Unexpected Effects of Alkyl Side Group Length on the Relaxation Rate in Disulfide Exchange Poly(acrylate) Networks

The concept of covalent adaptive networks (CANs) has garnered significant attention for potential applications as sustainable materials. The efficiency of macroscopic functions, such as healability and recyclability, correlates with the relaxation rate at the molecular level, which is determined by the frequency of bond exchange. While recent studies have extensively examined the influential network factors affecting the relaxation rate, the impact of glass transition temperature (T_g) has been rarely studied. In this work, we fabricated poly(acrylate)-based CANs with varying T_g values by utilizing acrylate monomers with different alkyl side lengths. In our design, bond exchange occurs via disulfide exchange, with tertiary amino groups chemically attached to serve as internal catalysts. We initially hypothesized that the relaxation rate would be higher for the samples with a low T_g due to increased strand mobility. Interestingly, this expectation is not valid at temperatures significantly higher than the T_g; the samples with a higher T_g exhibited a higher relaxation rate. This finding can be interpreted in terms of effects of steric hindrance and the polarity of the alkyl side groups on the diffusion of exchangeable units and the catalytic units. Overall, we offer insights into the unexpected influence of steric factors in strand polymers, which can sometimes outweigh the strand mobility determined by T_g.