Temperature-induced nucleation of poly(p-phenylene vinylene-co-2,5-dioctyloxy-m-phenylene vinylene) crystallization by HiPco single-walled carbon nanotubes

Hybrid systems of the conjugated organic polymer poly(p-phenylene vinylene-co-2,5-dioctyloxy-m-phenylene vinylene)(PmPV) and HiPco single-walled carbon nanatubes (SWNTs) are explored using spectroscopic and thermal techniques to determine specific interactions. Vibrational spectroscopy indicates a weak interaction, and this is further elucidated using differential scanning calorimetry (DSC), confocal laser scanning microscopy, temperature-dependent Raman spectroscopy, and temperature-dependent infrared spectroscopy of the raw materials and the composite. An endothermic transition is observed in the DSC of both the polymer and the 0.1% HiPco composite in the region of 50°C. Also observed in the DSC of the composite is a double-peaked endotherm at -39 and -49°C, which does not appear in the polymer. The Raman spectroscopy of the polymer upon increasing the temperature to 60°C shows a diminished cis-vinylene mode at 1575 cm^-1, with an increase in relative intensity of the trans-vinylene mode at 1630 cm ^-1. Partially irreversible change in isomerization suggests increased order in the polymer. This change in the polymer is also manifest in the Raman composite spectrum upon increase of the temperature to 60°C, where the spectrum becomes abruptly dominated by nanotubes. Raman spectroscopy of the composite shows no change at -35°C; however, infrared absorption measurements suggest that the transition at -35°C derives from the polymer side chains. Here the composite at -35°C shows a change in the absorbance of the polymer side chain aryl-oxide linkage at 1250 cm^-1 and alkyl-oxide stretch at 1050 cm^-1. Infrared spectra thus suggest that the transitions in the lower temperature region around -35°C are side chain-induced, while Raman spectra suggest that the transition at 60°C is backbone-induced. Furthermore, temperature cycling induces an irreversible decrease in the mean fluorescence intensity of the polymer, coupled with a further reduction in the mean fluorescence intensity of the composite. This suggests that an increase in crystallization of the composite is supported and enhanced by an increase in ordering of the polymer. Implications are discussed.