Thermal decomposition behavior of main-chain thermotropic liquid crystalline polymers, Vectra A-950, B-950, and Xydar SRT-900

Abstract

We investigated the thermal decomposition behavior of three commercially available liquid crystalline polymers (LCPs), Vectra A950, Vectra B950, and Xydar SRT-900. The apparent activation energies (Ea) associated with the thermal degradation processes were determined by the Ozawa and Kissinger methods, using data from dynamic thermogravimetric analysis (TGA) experiments. The magnitudes of the Ea for these LCPs follow the order: Xydar > Vectra A > Vectra B in both air and N2 environments. The stability of the samples at the beginning of the degradation processes follows the same order. This order may result from the kink naphthoyl units in Vectra A and a relatively weak bond dissociation energy of C-N in Vectra B. However, at 560°C the weight loss values of these three LCPs in N2 become close (around 37%). After 600°C, the stability order surprisingly changes to Vectra B950 > Vectra A950 > Xydar SRT-900. This suggests that the more stable the sample is at the beginning, the less stable the corresponding residue is. Fourier Transform Infra-red (FTIR) spectra imply that random chain scission and hydrogen abstraction are the degradation mechanisms in N2 atmosphere and the bands of CO stretching for all 3 LCPs decrease after 560°C, indicating the finish of the ester bond rupture process. Further increasing temperature mainly results in carbonization. For all three LCPs, CO2 is the dominant degradation product during the entire testing periods in both N2 and air environments and the change of CO2 amount is consistent with the degradation rate. Residues after TGA experiments in N2 were analyzed and found to have a relatively high percentage of oxygen element, indicating the formations of ether and ketone structures during the thermal degradation of these three LCPs. Forty-seven percent of the nitrogen element remaining in the case of Vectra B950 indicates the formation of the structures containing nitrogen. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2195–2207, 1999