以分子動力學研究二氧化矽基板表面疏水層對P3HT堆疊位向的影響

Abstract

我們利用全原子模型的分子動力學模擬及理論計算，探討導電高分子材料聚噻吩(PT)和聚3-己烷噻吩(P3HT)在四種矽烷分子鏈(OTS、ODTS、APS，以及AHS)表面的作用情形；四種矽烷分子鏈涵蓋了接枝鏈長度與末端官能基(甲基和胺基)對於PT或P3HT高分子鏈構形，以及吸附位向的影響。在各組系統中，隨著接枝密度越高，接枝鏈仰角越大，導因於接枝密度上升時，接枝鏈間距將隨之縮短，致使接枝鏈間的凡得瓦作用力提升，因而開始有聚集之型態產生。此外，接枝鏈較長的ODTS系統，分子鏈間凡得瓦吸引力較短鏈段OTS更為強烈，致使在任一接枝密度下，ODTS接枝鏈仰角均比OTS來得大。值得注意的是，OTS與ODTS在接枝密度達100%時為最接近實驗結果之系統，由模擬結果得知，接枝鏈均以全反式構形朝垂直於基板方向向上延伸，致使PT主鏈可受末端甲基上的氫原子所吸附而朝表面平躺。此外，P3HT烷基側鏈水平散佈於基板表面，形成共平面性較好的P3HT分子鏈。而對於含有胺基末端基的系統(APS和AHS)，也觀察到相同的接枝密度與接枝鏈長度效應，但含有極性胺基末端基的系統可能受到下方二氧化矽表面氫氧基的吸引(表面氫鍵效應)，亦或是受鄰近分子鏈末端的胺基官能基的吸引，致使接枝鏈較易彎折。在實驗系統中，APS末端胺基與表面之氫鍵效應將可能佔據了其他有效的吸附位置，無法形成較高之接枝密度，因而APS之接枝密度約為60%。此時末端胺基易受二氧化矽表面吸附而產生特殊之吸引位向，此位向將可誘導噻吩以垂直位向被吸附至表面，但P3HT烷基側鏈與接枝鏈末端胺基之排斥力，仍將造成P3HT主鏈產生較大之旋扭角，有較差之共平面性。此外，對於長鏈段AHS系統而言，PT主鏈有較差之吸附性，但P3HT可藉由烷基側鏈拉攏高分子鏈至帶有長碳鏈的AHS表面排列，因而部分噻吩仍有機會受到末端胺基的吸引，致使P3HT平均噻吩仰角仍較OTS與ODTS的系統來得大。

We employ the all-atom molecular dynamic simulation and the theoretical calculations to study the interaction among polythiophene(PT)、poly(3-hexylthiophene) and four kinds of organosilanes(OTS、ODTS、APS ans AHS)；These organosilanes having different chain lengths and terminal functional groups may affect the conformation and the adsorption orientation of PT and P3HT. While considering all these surface treating systems, the elevation angles of the hydrophobic chains grafting on the silicon oxide surface increase with increasing the grafting density and these grafting chains have self-assembled behavior because the van der Waals interaction will be strong with increasing the grafting density. Furthermore, the carbon chain of ODTS is longer than that of OTS, so the van der Waals interaction between ODTS is much stronger than OTS, which results in high elevation angle of ODTS than that of OTS. In our results, when the grafting density of OTS and ODTS reach 100%, OTS and ODTS chains have an all-trans conformation and the methyl group will extend out of the surface normal，and the morphology of OTS and ODTS monolayer is close to the experimental result. In this cases, the thiophene rings are parallel to the substrate and attracted by the hydrogen of methyl group, moreover, the alkyl side chains of P3HT also spread parallel to the substrate, hence P3HT maintains a coplanar structure. For the organosilanes having amino groups, the amino group may be attracted by the hydroxyl group on the silicon oxide surface, or attracted its surrounding amino groups, and therefore the grafting chain would like to bend and deviate from the all-trans conformation. The simulation result quite agree with the experimental data；The amino groups of APS like to be attracted by the hydroxyl group and occupy the active site on the surface, that result in lower grafting density about 60%. In this case, the amino group of APS have a special orientation due to the hydrogen bonding effect with the hydroxyl group on the silicon oxide surface, and the special orientation of the amino group will induce the thiophene rings normal to the substrate to be attracted by the amino group. Nevertheless, repulsive force between the Hydrophobic alkyl side chain of P3HT and the polar amino groups will cause the P3HT main chain to have large distortion and deviate from the coplanar structure. However, the adsorption behavior of PT on AHS surface is poor, because the longer carbon chains with polar amino groups of AHS can interfere the adsorption with thiophene rings. Nevertheless, the alkyl side chain of P3HT can attract with the long carbon chain of AHS and move the polymer chain close to the AHS monolayer, so the thiophene rings may still have an opportunity to interact with the amino groups, hence the average elevation angle of thiophene rings of AHS is still larger than that of OTS and ODTS.