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The effects of molecular architecture and conformational asymmetry on block copolymer morphology
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.
Pochan, Darrin John, "The effects of molecular architecture and conformational asymmetry on block copolymer morphology" (1997). Doctoral Dissertations Available from Proquest. AAI9809387.