Session A50: Liquid Crystalline Order in Polymers, Soft Matter, and Complex Fluids

Shape Memory as a Process: Optimizing Polymer Design for Shape Recovery

Shape memory is a process that enables the reversible storage and recovery of mechanical energy through a change in shape. Polymers provide a unique alternative to kinematic designs and other materials (e.g. metallic alloys) for applications requiring large deformation and novel control options. The effect control of storage and relaxation of strain energy associated with chain deformation depends on the nonlinear visco-elasitc behavior and glassy dynamics of the polymer network. Considering the molecular understanding of rubbery elasticity, chain entanglements in concentrated polymer liquids, affine deformation of networks, and glass fragility, heuristic guidelines can be formulated to optimize the molecular design of a polymer for shape memory. These are applied to the development of a polymer system for shape memory processes at high-temperature (200$^{o}$C). The low-crosslink density polyimide exhibits very rapid shape recovery, excellent fixity, high creep resistance, and good cyclability. Furthermore, the molecular design affords a very narrow temperature range for programming and triggering shape change that can also be accessed by photo-isomerization of the cross-link nodes.   

Shape and Memory in Liquid Crystalline Elastomers

As part of an ongoing effort to understand the origins of shape memory in liquid crystalline elastomers (LCE), we have synthesized and examined a series of SmC main-chain LCEs. Uniaxial stretching of these polydomain films at room temperature produces a monodomain structure that can, upon removal of load, retain the monodomain and a significant level of strain. Although these films show ordinary elastic response at temperatures near the isotropization (clearing) temperature, at room temperature -- far below the clearing temperature -- the mechanical response is anelastic. Experimental studies of isothermal strain recovery vs time after unloading will be presented along with details of the temperature profile for strain recovery of these LCEs. A rationale for the shape memory behavior is proposed that involves moving of crosslink points in the smectic lamellar arrangement during the stretching event and trapping of these crosslinks in different positions at low temperatures. This trapping is driven by the chemical segregation of the crosslink points from the mesogenic unit which can be thermally overcome at elevated temperatures allowing full elastic recovery.   

Phase Behavior of Disk-Coil Macromolecules

We explore the self-assembly of disk-coil macromolecules using Monte Carlo simulations in the NPT ensemble. Our study focuses on the role that coil length compared to the size of the disk has on the phase behavior of the system as well as the effect of stacking interactions between the disks. As a function of temperature T, we find a disordered phase at high T and lamellar, perforated lamellar, and cylinder phases at intermediate T. If we further lower the temperature, the disk-rich regions spontaneously order, and we find ordered lamellar, ordered perforated lamellar, and ordered cylinder phases depending on the strength of the stacking interactions. The appearance of any of these phases is, however, strongly dependent on the length of the coil. In addition to constructing a comprehensive phase diagram, we further analyze the correlations in the system, as well as the director vector field of the disks, and use it to construct an order parameter. We show that the latter changes drastically at the ordering transition points. We find that the ordered cylinder phase has a high degree of parallel packing. Our results are important to understand the self-assembly of supramolecular structures of disk-coil amphiphiles that are ubiquitous in nature, such as the chlorophyll molecule.   

Liquid Crystal Phases of Semiflexible Polymers

Liquid crystal polymers exhibit orientational order (nematic phase) and position order (smectic phase). Previous work on semiflexible polymers using self consistent field theory studied the isotropic-nematic and nematic-smectic transition for homogenous and diblock copolymers. The nematic phase is stabilized by excluded-volume effects between wormlike cylindrical segments. The smectic phase is further stabilized by excluded-volume effects between terminal end segments. Because models of semiflexible polymers include orientational degrees of freedom, in addition to the usual positional degrees of freedom, they are computationally more demanding to study. Spectral decomposition applied to segment orientations has previously been used to make computation feasible. However this method does not converge well for strongly ordered states, which arise in many real systems. I describe a Crank-Nicolson finite difference method applied to the orientations which is expected to converge well for highly ordered systems. This method also exhibits better numerical stability and accuracy and may thus serve as a better foundation for further studies of highly ordered systems. I also describe a modification to the spectral method which can compute the tilted Smectic C phase.   

