motion is not influenced by molecularweight polydispersity either, supporting the idea that string-like collective motion is not strongly correlated with chain connectivity.