Structural evolution of dimethacrylate networks studied by dielectric spectroscopy

In this work, the structural heterogeneity of copolymers formed by photopolymerizations of diethylene glycol dimethacrylate (DEGDMA) and poly(ethylene glycol 600) dimethacrylate (PEG600DMA) has been characterized as a function of double bond conversion and cross-linking density las measured by comonomer composition) by dielectric analysis (DEA). The heterogeneity has been characterized by a distribution parameter which represents the breadth of the polymer relaxation spectrum. It was found that the distribution parameter as measured by DEA matched well with those estimated previously by dynamic mechanical analysis. In addition to obtaining fundamental structural information from the alpha-transition (or glass transition) region, beta-transitions (or secondary transitions) were studied in the poly-(DEGDMA-co-PEG600DMA) networks studied. These secondary transitions are observed as a result of unreacted pendant as well as monomeric double bonds that are present in the samples. The heights of the damping factor peaks at the secondary transitions increase as the cross-linking density of the copolymer is increased (by increasing the DEGDMA composition). Further, the peak height decreases as the conversion increases in a given sample, strongly suggesting that the residual unsaturation in the copolymer network is responsible for the beta-transition. Evidence is also presented that precludes the possibility that the initiator or its fragments cause the secondary transitions.