Enhancing polymer T$_{\mathrm{g}}$ and tuning mechanical properties with stiff molecular additives

Small-molecule additives are commonly employed to alter glass formation, mechanical, and transport properties of polymers. For example, plasticizers are used to suppress polymer T$_{\mathrm{g}}$ and soften the glassy state, while antiplasticizers, which stiffen the glassy state of a polymer while suppressing its T$_{\mathrm{g}}$, are employed to enhance protein and tissue preservation. Recent advances in the understanding of additives' effects on glass formation suggest that additional combinations of temperature-dependent alterations to properties including T$_{\mathrm{g}}$, viscosity, and glassy moduli can be obtained via rational selection of additive properties. Here we employ coarse-grained molecular dynamics simulations to study the effect of introducing a stiff molecular additive to an unentangled polymer melt. Results indicate that, in contrast to plasticizer and classical antiplasticizer additives, these stiff molecular additives enhance the T$_{\mathrm{g}}$ of the matrix polymer. We further examine the impact of these additives on glassy moduli and yield stress of the polymer. These results highlight the importance of additive stiffness as a design parameter enabling more rational control of glass formation behavior.