Dynamic ¹H NMR Investigation of the Liquid Crystal MBBA Confined in Porous Silica

Abstract

The polymorphism and molecular dynamics of the liquid crystal MBBA [N-(p-methoxybenzylidene)-p-n-butylaniline], confined in controlled pores glasses (CPG) porous glasses of 82, 156, and 337 Å pore diameter, were investigated using ¹H nuclear magnetic resonance spectroscopy (NMR). The transition temperatures are determined from the analysis of NMR line-shapes and spin-lattice relaxation times (T₁). The results show that the depression of the phase-transition temperatures is linear versus inverse pore diameter. The motion parameters of the phases were derived from an isotropic rotational diffusion model (BPP). In the nematic and glassy nematic phases, the results show that the confinement weakly influences the dynamics. In the crystalline phases, the confinement results in a reduction of the motion activation energies and an important phase diagram modification. To interpret the experimental results with regard to the bulk characteristics properties, a Cole–Cole distribution model was introduced. The width distribution parameter was found to depend linearly on the inverse pore diameter in the crystalline phases.

Keywords:

• confined
• liquid crystal
• NMR

Notes

^a Pore size (Å).

^b Pore-size distribution (%).

^c Specific pore volume (cm³/g).

^d Specific surface (m²/g).