The polymorphism of 4-cyano-4 0 -pentylbiphenyl (5CB) was investigated by means of differential scanning calorimetry (DSC), X-ray diffraction, and proton nuclear magnetic resonance (NMR) experiments. Three new phases has been identified and characterized structurally and dynamically. By a slow cooling at a rate 0.2 K/min, the nematic phase crystallizes into a phase noted C1. Upon a fastest cooling ( ! 5 K/min) a glassy liquid crystal (GLC) phase with a glass transition temperature T g ¼ 208 K, is formed. Upon heating up both the solid state C1 and the GLC phase, a second crystalline phase C2 (DH ¼ 49 J/g.K) which is more stable than C1 (DH ¼ 36 J/g.K) can be obtained. The proposed phase diagram has been investigated by proton NMR lineshape analysis. The motional correlation time of end-chain methyl group motion was determined by means of spin-lattice relaxation time (T 1 ) analysis.

We report proton NMR experiments on the liquid crystal material N -( p-methoxybenzylid ene)-p-n-butylaniline (MBBA) at 100 MHz in the temperature range 110± 350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.

No 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.

No abstract