Xenon NMR Study of a Nematic Liquid Crystal Confined to Cylindrical Submicron Cavities

Abstract

NMR studies of xenon gas dissolved in the liquid crystal ZLI 1132 confined to submicron cylindrical cavities are reported. Spectra taken as a function of temperature yield a clear indication of the nematic to isotropic phase transition of the confined liquid crystals. In the nematic phase at 21 "C, the resonance line of dissolved 129Xe exhibits a chemical shift anisotropy of 15 ppm due to a random distribution of director axes in the plane perpendicular to the long axis of the cylinder. The anisotropy and temperature dependence of the confined system are compared to control experiments that use the bulk liquid crystal. The quadrupolar splitting observed in the I3'Xe NMR spectrum of the confined liquid crystalline solution of xenon gas is slightly greater than that found in the bulk. Two-dimensional exchange NMR demonstrates that the xenon atoms probe different average liquid crystal directors within a single cavity on a 20 ms time scale and that interpore exchange occurs on a time scale of 400 ms. The exchange data indicate that changes in the orientation of the director within individual cavities occur on a length scale of about 2 pm. IntroductionLiquid crystals are technologically important due to their widespread use in displays (LCDs) and their potential applicability in nonlinear optical devices.'-3 Liquid crystalline phases exhibit long-range molecular orientational order, in contrast to normal liquids which lack positional and orientational order. Devices based on liquid crystals are able to exploit the anisotropy of the medium for the control of alignment and switching of the director axis formed by the liquid crystalline phases. Characterization of the orientational order and the factors which govern it are therefore of considerable scientific and commercial interest.Nuclear magnetic resonance (NMR) is useful for studying liquid crystals4 due to the sensitivity of the NMR spectrum to orientational order. Anisotropic interactions such as chemical shift anisotropy, quadrupolar interactions, and magnetic dipolar couplings make it possible to measure the degree of orientational order. However, spectra can be extremely complicated and intractable for abundant spins in multiple sites, e.g., protons in the liquid crystal molecules. A common simplifying approach is to study small probe molecules dissolved in the liquid crystal or to use liquid crystal molecules which have been isotopically labeled at specific sites. For solute molecules within a liquid crystalline environment, the ordering of the solute by dispersive and steric forces can be described by an order tensor that relates the average alignment of the solute molecular frame to the liquid crystal director frame.5 A particularly simple example is the case of an atom dissolved in a liquid crystal solvent; in this case the principal axis system of the probe is completely determined by the liquid crystal director field. The use of xenon as a microscopic probe of various materials has rapidly developed since the pioneering work in the early

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Zeolite ZSM‐5 was studied with MAS 13 C NMR spectroscopy, and chemical exchange was seen for both slow rotation and for exchange from straight channels in the zeolite to zig‐zag channels. Pines et al . elegantly used 129 Xe‐ 29 Xe EXSY to study a nematic liquid crystal.…”

“…[141] Many solidse ngage in host-guest dynamics, and these can be studied by using solid exchange NMR spectroscopy.Z eolite ZSM-5 [142] was studied with MAS 13 CNMR spectroscopy,a nd chemicale xchange was seen for both slow rotation and fore xchange from straight channels in the zeolite to zig-zag channels. Pines et al [143] elegantly used 129 Xe- 29 Xe EXSY to study a nematicl iquid crystal.E xperiments showedc hemical exchange between the gas and the liquid crystal sites. Similar exchange in al iquid crystal is seen by Jokisaariand co-workers.…”

Characterization of the Cavity in Poly(4-methyl-1-pentene) Crystal by Gas Permeation and 129Xe NMR Measurements

“…In many studies of zeolites, [14][15][16][17][18][19][20][21][22][23][24] it has been reported that the δ(S) value is strongly correlated with the size and nature of a hole in which Xe is sorbed. For polymer systems, [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] in addition, some researches have also been reported about the relation between 129 Xe NMR chemical shift and size of hole that gives free volume or microvoids. Similarly it will be able to evaluate the size of the cavity in PMP crystal from δ(S) value.…”

Temperature dependence of the mean size of polyphenyleneoxide microvoids, as studied by Xe sorption and 129Xe NMR chemical shift analyses

“…[7][8][9][10][11][12][13][14][15][16] The 129 Xe NMR signal is sensitively influenced by its environment, and the induced 129 Xe NMR chemical shifts strongly correlate to the microvoid size because certain interactions with the host system disturb the Xe electron density. It has been reported by our research group that the mean microvoid size in glassy polymers can be determined based on the Xe density dependence of the 129 Xe NMR chemical shift.…”