The structure-property relationships in a new class of segmented copolymers, the smectic forming main chain liquid crystal polymers (LCPs), were systematically investigated. Soft segment length effects on polymer thermal phase behavior were studied. A series of segmented copolymers with different soft segment average lengths were prepared and characterized. Increasing soft segment length was found to favor the formation of smectic mesophases, irrespective of soft segment polydispersity. For interpreting the observations, a semi-quantitative thermodynamic theory was formulated, and a universal thermal phase diagram was proposed for segmented copolymers. Crystalline state structure and mesophase aggregation of these polymers were investigated. A crystalline state structure, in which the hard and soft segments are microphase separated, some of the soft segments have nearly fully extended conformation, and the hard segment crystal structure is independent of soft segment length, was proposed for smectic forming LCPs. The soft segment length was found to have significant effects on hard segment aggregation in the smectic phase. The observed thermal properties were interpreted. Morphology in the smectic state and melting behavior of a smectic forming LCP were studied. In the stable smectic state, the polymer showed a macrophase separated morphology, comprising smectic domains and an isotropic 'sea'. These smectic domains are cylindrical in shape and tens of microns in size. On cooling from smectic state, both smectic and isotropic phases crystallized and formed two different crystal structures. Consequently, the polymer showed dual melting endotherms, with their appearance strongly depending on sample thermal history. Crystallization kinetics of three different phases were investigated in a smectic forming LCP. The purely isotropic phase showed similar crystallization behavior as ordinary polymers. The crystallization of the purely smectic mesophase was found to be a local reorganization process and occurs within hard segment layers. The crystallization of the smectic-isotropic biphase showed complicated behavior, due to the simultaneous crystallization of both phases. The smectic phase was believed to nucleate the crystallization of the isotropic phase. Rheological behavior in the smectic state was investigated. A strong shear thinning at low shear rates was observed. On frequency sweeps, the polymer showed a two zone behavior, a 'solid-like' zone and 'liquid-like' zone. Further morphological investigations indicated that by low-rate shearing, the smectic domains were aligned along the shear direction. The domain alignment by flow was believed to be responsible for the observed rheological properties. By elongational flow, the polymer chains can also be oriented.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-9008 |
| Date | 01 January 1994 |
| Creators | Wu, Bing |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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