Structure and Property Control and Crystallite Influence on Segmental Dynamics in Hydrogenated Polynorbornene

Abstract

This dissertation investigates the properties of a model semicrystalline polymer, hydrogenated polynorbornene (hPN), with two distinct foci. The first focus is on structure-property relationships, and the effect of stereochemical changes in the cyclopentylene ring in the hPN backbone. The second focus examines the effect of hPN crystallites on the dynamics of polymer chain segments near the amorphous-crystal interface, to investigate the presence or absence of a rigid amorphous fraction in hPN. Polynorbornenes are easily synthesized via living ring-opening metathesis polymerization, allowing for tight control over the chain statistics and architecture. Hydrogenation of polynorbornene leads to a semicrystalline polymer even in the absence of strong tacticity (stereoregularity) along the chain, making hPN an ideal semicrystalline polymer system to study. Complete hydrogenation is achieved through a variety of routes. However, the different hydrogenation techniques do not produce identical products. Depending on the hydrogenation route, varying levels of epimerization (stereoinversion of the original cis-cyclopentylene configuration to the trans-cyclopentylene configuration) are achieved, ranging from 0 – 36% trans content. The relationships between epimerization, saturation, and time are investigated to determine the feasibility of tuning the trans content through the hydrogenation method. The influence of trans content on the properties of hPN is investigated. hPN’s major thermal transitions – the glass transition, the polymorphic crystal-crystal transition, and melting – all decrease in temperature with increased trans content. While the glass transition temperature and the melting temperature decrease modestly (< 20 °C) between 0% and 36% trans content, the crystal-crystal transition temperature shows a strong dependence on the trans content, dropping from 135 °C to 71 °C between 0% and 27% trans. At 34% trans content and above, only one crystal polymorph is observed. The room-temperature Young’s modulus shows no dependence on trans content, while the yield stress drops by 20% at 1% trans content, and slowly decreases with additional epimerization. The temperature dependence of the Young’s modulus differs for trans-containing vs. all-cis polymers, while the temperature dependence of the yield stress is set by the polymorph type (rotationally ordered vs. disordered). The crystallization of the all-cis hPN occurs faster than the trans-containing hPNs. Effective control over hPN’s material properties is achieved by blending polymers with different trans contents. Finally, the influence of the amorphous-crystal interface on nearby amorphous chain segments is investigated by fluorescence spectroscopy. The location of the region probed is systematically altered by using a block copolymerization scheme consisting of a polynorbornene block and a substituted polynorbornene block. Fluorescently-active polymers are obtained by termination of the synthesis with 1-pyrenecarboxaldehyde. The experimental parameters are optimized to generate the clearest signature of the glass transition. Agreement of the assigned glass transition temperature between doped homopolymers, labeled homopolymers, and differential scanning calorimetry measurements indicate that fluorescence is sensitive to the glass transition temperature in these systems, and determines the different fluorescence signatures of the transition, which differ according to pendant group flexibility. Polymers with flexible pendant groups exhibit two fluorescence transitions beyond the freezing point upon cooling, while polymers with stiffer pendant groups exhibit a single transition below the freezing point. Fluorescence measurements obtained using the block copolymerization scheme reveal only minor increases in the glass transition temperature with comonomer content, and the transition temperature rapidly converges to that for the bulk homopolymer, indicating that hPN contains at most a very small rigid amorphous fraction.