MICROSECOND TIME-RESOLVED INFRARED STUDY OF THE ELECTRIC-FIELD-INDUCED REORIENTATION OF NEMATIC LIQUID-CRYSTALS .2. DEPENDENCE OF THE MOTION OF LIQUID-CRYSTALS ON THE APPLIED ELECTRIC-FIELD

The electric-field dependence of the reorientation motion of a nematic liquid crystal, 5CB (4-n-pentyl-4'-cyanobiphenyl), has been studied b y microsecond time-resolved infrared spectroscopy. A rectangular pulse d electric field with a short pulse duration (2 ms) and a low repetiti on rate (5 Hz) was used to examine the liquid crystal (LC) response in a silicon cell. The motion of the rigid core part (the cyanobiphenyl group) of 5CB was monitored by the CN stretch band and that of the fle xible part (the pentyl group) by the pentyl CH stretch band. The respo nse of the LC to the pulsed electric field consists of two components, the slow component and the fast component. The slow component is comm on to the rigid core and the flexible parts of 5CB. The voltage depend ence of the slow component exhibits a clear threshold, indicating that this component corresponds to a cooperative motion of the 5CB molecul es. The fast component is specific to the flexible part and shows expo nential rise and decay behavior patterns. This observation suggests th at the fast component corresponds to some noncooperative motions which are characteristic of the pentyl group. It is most likely that the in ternal rotation around the C(biphenyl)-C(pentyl) bond is responsible f or the fast component. It is suggested that the LC molecules near the cell interface play a key role in the primary stage of the reorientati on motion under an applied electric field.