In samples of polymer-dispersed liquid crystals (PDLCs), submicron droplets of liquid crystal reside within a polymer matrix. By the use of the cross polarization technique for solid-state C-13 NMR spectroscopy, the NMR spectrum of the liquid crystal can be obtained without interference from the polymer spectrum, even though the two materials have some similar chemical structures. Both C-13 T1 and C-13 T1 rho relaxation experiments were performed on a PDLC system of 5CB in epoxy as a function of 5CB domain size. The C-13 T1 relaxation constants and the localized motions they measure showed no significant change over the liquid crystal domain size studied. However, the C-13 T1 rho relaxation constants and the segmental motions of molecules they measure revealed a significant change over the liquid crystal domain size studied. Therefore, the C-13 T1 rho values can be used to determine the change in mobility of the molecular segments of the 5CB molecules resulting from increased interaction between the liquid crystal and the polymer matrix and to further understand the importance of the molecular motions of the liquid crystal in the switching phenomenon for PDLC materials.
Molecular-Motion Analysis Of E7 In Pdlcs As A Function Of Droplet Size Using Solid-State C-13 Nmr Relaxation Spectroscopy01/01/1993
13C NMR relaxation spectroscopy as applied to polymer-dispersed liquid crystals (PDLCs) provides a method for measuring the molecular motions of the liquid crystal molecules within droplets dispersed throughout the material's polymer matrix. Because liquid crystal molecular motion may play a major role in the switching phenomenon of PDLC materials from an opaque film to a clear film, both T 1 and T 1ρ relaxation experiments are used to measure molecular mobility of the liquid crystal as a function of droplet size for PDLC materials made of E7 and epoxy. The segmental molecular motions measured by T 1ρ relaxation show a significant dependence on liquid crystal droplet size. Three models are provided which explain the observed restriction in segmental mobility of the liquid crystal molecules as the droplet size decreases and the polymer/liquid crystal interaction increases.