Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U24: Liquid-Crystalline Polymers |
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Sponsoring Units: DPOLY Chair: Matteo Pasquali, Rice University Room: Baltimore Convention Center 321 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U24.00001: Patterns and Defects in Nematic Elastomers Fangfu Ye, Tom Lubensky Nematic elastomers are materials that combine the orientational properties of nematic liquid crystals with the elastic properties of rubber. Ideal nematic elastomers, formed via a spontaneous symmetry breaking transition from the isotropic rubber state, exhibit soft elasticity in which one of the five elastic moduli of a uniaxial elastic medium vanishes. Monodomain samples crosslinked under imposed strain exhibit semi-soft elasticity in which that elastic modulus is small but nonzero. Applying linear stability analysis to the semi-soft elastic energy, we investigate two phenomena observed in experiments on nematic elastomers: (1) the formation, in experiments by Bob Meyer at Brandeis, of periodic modulations of the nematic director and elastic displacement (stripes) in cells subjected to a normal electric field in which the direction of stripe normals is at an oblique angle to the original nematic director and (2) the formation of +1 disclination defects at the surface of nanotube gel films [Islam, M. F., Nobili, M., Ye, Fangfu , Lubensky, T. C. and Yodh, A. G. , \textit{Phys. Rev. Lett }. \textbf{95}, 148301/1-4 (2005)]. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U24.00002: In situ synchrotron studies of structure development during injection molding of liquid crystalline polymers Stanley Rendon, Wesley Burghardt As in all polymer materials, the effect of polymer processing on the underlying molecular structure has a profound effect on the properties of liquid crystalline polymer products. While in situ scattering techniques have proven powerful for studying complex polymer structure during comparatively simple shearing or channel flows, their application to processing flows has largely been limited to in situ x-ray scattering/diffraction studies of structure development during fiber spinning. Here we report a new experiment in which a lab-scale injection molding machine has been modified to allow real-time, in situ measurements of molecular orientation development and subsequent crystallization during injection molding. The experiment requires high x-ray energy to reduce absorption in the aluminum mold wall, and high flux and a fast area detectors to achieve the necessary resolution to track time-dependent changes in fluid structure during mold filling. Hence it is ideally suited to the capabilites of the Advanced Photon Source. We report measurements injection molding of a commercial liquid crystalline copolyester (Vectra A) as a function of position in the mold and various process variables. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U24.00003: Liquid crystalline pattern formation in drying droplets of biopolymers Ivan Smalyukh, Olena Zribi, John Butler, Oleg Lavrentovich, Gerard Wong When a droplet of DNA in water dries out, a ring-like deposit is observed along the perimeter, similar to the stains in spilled drops of coffee. However, the dried ring of DNA is a self-similar birefringent pattern composed of extended molecules. We examine dynamics of the pattern formation at the droplet's rim. This gives us an insight into the underlining physics. During the major part of drying process the contact line is pinned so that DNA molecules are brought to the perimeter and extended by the radial capillary flow. Lyotropic nematic phase is formed in which highly concentrated DNA aligns along the triple line to minimize elastic energy. When the contact angle becomes small, the contact line starts to retract and the radial dilative stress causes buckling distortions at the rim which then propagate deep into the elastic liquid- crystalline medium and give rise to the pattern. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U24.00004: Orientational dynamics of dipolar or magnetized rigid nematic liquid crystal polymers and suspensions in imposed flow and external fields Qi Wang In this talk, I will present a scheme for computing the exact solution of the Smoluchowski equation under imposed flow and electric or magnetic external fields for rigid nematic liquid crystal polymers. This includes a suite of mathematical theorems that reveal the intrinsic relationship among the lower order moments of the pdf and the flow as well as the external field. Then, a simple mathematical transformation links the problem to some target model problems for which the solutions can be readily obtained using the existing numerical tools. The orientational dynamics is then studied from the exact solutions of the Smoluchowski equation. