Session D3: Polymers - Experiments

Deuteron NMR Study of the Isotropic-Smectic A transition in Liquid-Crystal-Aerosil Dispersions

In this work, we present a deuteron nuclear magnetic resonance (NMR) study of quenched disorder effects on the 12CB liquid crystal (LC) upon dispersion of silica nano particles (type A-300): hydrophilic silica spheres of diameter 7nm and surface area S=300m$^{2}$/g, with hydroxyl groups covering their surface. The LC-aerosil dispersions form a gel (network) if the aerosil density exceeds the gelation (percolation) threshold. The hydroxyl groups on the surface and the polar nature of the LC, likely yield a homeotropic alignment at the silica surfaces. For low densities of aerosil in the dispersions and while cooling the sample, the LC director in void volume is parallel to the external NMR field; a well defined and aligned LC configuration is established. When a complete silica network finally forms, if the dispersion orientation with respect to the NMR field is changed, a few silica links are broken by the field, re-aligning some LC molecules; effectively, the aerosil locks-in the LC configuration which exhibits a P$_{2}$ (Cos$\Theta )$ dependence. The external field anneals the random disorder introduced by aerosil up to a certain density beyond which, disordering effects dominate; for aerosil densities exceeding $\rho _{S} \quad \approx $ 0.055~g/cm$^{3 }$the NMR spectrum is a powder-pattern representing an isotropic distribution of smectic domains. The occurrence of quenching of 12CB Sm-A phase at $\rho _{S} \quad \approx $ 0.055~g/cm$^{3}$, is roughly one order of magnitude less than the density required for quenching of 8CB [1] [1] T. Jin and D. Finotello, \textit{Phys. Rev}. \textbf{E 69}, 041704 (2004); \textit{Phys. Rev Lett}. \textbf{86}, 818 (2001).   

Langmuir Layers of Bent-Core Liquid Crystals

Bent-core liquid crystals are confined at the air/water interface. They form discretely layered structures even where when the material does not form smectic liquid crystals in bulk samples. Structure and phase transitions are characterized by X-ray diffraction, surface pressure, Brewster angle microscopy, x-ray, and surface potential measurements. Optical anisotropy allows us to determine the tilt angle with respect to the film normal and to domain features, as well as the refractive index anisotropy. This material is based upon work supported by the National Science Foundation under Grant No.9984304. The surface potential work was supported by the Petroleum Research Fund, under grant ACS PRF{\#} 35293-G 7. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No W-31-109-Eng-38.   

Hole dynamics in polymer Langmuir films

This article develops a model for the closing of a gaseous hole in a liquid domain within a two-dimensional fluid layer, coupled to a fluid bulk substrate, and compares this model to experiments following hole dynamics in a polymer Langmuir monolayer. Closure of such a hole in a fluid layer is driven by the difference in pressure within the hole and far outside it, and by the line tension. The observed rate of hole closing is close to that predicted by our model and the line tension measured by other means, assuming that the pressure in the gas is negligible. This result both supports the model and suggests an independent means of determining the line tension. Unlike most previous hydrodynamics models of Langmuir films, the closing of a hole necessarily involves vertical motion of the underlying incompressible fluid: that fluid is dragged along with the liquid monolayer towards the center of the hole, and must plunge away from the surface. An explicit expression is found for this vertical fluid flow in the bulk substrate.   

Diffusion of biomolecules in nanoporous glasses

Two-Photon Fluorescence Correlation Spectroscopy (FCS) was used to measure the diffusion of simple and biomolecular fluorophores in nanoporous glasses, a candidate biosensor substrate. Diffusion of two fluorescent molecules of similar size, cationic rhodamine 6G in methanol and anionic fluorescein in sodium borate buffer, was measured in partially silanized porous Vycor 7930 glass (4 nm average pore size). The diffusion time constant in Vycor is measured to be 12.5 times slower than in bulk samples, for both fluorophores. Our data is consistent with a tortuosity factor correction for the diffusion of these molecules in Vycor glass. We also investigated diffusion of FITC labeled multiclonal E.coli antibodies in 50 nm and 100 nm Controlled Porosity Glass (CPG) samples. Both treated and untreated CPG samples were investigated, using an ELISA-based protocol for the treated sample. We find that the diffusion timescales depend on the pore size, and the functional form of correlation function deviates from the three dimensional diffusion model.   

Dynamics of Polyethylene Melts, Studied by Monte-Carlo Methods

In connection with our diffusion measurements in polyethylene (PE) melts we have performed coarse-grained Monte-Carlo simulations on the second-nearest-neighbor diamond lattice to study static and dynamic properties of PE from C40 to C324 (molecular weight M from 584 to 4538). The bridging method is based on beads combining neighboring moieties and now uses two- bead moves; it permits detailed reconstruction of the PE chain at any stage. It uses the short-range rotational isomeric state model and inter- and intrachain Lennard-Jones potentials in their discretized forms. Simulation results become reliable after extended equilibration. For static properties such as the radius of gyration Rg the assumption of Gaussian chain statistics fails for all but the longest chains as Rg scales anomalously with M; at low M chain-end effects become important. But autocorrelation functions of the end-to-end vectors are still interpretable in terms of Rouse and reptation theories for chains longer than C82. Above C100 the relaxation times, converted from Monte-Carlo steps to time units by comparing with experimental diffusion (D) results, scale with M in keeping with reptation theory. At the lowest M, D has a slightly steeper M- dependence than experiment.   

Zero-crossing Pattern and Scaling of Background Brain Dynamics and Alpha Rhythm

The scaling property of the broad-band fluctuation and the $\alpha$ rhythm of the brain dynamics is studied based on the zero-crossing of the local electroencephalographic (EEG) recording taken from six healthy young adults in eyes closed and eyes open. A two-component scenario, consisting of a broad- band fractal and narrow-band rhythm components, is assumed to model the EEG zero-crossing. Scaling is found in the power law distribution $p(\tau)\sim\tau^{-\nu}$ of the crossing time interval $\tau$ of the broad-band fluctuation. An inverse relationship between broad-brand fluctuation and the $\alpha$ rhythm is observed. The Hurst exponent was found to be higher in eyes closed compared to eyes open. In $\alpha$ dominant brain state, the $\alpha$ rhythm interval $L$ also exhibits scaling in the form of power law distribution: $p(L)\sim L ^\phi$. Our main result is the relationship $\nu+\phi\sim 3$ that characterizes the ``organization'' of these two prominent features of the brain dynamics. The possible role of self- organized criticality of punctuated equilibrium in this organization is argued.