Study of the properties of modified cyclic butylene terephthalate and its composites

Resum

The thesis at hand deals with the modification of cyclic butylene terephthalate (CBT) with the aim to improve its final properties. CBT polymerizes in an entropically-driven ring-opening polymerization into polymerized CBT (pCBT), which is typically brittle. To increase the toughness of pCBT, a series of chemical and physical modifications of CBT with toughening agents and nano-reinforcements have been investigated. The role of the used processing routes on the properties of unmodified pCBT has been studied and in all cases a brittle pCBT has been obtained. However, it was found that a small amount of tetrahydrofuran can toughen pCBT. Nevertheless, this toughening mechanism was found to be only temporarily. Reactive chain extension using low molecular weight, bi- or polyfunctional chain extenders such as epoxy resins or isocyanates was particularly effective and showed a remarkable toughening effect on pCBT. The epoxy- and isocyanatemodified pCBT polymers exhibited increased failure strain in tensile tests while modulus and strength were not significantly affected; contrary to other common toughening methods such as blending with other polymers, plasticization or copolymerization with soft segments. Nanocomposites of toughened CBT and nano-reinforcements such as organo-montmorillonite, graphene and polyhedral oligomeric silsesquioxanes (POSS) were prepared and thoroughly analysed. The organoclay nanocomposites showed an intercalated-flocculated structure with high stiffness and strength together with a semi-ductile deformation behaviour. The organoclay was further modified by tethering the chain extender to the clay surfactant which resulted in an intercalated organoclay. This chain extender-grafted organoclay reacted then with pCBT and therefore could be exfoliated and randomly dispersed in the pCBT matrix during polymerization. A similar strategy was followed in the synthesis of pCBT/graphene nanocomposites. However, graphene exfoliation could not be achieved due to the strong p¿p interactions between the graphene sheets. Binary blends containing a polyisocyanate- grafted graphene showed an improved stiffness and strength due to enhanced compatibility. pCBT/POSS nanocomposites showed a good potential for nano-modification. However, the challenge was to find a compatible POSS type which did not affect the ring-opening polymerization of CBT. Moreover, toughened carbon fibre fabric-reinforced pCBT composites have been prepared applying the developed toughening methods. Interlaminar shear strength, flexural strength and failure strain of the chemically modified composites increased up to 60% with respect to unmodified pCBT composites. This thesis has been prepared in the Centre Català del Plàstic (CCP) and the Department of Materials Science of the Polytechnic University of Catalonia (UPC) in Terrassa, Spain. A part of the experimental work has been accomplished in the Department of Mechanics and Industrial Production, Mondragon Unibertsitatea (Mondragón, Spain) and in the Department of Polymer Engineering of Budapest University of Technology and Economics (Budapest, Hungary).