以耗散粒子動力學法研究共軛硬桿—軟鏈團塊共聚高分子於選擇性溶劑中之自組裝行為

Abstract

硬桿-軟鏈團塊共聚高分子(rod-coil block copolymers)是過去四十年間奈米工業上的一項重大發展。此共聚高分子乃是由兩段或兩段以上具有不同物理性質之高分子或寡分子(oligomer)，經由化學鍵連接所形成的新穎材料。承襲了已被高度發展的軟鏈團塊共聚高分子，不同段鏈上的分子因存在著互不相容性而相互排斥，卻因彼此間具有鍵結而無法徹底分離，於是僅在略高於分子之層次產生微觀相分離；又因為其中硬桿段鏈不管是因自身排外體積效應或液晶特性之故，通常具有方向規整性(orientational ordering)，加上疏溶劑效應(solvophobic effect)，在趨向熱力學穩定態過程中，會展現所謂的”自組裝”行為(self-assembly behavior)，進而在微觀層次呈現許多前所未見的有趣型態及具潛力之應用。 近年來由於光電產業的蓬勃發展，共軛硬桿-軟鏈團塊共聚高分子或寡分子(conjugated rod-coils)因具有獨特的光電特性而被重視，不管在雷射、PLED全彩顯示器、省電能源或太陽能電板皆可看見其影子。不論透過理論預測或實驗操作皆發現，共軛硬桿-軟鏈共聚高分子所呈現的光電性質與其分子結構、呈現的介觀型態以及共軛硬桿排列行為有著強烈的關連性。一般在判定此類共軛高分子的光電性質會利用紫外線近光光譜(UV-vis)與螢光放射光譜(PL)來進行測定，其中不同型態的聚集模式會呈現不同的光譜行為。較常見的類型可分為兩種，一類是聚集體之吸收峰較分散相往藍端偏移，根據早期在染料分子中的研究，一般會將此現象歸類於H-聚集體(H-aggregates)的產生；第二類是聚集體之吸收波峰較分散相往紅端偏移，此現象則常歸類於J-聚集體(J-aggregates)的形成。這兩種聚集體皆是由互相平行排列之共軛硬桿分子所構成，其中H-聚集體中的共軛硬桿彼此錯位程度小，J-聚集體中的硬桿彼此錯位程度大，以致兩種聚集體所呈現的空間結構與光電行為皆有不同。 本研究利用耗散粒子動力學法(Dissipative Particle Dynamics)研究共軛硬桿-軟鏈團塊共聚高分子於稀薄選擇性溶液中之自組裝行為，其中硬桿為疏溶劑(solvophobic)，軟鏈為親溶劑(solvophilic)。我們在研究中詳細地探討了此類共聚高分子之分子結構(雙段鏈、三段鏈、星狀、梳狀、樹枝狀…等十二種)，硬桿、軟鏈長度(對應於真實分子之聚合度)，以及硬桿鏈段上之π-π作用力強度對溶液中高分子自組裝行為的影響。我們的研究結果發現，根據平衡態之硬桿排列行為可將不同的共聚高分子區分為兩大類結構，分別為硬桿-軟鏈形式(RC-type)與軟鏈-硬桿-軟鏈形式(CRC-type)；硬桿-軟鏈形式之分子結構可以兩端皆為硬桿，或一端硬桿一端軟鏈，而軟鏈-硬桿-軟鏈形式則必須兩端(或分子外圍)都為軟鏈結尾。硬桿-軟鏈形式之高分子多半傾向於層列型(smectic-like)聚集模式，此類聚集體在排列結構上可對應至傳統的H-聚集體；而軟鏈-硬桿-軟鏈形式之共聚高分子則多半傾向於特定角度-向列型(Preferred-nematic)聚集模式，此類聚集模式之排列結構則可對應到J-聚集體。為了實際印證錯位程度對吸收光譜造成的紅藍移現象，我們利用Material Studio中根據量子力學的模擬套件(VAMP)，計算了特定共軛高分子硬桿於不同錯位下的光譜性質，其結果與我們預期的相符，層列型排列造成藍偏移，特定角度-向列型造成紅偏移。另外在研究中也觀察到向列型(nematic-like)與等向型(isotropic)的聚集模式，前者所對應之光學性質較難由排列結構預測，後者則因硬桿不具平行排列的特性而無明顯光學特性。除了分子結構外，我們發現硬桿、軟鏈長度對此類共聚高分子的指向性排列(orientational order)與規整度排列(positional order)皆存在一定程度的影響，而π-π作用力強度則扮演了以上兩種排列規整性的關鍵角色。另外，我們也將各種結構經自組裝而成的型態(morphologies)做了系統性的整理分類，許多有趣的結構諸如微胞型態(micelle)、囊胞型態(vesicle)、柱狀(cylinder)、螺旋狀(helix)、蜂巢狀(honeycomb)…等，依據分子結構呈獻於各章之尾節。 我們的研究結果與諸多理論預測與實驗結果吻合，故此研究成果或可提供未來在調控此類共聚高分子之自組裝結構或光學性質時，於分子層次上的一些參考。

