Hierarchical Self-Assembly and Chemical Imaging of Nanoscale Domains in Polymer Blend Thin Films

Self-assembly of polymers driven by intermolecular and intramolecular interactions leads to rich structural variations in thin films of polymer blends. In this work, ordering of poly(3-hexylthiophene-2,5-diyl) (P3HT) to form nanoparticles (NPs) and the self-assembly pattern of the NPs in poly(4-vinylpyridine) (P4VP) matrix are revealed by using scattering-type scanning near-field optical microscopy (s-SNOM). The P3HT NPs are formed spontaneously when the P3HT/P4VP/chloroform solution is spin-cast on unpatterned silicon wafer at room temperature. We find that the P3HT NPs organize within the P4VP matrix forming continuous stripes along the parallel striations, depending on the solution composition and spin-coating angular speed. The spacing between the parallel P3HT stripes that runs along the center of the ridges varies from sample to sample within 4-12 mu m, whereas the width of the P3HT stripes confined within the ridges ranges from 0.2 to 1.1 mu m as determined from the s-SNOM infrared phase contrast. The sizes of the P3HT NPs range from 5 to over 100 nm depending on the composition of the blend as determined by using an atomic force microscope after selectively dissolving the P4VP matrix. The smallest size observed in our experiment is significantly smaller than the smallest diameter (larger than 20 nm) reported in the literature. The results presented here show that interaction between immiscible polymers during solvent evaporation can be manipulated to control hierarchical self-assembly from molecular ordering to mesoscale dimensions, and high-resolution chemical imaging is necessary to resolve nanoscale domains so that solution processes can be optimized.