Morphological Adjustment of Cationically Polymerized Epoxy Networks for the Implementation of Functional Properties

Material improvements in polymer science are often based on morphology adjustment through incorporation of additives, especially telechelics. For this, a view on natural materials, such as bone, spider silk, human enamel, or nacre reveals that crystallinity is a dominating factor for improved mechanical properties. Furthermore, synthetic materials, e.g. elastomers, thermoplastics, and shape-memory polymers have been pointed out to show functional as well as increased mechanical properties by a defined crystallinity. The use of crystalline domains in thermosets, such as epoxy resins, is only rarely discussed in the literature. This work shows how enhancements in both strength and toughness are achievable in cationically polymerized, epoxy based thermosets due to a semi-crystalline character of the resulting network. Furthermore, functional properties are generated by the materials morphology and composition. The investigations of these materials show combinations of versatile thermo-mechanical properties with moldability, shape-memory behavior, and stress relaxation performance which emphasize them as smart materials. Semi-crystalline polyester polyols, such as poly( -caprolactone) or poly(omega-pentadecalactone), are suitable telechelics to introduce crystalline domains in epoxy networks. Additionally, these telechelics are able to react with the epoxy resin due to a chain transfer reaction known as activated monomer (AM) mechanism. Crystallinity adjustment in epoxy resins is possible by the control of reaction mechanisms and reaction conditions during cationic polymerization. For this, an enhanced segregation into crystalline domains as well as nano-domain formation is observed by suppressing the AM mechanism e.g. by esterification of the polyester polyol end groups or by increased reaction rates due to high curing temperatures. In the last case, the polymerization by epoxide propagation proceeds preferably compared to the AM mechanism. Strong phase separation during polymerization leads to both enhanced strength and toughness, but also to superior adhesion properties when the crystal sizes are small; preferably below one micrometer. Moldability of those crosslinked networks has been observed under certain conditions due to the occurrence of a transesterification reaction. It is shown that reaction conditions and the choice of polyester are responsible for the occurrence of crystallinity and; furthermore, for its unique properties as universal smart material