Cooperative modes of molecular mobility in liquid crystalline comb-like oligoacrylates and oligomethacrylates with 4-cyanoazobenzene side groups

Abstract

Cooperative modes of molecular mobility in comb-like liquid crystalline oligoacrylates and oligomethacrylates containing mesogenic 4-cyanoazobenzene side groups and spacers of 3, 5, 7, 9, 10, and 11 CH2 groups were studied by means of dielectric spectroscopy. Near Tg the existence of two cooperative relaxation processes denoted as α and δ was revealed. The δ-relaxation is connected to the motion of mesogenic groups with respect to the short mesogenic axis and the α-relaxation is connected to the motion of mesogenic groups with respect to the long axis and with the segmental mobility of the main chains. The temperatures of α- and δ-transitions decrease with increasing spacer length. The liquid-crystalline structure formation increases main chain rigidity and temperatures of the α- and δ-transitions. For these oligomers aligned by an external electric field or by mechanical shear, the planar or homeotropic orientation of mesogenic groups is established. The choice of orientation conditions makes it possible to obtain the predetermined order parameters.