The development of rod-coil block copolymers in the past four decades is phenomenal to the field of nanoindustry. This type of block copolymers consists of two or more polymeric or oligomeric blocks which differ significantly from each other in physical properties, and are linked by chemical bonding. Rod-coils inherit the block copolymer characteristics of the highly-developed coil-coils; microphase separation occurs due to the incompatibility between the blocks and the spatial limitation from the chemical linkage. Moreover, the rigid block exhibits orientational ordering owing to the excluded volume effect and the mesogenic characters. The rod effect as well as molecular amphiphilic nature drives the block copolymers to self-assemble into a variety of intriguing morphologies with a number of potential nanoscaled applications. Due to the prosperity of the optoelectronic industry in recent years, conjugated rod-coil block copolymers gain increasingly attention on account of the distinct optoelectronic properties. The applications of conjugated rod-coils can be seen in Lasers, PLED full color displays, energy savers and solar panels, etc. Theoretical works and experiments show that the optoelectronic properties of conjugated rod-coils are highly correlated to the molecular structures, supramolecular morphologies and detailed arrangements of rods. UV-visible and Photoluminescence (PL) spectroscopies are commonly adopted to examine the photophysical properties for conjugated rod-coils. Different types of rod-coil aggregation result in different spectroscopic results. Two major types of aggregates are usually used to account for the distinct spectral phenomena. If the absorption wavelength of the aggregates has a hypsochromic shift (blue-shift) compared to that of the dispersed phase, it is usually attributed to the formation of H-aggregates. On the other hand, if the absorption wavelength has a bathochromic shift (red-shift), it is attributed to the formation of J-aggregates. Both models originate from the well-studied dye molecules. Rods are parallel to each other in both types of aggregates and with a relatively smaller slippage extent for H-aggregates while the slippage extent is larger for J-aggregates. The Dissipative Particle Dynamics was employed to study the self-assembly behavior of conjugated rod-coils in dilute selective solution, where the rods are solvophobic and the coils are solvophilic. The effects of the molecular structures (rod-coil, coil-rod-coil, rod-coil-rod, star, graft, and other 7 more structures), rod/coil length, and π-π strength on the final self-assembled results of the rod-coils are systematically investigated. It was found that the molecular structures of the rod-coils can be classified into two categories, the RC-type and the CRC-type structures, according to the molecular arrangement of rods in the equilibrium state. RC-type structures can be characterized by rods on both ends of the copolymer, or rod on the one end and coil on the other. CRC-type structures are characterized by coils at outer rims. RC-type rod-coils prefer to take the smectic-like patterns, which correspond to H-aggregates. CRC-type rod-coils tend to take the preferred-nematic patterns (nematic packing with preferred slippage extent), which correspond to J-aggregates. To justify the blue- and red-shift spectral phenomena caused by different types of aggregation, a routine based on quantum mechanics (VAMP) in the Material Studio was applied to estimate the absorption spectrum for several arrangements of conjugated rods, and the results were as what we have expected; blue-shift for the smectic–like and red-shift for the preferred-nematic. Nematic-like and isotropic patterns were also observed in the investigation, where the photophysical property of the former was unclear, and the latter should have no particular features due to the disordered arrangement of rods. In addition to the molecular structures, rod and coil length have certain degrees of effect on the orientational order as well as the positional order for the final organization of rods. On the other hand, π-π strength plays the crucial role in the above mentioned two properties. As to the self-assembled morphologies, many interesting supramolecular structures appear, such as spherical micelles, vesicles, cylinders, helices, honeycombs, and so forth. The classifications on the self-assembled morphologies are shown in the end of each chapter according to the molecular structures. Results from our investigation are consistent with most theoretical predictions as well as experiment findings. This work can provide the insights from the molecular level and should be a valuable reference for future applications of conjugated rod-coils.