GAMMA-RAY-INDUCED POST-POLYMERIZATION OF A MESOGENIC METHACRYLATE IN THE CRYSTALLINE, LIQUID-CRYSTALLINE AND MELT PHASE - REACTION-KINETICSAND MOLECULAR-WEIGHT DISTRIBUTION

The gamma-ray-induced post-polymerization of mesogenic 4-methoxy-4'-(6 -methacryloyl-oxyhexyloxy)biphenyl (1) was investigated. Reaction kine tics and molecular weight distribution of the polymer were studied. Mo lecular weights and weight distributions were analyzed using size excl usion chromatography. Especially, effects of radiation dose, polymeriz ation time and temperature were investigated. Post-polymerization was found in the crystalline, liquid crystalline and melt phase. Rate of p olymerization and Limiting conversion to polymer increase with radiati on dose, polymerization time and temperature. Samples exposed to a low gamma-ray dose of 5 kGy and subsequently polymerized in the crystalli ne state at 20 degrees C, for example, only reach a limiting conversio n of about 7% within 100 h, while samples exposed to a high dose of 20 kGy and polymerized in the melt at 79 degrees C reach a limiting conv ersion of 85.1% within 90 s. Molecular weights increase with polymeriz ation temperature and reach values comparable with in-source polymeriz ation. Different from in-source polymerization, they are additionally affected by the parameters radiation dose and polymerization time. Hig h molecular weights are obtained, if samples are exposed to a low radi ation dose and isothermally polymerized at high temperature for a long time, while low molecular weights result from high radiation dose and short time of polymerization at low temperature, e. g., at 20 degrees C. By carefully adjusting the reaction parameters, molecular weights can be reliably tailored in the range from 2 x 10(4) to 6 x 10(5). Pol ymerization at room temperature leads to a normal molecular weight dis tribution, while distributions are broader when the polymerization is carried out in the Liquid crystalline or melt phase. The origin of the different reaction rates and molecular weights obtained under the var ious conditions is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and terminat ion and thus the kinetic chain length.