聚方酸高分子薄膜之製備及其複合薄膜之 熱電性質研究

Abstract

兩性離子的π-共軛聚方酸高分子(polysquaraine)普遍具有本質的半導體性質及優異的光電特性,使其在許多不同的領域中都極具有發展的潛力。本論文的研究方向是針對結構中帶有推拉(donor-acceptor)電子基團之π-共軛聚方酸高分子(SQ)探討同樣具有兩性離子(zwitterionic)特性之離子液體(ionic liquid, IL)在SQ的聚合反應中所扮演之重要角色及其對產物高分子(SQIx)在物理,導電性及光學特性上所造成之影響。更進一步地,利用摻雜碘或混摻導電填料(如奈米碳管或中孔碳)等方式,使最適化條件所得之產物高分子薄膜(SQI0.1-based films)能有效提升其熱電性質。 本研究中,帶有強近紅外光(near-infrared, NIR)吸收及本質導電性之SQ是由二吡咯(bispyrrole)與方酸(squaric acid)在聚縮合反應條件下製備而得。聚方酸高分子SQ其分子結構由重複單位 zwitterionic 與二酮(diketonic)的結構組合而成。在SQIx聚合反應中, IL的存在有助於降低高分子之本質的聚集行為及改善沉澱的現象,主要是藉由SQ與IL間之離子與離子的溶合(ion-ion solvation)作用力,使得SQIx之分子鏈能穩定的分散於溶劑中。而IL在反應中含量的多寡,不只影響了SQIx之分子結構組成,同時亦影響著其物理及光學性質。同時也提供了製備具有金屬光澤,導電性質及可撓特性之薄膜的可能性。由於SQI0.1薄膜本質地具有較低之熱傳導係數及較高之Seebeck係數,具有應用於熱電材料之潛力。因此,將最適化條件所得之SQI0.1薄膜利用摻雜碘或混摻奈米碳管(Single-wall Nanotubes, SWNTs)及中孔碳(mesoporous carbon, CMK-3)等方式提升其在熱電(Thermoelectric, TE)的應用。藉由不同的分析方法去探討摻雜碘或導電填料(SWNTs, CMK-3)對於SQI0.1在物理及熱電特性上的影響。研究結果顯示,在SQI0.1薄膜中,碘的最佳摻雜量在8–12 wt.%時,導電性可由2.586×10-5 S/cm大幅提升至8.816×10-1 S/cm。有別於一般熱導係數隨著導電度增加而上升,此研究中,隨著碘摻雜程度之增加, SQI0.1的熱導係數隨著導電度的上升(10–5至10–1 S/cm)而下降(0.33至0.13 W/mK),此特性亦說明了在π-共軛系統中,適量的碘摻雜有助於提升熱電性質。在混摻有導電填料之碘摻雜的複合材料(20NT/5CMK/SQI0.1)薄膜中,SWNTs與CMK-3藉著與SQI0.1間的π-π作用力,能均勻地分散於SQI0.1主體,而具多孔性之CMK-3的存在能有助於碘分子更均勻地分散其中。這些特性使得碘摻雜的20NT/5CMK/SQI0.1導電性相較於碘摻雜的SQI0.1顯著地提升十倍至13.21 S/cm,而其熱電優值(the figure of merit, ZT)也有143%的提升達到4.563 × 10–3。

Owing to its unique intrinsic semiconducting and optoelectronic properties, π-conjugated polymer is one of the recent focuses in the field of material research. This dissertation focuses on donor-acceptor type π-conjugated zwitterionic polysquaraine (SQ), and describes the important role of zwitterionic ionic liquid (IL) in the SQ polymerization system and its influence on the structural, physical, conductivity and optical properties of SQIx. Additionally, zwitterionic polysquaraine (SQI0.1)-based films can through by doping with iodine and integration of conductive carbon filler (carbon nanotube and mesocarbon) enhances the potential of these films for thermoelectric (TE) applications. In this study, SQ with intense near-infrared (NIR) absorption and intrinsic conductivity was synthesized from bispyrrole with polycondensation of squaric acid. The SQ consists of two isomeric subunits: 1,3-addition (zwitterionic) and 1,2-addition (diketonic) moieties in the main chain structure. The IL was used in the preparation of bispyrrole-based SQIx during the polymerization leading to SQ polymers. Ion−ion solvation interactions between IL and SQ chains can lead to the stable dispersion of SQIx polymer chains in solution, thereby decreasing their intrinsic tendency to aggregate and precipitate. A proper content of IL in the solution not only affected the conformation of SQIx but also had an impact on its physical and optical properties. Moreover, IL presented in solution can make it possible to cast continuous free-standing films with conductivity, flexibility and hard metallic luster. Because the SQI0.1-based films exhibited favorable optoelectronic properties and good film formation, they were suitable for the design of TE applications. To enhance their TE properties, both single-walled carbon nanotubes (SWNTs) and mesoporous carbon (CMK-3) were integrated into the SQI0.1-based films and the effects of doping with iodine were also investigated. The electrical conductivity (σ) of SQI0.1 film greatly improved from 2.586×10-5 S/cm to 8.816×10-1 S/cm when in the presence of 8–12 wt.% of iodine. In most materials, electrical and thermal conductivity (κ) go hand in hand. However, this rule was not obeyed by the SQI0.1 film developed herein when the value of σ was increased after doping with iodine. The σ and κ were improved (from 10–5 to 10–1 S/cm) and decreased (from 0.33 to 0.13 W/mK), respectively, after doping with iodine. In iodine-doped 20NT/5CMK/SQI0.1 film, due to the π–π interaction between SQI0.1 matrix and carbon materials, SWNTs and CMK-3 can greatly improve the flexibility of the composite film and the dispersion of the doped iodine, respectively. Thus, the σ of 20NT/5CMK/SQI0.1 dramatically increased tenfold from 7.63 × 10–3 S/cm to 13.21 S/cm compared with that of the iodine-doped SQI0.1 film. The figure of merit (ZT) value was 143% of the optimized iodine-doped SQI0.1 film, increasing to 4.563 × 10–3.