γ-Ray-induced post-polymerization of a mesogenic methacrylate in the crystalline,liquid crystalline and melt phase: Reaction kinetics and molecular weight distribution

The γ-ray-induced post-polymerization of mesogenic 4-methoxy-4′-(6-methacryloyloxyhexyloxy)biphenyl (1) was investigated. Reaction kinetics and molecular weight distribution of the polymer were studied. Molecular weights and weight distributions were analyzed using size exclusion chromatography. Especially, effects of radiation dose, polymerization time and temperature were investigated. Post-polymerization was found in the crystalline, liquid crystalline and melt phase. Rate of polymerization and limiting conversion to polymer increase with radiation dose, polymerization time and temperature. Samples exposed to a low γ-ray dose of 5 kGy and subsequently polymerized in the crystalline state at 20°C, for example, only reach a limiting conversion of about 7% within 100 h, while samples exposed to a high dose of 20 kGy and polymerized in the melt at 79°C reach a limiting conversion of 85.1% within 90 s. Molecular weights increase with polymerization temperature and reach values comparable with in-source polymerization. Different from in-source polymerization, they are additionally affected by the parameters radiation dose and polymerization time. High molecular weights are obtained, if samples are exposed to a low radiation 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°C. By carefully adjusting the reaction parameters, molecular weights can be reliably tailored in the range from 2 × 10⁴ to 6 × 10⁵. Polymerization at room temperature leads to a normal molecular weight distribution, 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 various conditions is discussed in terms of the mobility of growing chain ends and residual monomer, which influences chain growth and termination and thus the kinetic chain length.