Session L20: Liquid Crystalline Polymers and Anisotropic Particles

Patterns on the iridescent beetle, \textit{Chrysina gloriosa}

The brilliant metallic color of a beetle \textit{Chrysina gloriosa} has been known to occur due to selective reflectance from a cholesteric structure on the exoskeleton. The surface also appears to have hexagonally packed structures. Crystallographic concepts and Voronoi analysis were used to determine the degree of order in different regions of the beetle. Along the hexagons in the Voronoi diagram, many clustered pentagons and heptagons were observed. Due to the surface curvature, the number of pentagons was found to be higher than the number of heptagons. The cells appear yellow in the center surrounded by a green region with a yellow edge. Confocal microscopy was used to image the underlying structure, which was found to consist of concentric arcs on a surface of a shallow cone. The observed structures resemble the defects on a cholesteric phase with a free surface, and provide an interesting explanation of structural color development in beetles, along with inspiration for the design of chiral photonic structures.   

Frustrated self-assembly of dendron and dendrimer-based supramolecular liquid crystals

A new inverted topological configuration is demonstrated both experimentally and theoretically for self-assembled dendron and dendrimer-based supramolecular liquid crystals in which the dendrons/dendrimers occupy the continuous domain and the ionically attached pendant chains are confined in discrete domains. All previous studies on dendrimer and dendron-based liquid crystals have reported normal liquid crystalline configurations in which the dendritic templates occupy discrete domains (in spherical or columnar phases) or continuous struts (in bicontinuous cubic phases), while the pendant chains occupy the continuous space-filling domain. These surprising results mandate a re-examination of the packing mechanisms for this important class of materials and open new routes to unique nanostructures of possible use in existing and emerging technologies. References: R. Mezzenga, J. Ruokolainen, N. Canilho, E. Kas\"{e}mi, D.A. Schl\"{u}ter, W.B. Lee, G. H. Fredrickson, \textit{Soft Matter}, in press (\textbf{DOI:} 10.1039/b814972k)   

Hydrogen-Bonding Assisted Supramolecular Self-Assembly of Double Discotic Supermolecues

Symmetric double discotic supermolecules based on porphine (Py) and triphenylene (Tp), Py(Tp)$_{4}$, have been successfully synthesized via PyBOP-catalyzed amidization reactions. The Tp moieties had either C$_{5}$- or C$_{12}$-arms, and the spacer linking between the central Py and peripheral Tp was either C$_{6}$ or C$_{10}$. Thermal properties of these supermolecules were studied by differential scanning calorimetry, and self-assembled crystalline and/or liquid crystalline textures were confirmed by polarized optical microscopy. For samples with C$_{5}$-arms in Tp, only a crystal-melt transition was observed. X-ray diffraction (XRD) on shear-oriented samples showed that Py was crystalline and Tp formed columnar liquid crystal. For samples with C$_{12}$-arms in Tp, sequential crystal-liquid crystal-isotropic melt transitions were observed. XRD results indicated that the crystalline unit cells were orthorhombic for all samples, and amide hydrogen-bonding was responsible for their supramolecular self-assembly.   

Glassy correlations in nematic elastomers

We address the physical properties of an isotropic melt or solution of nematogenic polymers that is then cross-linked beyond the vulcanization point. To do this, we construct a replica Landau theory involving a coupled pair of order- parameter fields: one describing vulcanization, the other describing local nematic order. Thermal nematic fluctuations, present at the time of cross-linking, are trapped by cross- linking into the vulcanized network. The resulting glassy nematic fluctuations are analyzed in the Gaussian approximation in two regimes. When the localization length is shorter than the thermal nematic correlation length, the nematic correlations are well captured as glassy correlations. In the opposite regime, fluctuations in the positions of the localized polymers partially wash out the glassy nematic correlations.   

Orientation distribution and process modeling of thermotropic liquid crystalline copolyester (TLCP) injection-moldings

The influence of melt processing conditions upon mechanical properties and degrees of compound molecular orientation have been thoroughly studied for a series of well-defined injection molded samples fabricated from VECTRA (TM) A950 and 4,4'-dihydroxy-a-methylstilbene TLCPs. Fracture and tensile data were correlated with processing conditions, orientation, and molecular weight. Mechanical properties for both TLCPs were found to follow a ``universal'' Anisotropy Factor (AF) associated with the bimodal orientation states in the plaques determined from 2-D WAXS. Surface orientations were globally surveyed using Attenuated Total Reflectance -- Fourier Transform Infrared (ATR-FTIR) spectroscopy and C K edge Near-Edge X-ray Absorption Fine Structure (NEXAFS). The results derived from the two spectroscopy techniques confirmed each other well. These results along with those from 2-D WAXS in transmission were compared with the results of process modeling using a commercial program, MOLDFLOW(TM). The agreement between model predictions and the measured orientation states was gratifyingly good.   

