Dissolution behaviour of thermotropic liquid-crystalline polyesters in 3,5-bis(trifluoromethyl)phenol and pentafluorophenol

Determining η sp c as a function of time has been used to investigate rates of dissolution and stabilities of solutions of liquid-crystalline polyesters (LCPs). The dissolution behaviour of commercial LCPs and prepolymers, post-condensates and pelletized LCPs prepared in-house has been studied in 3,5-bis(trifluoromethyl)phenol (BTFMP) and pentafluorophenol (PFPh). All of the investigated polyesters dissolved in both solvents. Optimum temperatures for dissolution appeared to be 25°C for BTFMP and 40°C for PFPh. Dissolution times ranged from 10 to 40 h. The solutions remained stable for at least 80 h. Dissolution of post-condensates is very difficult. Fortunately, by reducing the crystallinity, they dissolved more readily in both solvents without modifications of the molecular structure. Therefore, determining reliable molecular parameters of LCPs seems to be possible in these solvents.