Composition-Dependent Thermomechanical Stability of Epoxy–POSS Nanocomposites in their Glassy and Rubbery States

Shape memory polymers (SMPs) are frequently exposed to coupled thermal and mechanical loads across the glassy-to-rubbery transition, where their load-bearing capability is significantly reduced. This study systematically investigates the effects of the type and content of polyhedral oligomeric silsesquioxane (POSS) on the thermomechanical stability of epoxy–POSS (EPOSS) SMP nanocomposites in both glassy and rubbery states. Epoxide-functionalized (EP–POSS) and amine-functionalized (AM–POSS) molecules were incorporated into an epoxy resin at loadings up to 73 wt.%, thereby enabling controlled modification of the network architecture and intermolecular interactions. In the glassy state, EP–POSS leads to a deterioration in the mechanical performance, attributed to plasticization and increased chain mobility resulting from flexible ether linkages and unreacted molecules. In contrast, AM–POSS leads to moderate reinforcement, attributed to rigid phenyl groups, reduced flexible linkages, and filler-like effects. In the rubbery state, a fundamentally different behavior is observed: EP–POSS significantly increases the effective crosslink density, resulting in an exceptionally high rubbery state Young’s modulus of up to ∼250 MPa, whereas AM–POSS exhibits only a limited effect. Thermal properties, including thermal expansion, thermal diffusivity, thermal conductivity, and specific heat capacity, are only moderately affected by POSS incorporation, indicating that the observed thermomechanical behavior is governed primarily by molecular structure. A schematic structure–property model is proposed to rationalize these trends and provide insights into the design of SMP nanocomposites for demanding environments.