A computationally-informed unified view on the effect of polarity and sterics on the glass transition in vinyl-based polymer melts

We unveil a unified view on the effect of side chains on the glass transition temperatures (T_g) in polymer melts using molecular dynamics simulations, density functional theory calculations, and available experimental data. We use acrylates as a model system and evaluate the effect of n-alkyl side-chains on T_g. We find that backbone dihedral angle fluctuations follow established patterns due to sterics, as expected. However, we also find that the dihedral angle orthogonal to the backbone, which normally is neglected when discussing the effect on T_g, introduces a secondary rotational degree of freedom which impacts strongly T_g. These results are in agreement with experiments, and generalize to multiple other polymer systems, as is demonstrated using available experimental data. Conversely, n-alkyl pendant groups attached to the side group reduce T_g. Our work establishes a coherent framework that unifies previously established trends, emphasizing the polarity and size effects of n-alkyl chains on T_g.