γ-ray polymerization of a mesogenic methacrylate in the crystalline, liquid crystalline and liquid state: kinetics, molecular weight distribution and conversion-dependent phase behaviour

The γ‐ray polymerization of mesogenic 4‐methoxy‐4′‐(6‐methacryloyloxyhexyloxy)biphenyl (1) was investigated. 1 can be polymerized in the crystalline, in the liquid crystalline and in the molten state. The rate of polymerization increases from the crystalline to the melt phase. In order to reach 90% conversion to polymer in melt, smectic and crystalline phase, radiation doses of 10, 40 and more than 120 kGy are necessary. Melt polymerization leads to a weight‐average molecular weight of about 1 × 106 independent of the conversion to polymer. In the smectic and crystalline phase the corresponding molecular weights are about 3.8 × 105 and 4.1 × 104 at low conversion, and shifted to 2.6 × 105 and 3.9 × 104 at high conversion. The polydispersity generally increases with the γ‐ray dose and the reaction temperature. The origin of the different reaction rates and molecular weight obtained in the various phases 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. Differential scanning calorimetry (DSC) of partially polymerized samples was used to describe the conversion‐dependent phase behaviour. Optical microscopy showed that the melt polymerization at 78.5°C proceeds under phase separation into a liquid, monomer‐rich and a solid, polymer‐rich phase. As a consequence, liquid monomer samples solidify upon γ‐irradiation once a higher conversion to polymer is reached.