Phase separation-induced glass transition under critical miscible conditions†

Plasticizers have been widely utilized to adjust the glass transition temperature (T_g) of glassy polymeric materials. To optimize performance while minimizing volume, plasticizers with a strong affinity for the target polymer are typically chosen. If we consider a combination of a glassy polymer and a plasticizer with a critical miscibility condition, where the miscible/immiscible states are altered by changing the temperature, phase separation induced by temperature variations will trigger the glass transition. In this study, we report on a polymeric material synthesized from a blend of a high T_g polymer and a plasticizer, exhibiting a phase separation-induced glass transition around the upper critical solution temperature (UCST). It is expected from a crossover point of the T_g curve and the demixing curve in a thermodynamic phase diagram, corresponding to the Berghmann point. Poly(isobornyl acrylate) (PIBXA) with an original T_g of ∼100 °C and triethyl phosphate (TEP) were employed as the glassy polymer and plasticizer, respectively. When the TEP fraction was relatively small (∼10 wt%), the sample showed no phase separation and a decrease in T_g compared to that of the pristine PIBXA, following the conventional trend of plasticizer addition. Conversely, at 20 wt% or higher fractions, the samples displayed UCST-type phase separation and an abnormal increase in T_g with increasing plasticizer content. Furthermore, this miscible/immiscible transition can be predicted through an analysis of the temperature-corrected Hansen solubility parameter (HSP). This report proposes a novel role for plasticizers in adjusting T_g and prediction of objective combinations that satisfy the critical miscible condition.