Morphological control in nanostructured polymers
This dissertation investigates several aspects of an important research topic in polymer science, that is, morphological control in polymer nanostructures. A brief introduction is given in Chapter 1. In Chapter 2, the central issue is defect control in novel noncentrosymmetric (NCS) lamellar block copolymer blends, made from poly(styrene-b-butadiene-b-isoprene- b-cyclohexadiene) and poly(styrene-b-cyclohexadiene). Coexisting centrosymmetric (CS) and NCS lamellar morphologies were observed, as predicted by mean-field theory. Morphologies of NCS tilt and twist grain boundary defects are studied, and detailed analysis of interfacial energies associated with NCS twist grain boundaries are performed. Defects unique to NCS layered systems, such as versatile NCS kink bands and NCS polarity reversals, are also studied in detail. Based on these results, strategies for reducing certain NCS defects are proposed.
In Chapter 3, the issue becomes morphological characterization of a new ABA triblock copolymer, poly(tert-butyl vinyl ether- b-isobutylene-b-tert-butyl vinyl ether) (P tBVE-PIB-PtBVE), a thermoplastic elastomer (TPE) and precursor for poly(vinyl alcohol-b-isobutylene- b-vinyl alcohol) (PVA-PIB-PVA). PtBVE-PIB-P tBVE samples with 23--39 wt% PtBVE content exhibited typical characteristics of TPE with tensile strengths of 9--15 MPa and elongations at break of 760--1300%, and formed lamellar or cylindrical morphologies, depending on molecular compositions.
In Chapter 4, the subject shifts to crystal morphology characterization of a porous crystalline polymer (high density polyethylene, HDPE) prepared through a novel crosslinking-swelling-crystallization (CSX) process. Open pores with oriented lamellar structures in the pore walls were observed, much different from structures found in typical bulk HDPE. Based on the crystal morphology, possible mechanisms for the pore formation in the CSX process are proposed.
In Chapter 5, results from two other polymer systems studied are summarized. For the first system, poly(styrene-b-ethylene butylene- b-styrene), the effects of solvent selectivity and annealing temperature on resulting morphologies are studied systematically. Spherical or lamellar morphologies were initially obtained from three selective solvents; upon thermal annealing all changed into cylindrical morphology. The second system is polydiacetylene (PDA) single crystals of bulk, micron, and submicron sizes. For both PDAs studied, the crystal lattice parameters do not change markedly with decreasing crystal size, suggesting other factors may contribute to the observed change in optical properties.