THE THERMAL STABILITY AND DEGRADATION BEHAVIOR OF THERMOTROPIC LIQUID CRYSTALLINE POLYMERS

Abstract

We have investigated the thermal decomposition behaviors and related properties of six commercially available thermotropic liquid crystalline polymers (TLCPs): Vectra A950, B950, Xydar SRT-900, SRT1000, and Zenite 6000, 8000B. The detailed degradation mechanisms of the above TLCPs were investigated through FTIR (Fourier Transform Infra-Red) analysis of the thermogravimetric (TGA) gas products and elemental analysis of the final solid products. The apparent activation energies (Ea) associated with the thermal degradation processes were determined with the Ozawa-Flynn and Kissinger methods, using data from dynamic TGA analysis experiments. For the three most common wholly aromatic TLCPs: Vectra A950, B950, and Xydar SRT900, the magnitudes of the Ea follow the order: Xydar SRT900 > 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. FTIR spectra of the gas products from TGA in N₂ imply that 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. 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. 47% of nitrogen element remaining in the case of Vectra B950 indicates the formation of the structures containing nitrogen. The Xydar SRT1000 and Zenite TLCPs are absolutely new materials. Their chemical structures were identified by FTIR spectroscopy, and phase transition investigated by Differential Scanning Calorimetry (DSC), Polarized Light Microscopy (PLM). The FTIR spectrum of fresh sample of Zenite 8000B is found to be more similar to that of Vectra A950 than those of the other three LCPs. Xydar SRT900 and 1000 have almost the same FTIR spectra. Zenite LCPs have broader crystal-mesophase transition (TcM) and less clear liquid crystalline textures at TcM than those of Xydar LCPs. Xydar SRT1000 and Zenite 8000B have lower TcM than Xydar SRT900 and Zenite 6000, respectively. For all the four LCPs of Xydar and Zenite brands, the glass transition can not be easily observed in DSC without the aid of annealing, and the isotropic phase does not appear before thermal degradation. Xydar SRT1000 and Zenite 8000B have lower thermal stability as well as lower TcM than Xydar SRT900 and Zenite 6000, respectively. It is interesting to find that Ea curves of Xydar SRT900 and SRT1000 have the similar shapes as those of Zenite 6000 and 8000B, respectively. For the first time, we have found that there is a minor degradation maximum for Zenite 8000 at about 2.5% weight loss in N₂· For all the four TLCPs of Xydar and Zenite brands, the Ea values in air atmosphere begin to decrease at a temperature close to the deflection points on the first derivative curves. All the six TLCPs are more stable in N₂ than in air. Random chain scission, crosslinking and hydrogen abstraction are the main mechanisms in the beginning stage of degradation N₂· From the structure formed, CO₂, CO, phenol, and aromatic ketone etc. can be produced in the next steps. The flrst stage of the thermal degradation in air atmosphere has similar mechanisms as that in N₂· CO₂ is the dominant degradation product throughout the degradation processes both in air and N₂, and the amount of CO₂ release is proportional to the thermal decomposition rate.