Correlating solubility parameters and solvatochromic parameters with the self-assembly of poly(3-hexylthiophene) in mixtures of organic solvents

We have studied the assembly and crystallinity of poly(3-hexylthiophene) (P3HT) (M$_{\mathrm{n}} \approx $ 28.2 kDa, regioregularity $>$ 96$\% $, PDI $\approx $ 1.3) in $>$100 binary solvent mixtures using UV-Vis absorption spectroscopy, and it is clear that the identity of the poor solvent used to drive aggregation has a significant impact on the structural order and crystallinity of the P3HT aggregates in solution. Here we report our findings using Hansen solubility parameters (HSPs), specifically the solubility distance vector, R$_{\mathrm{a}}$, and the Kamlet-Taft solvatochromic parameters of the solvent mixtures to better understand the dominant solvent forces driving the self assembly of P3HT. We find that the \textit{directionality} of the R$_{\mathrm{a}}$ vector provides a better measure of the crystallinity of the P3HT assemblies formed in the solvent mixtures than does the \textit{magnitude} of the R$_{\mathrm{a}}$ vector. Our analysis of the Kamlet-Taft ($\alpha $, $\beta $, $\pi $*) and E$_{\mathrm{T}}$(30) solvatochromic parameters reveals that the $\beta $ parameter correlates best with the crystallinity of P3HT and that, in general, assemblies having higher structural order are formed in solvent mixtures with lower values of $\beta$.