Kerr Effect in the Isotropic Phase of Copolymers with Mesogenic Side Groups

Abstract

One of the promising methods for producing novel high-molecular-weight compounds that form a liquidcrystalline (LC) phase [1] is the synthesis of functionalized copolymers from monomers with distinct chemical structures. In this case, one type of monomer (mesogen) is responsible for the formation of the LC phase, while the other type of monomer (functional) imparts the desired physicochemical properties to copolymers. Recently, we synthesized comb-shaped random copolymers eC-AA6- x with mesogenic alkoxycyanobiphenyl (CB) and isophthalic acid (AA6) side groups (Fig. 1). The synthesis and phase behavior of the eC-AA6- x copolymers were described in [2]. Homopolymers CB and AA6 form a thermotropic LC phase. However, incorporation of 18 or 80 mol % of AA6 acid groups leads to the complete destruction of the LC phase. The CB-AA6-18 and CB-AA6-80 copolymers are amorphous and transform directly from the glass state to the isotropic liquid phase. At the same time, for copolymers in which the concentration of CB and AA6 groups is close to the equimolar one (at x = 37 and 58, curves 3 and 4 in Fig. 2), two optically isotropic phases were detected, high-temperature I 1 and lowtemperature I 2 , which are separated by a first-order phase transition with the transition heat 1.5‐2.3 kJ/mol [2]. This phase transition inside the isotropic state is likely caused by the structural organization of the melt, in which both the mesogenic CB and the acid AA6 groups play an important role. The copolymers were characterized by calorimetry, polarizing microscopy, IR spectroscopy, and SAXS data. However, these data did not answer the fundamental question of whether this transition is an isotropic liquid‐liquid crystal transformation i.e., what happens with orientational ordering of mesogenic CB groups upon such a transition. Therefore, we studied the isotropic solution by the transient electric birefringence (TEB, the Kerr effect) method. This method is efficient for studying the pretransitional phenomena caused by orientational order fluctuations in the isotropic phase of mesogens near the phase transition to a LC phase [3‐6]. In the present work, we identified, for the first time, the novel I 2 phase as a true isotropic one and compared the properties of the I 1 and I 2 phases. Two important facts were established: the magnitude of the local orientational order does not change abruptly upon the I 1 ↔ I 2 phase transition, and a temperature dependence of the orientational order in the I 2 phase is missing. This implies that, in the I 2 phase, changes in the short-range orientational order are spatially limited, which is associated with the microphase segregation of mesogenic and acid groups. In this work, eC-AA6- x copolymers containing 18 and 37 mol % of isophthalic acid groups were investigated. According to differential scanning calorimetry (DSC) data (Fig. 2), the CB-AA6-18 copolymer is amorphous and has the glass transition temperature T g = 56°e . The DSC curve for the CB-AA6-37 copoly