Shear-Induced Solution Crystallization of Poly(3-hexylthiophene) (P3HT)

The rheological properties of poly(3-hexylthiophene) (P3HT) solutions were investigated in a nonvolatile solvent, 2-ethylnaphthalene, at various P3HT concentrations. A mild viscosity increase was noted during shear for lower concentration P3HT solutions over 24 h, yet the viscosity increase was greater than if the sample was not sheared at all over the same time period. In this case, crystalline fibrils with large aspect ratios were formed during shear. Crystallization was determined to be dictated by Brownian motion and mediated by shear, or in other words Brownian motion brought the molecules together while shear changed their conformation to allow crystallization. Higher concentration P3HT solutions produced a significant viscosity increase, up to 2 orders of magnitude, during shear. Again, if a similar solution was merely aged without shear, a much lower viscosity increase was noted. Simple calculations show that the fibril concentration was above the percolation threshold at the higher concentration accounting for the large viscosity increase. Finally, the crystal structure of P3HT after shear was found to have pi-pi. stacking (b-axis) direction parallel to the crystalline needle axis, as seen for crystals formed under quiescent conditions, yet the crystalline fibrils are micrometers long. The proposed mechanism for crystal formation is that Brownian motion brings the molecules to a growing crystal face while shear removes improperly incorporated chains, allowing longer crystals to form. This phenomenon is expected to apply to other systems, allowing formation of longer, more perfect crystalline fibrils.