In Situ X-ray Scattering Measurements and Polydomain Simulations of Molecular Orientation Development during Injection Molding of Liquid Crystalline Polymers

We report on a coordinated experimental/computational study of injection molding of commercial thermotropic LCPs. In situ synchrotron x-ray scattering, combined with a customized injection molding apparatus, is used to track development of molecular orientation during the mold filling process for a commercial LCP, Vectra A950, in two simple plaque mold geometries: square and T-shaped. Use of high brilliance undulator radiation at the Advanced Photon Source, coupled with a high speed CCD detector provides sufficient time resolution (~ 12 frames per second) to resolve the transient orientation dynamics during and following mold filling. In addition to in- situ scattering measurements, ex-situ 2-D wide angle X-ray scattering measurements are conducted on the same injection molded plaques. The experiments are complemented by process simulations performed using commercial mold filling software. A very close analogy between the Folgar-Tucker fiber orientation model and the Larson-Doi polydomain model for textured liquid crystalline polymers is exploited to allow for the first tests of Larson-Doi model predictions in injection molding processing.   

Dynamics and rheology of active polar liquid crystalline films

I will discuss the dynamical and rheological properties of active polar liquid crystalline films. Like active nematic films, active polar films undergo a dynamical transitions to spontaneously flowing steady-states. Spontaneous flow in polar fluids is, however, always accompanied by strong concentration inhomogeneities or ``banding'' not seen in nematics. In addition, a spectacular property unique to polar active films is their ability to generate spontaneously oscillating and banded flows even at low activity. The oscillatory flows become increasingly complicated for strong polarity.   

Influence of nanorods on the properties of polymeric materials

Nanoscopic additions, such as metallic nanoparticles or carbon nanotubes, can dramatically impact the mechanical properties of polymeric materials, such as the plateau modulus, which is intimately related to the entanglement length of the polymer. To explore the connection between nanocomposite configurations and the dynamic mechanical effects that are difficult to probe experimentally, due to challenges associated with sample preparation and particle dispersion, we have performed extensive Molecular Dynamics and Monte Carlo simulations of polymer nanocomposites with nanoparticles whose size, shape, and concentration have been varied systematically. Calculations of the entanglement network through primitive path analysis of these composites have enabled us to connect nanorod effects on the entanglement network structure and density to the system's dynamic properties. The main outcome of our study is a better understanding of how inclusions alter entanglements and how those entanglements are magnified in macroscopic observables.   

Interactions between rod-like cellulose nanocrystals and xylan derivatives: A light scattering study

Interactions between rod-like cellulose nanocrystals and 2-hydroxypropyl-trimethylammonium (HPMA) xylan were investigated by polarized (DLS) and depolarized dynamic light scattering (DDLS). Cellulose nanocrystals were prepared by the controlled hydrolysis of black spruce pulp. Binary rod-like cellulose nanocrystal/water and ternary HPMA xylan/rod-like cellulose nanocrystal/water systems with different concentrations of cellulose nanocrystals were probed. Translational and rotational diffusion coefficients of cellulose nanocrystals in water are (4.8 $\pm $ 0.4) $\times $ 10$^{-8}$ cm$^{2}$s$^{-1}$ and (526 $\pm $ 20) s$^{-1}$, respectively, and calculated lengths and diameters for nanocrystals are comparable to those of cellulose whiskers from cotton. At high cellulose nanocrystal concentrations, DDLS studies in ternary systems provide translational and rotational diffusion coefficients. However, at low cellulose nanocrystal concentrations, DDLS studies of ternary systems do not yield rotational diffusion coefficients. This behavior is attributed to bridging between polymer chains that causes non-linear deviation on standard decay rate ($\Gamma )$ versus scattering vector magnitude (q$^{2})$ plots.   

Polymerization-Enhanced Alignment Order in Carbon Nanotube Composites

Polymer nanocomposites containing vertically aligned carbon nanotubes (VACNTs) have been synthesized via vacuum infiltration of monomers into confined VACNT arrays followed by in situ polymerization. The alignment order of VACNTs before and after polymerization has been quantitatively assessed using small angle neutron and x-ray scattering. The trend of continuous variation of alignment order along the height of VACNTs remains unaltered whereas the degree of order is enhanced upon polymerization. Polymerization-enhanced alignment order may assist preparing better carbon nanotube composites.   

Perylene diimide liquid crystals: A density functional study

Organic semiconductors (OSs) are getting each time more space in the field of electronic materials mainly due to its low manufacturing costs, the relative facility of manufacturing in the desired way. A model for the crystal structure of perylene diimide PPEEB has been proposed, partially based on experimental observations. In this structural model, we've performed an {\it ab initio} calculations on the electronic structure of this material. Due to the strongly localized nature of the Oxygen atoms on the side chains, is necessary to go beyond the standard LDA and GGA calculations. With the PBE0 approach, the electronic structure becomes in agreement with previous results. The tails of the molecular crystal not only is responsible for its structural conformation, but also can be used for tuning the electronic and optical properties of the material.