Self -assembly of amphiphilic and nanoparticle containing block copolymer films

Chen Xu, University of Pennsylvania

Abstract

The self-assembly of amphiphilic and nanoparticle containing block copolymer films is explored in this thesis. We first present the stimuli-responsive nanostructures assembled from amphiphilic block copolymers. Amphiphilic block copolymer poly(styrene-b-acrylic acid) (PS-b-PAA) or poly(styrene-b-acrylic anhydride) (PS-b-PAAn) are derived via thermochemical evolution of the tert-butyl groups in poly(styrene-b-tert-butyl acrylate) (PS- b-PtBA). This novel approach leads to the formation of nanostructures containing perpendicular hydrophilic cylinders or ordered spherical domains on silicon substrates. The surface morphology and properties of the nanostructured PS-b-PAA films depend on external stimuli, i.e., selective solvents (water and toluene) and pH. Upon exposure to water, the hydrophilic PAA cylinders swell above the surface and transform to PAA mushrooms that render the entire surface hydrophilic. Upon exposure to toluene, the hydrophobic PS matrix swells above the surface and results in a hydrophobic surface. Because PAA is a weak polyelectrolyte, the films exhibit three unique nanostructured morphologies across three pH regimes. Film wettability can be tuned by directing the arrangement of PAA chains via external stimulation, demonstrating that the nanostructured PS-b-PAA films can display self-adaptive surface properties. This dissertation also explores the dispersion of poly(methyl methacrylate) (PMMA)-grafted magnetic magnetite (Fe3O4) NPs in PMMA and poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films, as well as the self-assembly of PS-b-PMMA/Fe 3O4 NP nanocomposite films. The Fe3O4 NPs are grafted with PMMA brushes of three different molecular weights, i.e., 2700, 13300 and 35700 g/mol (i.e., Fe3O4-2.7K, Fe 3O4-13.3K and Fe3O4-35.7K). The Fe 3O4-35.7K NPs are uniformly dispersed in PMMA films, whereas the Fe3O4 NPs with short brushes form aggregates. This behavior is consistent with the wet and dry brush theory. However, for NPs in PS-b-PMMA films an opposite trend is observed whereby aggregation increases as the brush length increases. This behavior is described by the multi-component Flory-Huggins theory. The morphology of PS- b-PMMA also depends on the molecular weight of the PMMA brush. For the Fe3O4-2.7K NP, the nanocomposite films self-assemble into mixed morphology of perpendicular and parallel lamellae at low NP concentration (≤ 4 wt%), whereas the lamellar structure is frustrated at high NP concentration (i.e., 10 wt%). As the molecular weight of the PMMA brush increases, the Fe 3O4 NPs form aggregates in the as-cast nanocomposite films. Since the aggregate size is larger than the PMMA domain size, the block copolymer has to self-assemble around these aggregates.

Subject Area

Materials science

Recommended Citation

Xu, Chen, "Self -assembly of amphiphilic and nanoparticle containing block copolymer films" (2008). Dissertations available from ProQuest. AAI3310383.
https://repository.upenn.edu/dissertations/AAI3310383

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