Network dynamics in cationically polymerized, crosslinked epoxy resins and its influence on crystallinity and toughness

- Crosslinked epoxy resin with molecular dynamics.
- Transesterification reaction leads to formable thermosets.
- Concomitantly enhanced stiffness, strength, and toughness.
- Adjustment of crystallinity in crosslinked epoxy networks.

Crystallinity in crosslinked epoxy resins is a rarely observed phenomenon. If crystallinity is properly adjusted, it can lead to enhanced toughness in cationically polymerized epoxy resins. This work demonstrates the controlled crystallization of partially crystalline poly(ɛ-caprolactone) (PCL) in a model system based on a cycloaliphatic epoxy matrix polymerized with a thermo-latent, cationic initiator. Samples with the same composition were polymerized under varied conditions leading to different degrees of crystallinity and furthermore to differences in morphology and mechanical behavior. Mild curing conditions lead to a sufficient flexibilization of the epoxy matrix and the epoxide consumption is caused mainly by the activated monomer mechanism. On the other hand, more intense polymerization conditions result in a higher amount of transesterification compared to mild conditions which is evidenced by decreasing crystallinity and higher gel content despite a complete polymerization reaction. The PCL chains are shortened by transesterification while the smaller PCL chains are integrated into the network. Additionally, stress relaxation experiments measured with DMA and deformation-fixation-relaxation experiments reveal a type of flowing and stress reduction of the crosslinked polymers caused by rearrangement reactions. SEM micrographs show that increasing crystallinity leads to a raised roughness of fracture surfaces. Examination of the mechanical properties shows that enhanced crystallinity leads to higher stiffness, mechanical strength and also to superior toughness even if the network density does not significantly change. This work shows how the morphology and properties of the epoxy resin can be adjusted by just small variations in the polymerization conditions.