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Growth and self-assembly of macromolecular systems
Abstract
In the first part of this thesis, we consider the physical phenomena accompanying the growth of highly branched polymers through computer simulation. The resultant dendritic molecules have unique properties arising from their "modified" Cayley tree branching pattern. We first consider growth of starburst molecules with flexible spacers separating tri-functional branch points. A self-avoiding walk algorithm is employed to kinetically grow the molecules. From the intramolecular density profiles of these structures, we find that the branch ends traverse the molecule throughout growth and are not confined to its surface. Further we observe that the branches are highly folded at all stages of growth. We observe power law relationships correlating the radius of gyration of the molecule to its molecular weight, M, and spacer length, P, finding in general: $R\sb{g}\sim M\sp\rho P\sp\nu$ with $\rho=0.22\pm 0.05$ and $\nu=0.50\pm 0.05$ at high molecular weights. From this we predict the hydrodynamic characteristics of the molecule. We then explore generalizations of the starburst structure by considering first the effect of branch stiffening, and second, the effect of changes in dendrimer connectivity by considering a related structure, the comb-burst. We repeat our study described above for these structures. In general we observe similar behavior to that described above, however slightly modified due to the structural modifications employed. The second part of this thesis addresses polymeric systems exhibiting the phenomenon of self-assembly. The specific problem under consideration is the characterization of phase transitions in diblock copolymer systems using density functional theory. We present a comprehensive, general scheme which allows the characterization of microphase separation of A-B diblock copolymer systems in terms of observed physical phenomena at all degrees of segregation. Our method is based on the density functional theory of Melenkevitz and Muthukumar and uses the technique of density profile parameterization to greatly reduce the technical complexity of the solution. We find that the microphase separated systems pass through three stages of ordering as the system is quenched. These are the weak, intermediate, and strong segregation regimes. We have calculated the phase diagram for three ordered morphologies: lamellae, hexagonally-packed cylinders, and body-centered-cubic spheres. We also characterize these microphases by the dependence of the lattice constant, D, and the interfacial width, $\sigma\sb{o},$ on the quench parameter $\chi N.$ We correctly reproduce the behavior predicted by previous theories describing the weak and strong segregation regimes. Through investigation of $D\sim N\sp\alpha,$ we find that $\alpha$ depends on both block length and morphology in the intermediate segregation regime. We attribute this behavior to chain stretching arising from the phenomenon of localization.
Subject Area
Polymer chemistry
Recommended Citation
Lescanec, Robert Louis, "Growth and self-assembly of macromolecular systems" (1992). Doctoral Dissertations Available from Proquest. AAI9305861.
https://scholarworks.umass.edu/dissertations/AAI9305861