Structural Evolution and Heterogeneities Studied by Frequency-Dependent Dielectric Sensing in a Styrene/Dimethacrylate Network

Frequency-dependent dielectric sensing and differential scanning calorimetry (DSC) are used to observe the formation of a spatial heterogenety in a dimethacrylate-based network system during polymerization. The presence of two α-relaxation processes as observed in the dielectric sensing measurements very early during the polymerization supports the existence of two cooperative regions of sufficient size to create two T g 's. The two regions do exhibit two T g 's detected by DSC as well as the separately identifiable dielectric α-relaxations. This heterogeneity in the styrene/dimethacrylate system is due to the formation of microgels shortly after reaction initiation. The microgels and their agglomeration form cross-linking regions in a pool of unreacted monomers and oligomers. With the advancement of the reaction, the microgels become larger and start to grow into the monomer−oligomer region. Near the end of the reaction, the low-frequency tail of the microgel and high-frequency portion of the oligomer regions overlap, forming a very broad distribution in the dielectric response and a very broad T g temperature transition spanning 200 °C in the DSC profile.