Crystal Phase Transitions In Hydrogenated Norbornene-Hexylnorbornene Copolymers

Abstract

Ring-opening metathesis polymerization (ROMP) is an area of interest in polymer science due to the range of polymer classes which it can access from semicrystalline to rubbery, its usefulness in forming block copolymers, its retention of backbone double bonds, and in some cases, its ability to control tacticity. Norbornene is the prototypical monomer used in studies of ROMP due to its reactivity and chemical tunability. Hydrogenated polynorbornene exhibits a highly crystalline structure with both a melting temperature (Tm) and a crystal-to-crystal transition temperature (Tcc) above which polymer chains in the crystallites exhibit rotational disorder. This thesis asks the question: what happens to the crystal structure and the phase transition temperatures of semicrystalline polymers produced by ROMP when a bulky comonomer is added? To address this question, a series of norbornene-hexylnorbornene copolymers are synthesized, hydrogenated, and characterized. The results of this research show that norbornene-hexylnorbornene (NB-HNB) copolymers also exhibit a Tm and a Tcc, and that depression of the Tm and Tcc is directly proportional to hexylnorbornene content. The Tcc is depressed more than twice as much as the Tm for a given hexylnorbornene content, leading to the disappearance of the crystal-to-crystal transition altogether in NB-HNB copolymers of more than ~3 mol% hexylnorbornene. These copolymers exhibit only the rotationally disordered phase at room temperature, as is confirmed by abrupt changes in x-ray diffraction data showing the disappearance of several crystal reflections and a discontinuity in the principle interplanar spacing.