The phenomenon that crystallinity leads to enhanced toughness and strength in cationically polymerized, crosslinked epoxy resins is up to now only rarely discussed. We demonstrate the controlled crystallization of partially crystalline poly(ε-caprolactone) (PCL) in a model system based on a cycloaliphatic epoxy matrix and a thermolatent super acid as initiator. To adjust the morphology and the degree of crystallinity in such epoxy resins, the manipulation of the polymerization mechanisms as well as occurring transesterification is a proper way. To illustrate the interplay of the different occurring mechanisms and reactions, samples with the same composition were polymerized under varied conditions leading to different degrees of crystallinity XC (XC: B &gt; A ≈ D &gt; C; see figure 1) and furthermore to differences in morphology and mechanical behavior. It has been shown that mild curing conditions lead to a sufficient flexibilization of the epoxy matrix due to an exchange reaction between the epoxide and the polyester polyol, named activated monomer mechanism (AM). Finally, such a flexibilization enables phase separation in combination with a high degree of crystallinity. Furthermore, annealing the already completely cured polymer under more intense conditions results in a higher amount of transesterification compared to mild conditions which could be evidenced by model experiments. The super acid which is generated during the thermal treatment acts as transesterification catalyst so that parts of the PCL chains are integrated into the crosslinked network going hand in hand with a decreased degree in crystallinity. This means, to preserve crystallinity and therefore the polymer’s superior mechanical properties, the transesterification has to be prevented. Otherwise, the ability of transesterification in crosslinked polymers can be used for stress relaxation. Additionally, such a phenomenon can be accelerated by using a typical transesterification catalyst like titanium(IV) butoxide.

Partially crystalline epoxy resins: Investigations on dynamics, morphology, and mechanical behavior

250th National Meeting (2015)

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