Electron paramagnetic resonance application to polymers: 4. The segmental volume of poly(phenylacetylene) using different nitroxide spin probes

Electron paramagnetic resonance (e.p.r.) studies of T_50G of poly(phenylacetylene) have been carried out at X-band as a function of spin probe size over a wide range of temperatures. It was found that T_50G increased with increasing probe size. The polymer glass transition temperature T_g, determined from the break on the plot of ln T_R versus 1 T for tempone in poly(phenylacetylene), was 296 K. The apparent activation enthalpy for the T_g relaxation process increased with increasing probe size, while the segmental size of the polymer appeared to be independent of the nitroxide used, and has a range of 898-922 ų. The segmental volume measured by the spin probe technique is in excellent agreement with the volume of 970 ų of 10 repeat units on an average chain based on number-average molecular weight of 960. Thus our results showed that there are 17-20 backbone atoms in the segment of poly(phenylacetylene). Assuming the polymer segment to be of spherical shape, the diameter of the sphere was found to have a range of 11.7-12.4 Å.