掌性團聯共聚物之自組裝於螺旋形態之建構

Helical Architectures from Self-assembly of Chiral Block Copolymers

陳俊谷(Chun-Ku Chen)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00774

摘要

A new helical phase (H* phase) has been found in the self-assembly of poly(styrene)-b-poly(L-lactide) (PS-PLLA) chiral block copolymers (BCPs*) with PS-rich fractions in our laboratory. The formation of the H* phase is attributed to the chiral effect on the self-assembly of the BCPs*. Also, various interesting crystalline PS-PLLA nanostructures (e.g. crystalline helices and crystalline cylinders) could be obtained by controlling the crystallization temperature of PLLA (Tc,PLLA) to adjust environmental conditions (i.e., under soft or hard confinement). The crystallization would affect the self-assembled nanostructures and create various interesting morphologies. Accordingly, the self-assembled morphologies of the PS-PLLA BCPs* are resulted from the mutual interaction of crystallization and microphase separation. The examination of the competition between crystallization and microphase separation from the self-assembly of PS-PLLA BCPs* in solution was thus carried out. In contrast to the self-assembly of BCP* in melt at which the formation of ordered textures is supercooling dependence, a kinetically controlled process by simply adjusting the supersaturation rate was utilized for the control of self-assembly for ordering. Single-crystal lozenge superstructures can be obtained from the slow self-assembly (i.e., low supersaturation rate) of PS-PLLA BCP* with long PS chain (i.e., PS-rich PS-PLLA BCP*) whereas amorphous helical ribbon superstructures were obtained from the fast self-assembly (i.e., high supersaturation rate). Moreover, amorphous flat ribbon superstructures were found in the poly(styrene)-b-poly(D,L-lactide) PS-PLA achiral BCPs, suggesting that the chirality plays an important role in the formation of the helical ribbon superstructures. By contrast, the self-assembly of PS-PLLA BCP* with short PS chain (i.e., PLLA-rich PS-PLLA BCP*) was dominated by the PLLA crystallization regardless of the supersaturation rate, indicating that the length of PS chain in the PS-PLLA BCPs* justifies the relaxation time for PLLA crystallization rate. As a result, the formation of the helical architectures from the self-assembly of the PS-PLLA BCP* is attributed to the effect of chirality from microphase separation but the chiral effect might be overwhelmed by crystallization.
Recently, a new method, TEM tomography, for imaging the morphologies of soft materials in three-dimension (3D) space, in particular for nanoscale features, has been developed. Through the image reconstruction from a series of tilted projections, a direct imaging of 3D nanoscale objects can be achieved. By taking the advantage of the degradable character of polylactide, PS with helical nanochannels can be prepared first from the self-assembled H* phase after hydrolysis, and then used as a template. Sol-gel reaction was then carried out within this template so as to fabricate helical nanocomposites. As a result, in contrast to the inaccessibility for the reconstruction of projection images from RuO4 stained PS-PLLA samples, the PS/SiO2 nanocomposites can be directly visualized by TEM without staining due to the significant increase in the contrast of the SiO2 helices. Hexagonally packed SiO2 helices were clearly observed through 3D direct visualization. A new H* phase at which hexagonally packed PLLA helices are embedded in a PS matrix with interdigitated character from the self-assembly of PS-PLLA BCPs* was identified.
An order-order phase transition of chiral block copolymers (BCPs*) from single helix to double gyroid (H*→G) through a nucleation and growth process was demonstrated. The H* and G phases can be obtained by solution casting from fast and slow solvent evaporation, respectively, suggesting that the H* phase is a metasable phase. Consequently, the coexistence of H* and G phase can be found in the solution-cast samples from intermediate evaporation. To truly examine the transition mechanism of the H*→G, electron tomography was carried out to directly visualize the morphological evolution in real space, in particular, the transition zone at the interface. Unlike the mechanisms for the transitions of block copolymers (BCPs) by considering the inter-domain spacing matching, a significant mismatch in the lattices for the H*→G was found. Consequently, the transition may require the lattice rotation along the helical central axis so as to justify the corresponding lattice mismatch. As a result, the morphological observations from electron tomography offer new insights into the BCP phase transitions.
In contrast to the chiral effect on the self-assembly of PS-rich PS-PLLA BCPs*, the chirality driven core-shell cylinder phase (CS* phase) can be obtained in PS-PLLA BCPs* with PLLA-rich fractions. We suggest that the observed core-shell cylinder nanostructure is the consequence of chirality driven twisting and bending origins from bilayered nanostructure through the scrolling of helical microdomains. As a result, the chirality driven nanostructures with helical sense for achiral components provides crucial information for scientific research. Moreover, tubular nanostructures can be simply obtained by hydrolyzing the degradable PLLA block of the CS* phase; it provides an easy and convenient way of fabricating tubular nanostructures with unique applications in nanotechnologies.
To investigate the mechanisms of chiral information transfer from molecular level to macroscopic level through self-assembly, a serious of PS-PLLA and PS-PDLA BCPs* were synthesized. Circular dichroism (CD), scanning probe microscopy (SPM) and electron tomography were used to examine the handedness of helical chain conformations, helical superstructures and helical phases (H* phases), respectively. On the basis of the CD results, the handedness of the helical chain conformation can be characterized in solution. The results indicated that the PS-PLLA and PS-PDLA BCPs* possess left- and right- handed helical chain conformations, respectively. Subsequently, left-handed helical superstructures were found from both the self-assembly of PS-PLLA and PS-PDLA BCPs* in solution. For the identification of the handedness of the H* phases in bulk, it is noted that the handedness of helices is difficult to be directly determined from 2D TEM projection. Therefore, a three-dimensional (3D) TEM imaging was carried out to examine the handedness of the H* phases. The results of electron tomography indicated that both the PS-PLLA and PS-PDLA BCPs* self-assemble into left-handed H* phases. As a result, the chiral information transfer may be missing as the BCPs* self-assemble into higher organizational levels.

關鍵字

高分子團聯共聚物 ； 掌性 ； 螺旋 ； 三維電子顯微鏡

並列摘要

無資料

並列關鍵字

block copolymer ； chiral ； helical ； electron tomography

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