Influence of Side-Chain Length and Relative Rigidities of Backbone and Side Chains on Glass Formation of Branched Polymers

Molecular branching is a common characteristic of both synthetic and natural polymers that plays a key role in determining their physical properties, but a fundamental understanding of how this molecular factor influences polymer glass formation remains incomplete. Here, we utilize the generalized entropy theory (GET) and molecular dynamics simulation to investigate the influence of side-chain length on the glass formation of general classes of branched polymers with a fixed molecular mass, as characterized by the relative rigidities of the backbone and side chains. We consider the crossover from linear polymers to comb polymers with side chains of moderate length to bottlebrush polymers. The relative rigidities of the backbone and side chains are shown to be key factors in dictating the general trends of the characteristic properties of polymer glass formation in variation with the side-chain length. Both the glass transition temperature and fragility are predicted by the GET to decrease on increasing the side-chain length in the polymer class with a relatively stiff backbone and flexible side chains, whereas the trends are reversed in the other classes of polymers. Simulation results of a coarse-grained polymer model are shown to be largely consistent with the predictions from the GET, although some differences between the GET and simulation are also observed. An analysis based on simulation results reveals the presence of a power-law relationship between the average segmental structural relaxation time and the structural relaxation times corresponding to different parts in the monomers of branched polymers, indicating that it is adequate to utilize the average segmental structural relaxation time to discuss many aspects of glass formation in branched polymers. Our work demonstrates that the GET can be a convenient theoretical vehicle for studying the glass formation of branched polymers.

Automated summary

This study investigates the influence of side-chain length on the glass formation of branched polymers using generalized entropy theory and molecular dynamics simulation. The relative rigidities of the backbone and side chains are key factors in determining the characteristic properties of polymer glass formation. The predictions from the theory are largely consistent with simulation results, suggesting that the theory can be a useful tool for studying the glass formation of branched polymers.