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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

The effects of molecular architecture and conformational asymmetry on block copolymer morphology

Pochan, Darrin John 01 January 1997 (has links)
The effects of molecular architecture and block conformational asymmetry on the equilibrium bulk morphological behavior of strongly phase-separated, amorphous block copolymers have been studied. Transmission electron microscopy techniques and small angle x-ray scattering, as well as small angle neutron scattering, were primarily utilized to characterize the block copolymer morphologies. Both architecture and block conformational characteristics are found to be molecular parameters, in addition to the relative volume fractions of the constituent blocks, with which one can controllably manipulate the bulk morphological behavior. The effects of novel molecular architecture were discerned via a systematic morphological study of a series of simple graft A$\sb2$B, or "Y", block copolymers where A=polyisoprene (PI) and B=polystyrene (PS). In the microphase separated state a 2:1 A to B arm number asymmetry is introduced across the AB interface due to the simple graft architecture. This arm number asymmetry causes significant deviations in the volume fraction dependence of the morphologies formed by the A$\sb2$B series as compared to the volume fraction dependence of linear diblock morphology. In addition, at a unique volume fraction in the A$\sb2$B series where the two PI arms per molecule are first forced to the concave side of the interface, a new morphology in neat block copolymers is observed which has not been predicted by theory. The bulk morphological behavior of a series of poly(isoprene-block-tert-butylmethacrylate) linear block copolymers was characterized. The larger unperturbed dimension of PtBMA, due to its larger statistical segment length relative to PI, provides for a lower PtBMA entropic chain stretching penalty in the microphase separated state. This also causes the relative volume fraction windows in which morphologies are observed to shift to higher relative volume fractions of the more easily stretched PtBMA block than found in conformationally symmetric AB linear diblocks. In addition, initial morphology studies on more complicated graft architectures and linear diblocks with tunable conformational asymmetry are presented.

Direct imaging of deformation and disorder in extended-chain polymers

Martin, David Charles 01 January 1990 (has links)
The rigid-rod polymers poly(paraphenylne benzobisthiazole) (PBZT) and poly(paraphenylene benzobisoxazole) (PBZO) can be spun from lyotropic liquid-crystalline solutions into solid fibers with extraordinary mechanical properties. However, these fibers are comparatively weak in compression, with deformation occurring by strain localization into kink bands. Here, we examine the ultrastructure of PBZT and PBZO fibers as a function of processing condition. In particular, High Resolution Electron Microscopy (HREM) is used to directly image structural features such as grain boundaries, dislocations, and the molecular level details of deformation processes. HREM images of PBZT and PBZO enable the quantitative determination of crystallite size, shape, orientation, and internal perfection as a function of processing condition. The nature of the disorder present within and between PBZT and PBZO crystallites is analyzed, modeled, and compared to analyzed, modeled, and compared to experimental Wide Angle X-ray Scattering (WAXS) and Selected Area Electron Diffraction (SAED) data. HREM images in and around PBZT and PBZO kink bands reveal that local, sharp bending and/or breaking of covalently bonded molecules is involved in compressive failure. A model for kinking is proposed which involves the nucleation and growth of a region of shear deformation bounded by partial edge dislocations. A stress analysis using this model is used to correlate systematic morphological features of kinks with local material instabilities. A quantitative analysis of the linear density of kinks with a fiber as a function of applied plastic strain enables the energy required to form a kink to be determined. The geometry and energetics of grain boundaries in extended chain polymer solids is discussed. Possible mechanisms for grain boundary motion are presented. Comparisons between different grain boundary structural models and experimental HREM data are shown.

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