Photomechanical mechanism and structure-property considerations in the generation of photomechanical work in glassy, azobenzene liquid crystal polymer networks

Azobenzene-containing polymeric materials have shown shape adaptive responses when irradiated with light. We contrast the photogenerated mechanical response of glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) upon exposure to either UV or blue-green irradiation. The profound differences in the fundamental photomechanical response to exposure to light in these wavelength regimes are dictated by distinctive photochemical mechanisms, elucidated through UV/VIS spectroscopic examination of the materials before and after irradiation with UV and blue-green light. The glassy, photoresponsive polymeric materials were subjected to structure-property examination to ascertain the role of crosslink density, azobenzene concentration, and azobenzene connectivity (crosslinked or pendant) on the photomechanical output.   

Surface wrinkling in liquid crystal elastomer bilayers

Mechanically-induced surface wrinkling patterns, also known as strain-induced elastic buckling instability for mechanical measurements (SIEBIMM), represent a versatile and high throughput technique for thin film metrology. However, the technique requires clamping and mechanically straining bilayer samples, which can introduce errors and present challenges with small samples. Here, we present a modified approach in which thin films are deposited on top of a stimuli-responsive liquid crystal elastomer (LCE). Temperature changes induce a spontaneous and controllable shape-change in the LCE substrate, without the need for clamping or mechanically straining. We show that LCE bilayers can be used to accurately measure the modulus of nanoscale poly(styrene) films, down to 20 nm thick films. Furthermore, the surface wrinkle orientation can be controlled using different preparation methods and reoriented in a single sample with temperature changes only. In the case of thick (over 500 nm) PS films, the bilayer flexes in response to temperature changes. This work shows that LCE bilayers are useful systems for thin film metrology and controlled assembly using well-defined surface structures.   

Field-induced orientational order of liquid crystals in random environments

Over the last twenty years, there has been extensive theoretical and experimental work on liquid crystals in disordered polymer networks and other random environments. It was shown that the disordered environment disrupts the long-range order of the liquid crystal. Recently, D.-K. Yang has performed new experiments, in which an electric field is applied to the polymer-disordered liquid crystal, leading to a large Kerr effect, i.e. field-induced long-range orientational order [1]. This experimental approach offers new opportunities for liquid-crystal displays. To understand the experiments and improve the applications, we perform Monte Carlo simulations of a nematic liquid crystal in a disordered polymer network. These simulations show the formation of randomly oriented domains of uniform directors. We study the response to an applied field by calculating the Kerr coefficient for variable system parameters. Furthermore, using an Imry-Ma-like approach we predict the domain size as a function of temperature and material properties of the system, and estimate the induced orientational order parameter due to an electric field. The simulations and analytic results agree well with each other and with the experiments. \\[4pt] [1] Y.-C. Yang and D.-K. Yang, Appl. Phys. Lett. 98, 023502 (2011).   

Highly viscous liquid crystalline mixtures: the alternative to liquid crystalline elastomers

Novel highly viscous liquid crystalline materials based on mixtures of glass forming oligomers and low molar mass liquid crystals were recently designed [1, 2] and studied. In this communication the novel data are presented, the analysis and discussion are extended. It is shown that viscoelastic properties of the materials are due to the physical entanglements between cyclic oligomers and low molar mass mesogens, not due to the chemical crosslinks between molecular moities. However, the mechanical properties of these viscoelastic materials resemble those of chemically crosslinked elastomers (elasticity and reversibility of deformations). The properties of chiral and non-chiral materials loaded with ferromagnetic nanoparticles are discussed in detail. Cholesteric materials undergo gigantic color changes in the wide spectral range under the deformation that allows distant detection of deformation and determination the anisotropy of deformation and its type. The materials doped with laser dyes become mechanically tunable lasers themselves and emit coherent light while pumped by external laser. A simple model is suggested to account for the observed effects; physical properties of the novel materials and liquid crystalline elastomers are compared and discussed. \\[4pt] [1] P.V. Shibaev, C. Schlesier, R. Uhrlass, S. Woodward, E. Hanelt, Liquid Crystals, 37:12, 1601-1604 \\[0pt] [2] P.V. Shibaev, P. Riverra, D. Teter, S. Marsico, M. Sanzari, V. Ramakrishnan, E. Hanelt, Optics Express, 16, 2965 (2008)   

