Molecular Packing Orientations of Poly(3-hexylthiophene) Thin Films on Hydrophobized Surfaces：A Molecular Dynamics Study
Date Issued
2009
Date
2009
Author(s)
Chiu, Chu-Han
Abstract
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.
Subjects
Surface modification
P3HT
OFETs
Type
thesis
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