GAMMA-RAY POLYMERIZATION OF A MESOGENIC METHACRYLATE IN THE CRYSTALLINE, LIQUID-CRYSTALLINE AND LIQUID-STATE - KINETICS, MOLECULAR-WEIGHT DISTRIBUTION AND CONVERSION-DEPENDENT PHASE-BEHAVIOR

The gamma-ray polymerization of mesogenic 4-methoxy-4'-(6-methacryloyl oxyhexyloxy)biphenyl (1) was investigated. 1 can be polymerized in the crystalline, in the liquid crystalline and in the molten state. The r ate of polymerization increases from the crystalline to the melt phase . In order to reach 90% conversion to polymer in melt, smectic and cry stalline phase, radiation doses of 10, 40 and more than 120 kGy are ne cessary. Melt polymerization leads to a weight-average molecular weigh t of about 1 x 10(6) independent of the conversion to polymer. In the smectic and crystalline phase the corresponding molecular weights are about 3.8 x 10(5) and 4.1 x 10(4) at low conversion, and shifted to 2. 6 x 10(5) and 3.9 x 10(4) at high conversion. The polydispersity gener ally increases with the gamma-ray dose and the reaction temperature. T he origin of the different reaction rates and molecular weights obtain ed in the various phases is discussed in terms of the mobility of grow ing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length. Differential scanning calorimetry (DSC) of partially polymerized samples was used to describ e the conversion-dependent phase behaviour. Optical microscopy showed that the melt polymerization at 78.5 degrees C proceeds under phase se paration into a liquid, monomer-rich and a solid, polymer-rich phase. As a consequence, liquid monomer samples solidify upon gamma-irradiati on once a higher conversion to polymer is reached.