Tuning Polymer Glass Formation Behavior and Mechanical Properties with Oligomeric Diluents of Varying Stiffness

Small-molecule diluents are important tools in the control of polymers' glass formation, transport, and mechanical properties. While recent work has indicated that these diluents can impose a more diverse range of effects than previously appreciated, use of these additives to rationally control polymer properties requires a predictive understanding of their effects. Here we employ molecular dynamics simulations to show that diluent-induced changes in a polymer's glass transition temperature T_g can be predicted based on the diluent's Debye-Waller factor ⟨u²⟩, a measure of picosecond time scale rattle-space, via a functional form previously found to predict nanoconfinement-induced shifts in polymer T_g. Moreover, we show that diluent-induced alterations in polymer segmental relaxation time are related to changes in modulus and ⟨u²⟩ via the Generalized Localization Model of relaxation. These results provide new design principles for the use of oligomeric diluents in achieving independent, targeted control of structural relaxation and glassy moduli.