EFFECT OF HIGH-PRESSURE ON VIBRATIONAL-MODES OF POLYIODIDES IN POLY(VINYL ALCOHOL) FILMS

We have used resonance-Raman scattering to study the effects of hydros tatic pressure (to 80 kbar) on polyiodide chain vibrations in iodine-d oped poly(vinyl alcohol) films. Two strong fundamental bands are obser ved along with overtone and combination bands. The low-energy band at 107 cm(-1) is attributed to symmetric stretching of I-3(-). The higher lying band at 154 cm(-1) is from symmetric stretching of the two end I-2 units in I-5(-). The I-3(-) and I-5(-) are intermixed within the p olyiodide chains. The combination band arises from exciton coupling be tween molecules within the chains. Three primary effects are observed with increasing pressure. First, the Raman band at 107 cm(-1) blue shi fts under pressure, while the band at 154 cm(-1) shifts weakly. This l atter is attributed to the competing effects of compression, enhanced interaction of the outer I-2 with the central I- (which weakens the ou ter I-I bond), and attractive host-guest interactions which increase i n strength with compression. The second effect observed in the Raman b ands is dramatically increasing line widths. This signifies locally no nhydrostatic conditions due to the variations in local microstructure within the polymer. Third, the relative intensities reverse in dominan ce as the pressure exceeds approximate to 18 kbar. We suggest that thi s is due to breaking of the I-5(-) chain elements into I-2 and I-3(-), which is caused by the locally inhomogeneous strain conditions. The i ntensities reflect the changing populations in I-3(-) and I-5(-). Thes e three effects are also observed in the overtone and combination band s of the Raman spectra. High-pressure absorption measurements in the v isible support our interpretation and confirm the important role playe d by attractive host-guest interactions.