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The mechanical and adhesion behavior of cationically polymerized, partially crystalline epoxy networks is presented. For this, a reactive and a nonreactive poly(ɛ-caprolactone) (PCL) were used as the crystalline component for the formation of copolymers and polymer alloys, respectively. The trade-off between toughness on the one hand and glass transition temperature and mechanical strength on the other can be reduced by the presence of nanostructures in combination with small crystalline domains (< 1 μm), as in the case of epoxy/PCL-based polymer alloys. This structure–property relationship reveals that a certain degree of crystallinity leads to enhanced toughness in cationically polymerized epoxy networks

Structure–property relations of cationically polymerized epoxy thermosets with different morphologies are examined. The morphology adjustment of amorphous epoxy based copolymers and partially crystalline polymer alloys is carried out with star-shaped poly(ε-caprolactone) (SPCL) bearing various numbers of hydroxyl end groups. These hydroxyl groups are known for their reactivity toward epoxides following the activated monomer (AM) mechanism. For this reason, four-armed SPCL was synthesized with four hydroxyl end groups (SPCL-tetraol) and, in addition, with successively esterified ones down to a SPCL with four ester end groups (SPCL-tetraester). SPCL species bearing fewer or no hydroxyl end groups segregate into needle-like nanodomains within the epoxy networks and, if the concentration is high enough, also into crystalline domains. The stronger phase separation of SPCL-tetraester within the epoxy network compared with SPCL-tetraol is due to a reduction of the AM mechanism. The mechanical properties resulting from different morphologies lead to a trade-off between higher storage moduli and Tg values in the case of the more phase separated (and partially crystalline) polymer alloys and higher strain at break in the case of the amorphous copolymers. Nevertheless, in both cases toughness is improved or at least kept on the same level as for the pure epoxy resin

The structure–property relationship of polyester polyols in cationically polymerized, amorphous epoxy-based copolymers is investigated. An epoxy resin is polymerized in the presence of structurally different polyesters. These resulting copolymers show improved tensile strength and toughness. The optimal epoxide/polyester ratio depends on the structure of the polyesters. Poly(d-valerolactone)(PVL) reveals the highest ester group density of the investigated polyesters, which enhances physical interactions with the epoxide during polymerization as well as in the network. Furthermore, PVL leads to outstanding tensile strength, strain at break, and toughness. Among all polyester polyols examined, PVL leads to the highest gel fraction or, in other words, the most complete integration into the epoxy network. This work shows that polyesters that are present in the reactive system should be covalently integrated into the polymer network as completely as possible to obtain good mechanical properties of the amorphous copolymer

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