Deuteron spin relaxation and molecular dynamics of a nematic liquid crystal (5CB) in cylindrical microcavities

Deuteron NMR study of molecular ordering in a holographic-polymer-dispersed liquid crystal

Using deuteron nuclear magnetic resonance (NMR) and dynamic light scattering, we study the orientational order and dynamics of a BL038-5CB liquid-crystal mixture in a holographic polymer dispersed liquid-crystal material (HPDLC) as used for switchable diffractive optical elements. At high temperatures, where the liquid crystal is predominantly in the isotropic phase, the HPDLC deuteron NMR linewidth and transverse spin-relaxation rate T-12 are two orders of magnitude larger than in the bulk. The analysis shows that the surface-induced order parameter in HPDLC is significantly larger than in similar confining systems and that translational diffusion of molecules in the surface layer is at least two orders of magnitude slower than in the rest of the cavity. The unusual temperature dependence of T-12 upon cooling suggests the possibility of a partial separation of the 5CB component in the liquid-crystal mixture. The onset of the nematic phase in HPDLC occurs at considerably lower temperature than in the bulk and takes place gradually due to different sizes and different content of non-liquid-crystalline ingredients in droplets. Parts of the droplets are found isotropic even at room temperature and the structure of the nematic director field in the droplets is only slightly anisotropic. We point out the capability of NMR to detect the actual state of liquid-crystalline order in HPDLCs and to contribute in this way to the improvement of the switching efficiency of diffraction gratings.

Measurement of the surface-induced order in polymer dispersed liquid crystals: an approach by NMR relaxometry

A method based on NMR relaxometry is introduced to study surface-induced order in the isotropic phase of confined liquid crystals. We show that the magnitude of the surface order parameter S0 can be obtained from an increase in the deuteron transverse relaxation rate T-12. The increase originates in the molecular diffusion between the weakly ordered surface region and disordered area in the rest of the cavity. No assumptions concerning the residence time of molecules at the surface are needed. We apply the approach to a polymer dispersed liquid crystal and find a temperature independent S0 of magnitude congruent with 0.08. This indicates that short range interactions at the interface dominate the behavior of S0, and that only a partial orientational wetting occurs.

Investigation of molecular order and dynamics in liquid crystals confined in porous media using the dipolar-correlation effect on the stimulated echo

A new application of the stimulated echo pulse sequence is presented that permits the elucidation of molecular order and dynamics in a time scale between about 100 microseconds and the spin-lattice relaxation time. The technique exploits the influence of dipolar coupling on the quotient of the stimulated and primary echoes produced by the standard three 90 degrees-pulse sequence. Results obtained for a nematic liquid crystal in bulk and confined in porous glass (mean pore diameter 4 nm) are compared. In both cases the echo amplitude quotient oscillates as a function of the pulse spacing. In a bulk nematic crystal these oscillations originate from strong unaveraged dipolar interactions and directly reflect the molecular order in the material. In porous glass a real nematic order is absent. In this case, the oscillations can be attributed to spin exchange between inequivalent protons. Exchange rates are estimated.