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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.
1

Controlling polymer thin film structures by tuning interfacial interactions

Huang, Elbert E 01 January 2000 (has links)
The utilization of poly(styrene-random-methyl methacrylate), P(S-r-MMA), random copolymer brush surfaces to control the structure of polymer films is demonstrated. Random copolymer brush layers were generated by end anchoring the copolymers onto silicon substrates. By dictating the chemical composition of the random copolymer, the resultant brush layer could be precisely tuned to have properties ranging from pure PS to pure PMMA. This was determined by experiments where PS and PMMA homopolymer films showed dewetting behavior characteristic of the interactions between the homopolymer film and the underlying brush layer. Brushes having a styrene fraction of 0.58 have balanced interactions with PS and PMMA and constitute surfaces that are nonpreferential or neutral to PS and PMMA. The influence of interactions between polymer films and random copolymer brush layers is addressed for three systems. First, the interfacial structures of dPS and dPMMA homopolymer films with the brush layer were examined with neutron reflectivity (NR). The interfacial width between the two layers strongly depended upon the interactions between them. Furthermore, it is observed that penetration of dPMMA to the silicon substrate can occur for cases where the brush layer does not provide a sufficient enthalpic or entropic barrier. Second, atomic force microscopy studies (AFM) show that PS/PMMA demixed films had morphologies that varied greatly with substrate interactions. With annealing, some of these structures rearranged significantly while others remained relatively unchanged. The third type of polymer film examined in this study, diblock copolymers constitutes the main focus of this work. Normally, preferential interactions of one block at an interface induce a parallel orientation of the block copolymer domains. By utilizing neutral random copolymers, preferential segregation of each block is eliminated resulting in a perpendicular orientation of block copolymer domains. This was shown for poly(styrene-block-methyl methacrylate), P(S-b-MMA), block copolymers having lamellar and cylindrical morphologies. Using a wide variety of techniques the structural dependence of these films with annealing time and the effects of commensurability were examined. By controlling the growth of these domains, the generation of novel film structures comprised of perpendicular lamellar and cylindrical domains was achieved.
2

Hierarchical organization in polymeric systems

Shin, Dongseok 01 January 2007 (has links)
Hierarchical assembly of materials has attracted significant interest, since it provides opportunities to fabricate novel materials. In this thesis, we investigated three different systems where polymer chains organize hierarchically. First, a semicrystalline triblock copolymer, poly(L-lactic acid- b-ethylene oxide-b-L-lactic acid) (PLLA- b-PEO-b-PLLA), was prepared and the effect of the block-wise construction on the sequential crystallization was investigated by comparison to the corresponding homopolymer blend. In the resultant spherulitic morphology, the crystallization of PEO occurred within the framework established by the PLLA crystals. The preformed PLLA crystals biased the PEO chain orientation and the effect was more significant in the block copolymer system, where PEO chains were covalently anchored to PLLA. Secondly, the influence of the microenvironment of multifunctional chains on their organization was studied. For this investigation, styrene-based linear polymers having two different pendant groups, a carboxylic acid and a neutral group, on every repeat unit were prepared. With alkyl (n-C 10H21-) groups as the neutral pendant, the linear macromolecules assembled into thermally reversible globular aggregates through non-covalent interaction with multifunctional tertiary amines. The aggregates had a structural hierarchy and remained stable without inter-particle crosslinking. In the absence of the alkyl pendant groups, control over the structure and properties of the aggregates was lost. In a third system, the coupled self-assembly of bionanoparticles and block copolymers was investigated. A simple way to incorporate bionanoparticles into a thin film of water-insoluble block copolymer was developed by combining the bionanoparticle adsorption on a polymer film and subsequent annealing under solvent vapor. Through the use of a block copolymer having a positively charged component, the loading of bionanoparticles increased significantly. When highly loaded, a hierarchical co-assembly of the block copolymer and the bionanoparticle was observed where the microphase separation of the block copolymer forced a segregation of bionanoparticles to the grain boundaries, forming a much larger scale structure.
3

Polymers on nanoperiodic, heterogeneous surfaces

Rockford, Lee David 01 January 2001 (has links)
Herein we establish a relationship between controlled nanoscale surface interactions and subsequent macromolecular ordering. Chemically heterogeneous striped surfaces of polar silicon oxide and non polar gold are generated over large areas, via glancing angle evaporation on facetted silicon substrates. The processing conditions required for generation of stripe widths comparable to the size of a polymer molecule are outlined. Substrates with 20–30 nm metal linewidths and 40–60 rim stripe periods are prepared. Spin and solution casting of incompatible polymer mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) on heterogeneous surfaces are found to generate films with unique, substrate directed morphologies dependant on the kinetics of the casting process. Spin cast films posses a surface adsorbed layer of blended composition due to rapid polymer adsorption from solution, while solution cast films phase separate at the substrate/polymer interface on a molecular level. Preferential adsorption of PS to the non polar gold stripes and PMMA to polar silicon oxide stripes is observed at the substrate beneath the macroscopically phase separated domains of the blend components. Preferential adsorption occurs over a large molecular weight range, with a molecular weight dependence on the morphology of the adsorbed polymer lines found. Solution cast films of the symmetric copolymer poly(styrene-block-methylmethacrylate), P(S-b-MMA), on heterogeneous surfaces show lamellar microdomain orientations perpendicular to the substrate plane, parallel to the striping. Commensurability of the block copolymer and substrate stripe periods is found to be essential for producing such a surface directed morphology. The commensurability window depends inversely on the degree of confinement of the morphology, with unconfined films requiring more stringent conditions for surface directed morphology reorientation. The distance over which the orientation of the microdomains persists in thick films is found to depend on the ordering kinetics, scaling with copolymer molecular weight. Confinement effects such as tension and compression and defects in the lateral long range orientation of surface directed lamellar morphologies are observed for slightly incommensurate morphologies, with the amount of strain and defect concentration found to increase with the loss of commensurability.

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