Spatially heterogeneous dynamics in thermoset resins below the glass-transition temperature: Effect of temperature and composition

A photobleaching technique was used to measure the rotational dynamics of rubrene dispersed in thermoset resins. The matrices were polymerized from mixtures of two monomers with five different compositions. At temperatures below the glass-transition temperature, probe rotational correlation times were shorter and showed a much weaker temperature dependence than those observed in glassy homopolymers. The probe correlation functions became increasingly nonexponential as the amount of the minor component in the matrix increased, presumably because a more heterogeneous set of environments resulted. Dynamics in the single-component sample were quite homogeneous at room temperature. In contrast to homopolymer systems, a bimodal distribution of local relaxation times developed with the addition of the second component. At a given polymer composition this bimodal distribution changed shape with temperature in a reversible manner.