Author: Mattu, Jusroop Singh
The fabrication and characterization of highly-ordered thin films made from an amphiphilic, regioregular (RR) polythiophene (PT) derivative are described. The effects of structural conformation and order on materials properties and reactivity are discussed. High optical quality films of poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene) (PTHPUDT) were prepared by the Langmuir-Blodgett (LB) technique. The amphiphilic nature of the polymer affords layer-by-layer deposition and the formation of multilayer films of head-to-head and tail-to-tail, Y-type structure. X-ray diffraction (XRD) studies indicate the formation of a semi-crystalline film with bilayer separations of ~30 Å. Anisotropic optical absorption in the plane of the film signify that the thiophene backbones are preferentially oriented along the dipping direction. Further, polarized light microscopy studies reveal that these films are highly birefringent. The nonlinear optical properties of PTHPUDT films were explored. Optical second harmonic generation (SHG) studies of multilayer films provide information regarding both the thiophene orientation within the film and the anisotropic distribution of chromophores in the surface plane. The third order nonlinear optical response of the polymer films was probed using z-scan methods. Analysis of the resulting z-scan data yielded values consistent with previous studies on PT-based polymer systems. However, even with relatively low intensities incident on pristine PTHPUDT polymer films, the required power to achieve a nonlinear response in the film is sufficient to produce photoinduced changes in the original polymer. Although our observations pertain specifically to PTHPUDT, our results may extend to the broader class of PT-based materials more generally. These observations put into question whether the nonlinear optical susceptibility values of PT-based materials currently being reported in the literature result from pristine polymers or from their photoconverted products. The chemistry occurring in the ordered films was examined. Large differences in both the thermal reactivity (thermochromism) and physical properties (solvatochromism) of the films were observed after heat-treatment. Taken together, this data offers strong evidence of highly-ordered films. As such, PTHPUDT is a model polymer system with potential of elucidating the connection between polymer morphology and physical properties in materials that are otherwise subject to a sufficiently complex distribution of morphologies that such a correspondence is precluded.
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