Exploiting the heterogeneity of cross-linked photopolymers to create High-T-g polymers from polymerizations performed at ambient conditions

There has been a significant debate related to whether a material with a hi gh glass transition temperature can be obtained from a polymerization perfo rmed at a significantly lower temperature (e.g., ambient temperature). To i nvestigate this issue and understand the factors that play a role in determ ining the relationship between glass transition temperature (T-g) and cure temperature (T-cure) three model systems with different degrees of heteroge neities were studied: triethylene glycol dimethacrylate (TEGDMA), diethylen e glycol dimethacrylate (DEGDMA) (more heterogeneous), and styrene-divinyl- benzene copolymer (St-co-DVB) (more homogeneous). These systems were photop olymerized in a temperature cell while simultaneously monitoring the sample temperature with the T-cure varying from 25 to 95 degreesC. The polymeriza tion rate and final double-bond conversion were monitored using near-infrar ed (NIR) spectroscopy. The T-g 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 a ssociated with characterizing chain polymerizations (specifically, trapped radicals that persist after cure) were eliminated. It was found that the di fferences between T-g and T-cure were significantly greater for the more he terogeneous multi (EG)DMA system than for the less heterogeneous St-co-DVB system with the T-g - T-cure being as large as 100 degreesC. Therefore, het erogeneous networks with broad distributions of relaxation times like DEGDM A exhibit unique cure behavior and facilitate obtaining a higher T-g as a f unction of T-cure than is possible in a comparable, more homogeneous networ k.