以選擇性溶劑調控聚三-己基噻吩-聚異戊二烯嵌段共聚高分子之微結構

Abstract

此研究的主要目的是以改變混合溶劑的組成來調控聚(3-己基噻吩)嵌段共聚高分子(P3HT block copolymer)在溶液中的自組裝行為，進而達到利用低分子量的聚(3-己基噻吩)(P3HT)及較為環保的無鹵溶劑製備具有良好電子傳導特性的P3HT長纖維薄膜。研究中所使用的嵌段共聚高分子是由具有高度立體規則性，分子量為8000的P3HT與柔軟且具有低玻璃轉移溫度、分子量為3000的聚異戊二烯(PI)所組成的雙嵌段共聚高分子P3HT-b-PI；藉由GRIM聚合及後續的官能基轉換可得到末端官能基為醛基的(P3HT-CHO），並以陰離子聚合得到PI的活性陰離子，再將此兩者進行耦合成功合成出P3HT-b-PI。所使用的溶劑系統是以一P3HT的溶劑(如tetrahydrofuran或toluene)，以及一P3HT的非溶劑(non-solvent, 如hexane或decane)進行搭配，而此四種溶劑皆對PI有良好的溶解度。我們系統化改變P3HT溶劑與非溶劑的組成與比例, 及溶液製備的方式以探討影響其自組裝行為的因素。 我們發現非溶劑比例超過70%時，純P3HT會形成大量的沉澱，但PI鏈段的導入可以使P3HT-b-PI在P3HT已經形成聚集的情形下仍良好的分散在混和溶劑中，隨著靜置時間的增加可以觀察到長纖維的產生。推測其自組裝行為是P3HT鏈段首先形成小晶粒或聚集，之後溶解良好的PI鏈段可以引導小晶粒接合使其成長為長纖維以減低高分子與溶液間的界面面積。而不同的溶劑組合，可以產生不同長度和寬度的纖維；有趣的是，當使用decane為非溶劑與不同的溶劑進行搭配時，P3HT-b-PI皆可形成較為長直的纖維；而以drop casting製備薄膜時，高沸點溶劑的緩慢揮發使P3HT-b-PI可以形成更長的纖維。溶液的靜置時間對於纖維的生長與結晶度亦會有所影響，在toluene/decane (v/v=3/7)的溶劑體系中，將溶液靜置7天後，P3HT晶粒的大小可以從7.5奈米成長到12.1奈米，纖維的長度可以超過1微米且顯示出更緊密的堆疊並具有垂直於基材位向(edge-on)的方向性，這些特點都有利於有機薄膜電晶體的後續應用。我們也初步探討了此選擇性溶劑系統對分子量較大的P3HT-b-PI的影響, 發現高分子量P3HT的結晶會主導自組裝行為; 而具相近P3HT嵌段長度的P3HT-b-PI-b-P3HT三嵌段共聚高分子的亦無法如雙嵌段共聚高分子展現出良好的結晶性。

This research aims to manipulate the self-assembly behavior of poly(3-hexylthiophene) block copolymers in solution by employing mixed solvents with different compositions. The ultimate goal is to prepare P3HT long fiber thin films with good electronic conducting properties by utilizing low molecular weight P3HT and environmentally friendly, halogen-free solvents. The block copolymer used in the study is a P3HT-b-PI diblock copolymer, composed of a highly regioregular P3HT segment with a molecular weight of 8000, and a flexible polyisoprene (PI) with low glass transition temperature and a molecular weight of 3000. By employing GRIM polymerization and subsequent functional group transformation, we can obtain an aldehyde terminated P3HT, (P3HT-CHO). Anionic polymerization is adopted to synthesize living polyisoprene anions and then coupling with P3HT-CHO would afford the P3HT-b-PI block copolymer. The mix solvent system consists of a solvent for P3HT, such as tetrahydrofuran or toluene, and a non-solvent for P3HT, such as hexane or decane. All these four solvents have good solubility for polyisoprene(PI). By systematically varying the composition and ratio of P3HT solvent and non-solvent as well as the solution preparation process, we can explore important factors influencing the self-assembly behavior of P3HT-b-PI in the mixed solvent system. It is observed that when the non-solvent ratio exceeds 70 vol%, P3HT homopolymers tend to precipitate seriously. However, the incorporation of PI segments allows P3HT-b-PI to disperse homogenously in the mixed solvent even when P3HT has already form certain aggregates. With increasing standing time of the solution, long fibers could be observed. We propose a model suggesting that, P3HT segments initially forms small crystallites or aggregates in the solution; subsequently, the well-dissolved PI segments could guide these small crystallites to join together and facilitate their growths into long fibers. This process could be thermodynamic driving as the interfacial areas between the aggregates and the solvents are reduced with the formation of larger structures. Furthermore, different solvent combinations could result in fibers with varying lengths and widths. It’s noted that when using decane as the non-solvent in combination with different solvents, P3HT-b-PI consistently forms longer and straighter fibers. Additionally, drop casting films prepared from high boiling point solvents, slow evaporation allows P3HT-b-PI to form even longer and wider fibers. These observations suggest that solvent selection and the preparation method play critical roles in the microstructure of the resulting films. We also observe that the solution's standing time plays an important role in fiber growth and the crystallinity of the resulting fiber. In the toluene/decane (v/v=3/7) solvent system, the fibers obtained from the solution standing for 7 days exhibit an increase in P3HT crystallite size from 7.5 to 12.1 nanometers and a denser packing in comparison with the fibers from the same solution standing for 1 day. The fibers grow to lengths exceeding 1 micron with edge-on orientation. These features are favorable for the applications of organic thin-film transistors. The preliminary exploration of the impact of the selective solvent system on higher molecular weight P3HT-b-PI reveals that the self-assembly behavior is dominated by the crystallization of high molecular weight P3HT. Additionally, the tri-block copolymer P3HT-b-PI-b-P3HT, with similar P3HT segment lengths, does not exhibit the same level of crystallinity as the diblock copolymer counterpart. This information provides insights into how the molecular weight and block arrangement can influence the self-assembly behavior in the selective solvent system.