Curvature Fluctuations of Asymmetric Photopolymerized Networks: Impact of Solvent-Mediated Interfacial Exchanges

Spontaneous curvature fluctuations develop in asymmetric polymer networks fabricated by frontal photopolymerization (FPP) following "development", the immersion into a selective solvent. We investigate the impact of the choice of solvent on the spatiotemporal response of the gradient networks, employing a model system of UV-cross-linked poly-(ethylene glycol) diacrylate. Experimentally, we find that FPP networks can remain planar or exhibit one or two curvature changes over time depending on the solvent. We quantify the solvent and monomer mass exchanges during development and the overall mass uptake or loss and find a correlation between solvent polarity and the propensity for curvature fluctuations. A simple FPP model accounting for interfacial diffusive mass transfer and the transient formation of asymmetric surface shrinkage and/or swelling boundary layers is found to account for all experimental data. Based on our results, we demonstrate a simple concept for the autonomous propulsion of a submerged device that exploits these spontaneous fluctuations with an appropriate solvent medium.