Enhancing the Solvent Resistance of Random Copolymer Films via Sequential Infiltration Synthesis: Low Functional Group Density Can Make a Large Impact

The resistance of polymers to solvents is a critical property for their integration into applications such as medical devices, energy storage, and membranes. Sequential infiltration synthesis (SIS) is a promising technique for enhancing the chemical stability of polymers, minimizing their solvent dissolution, through vapor phase-based growth of inorganic materials within the polymers. SIS is effective in polymers with functional groups that are reactive with SIS precursors, such as polymethylmethacrylate (PMMA), but is ineffective in polymers that lack reactive groups, such as polystyrene (PS). By combining these two materials as a random copolymer (P(S-r-MMA)), polystyrene-based materials may be modified with SIS according to the functional group density, which scales with methacrylate (MMA) concentration. Herein, we use various compositions of random copolymers to explore how solvent resistance is affected by SIS cycles and functional group density. A series of P(S-r-MMA) samples were modified with one or five SIS cycles using trimethylaluminum and water, then exposed to liquid and vapor toluene environments. Changes to film mass and appearance were monitored with a quartz crystal microbalance (QCM), infrared spectroscopy, and optical microscopy. We found that a single SIS cycle led to mass and thickness retention as high as ∼70% with increasing MMA content, indicating the presence of inorganic crosslinks. With five SIS cycles, films with as little as ~10% MMA and above it had near complete thickness retention, in agreement with lower growth rates in a high number of cycles, evidenced by in situ QCM measurements. This demonstrates the role of diffusion barriers in SIS processes. Thus, a combination of crosslinking and transport resistance led to tunable solvent resistance according to the number of SIS cycles and the MMA content of the copolymer.