Reorientation instabilities and viscoelastic measurements in a main chain thermotropic nematic polymer Optical and NMR studies

NMR study of the long time, defect-controlled, magnetic reorientation of a nematic polymer liquid crystal

“…The proton NMR spectra were taken at 60 MHz (H N 14 kG) and the sample temperature was kept at 101°C. We followed the experimental procedure described in [2,3]. The equilibrium spectrum (see figure 5{a)) shows a central line with some structure and a symmetric doublet.…”

“…Now we turn to the main chain nematic thermotropic polymer studied in [3], a polyester labelled AZA9, and consider the last experimental NMR spectrum wellsimulated with the hydrodynamic model in equation (l), corresponding to t = 1150 s after rotating the sample. Figure 2 shows QF versus u , , ,…”

“…The proton NMR line shape of a nematic sample is strongly affected by the director spatial pattern [3]. For a given director distribution O(z, t) we can get the corresponding spectrum f (v, t ) from the spectrum of the aligned nematic monodomain fo(v) using where P,(cos a) is the second Legendre polynomial and a = 4 2 -8(u) [3].…”

“…For a given director distribution O(z, t) we can get the corresponding spectrum f (v, t ) from the spectrum of the aligned nematic monodomain fo(v) using where P,(cos a) is the second Legendre polynomial and a = 4 2 -8(u) [3]. Here u is a reduced distance (for example, u=qz for a sinusoidal distortion) and 8 is the angle between the local director n(r) and the initial orientation.…”

“…Equation (1) includes the effect of backflow through the substitution of y1 by yerf(0) in the first term and the effect of director field distortions (for example, banded structure) through the form of K(O), as shown in [2]. In figure 1 (curve c) we show the numerical solution of equation (1) for very long times with the parameters of the PLC studied in [3]. In the limit t+ 00 this equation does not lead to a uniform director pattern (O(z, t = co) 1: 90°) but, instead, to a pattern of splay-bend inversion walls corresponding to a banded texture with a wavelength A =4<f(Omax), where f(fImax) is a function of the maximum distortion angle Omax and t 2 = r?i/1,H2 is the magnetic coherence length, with K = ( K , + K J 2 an average elastic constant.…”