| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 95 === Block copolymers containing aliphatic polyesters draw extensive attention in the preparation of nanoporous polymers attributed to the easy degradation of ester groups by hydrolysis. The ordered nanoporous polymer is very suitable to be the template for fabrication of nanomaterials due to the nanoreactor concept. In this study, a series of degradable block copolymers, poly(styrene)-b-poly(L-lactide) (PS-PLLA), has been synthesized. We try to control the orientations of lamellar and cylindrical nanostructures in thin-film state.
For lamellar thin-film system, well-oriented lamellar nanostructures of PS-PLLA thin films were produced by using the crystallizable solvents: benzoic acid (BA) and hexamethylbenzene (HMB). In strongly segregated PS-PLLA (PS130-PLLA106 (fPLLAv=0.49)), the oriented nanostructures were obtained through the PS-PLLA microphase separation due to directional crystallization of crystallizable solvents at the eutectic point (directional eutectic solidification) regardless of the PLLA crystallization. Lattice matching between crystalline substrates and PLLA was found to be non-essential for inducing nanostructure orientation although it may improve its orientation order. In weakly segregated PS-PLLA (PS36-PLLA32 (fPLLAv=0.49)), the oriented nanostructures were formed through the directional crystallization of PLLA on the substrates (crystallization-induced orientation) regardless of the directional eutectic solidification and lattice matching. Two mechanisms: mechanisms of directional eutectic solidification and of crystallization-induced oriented nanostructure have thus been identified. Oriented, defined lamellar trenches can be prepared by hydrolysis of PLLA component; providing a possible path to prepare nanopatterned templates with lamellar nanochannels.
For cylindrical thin-film system, well-oriented, perpendicular PLLA cylinders of PS-PLLA thin films were efficiently achieved by spin coating using appropriate solvents regardless of the use of substrates. After hydrolysis of PLLA, well-oriented HC nanochannel arrays over large area in addition to uniform surface with controlled thickness and domain size were obtained; providing a simple and efficient path to prepare nanopatterned templates for applications. The induced orientation of PS-PLLA nanostructure was strongly dependent upon the evaporation rate of solvent and its solubility between constituted blocks. The origins for the formed perpendicular HC morphology were also systematically studied. The primary concern of controlled morphology for nanopatterning is to develop ordered microphase-separated morphology by considering the time scale for segregation, namely segregation strength during solvent evaporation. The induced orientation is attributed to the permeation discrepancy between phase-separated microdomains. The perpendicular morphology is initiated from the air surface, and formed in order to create an optimized condition (i.e., the fastest path) for solvent evaporation whereas parallel morphology may impede the evaporation of solvent molecules. Following the nucleation of microphase separation, the perpendicular morphology can be kinetically induced by solvent evaporation.
For application approach, we demonstrate the possibility to fabricate the inorganic nanohelices and helical nanocomposites by the combination of chiral degradable block copolymer and sol-gel chemistry. Inorganic nanohelices have attractive applications in nanomechanical, -sensing, -electronic, -electromagnetic, -optoelectronic devices, and composite materials. In addition, the well-oriented inorganic helical nanostructures can be used as photonic crystals, alignment films for liquid crystal and so on. 3D hexagonally packed helical nanostructure of PLLA in the PS matrix was formed by self-assembly of chiral block copolymer PS-PLLA with specific volume fraction (PS267-PLLA118 (fPLLAv=0.33)). After hydrolysis of PLLA blocks, helical nanoporous PS template was obtained. Subsequently, the silica precursor mixture was introduced in PS template by pore-filling process at room temperature. After aging under controlled humidity and then drying under atmosphere, the PS/SiO2 helical nanocomposite was successfully prepared. In addition, we also can prepare the SiO2 nanohelices by degradation of the PS template by UV exposure. As a result, the formation of inorganic nanoobjects and inorganic/organic nanocomposites from the degradable block copolymers represents an excellent way for the manufacturing of materials in nanoscale through templation.
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