Exploiting the Heterogeneity of Cross-Linked Photopolymers To Create High-<i>T</i>_g Polymers from Polymerizations Performed at Ambient Conditions

There has been a significant debate related to whether a material with a high glass transition temperature can be obtained from a polymerization performed at a significantly lower temperature (e.g., ambient temperature). To investigate this issue and understand the factors that play a role in determining the relationship between glass transition temperature (Tg) and cure temperature (Tcure), three model systems with different degrees of heterogeneities were studied: triethylene glycol dimethacrylate (TEGDMA), diethylene glycol dimethacrylate (DEGDMA) (more heterogeneous), and styrene−divinylbenzene copolymer (St-co-DVB) (more homogeneous). These systems were photopolymerized in a temperature cell while simultaneously monitoring the sample temperature with the Tcure varying from 25 to 95 °C. The polymerization rate and final double-bond conversion were monitored using near-infrared (NIR) spectroscopy. The Tg of the exact sample cured within the NIR was subsequently measured using dynamic mechanical analysis (DMA). Since this work utilized living radical photopolymerizations, the traditional issues associated with characterizing chain polymerizations (specifically, trapped radicals that persist after cure) were eliminated. It was found that the differences between Tg and Tcure were significantly greater for the more heterogeneous multi(EG)DMA system than for the less heterogeneous St-co-DVB system with the Tg − Tcure being as large as 100 °C. Therefore, heterogeneous networks with broad distributions of relaxation times like DEGDMA exhibit unique cure behavior and facilitate obtaining a higher Tg as a function of Tcure than is possible in a comparable, more homogeneous network.