Surface alignment of P3HT and other conductive polymers

[Tom_Bristol]

Hi all,

It is well known that poly(3-hexyl thiophene) and other conductive polymers with alkyl substituents align when deposited on a surface (e.g. by spin coating or drop casting). The common orientations for P3HT on silicon/silicon dioxide are "edge-on" in which the alkyl side-chains point normal to the surface, and "face-on" in which the thiophene rings lie parallel to the surface. I have read quite a few papers that discuss this observation, for example these two highly cited Nature articles:

[1] http://www.nature.com/nature/journal/v401/n6754/full/401685a0.html
[2] http://www.nature.com/nmat/journal/v5/n3/full/nmat1590.html

My question is: what is the exact energetic reason for this alignment. The "edge-on" alignment is most common and my rationale for this is that the SiO2 surface is quite polar. The aromatic thiophene rings are more hydrophobic than the alkyl tails, thus it is favourable for the polymers to stand up such that the thiophene rings do not interact with the substrate. The second article I posted does discuss the mechanism a little but I am looking for a good reference that explains this phenomenon.

Many thanks.

 

[guyknowslittle]

in my view, in the case of thin polymer film assembly, alignment will be strongly governed by the type of interaction with the substrate. If this is agreed upon, then the energetic explanation depends on the type of interaction. Is it chemical (covalent), with a functionalized substrate? Then the driving force for self assembly is probably enthalpic. Is it based on ion exchange? Then it is most likely entropic.