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We employ spin probe Electron Spin Resonance (ESR) spectroscopy to investigate the two phase Holographic Polymer Dispersed Liquid Crystal (HPDLC) system made of liquid crystal (LC) confined periodically in a polymer matrix. The spectroscopic analysis is conducted for various temperatures from the nematic to the isotropic phase of the LC to understand the orientation and behavior of the HPDLC. HPDLC are prepared using two beam laser interference set up of 532nm wavelength on the prepolymer mix [1] consisting of LC doped with a spin label,a monomer and a light sensitive photoinitiation dye. Polymerization at the bright regions of the interference pattern causes the diffusion of the liquid crystal to the dark regions forming a nanosize droplet structure. A beam of light shone across the HPDLC reflects specific wavelengths of light, depending on the periodicity of the grating structure. These systems have extensive application in the fields of telecom and spectroscopy, e.g. they can be used as switches to filter particular wavelengths of light in optical telecommunication applications [2]. The morphology and droplet behavior of HPDLC have been studied earlier employing Nuclear Magnetic Resonance using deuterated 5CB LC [3] and protonated LC by Crawford et al. Here we further extend the study by conducting ESR spectroscopy [4] on HPDLC based on commercially viable LC BL038 by doping with a stable nitroxide free radical spin probe and comparing it to its bulk state. We have determined the orientational order and the dynamics of the spin probe dissolved in the LC droplets, finding more order in the confined state with order reduction as the temperature is increased as seen in figure 1. Analysis of the droplet size, which varies from a range of 200nm to 300nm, and the interaction of the LC with the polymer matrix is also conducted. With this method we hope to further our understanding of the LC polymer interaction and improve its efficiency. References: [1] M. L. Ermold, K. Rai, A. K Fontecchio, J. Appl. Phys., 97, 104905 (2005) [2] T. J. Bunning et al., Annu. Rev. Mater. Sci., 30, 83 (2000) [3] M. Vilfan, B. Zalar et al., Phys. Rev. E, 66, 021710 (2002) [4] A. Arcioni, C. Bacchiocchi, I. Vecchi, G. Venditti, C. Zannoni, Chem. Phys. Lett., 396, 433 (2004). |
