Temperature dependence of the activation volume in a nonlinear optical polymer: Evidence for chromophore reorientation induced by sub-T-g relaxations

摘要

Second harmonic generation (SHG) was used to measure the temperature dependence of the reorientation activation volume of 4-(diethylamino)-4'-nitrotolane (DEANT) in poly(methyl methacrylate) (PMMA). The decay of the SHG signal from films of DEANT/PMMA was recorded at hydrostatic pressures up to 3060 atm and at different temperatures between 25 degrees C below the glass transition temperature to 35 degrees C above it. The activation volume, Delta V-alpha beta*, associated with the long range or-type motion of the polymer remained constant at 213 +/- 10 Angstrom(3) between T-g -25 degrees C and T-g + 10 degrees C. At higher temperatures, Delta V (alpha beta)* decreased linearly with increasing temperature. The activation volume, Delta V-beta*, associated with short range secondary relaxations was constant over the entire temperature range with a value of 77 +/- 10 Angstrom(3). The data suggest that above T-g chromophore reorientation is coupled to both the long range and local motions of the polymer; whereas, well below T-g chromophore reorientation is closely coupled to the local relaxations of the polymer. (C) 1998 John Wiley & Sons, Inc. [References: 40]