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U24.00005: Spacer length controlled lamello-columnar to oblique-columnar mesophase transition in liquid crystalline DNA - discotic cationic lipid complexes Lei Zhu, Li Cui, Jianjun Miao A series of asymmetric triphenylene imidazolium salts with different spacer lengths (C5, C8, and C11) were synthesized and their ionic complexes with double-strand DNA were prepared in aqueous solution. The molecular composition of the complexes was determined by FTIR analysis. The liquid crystalline morphology was characterized by polarized light microscopy, X-ray diffraction (XRD), and transmission electron microscope. 2D XRD results indicated an oblique columnar phase for the complex with a short spacer length of C5, while lamello-columnar phases for those with longer spacer lengths (C8 and C11). Thin film circular dichroism results showed the disappearing of any helical conformation in the DNA in all the complexes. Instead, the complexation between single-strand RNA and discotic cationic lipids did not show columnar morphology; therefore, the columnar liquid crystalline morphology in the DNA-discotic cationic lipid complexes was attributed to the DNA double-strand chain rigidity. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U24.00006: Binary phase diagrams of liquid crystal/polymer systems exhibiting crystal, smectic, and nematic transitions P. Dayal, R. Matkar, V. Ginzburg, T. Kyu Current theories describing the phase diagrams of nematic-nematic mixtures and polymer-nematic liquid crystal mixtures are based on the combination of classical Flory-Huggins (FH) theory of isotropic mixing and the Maier-Saupe (MS) free energy of nematic ordering. This combined FH-MS theory was extended by Kyu and Chiu to polymer-smectic-A systems by incorporating McMillan free energy of the nematic-smectic-A transition. We have developed a generalized model for an LC system undergoing crystallization, extending the Maier-Saupe-McMillan theory in conjunction with the Landau-Brazowiskii model of weak first order phase transition such as solidification. The generalized FH-extended MSM free energy is then minimized with respect to all order parameters and the phase diagram is constructed by balancing the chemical potentials of corresponding phases. This form of the free energy could be valuable in the time evolution studies of phase transitions in polymer/liquid crystal mixtures. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U24.00007: Interfacial Characteristics of Semi Fluorinated Polymeric and Polymeric Liquid Crystalline Surfaces Gang Cheng, Bryan Spraul, Dennis Smith, Dvora Perahia The interfacial interactions of semi fluorinated perfluorocyclobutane (PFCB) liquid crystalline, and semi crystalline polymers have been investigated. The interfacial characteristics are critical in any of their applications in which a well define stable interface is required from LCD technology to wave-guides and lithography. The inherent segregation between fluorine rich and hydrogen rich segments results in induced liquid crystallinity within the polymers themselves, even though these groups are small. The segregation to the interfaces controls the capability of the surface energy of the system. In addition of interfacial tension measurements, we used their alignment of small LC molecules using of 4, 4'-octylcycanobiphenyl (8CB) as a model system to further explore their interfacial characteristics. The correlation between surface energies, dynamics of the interface and their effects of the orientation of small LC will be discussed. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U24.00008: The Origin of Supra-Molecular Columnar Structures from Symmetrically Tapered Bisamides Kwang-Un Jeong, Hong Shen, Mattew J. Graham, Huabing Wang, Mingming Guo, Frank W. Harris, Stephen Z. D. Cheng A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CnPhBA) were synthesized to study the effect of alkyl chain length on supra-molecular structures. Phase transitions were studied with DSC, PLM, WAXD, IR, SAED, solid-state $^{13}$C NMR and computer simulations. All of the CnPhBAs formed a highly-ordered oblique columnar ($\Phi _{OK})$ phase and a low-ordered oblique columnar ($\Phi _{OB})$ phase. The two main driving forces to form these supra-molecular structures were identified: One is the $H$-bond between N-H and C=O groups, and the other is the micro-phase separation of the bisamide cores and the alkyl chains. With increasing alkyl length, the isotropization temperature decreased, while the alkyl chain disordering temperature increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing alkyl length. However, the alkyl length did not have a significant influence on the packing along the columnar axis. