Session X25: Liquid Crystalline And Amorphous Polymers

Swelling and Shrinking Dynamics of Monodomain Nematic Elastomers

We demonstrate that the swelling and shrinking of monodomain nematic elastomers in solvents exhibit unusual dynamics because of the presence of shape and volume variation modes with markedly different rates. A variation in the degree of orientational order induced by temperature (T) jumps causes a spontaneous deformation along the director as well as a change in the chemical potential of the solvent inside the gel. The former effect results in an almost instantaneous shape change, whereas the latter drives a slow volume change governed by the diffusion of polymer networks. The markedly different rates of these two modes cause unique dynamics: (i) a pronounced over- or undershoot of the specimen dimensions occurs in the direction where the shape and volume variations act to change the dimensions in the opposite manner, and (ii) a large dimensional change (more than 50 percent of the total change) takes place with almost no delay after the T-jumps in the direction where these two effects on the dimension synchronize.   

Effects of Long Wave-length Thermal Fluctuations on the Elasticity of Nematic Elastomers

We study the long wavelength fluctuations of the nematic director as well as the phonon fields in nematic elastomers. These fluctuations have important effects on the elasticity in the large deformation regime. We calculate the nonlinear stress-strain relations for several simple geometry of deformation. We also analize the correlation functions of the nematic director and phonon fields in the presence of large uniform strain deformations. All of these results can be directly measured by experiments.   

Synthesis of Optimal and Imperfect Main Chain Smectic Elastomers

Liquid crystalline polymers (LCPs) and elastomers (LCEs) are mesomorphic polymers that exhibit unique rheological characteristics. Smectic LCE, rubber-like materials which exhibit lamellar mesophases, exhibit high values of the mechanical loss factor (tan delta) over several decades of frequency. We are designing a model LCE system for study of the underlying molecular-level relaxation mechanisms responsible for this broad spectrum of relaxation times. We seek to distinguish to what extent the dynamic evolution of defects (e.g. focal conic defects and dislocations) contributes to the broad loss spectrum, as opposed to other ordinary processes such as relaxation of dangling and free chains. We are studying main chain, smectic LCPs and LCEs consisting of alternating flexible siloxane segments and mesogens. Elastomers are prepared by a three-monomer (A2 + B2 + A4) non-linear polycondensation. Synthetic methods are needed to prepare ``optimal'' and ``imperfect'' networks by introducing controlled amounts of dangling and free chains, and to control the phase behavior of the networks through mesogen chemical structure and crosslinking. Dynamic mechanical behavior of model networks, in conjunction with X-ray and neutron scattering studies, will distinguish the underlying physical processes responsible for the broad loss spectrum in smectic LCE.   

Construction of Chiral Propeller Architectures from Achiral Molecules

Achiral BPCA-Cn-PmOHs construct chiral propeller structures in an N phase. The origin of chiral N phases in these achiral molecules comes from the twisted conformation of head-to-head dimers, indicating that neither molecular chirality, nor molecular bends, nor molecular tilting is necessary to form a chiral phase. The Frank-Pryce spherulitic N droplets and finger-print textures result from the single-twisting of chiral conformers, while the first-time observed propeller-patterned chiral N droplets are attributed to the double-twisting of chiral conformers in the N phase.   

Small angle X-ray scattering studies of side chain liquid crystalline block copolymers.

A series of well defined smectic side chain liquid crystalline (LC) block copolymers with a low T$_{g}$ siloxane center block has been synthesized via anionic polymerization. The presence of a smectic liquid crystalline phase and the block copolymer mesophase are observed across various temperature ranges via Small Angle X-ray Scattering (SAXS) and Grazing Incidence Small Angle X-ray Scattering (GISAXS). The influence of various types of confinement and mechanical deformation upon the morphologies of the liquid crystalline and block copolymer mesophases was investigated. The interactions between the smectic LC and the block copolymer morphologies and their influence upon their respective orientations in response to the various confinement and mechanical deformations are detailed. Additionally, it was found that modifications to the liquid crystalline moiety were key in the clearing points for the smectic liquid crystalline phase, as well as significantly influencing the nanophase segregation of the block copolymer.   

Solvent induced shape changes in liquid crystal elastomers

Liquid crystal elastomers are exceptionally responsive due to coupling between orientational order and mechanical strain.~ Changes in orientational order can give rise to mechanical deformations. Orientational order can be changed by a variety of excitations, including chemical concentration fields.~We have studied the dynamics of shape changes in LCE samples due to exposure to organic solvents and solvent vapors.~Unlike isotropic elastomers, which simply swell, LCEs show dramatic anisotropic shape changes when exposed to solvents.~We present results for the excitation and relaxation dynamics of shape changes for a variety of materials in response to the presence of different solvents.~ The absorption of solvents can cause a nematic-paranematic phase transition.~ We discuss possible applications   

Bloch wall defects in nematic thin films: experiments and simulations

We study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. We show direct visualization of two types of Bloch walls, pure twist walls and diffuse walls, using fluorescence confocal polarizing microscopy (FCPM) technique. We describe the simulations of the evolution of the Bloch walls with varying anchoring strengths using Frank elasticity, which agree remarkably well with the experimental FCPM observation.A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1.   

