DENSITY-FLUCTUATIONS IN BISPHENOL-A POLYCARBONATE AND TETRAMETHYL-BISPHENOL-A POLYCARBONATE AS STUDIED BY X-RAY-DIFFRACTION

Small-angle X-rav scattering (SAXS) is employed to measure the density fluctuations in Bisphenol A polycarbonate (BPA-PC) and tetramethyl-Bisphenol A polycarbonate (TMPC) as a function of temperature (T) in the range 300-510 K. Pressure-volume-temperature (PVT) measurements are made for TMPC in the range 300-539 K and for applied pressures up to 200 MPa. Wide-angle X-ray (WAXS) measurements are performed for both polymers in the range 300-480 K. Using these techniques we characterize the state of order in the two polymers. Whereas from specific volume and WAXS measurements an estimate of the average free volume is obtained, SAXS provides the fluctuations in the free volume. Our results demonstrate that TMPC possesses a higher average free volume and free volume fluctuations as compared to BPA-PC. This result is discussed in terms of the differences in packing and stiffness of chains. By extending this conclusion to other polymers, we suggest that the average free volume and its fluctuations are not the same for different polymers-as it is frequently assumed.