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-(pmethoxybenzylidene)-p-n-butylaniline], confined in controlled pores glasses (CPG) porous glasses of 82, 156, and 337 Å pore diameter, were investigated using 1 H nuclear magnetic resonance spectroscopy (NMR). The transition temperatures are determined from the analysis of NMR line-shapes and spin-lattice relaxation times (T 1 ). 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.

Molecular Dynamics Simulations of Nimodipine Confined in an Ordered Mesoporous Silica Matrix

“…The activation energy derived from this slope for crystalline nimodipine is 9.8 kJ/mol, while for the confined system one gets 3.7 kJ/mol. Similar temperature dependences of the relaxation-time T 1 were detected for other confined systems: nonpolar liquids in porous silica glasses, , benzene confined in cylindrical mesoporous materials MCM-41 and SBA-15, liquid crystals confined in controlled pore glasses, or even polymers in nanoscopic pores. , …”

“…Similar temperature dependences of the relaxation-time T 1 were detected for other confined systems: nonpolar liquids in porous silica glasses, 38,39 benzene confined in cylindrical mesoporous materials MCM-41 and SBA-15, 5 liquid crystals confined in controlled pore glasses, or even polymers in nanoscopic pores. 40,41 It is now interesting to compare the experimental values with estimations of T 1 obtained from the calculations. Spin−lattice relaxation times T 1 were calculated as linear combinations of spectral densities J(w), according to the following equation: 42 ∑ ∑…”

Electrospun Polymer Fibers Containing a Liquid Crystal Core: Insights into Semiflexible Confinement

“…Past studies on the confinement effects of MBBA show that its phase transitions can be affected by cavity size on the nanometer scale although most of these studies employed rigid cavities. It has been shown that the phase-transition temperatures of MBBA decrease with decreasing pore size and have a tendency to broaden. , In some cases, very small pores, on the angstrom scale, have led to phase transitions, which cannot be resolved by DSC measurements . Thus, the rigidity of these cavities provided strong confinement by steric interactions limiting the ability of MBBA to align.…”

“…It has been shown that the phase-transition temperatures of MBBA decrease with decreasing pore size and have a tendency to broaden. 40,41 In some cases, very small pores, on the angstrom scale, have led to phase transitions, which cannot be resolved by DSC measurements. 40 Thus, the rigidity of these cavities provided strong confinement by steric interactions limiting the ability of MBBA to align.…”

Probing the phase transition behavior of acetonitrile confined in mesoporous silica by FT Raman spectroscopy

“…Confined phase transitions were previously studied mainly using X‐ray diffraction (XRD),16 neutron diffraction (ND),16–19 differential scanning calorimetry (DSC),19–22 and nuclear magnetic resonance (NMR),19, 23–26 but few investigations were conducted using Raman spectroscopy 27…”