Synthesis and characterization of a nematic liquid crystalline network, 2. Monitoring of the network formation process using thermal analysis methods

Abstract

The network formation process of a main-chain liquid crystalline prepolymer consisting of statistically distributed segments of tert-butylhydroquinone, sebacic acid, 4-hydroxybenzoic acid and itaconic acid was investigated by thermal analysis methods. The DSC heat run shows two exothermic peaks. The crosslinking reaction starts at 180°C. For confirmation of the radical mechanism of the crosslinking process, we used the radical trapping agent Irganox 1010. One of the exothermic peaks disappeared. The crosslinking reaction proceeds via the double bond of the itaconic acid segment. The decreasing concentration of double bonds with reaction time is correlated with the increase in the glass transition temperature. Due to the superposition of the glass transition and the exothermic peaks, we used modulated DSC in order to separate these contributions. Thermal treatment of the networks changes the glass transition temperature and results in two different glass temperatures, due to two different non-miscible network phases. The crosslinking process was pursued with DMA, and an increase in the modulus, evidence for network formation, was found. Time-dependent isothermal heat capacity related experiments were done in order to obtain evidence for crosslinking. A decrease in the estimated values was found.