Exploring glass transition in polyethylene via molecular dynamics: From bulk to isolated chain

In this study, we investigate the glass transition behavior of polyethylene (PE) chains in the bulk and isolated, using molecular dynamics (MD) simulations. Leveraging both simulated dilatometry, Arrhenius analysis, and a procedure based on the evolution of percentage of trans states, we identified three distinct regimes with different behaviors, proposing a glass transition domain delimited by glass transition temperatures ($T_g^l$ and $T_g^u$ delimiting the transition domain, and $T_g^d$ extracted from dilatometry) for bulk polymers. The two latter methods were then used to characterize this domain for isolated chains, allowing us to compare with data stemming from the bulk polymer. Our findings reveal that T_gs of an isolated chain are generally lower than that of the bulk except for $T_g^l$ which remains unchanged. This observation aligns with previous experimental and simulation studies. The study further investigates the dynamic and static flexibilities of the polymer, correlating the potential energy barriers associated with dihedral transitions to the observed $T_g^l$ and $T_g^u$ values. We propose that $T_g^l$ is an intrinsic property of the polymer, as it depends on the potential energy barrier required to escape from the trans state. In contrast, $T_g^u$ is influenced by more complex interactions and is lower for the isolated chain.