Reorientation of Nematic Liquid-Crystals and Liquid-Crystalline Polymers in an Electric Field Studied by FT-IR Time-Resolved Spectroscopy and 2D-Correlation Analysis

Abstract

Fourier-transform infared (FT-IR) time-resolved spectroscopy was used to study the segmental mobility of two low molecular weight nematic liquid-crystals (p-cyanophenyl p-(n-hexyl)benzoate, 6CPB, p-cyanophenyl p-(n-heptyl)benzoate, 7CPB) and a nematic liquid crystalline polymer (poly[p-(n-hexyl)(p-(cyanophenyl)benzoate) acrylate], NLCP), which has a 6CPB unit in the side chain. Of particular interest was the comparison between the reorientation of free 6CPB and 6CPB integrated in the side chain of the polymer. The samples were studied under different experimental conditions of temperature, voltage, and prealignment. To accentuate and derive more useful information from the original time-resolved spectra, we applied 2D-correlation analysis. It has been shown that molecules of 6CPB and 7CPB reorient as rigid units. In the case of NLCP only the mesogen and a small part of the spacer, directly attached to the mesogen, participate in the reorientational movement; the rest of the spacer and the main chain do not move under the influence of the electric field. The temperature, in a narrow range, and the prealignment do not have a noticeable effect on the rate of reorientation of the nematic low molecular weight liquid-crystals, whereas increasing temperature and prealignment significantly shorten the relaxation times of the NLCP.

Polarized IR spectra of resonance assisted hydrogen bond (RAHB) in 2-hydroxyazobenzenes

“…As shows already rich literature [1][2][3][4][5][6][7][8][9][10] the polarized IR spectra of liquid crystals (LCs) yield interesting information from the point of view of both the degree of ordering in mesophases and characteristics of particular vibrational modes. Moreover, in the case of liquid crystals there are possibilities to prepare oriented samples, and applying the polarized light, to measure the direction of the transition dipole moment [2].…”

Two-Dimensional Correlation Spectroscopy: The Power of Power Spectra

“…In contrast, in the case of the polymeric LC an electric field reorients only the mesogen and a small part of flexible spacer, while the remaining part of the spacer and the backbone (main chain) do not participate in the reorientation. 15 Hence, the bands from the methyl and methylene groups (Fig. 7b) are hardly seen in the power spectrum, since the spectral changes for these groups are small.…”

“…More detailed information on the experimental conditions and sample preparation one can find in Czarnecki et al. 15…”

Concerning the spectral resolution enhancement of dynamic 2D-IR

“…Recently, however, in-phase and quadrature spectra were obtained with the IR beam parallel or perpendicular to the stretching direction only [33,[36][37][38][39]. This offers another opportunity for measuring dynamic spectra when orientation analysis is not a concern.…”