| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 96 === The self-assembly of synthetic supramolecules has been inspired by using the secondary interactions, and has already created a large number of nanoscale architectures. Among self-assembled architectures, helical morphology is probably the most fascinating morphologies in nature. Chirality of compounds has been referred to one of the main origins for the formation of helical textures. For the self-assembly of block copolymers (BCPs), one-, two-, or three-dimensional periodic nanostructures can be easily tailored by molecular engineering of synthetic BCPs. By taking the advantage of the BCP self-assembly and the specific configuration of chirality, chiral block copolymers (BCP*), poly(styrene)-block-poly(L-lactide) (PS-PLLA), containing both achiral PS and chiral PLLA blocks was designed for self-assembly. A unique transmission electron microscopy observation and small-angle X-ray scattering pattern for the self-assembled PS-PLLA nanostructure revealed the novel morphology of a hexagonally packed nanohelical phase. On the basis of the geometric features, the space group of the new nanohelical phase was identified as P622. The formation of the nanohelical phase from the BCP* self-assembly is thus referred to the contribution of chiral entities and might provide a new concept for design of the helical morphology in bulk. Because of large-size polymeric chains, the molecular chain dynamics is critical for the formation of stable and equilibrium morphology. Thermally induced transitions between the microphase-separeated morphologies are well known such as a HEX to BCC, a HPL to gyriod and a gyroid to HEX phase by varying the Flory interaction parameter x (a function of 1/T) at fixed block copolymer composition. In this study, to finely control formation of the helical morphology, metastability and order-disorder transition temperature for the nanohelical phase was examined by using time-resolved small-angle X-ray scattering and corresponding transmission electron microscopy. The phase transition of PS-PLLA after solution casting from a HPL to nanohelix was obtained by thermal annealing. With substantial time for annealing, the formed nanohelical phase might transform to a HEX phase; suggesting that the nanohelix is a metastable phase. Also, molecular weight effect on the formation of the nanohelical morphology was examined at a constant composition. The appearance of a gyriod instead of nanohelical phase for low-molecular-weight PS-PLLA indicates the formation of nanohelical phase is dependent upon segregation strength. To manipulate the three-dimensionally packed nanohelical morphology from BCP* in bulk, various stimuli were applied such as crystallization and shear stress. The self-assembled nanohelices can transform into crystallization- and shear-induced cylinders. The stress-induced cylinders (stretched nanohelices) were found thermally reversible upon annealing through undulation. As a result, the hexagonally packed PLLA nanohelices in PS matrix of chiral PS-PLLA block copolymers appear as spring-like behavior in response to the applied stimuli. This unique phase behavior thus creates a possible way for manipulating switchable nanohelical structures in practical applications. Since the crystallization-induced cylinder from nanohelical phase can be achieved by control of crystallization temperature, comprehension of the crystalline details within the nanohelical microdomain is critical. Various crystalline PS-PLLA nanostructures were obtained by controlling the crystallization temperatures of PLLA (Tc,PLLA) at which crystalline nanohelices (PLLA crystallization directed by helical confined microdomain) and crystalline cylinders (stretching of helical nanostructure dictated by crystallization) occur while Tc,PLLA<Tg,PS (the glass transition temperature of PS) and Tc,PLLA≧Tg,PS, respectively. As evidenced by simultaneous two-dimensional small-angle X-ray scattering and wide-angle X-ray diffraction as well as selected area electron diffraction, while Tc,PLLA<Tg,PS, owing to the directed crystallization by helical confinement, the preferred crystalline growth leads the crystallization following the helical track with growth direction parallel to the central axes of nanohelices through twisting mechanism. By contrast, while Tc,PLLA≧Tg,PS, the preferred growth may modulate the curvature of microdomains by shifting the molecular chains to access the fast path for crystalline growth due to the increase in chain mobility so as to dictate the stretching of nanohelices. It is known that aliphatic polyesters can be hydrolytically degraded owing to unstable character of the ester group. For application, the helical channels can be fabricated by elimination of PLLA through hydrolysis. By using field emission scanning electron microscopy and scanning probe microscopy, etched morphology of nanohelical structure can be identified. Stimulated by the idea of the nanoreactor, fabrication of nanohelical composites and inorganic nanohelices are successfully achieved by using the nanohelical channels as templates.
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