Melt Properties and String Model Description of Glass Formation in Graft Polymers of Different Side-Chain Lengths

Abstract

Graft polymers have attracted growing interest due to their unique properties, but a fundamental understanding of their glass formation remains to be established. We perform extensive molecular dynamics simulations of a coarse-grained bead–spring polymer model to investigate the influence of side-chain length on the glass formation of general classes of graft polymers having a fixed reduced molecular mass, which are classified based on the relative rigidities of the backbone and side chains. We first examine the basic properties of the graft polymers in a melt to understand their conformational and structural properties. Our analysis indicates that the overall average size and shape of our model graft polymers exhibit a nonmonotonic dependence on the side-chain length, while the side chains undergo a smooth transition from rod-like to coiled chain conformations near a side-chain length of 10 statistical segments. We then utilize the string model to understand the structural relaxation of graft polymers in the low-temperature non-Arrhenius regime─a model that takes the average string length as a key quantity in the description of the dynamics of glass-forming liquids, along with the high-temperature activation free energy. We find the string model to be capable of quantitatively describing the structural relaxation time over the range of temperatures considered in our model graft polymers. The string model also allows us to discuss issues related to the energetic parameters of high temperature activation, and our analysis indicates that the entropic contribution to the high-temperature activation free energy is not generally negligible for polymeric glass-forming liquids, in contrast to the assumption in the entropy theory of glass formation. We expect our work to motivate further studies of the glass formation of graft polymers based on the models in which the fluid entropy and collective motion are of central importance.