掌性團聯共聚合物PS-PLLA之自組裝

Self-assembly of Chiral PS-PLLA Block Copolymers

蔣酉旺(Yeo-Wan Chiang)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00018

摘要

本研究利用分子設計概念，仿效自然，由一掌性(chiral)分子與一非掌性分子(achiral)形成掌性團聯共聚合物(chiral block copolymer)，成功利用自組裝(self-assembly)建構具特定掌性之奈米螺旋結構(nanohelical structure)，此一奈米結構為一全新的自組裝形態，乃是團聯共聚合物之自組裝研究的重大發現，對傳統團聯共聚合物之理論計算將造成相當大震撼，預期開創一嶄新之研究領域方向。本計畫的結果顯示奈米螺旋結構呈現出一左旋形態與具左旋之組成單體有著相同之掌性，換言之，利用具分子掌性鏈段的團聯共聚物將可由自組裝的方式，而複製放大分子掌性形成奈米尺寸之螺旋微結構，並形成一奈米螺旋相。利用內部之結晶驅動力(Crystallization)或外部之剪切應力(Shearing force)調控掌性團聯共聚物之奈米螺旋結構之幾何形態，使其奈米螺旋結構經內部或外部趨動力轉變型態為結晶性或非晶態之圓柱新穎結構，並藉由熱退火的方式還原至初始的奈米螺旋結構，成為一新穎可調控式之奈米微結構。同時由於可進行螺旋微結構之排整，且由於所選擇之掌性鏈段為具可分解性，故將其奈米螺旋結構進行選擇性分解，輕易地製造出具有掌性之奈米螺旋孔道。此方式所建構之奈米螺旋微結構，將可提供奈米科技於材料應用開發上的一大突破。

關鍵字

掌性 ； 團聯共聚合物 ； 自組裝 ； 聚乙烯 ； 聚乳酸 ； 螺旋

並列摘要

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

並列關鍵字

Chiral ； PS-PLLA ； Block Copolymer ； Self-assembly ； Helical ； Nanohelical

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