Formation and Thermally-Induced Disruption of Nanowhiskers in Poly(3-hexylthiophene)/Xylene Gel Studied by Small-Angle X-ray Scattering

We identified the formation of nanowhiskers of poly(3-hexylthiophene) (P3HT) in the gel with xylene in situ by means of small-angle X-ray scattering (SAXS). In the freshly prepared solution, the rodlike segments of P3HT were found to form local network aggregates characterized by the mass fractal dimension of ca. 2.5. Upon prolonged aging at room temperature, the originally viscous liquid solution transformed into a gel, in which a rodlike entity with the cross-sectional radius of gyration of 7.31 nm was disclosed. Real-space observation of the morphology of the film cast from the gel revealed the presence of long nanowhiskers. The fact that the width and height of these whiskers closely agreed with those of the rodlike entity in the gel deduced from model fitting of the SAXS profile indicated that the nanowhiskers were present in the wet gel. Time-resolved SAXS experiment revealed that the nanowhisker formation was accompanied by the crystallization of P3HT. The ultimate crystallinity attained in the nanowhiskers was only 32%, and the limited crystallizability was attributed to the network aggregate structure formed by P3HT prior to the gelation. The integrity of the nanowhiskers was largely maintained by the crystallinity of P3HT, as the dissipation of the nanowhisker morphology upon heating was found to occur concurrently with the melting of P3HT crystallites that led to homogenization of the solution.