ESR study of nematic order and dynamics inside HPDLC droplets

Electron Spin Resonance (ESR) spin probe spectroscopy technique is used to investigate the two phase Holographic Polymer Dispersed Liquid Crystal (HPDLC) system made of liquid crystal (LC) confined periodically in a polymer matrix. HPDLCs are prepared using two beam laser interference set up at 532 nm wavelength on the prepolymer mix [1] consisting of LC, 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 regular array of nanosized droplets embedded in a polymer matrix. 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 [2], and in reflective displays [3]. Several microscopic and spectroscopic techniques were used to understand both droplets morphology and behaviour, ranging from SEM and AFM microscopy [2] to Nuclear Magnetic Resonance using deuterated 5CB LC [4] and protonated LC; however a better understanding of the director configuration inside the droplets is fundamental to improve the diffraction properties and the switching performance of HPDLCs. To this purpose the study was further extended by conducting ESR spectroscopy [5] on HPDLCs based on commercially viable LC BL038 by doping with a stable nitroxide free radical spin probe. The analysis was conducted for various temperatures ranging from the nematic to the isotropic phase of the LC to evaluate the effect of confinement on HPDLC local order and dynamics compared to bulk characteristics. At 283 K, below the LC nematic-isotropic transition temperature (TNI), the sample shows a peculiar equilibrium between a rigid-limit-like and an isotropic phase in approximately a 50:50 proportion. Increasing the temperature, the rigid-limit fraction decreases rapidly to about 5% while an ordered contribution, in the form of a relatively narrow distribution of nematic domains (quasi-monodomain) aligned along the magnetic field, begins to appear and can still be observed by a signature lineshape up to 373.2 K (the bulk BL038 TNI). The fractional contribution of this phase increases by field cooling the sample and decreases upon a zero field cooling treatment, with good reproducibility, and it occurs only along a direction parallel to the HPDLC droplet planes. The results are in agreement with the assumption of droplets stretched along the grating vector with homeotropic LC alignment at the polymer interface which induces a cylindrical (2D) distribution of local LC domain directors. Only when the uniform magnetic field is normal to the cylinder axis a fraction of this 2D distribution can be aligned to form the observed quasi-monodomain. 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] J. Qi and G. P. Crawford, Displays, 25, 177 (2004). [4] M. Vilfan, B. Zalar et al., Phys. Rev. E, 66, 021710 (2002). [5] A. Arcioni, C. Bacchiocchi, I. Vecchi, G. Venditti, C. Zannoni, Chem. Phys. Lett., 396, 433 (2004).