利用分子動力學探討苯基三氯矽烷分子在二氧化矽基板之排列型態及其對聚噻吩與聚3-己烷噻吩堆疊位向之影響

Abstract

我們利用全原子模型的分子動力學電腦模擬，研究苯基三氯矽烷分子以四種接枝密度(25%、50%、75%、100%)接枝於二氧化矽表面後，探討接枝鏈的排列型態以及接枝鏈對於導電高分子材料聚噻吩(PT)和聚3-己烷噻吩(P3HT)之構形與吸引位向之影響。我們先利用蒙地卡羅法計算苯環、噻吩單體與3-己烷噻吩單體兩兩之間可能出現的吸附位向，以幫助我們之後的分析。在第二部分中我們將苯基三氯矽烷分子接枝於二氧化矽基板上並形成四種接枝密度，對其排列型態進行探討。我們發現到隨著接枝密度上升，接枝鏈之間的距離縮短，彼此間的吸引力(π-π interaction & π-H interaction)會增強，使得接枝鏈形成規整的排列型態。第三部分為探討聚噻吩(PT)與聚3-己烷噻吩(P3HT)與各組接枝密度產生交互作用的系統。在低接枝密度(25%)時，由於接枝鏈間空隙較大，所以PT會插入接枝鏈間，此時主鏈會與基板表面產生一傾斜角。隨著接枝密度上升至50%，PT會以垂直的構型插入接枝鏈間，此時的傾斜角會上升，而主鏈因經驗到較多的翻轉使得共平面性下降。當高接枝密度(75% & 100%)時，接枝鏈間的縫隙太小導致PT已經無法插入接枝鏈之間，所以PT會平躺於接枝鏈表面，傾斜角因而下降，共平面性因此上升。最後我們將範圍延伸至含有烷基側鏈之聚3-己烷噻吩，探討側鏈存在之影響。在低接枝密度時，P3HT的側鏈會先往接枝鏈間的空隙作伸展，此時主鏈上的噻吩環依舊能夠與苯環產生π-π interaction，然而側鏈的擾動會導致此時的傾斜角比PT來的小，而主鏈也因為側鏈進入接枝鏈間而有較大的扭轉，使得共平面性不高。當接枝密度上升，接枝鏈間的距離已經縮小到無法讓含有側鏈的P3HT插入，因此大多數噻吩環會以平躺的構型座落在表面上，因此傾斜角沒有大幅度的上升，而共平面性則會上升。當達到高接枝密度時，由於接枝鏈間的空隙太小，所以不論主鏈或是側鏈都是在接枝鏈上呈現平躺的構形，傾斜角因此而下降，且共平面性依舊相當良好。

We employ the all-atom molecular dynamic simulation and the theoretical calculations to study the conformation of four different grafting density (25%、50%、75% and 100%) of phenyltrichlorosilane (PTS) and its interaction among polythiophene (PT) and poly(3-hexylthiophene). In the beginning ,we use Monte Carlo method to find the absorption site between benzene ring, thiophene ring, and 3-hexylthiophene ring to support our discussion later. In the second part, we graft four grafting density PTS on the SiO2 substrate and studied their conformation. We find that the conformation of PTS becoming dense and packing well-ordered because the π-π interaction and π-H interaction will be strong with increasing of grafting density. In the third part we investigate the interaction between PT/P3HT and grafted system. When grafting density is 25%, we find that PT will insert the vacant space between grafting silanes and there is an elevation angle between PT and substrate. When grafting density reaching 50%, we find PT will insert the vacant space by perpendicular to the substrate, and the elevation angle will increase, the coplanarity will decrease. At high grafting density, the vacant space between grafting silanes is too narrow to let PT insert, so PT will lay on the surface of grafting silanes, the elevation angle will decrease, the coplanarity will increase. At last, we investigate the interaction between P3HT and grafted system. At low grafting density, alkyl chain will extend into the vacant space, the thiophene ring can still make π-π interaction with benzene ring. However, the disturbance of alkyl chain will cause elevation angle smaller than PT’s, and coplanarity is also small because of the insertion of alkyl chain. With the increasing of grafting density, the vacant space is already too narrow to let P3HT insert, so P3HT will lay on the surface of grafting silanes, the elevation angle will decrease, the coplanarity will increase. At the highest grafting density, P3HT still lay on the surface of grafting silanes, so the elevation angle becoming smaller, and the coplanarity is still fine.