Adsorption-desorption effect on physical aging in PMMA-silica nanocomposites.

Poly(methyl methacrylate) (PMMA)-silica nanocomposite was used to study the adsorption-desorption of polymer chains on the surface of silica nanoparticles. We demonstrate that the adsorption-desorption is a dynamic process with respect to temperature and time. The adsorption condition could be changed after annealing at T g + 16 °C. Then, the physical aging under confinement was investigated under different adsorption conditions. At relatively high aging temperatures, we reveal a one-step equilibration with reduced enthalpy recovery of nanocomposite compared with neat PMMA. In contrast, the aging rate of composite is decelerated and then accelerated after a long period of aging at a relatively low temperature due to the emergence of two-step equilibration. Furthermore, the material system with a higher adsorption fraction exhibits a stronger confinement effect on aging. We speculate that the transition from one-step to two-step equilibration corresponds to the activation of segmental α′ relaxation in the interfacial region. The adsorbed chains result in restrained segmental α′ and β′ relaxation and low enthalpy recovery at relatively high aging temperatures. At a relatively low temperature, the interfacial α′ relaxation with high released enthalpy comes to the surface after a long aging time, inducing the elevated aging rate. [Display omitted] • The effect of heat treatment on adsorption-desorption was examined. • A change from one-step to two-step equilibration was found in PMMA/silica nanocomposites with decreasing aging temperatures. • The underlying mechanism was revealed by broadband dielectric relaxation. [ABSTRACT FROM AUTHOR]