Linear aggregation and liquid crystalline ordering: from semi-flexible polymers to rigid rods

Reversible self-assembly of filamentous aggregates is ubiquitous in soft matter and biophysics. Examples include worm-like micelles, patchy colloids, chromonic liquid crystals, DNA and RNA, and protein polymers and fibrils. These rod-like aggregates can form liquid-crystal (LC) phases; the liquid-crystal order then couples to the aggregation, promoting the formation of longer aggregates in the LC phases. We study the coupled aggregation and liquid crystalline ordering of a minimal system of sticky cylinders that interact primarily by hard-core interactions but can stack and bind end to end, making use of both analytic theory and Monte Carlo simulation. Accurate treatment of aggregate flexibility is essential for quantitative comparison of theoretical and experimental phase diagrams and other properties. Our analytic model describes aggregates as wormlike chains in an effective aligning nematic field, and allows self-consistent determination of this field within a density functional theory formalism over a broad range of aggregate flexibilities. We compare the analytic results for isotropic-nematic phase equilibrium with simulations performed in our group as well as similar work from the Sciortino group [De Michele et al., arXiv:1108.6135].   

Liquid Crystal Elastomer Motors

Motors produce motion due to the transfer of energy, but not of momentum, to the device. In LCE motors, motion arises due to changes in the shapes of solid samples. Here we consider motors where the shape change is a bend, rather than an elongation or contraction. We focus on the light-driven motor of Ikeda et al.; we analyze in detail the physical mechanisms which bring about the motion, and discuss the momentum current which is generated. We present the results of numerical simulations, and compare these with experimental observations.   

Orientational fluctuations of amorphous nematogenic solids

Amorphous nematogenic solids (ANS) are media comprising rod-like nematogens that have been randomly linked to form elastically deformable macroscopic networks. ~Classes of ANS include chemical nematogen gels (i.e., networks of small molecules) and liquid crystalline elastomers (built from crosslinked nematogen-containing macromolecules), as well as biophysical networks such as those composed of actin filaments. ~We use a method inspired by the cavity approach to construct a replica free energy for these random systems, and investigate the correlations of the thermal fluctuations of the orientational alignment of the nematogens at spatially separated points. ~We identify two qualitatively distinct regimes: (a) a weakly localized regime, in which the correlations decay exponentially with separation; and (b) a strongly localized regime, characterized by correlations that also decay but oscillate as they do.   

Non-Monotonic Concentration Effects in the Phase Behavior and Nematic Orders: Mixtures of Side-Chain Liquid Crystalline Polymers and Low-Molecular-Weight Liquid Crystals

Mixtures of side-chain liquid crystal polymers (SCLCPs) and low-molecular-weight liquid crystals (LMWLCs) are novel materials with applications such as optical data storage, non-linear optics, solid polymer electrolytes, chromatography and display materials. Recent experiments showed that the nematic-isotropic transition temperature and the nematic orders of each component vary non-monotonically with concentration. Existing theories, which combine the Flory-Huggins theory for isotropic mixing and the Maier-Saupe theory for nematic order, cannot explain such non-monotonicity. Here, we extend the existing theories by, first, incorporating the local steric constraints between the side-chain and the polymer backbone on the SCLCPs, and second, accounting for the crowding effects at high SCLCP concentrations. The new extended theory is able to resolve the discrepancies between the predictions of existing theories and the experimental observations.   

Effect of blending on nematic order in semiflexible polymers

Semiflexible polymers of sufficient stiffness exhibit liquid crystalline order at sufficient polymer concentrations. In this work, we investigate blends of flexible and semiflexible polymers with the aid of Monte Carlo simulations of a bond-fluctuation model. The model is an extension of Shaffer's bond-fluctuation model, where chain stiffness is controlled by including different forms of bending penalties, and includes attractive interactions between monomers. From simulations for a range of values of the bending energy, density, and temperature, we determine the effect of concentration of the flexible polymer on liquid crystalline order and domain formation.