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U24.00009: Crystallization, the gel point and DQ NMR in PDMS networks Moshe Gottlieb, Yoav Hayoun, Inbal Preker, Rachel Yerushalmi-Rozen, Kay Saalwaechter In the framework of classical models, the presence of constraints that reduce the mobility of the chains should lead to a reduction in the crystallization rate of polymer melts and result in a lower degree of crystallinity at a given cooling rate. An increasing number of experimental observations which seem to contradict the basic premises of the classical picture have been reported in the last couple of years. In particular, recent experiments findings suggest that in some cases crystallization from a crosslinked melt is more efficient than that from the non crosslinked analogue. In this work we report on a detailed study carried out in order to examine the relation between crystallization and the crosslinked network parameters. The effect of precursor molecular weight, crosslinker functionality and degree of crosslinking were examined. The thermal characteristics of the system were obtained by DSC. These were complemented by DQ NMR (dynamic order parameter) and rheological measurements. The use of DQ NMR as means to determine the gel point is also discussed.. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U24.00010: Phase Separation of Model Segmented Poly(Carbonate Urethanes) Rebeca Hernandez, Elena Hung, James Runt The present paper focuses on the phase separated morphology and segment demixing of model poly(carbonate urethanes) [PCU] with hard segment contents ranging from 30 -- 65{\%} and soft segments composed of 1,6 poly(hexamethylene carbonate) [MW = 1K]. Hard segments were formed from 4,4'-methylenediphenyl diisocyanate and 1,4 butanediol. This family of materials represents a recent approach in the development of polyurethanes with improved long-term biostability, and is under clinical investigation in a number of biomedical devices. Only a single glass transition temperature was observed for each copolymer, increasing in temperature with increasing hard segment content. However, loss spectra from dynamic mechanical analysis showed clear evidence of two mixed phases. The results of small-angle X-ray scattering and tapping mode AFM experiments were consistent with these observations and will be discussed. Finally, these results will be compared with initial findings on phase separation in another family of polyurethane copolymers of current interest as blood-contact materials in biomedical devices having mixed poly(dimethylsiloxane) -- poly(hexamethyleneoxide) soft segments. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U24.00011: How Does Rubber Crystallizes Upon Application of Strain Mindaugas Rackaitis, Xiaorong Wang Strain induced crystallization of natural and synthetic rubbers is one of the most interesting phenomena in the field of rubber science and technology. Most of the research in the field was done using X-ray diffraction techniques. However up to this day there are no reports of direct observation of strain induced crystallization of rubber. This presentation reports first atomic force micrographs of high cis polybutadiene rubber surface at various strains. The crystallization of the rubber was confirmed by X-ray diffraction and crystallite orientation is discussed. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U24.00012: Melt-processable high acrylonitrile copolymers Shawn R. Hutchinson, Juan Hinestroza, Bhupender S. Gupta, David R. Buchanan, Alan E. Tonelli High acrylonitrile homo- and copolymers (PAN) are unique because of chemical, ultraviolet, and corrosion resistances. Historically, because of the atypical processing and thermal behavior of solution-processable PAN, consensus regarding the actual microstructure and paracrystalline order was elusive -- it has been described as `two-dimensional liquid crystalline-like structure with many defects.' New, sequence-structured copolymers rendered PAN for the first time melt-processable before degradation and are providing new insight into the solid state conformation. Solution $^{13}$C nuclear magnetic resonance was used to compare the new and historical comonomer sequence lengths. Optimal processing conditions were obtained using capillary rheometry as a function of dwell time and melt temperature. A filament extrusion investigation was conducted and wide angle x-ray diffraction, differential scanning calorimetry thermograms and mechanical properties of the filaments were used to characterize the nonequilibrium melt transitions and paracrystal morphology as a function of processing parameters. [Preview Abstract] |
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