Increasing Ductility and Toughness of Photocurable Thermosets with Microstructures Controlled by Phase Separation

In this work, photopolymerization-induced phase separation (photo-PIPS) is implemented into a polymer network consisting of both acrylic and epoxy-based chemistries to allow for the development of an acrylic network through photopolymerization and for enhanced strength and stiffness to be obtained via thermal cross-linking of the epoxy-based chemistries. Two epoxy-based end-group terminated polydimethylsiloxane (PDMS) polymer additives hydroxy-terminated PDMS (OH-PDMS) and diglycidyl ether-terminated PDMS (epoxy-PDMS) are added to enable phase separation, creating a heterogeneous polymeric material. During thermal treatment applied after photopolymerization, these additives can cross link together, concurrently with the cross-linking of the epoxy-based monomers which are part of the network, therefore creating elastomeric, phase-separated subdomains. Mixtures of OH-PDMS and epoxy-PDMS of various molar ratios are used to alter the extent of phase separation and the amount of cross-linking that occurs within the subdomains. It is observed that OH-PDMS does not phase separate when present alone, while it separates in the presence of epoxy-PDMS. Cross-linking within the phase-separated domains containing both types of PDMS leads to enhanced ductility, with the optimal ductility and toughness enhancement being obtained for the PDMS stoichiometry (OH-PDMS:epoxy-PDMS 80:20 by mol %). This composition retains good creep resistance and has only a slightly smaller strength than the neat resin.