In Situ Investigations of Microstructure Formation in Interpenetrating Polymer Networks

Interpenetrating polymer networks (IPNs) represent an effective strategy for compatibilizing immiscible polymers to enhance the mechanical properties of the final material. While it has been established that the macroscopic properties are dependent on the microstructure, it is unknown why various microstructures are formed in IPNs because the microstructure is often trapped in a nonequilibrium state. To explore this, we conducted a study to establish a relationship between polymerization kinetics and microstructure formation in polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA) IPNs. By manipulating the UV curing intensity, we observed three distinct morphologies: isolated PMMA-rich spheres within a PDMS matrix with a monomodal domain size distribution, spheres with a bimodal size distribution, and a clustered domain microstructure. To investigate the different phase separation mechanisms, we correlated in situ small-angle X-ray scattering (SAXS) to track microstructure formation and Fourier transform infrared spectroscopy (FT-IR) to track polymerization kinetics. Based on our findings, we propose that the monomodal sphere microstructure formed via spinodal decomposition. The positions of the domains are kinetically trapped in the PDMS network, preventing macrophase separation. Similarly, the clustered domain microstructure also arises from spinodal decomposition, but increased mobility within the PDMS matrix enables domains to aggregate after network percolation. In contrast, the bimodal spherical morphology is attributed to a combination of nucleation and growth, and spinodal decomposition. We postulate that these different mechanisms are dictated by changes in the PMMA molecular weight during polymerization. Through the examination of polymerization kinetics and microstructure formation, we have proposed multiple mechanisms that explain the microstructure formation in IPNs.