Abnormal Slowdown of Longitudinal Diffusion of F-actin across Isotropic to Nematic Phase Transition

F-actin is a semi-flexible macromolecule. Above a few tenths of a percent in volume fraction, F-actin solution undergoes an isotropic (I) to nematic (N) phase transition. By tracking fluorescently labeled F-actin in a network of unlabeled filaments, we studied the diffusion behavior of F-actin across the I-N phase transition and found an abnormal slowdown of longitudinal diffusion after the system enters the transition region. In contrast, for an ordinary liquid crystalline I-N phase transition, there is an abrupt increase of longitudinal diffusion coefficient at the transition point. By comparing the diffusion behaviors of F-actin, microtubule and fd virus in F-actin solution and studying the apparent viscosity dependence on divalent counter-ion concentration, we attribute this counter-intuitive phenomenon to counter-ion condensation induced weak attraction between filaments in nematic phase.   

Dynamic Fragility and the Glass Transition: Is there a relationship?

There have been multiple efforts over the years to correlate dynamic fragility, i.e., a Tg normalized temperature dependence of the dynamics, with various thermodynamic and dynamic parameters. Here we make a case that the dynamic fragility m=dln(viscosity)/d(Tg/T) evaluated at T=Tg is in fact strongly correlated to the glass transition Tg itself except for inorganic network glasses. We compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers and find that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and Tg, a correlation not previously examined. For hydrogen bonding organics, polymeric and metallic glass formers, there is a near linear increase in m with increasing Tg. For inorganic glass formers, m appears almost independent of Tg, remaining nearly constant over a wide range in Tg. We also investigated the apparent activation energy Eg at Tg and found that Eg increases with the square of Tg for hydrogen bonding organics, polymeric and metallic glass forming liquids, while Eg of the inorganics has a linear dependence on Tg.   

Prediction of Creep Behavior in PMMA

Recently proposed thermoviscoelatic constitutive model (TVEM) of Caruthers et al. [1] has shown promise in being able to describe within a single set of material parameters a wide range of experiments including yield, stress and enthalpy relaxation, and nonlinear stress-strain behavior under complex loading histories. The TVEM program consists in performing a number of linear relaxation experiments (i.e. for small deviations from equilibrium in relaxing quantity) in order to determine the shear, bulk, and enthalpy relaxation spectra which serve as input to the TVEM constitutive equations. Once these memory functions have been set, TVEM must be able to predict results of a non-linear experiment under an arbitrary thermal and loading history without any further adjustment of model parameters. Following this program we carried out an extensive study of relaxation behavior of lightly cross-linked PMMA using TMA, DMA, and DSC techniques. The nonlinear experiments chosen for validation of TVEM were the creep experiments below Tg where we studied the dependencies on load, temperature, and aging time. We also performed multi-step loading-unloading experiments in both linear and non-linear regimes. In this report the predictive capabilities of TVEM are presented and critically analyzed. 1. J.M. Caruthers, D.B. Adolf, R.S. Chambers, P. Shrikhande - Polymer, 45, 4577 (2004)   

Strain Hardening and Plastic Deformation in Polymer Glasses

Although entropic network models are often used to fit stress-strain curves for polymer glasses, we show that these models do not correctly capture the physics of glassy strain hardening. We examine the relationship between strain hardening and plastic deformation in model polymer glasses over a wide range of entanglement densities and temperatures. While the total stress in densely entangled samples is well fit by the Langevin model, the dissipative component of the stress is always Neohookean. This indicates that the nonlinear corrections to the stress are associated with internal energy storage, contrary to entropic models. In the athermal limit, plastic dissipation is directly proportional to the rate of damage of van der Waals bonds. At large strains, both are proportional to entanglement density. The energy dissipated per damaged bond is independent of entanglement density and strain at moderate strains, but increases at high strains for the more densely entangled systems due to increased energy barriers. The partitioning of plastic events into strain-activated and thermally-activated events is discussed.   

The influence of nonlinearity on the timescale of volume relaxation

The relationship between the timescales of volume and enthalpy relaxation has been studied extensively in the literature with differing results. Based on volume, enthalpy, and creep relaxation studies for polyetherimide, polystyrene, and selenium, a general picture was developed for the relationship between the relative timescales of different properties which was consistent with the data in the literature. According to the general picture, the timescales of different properties are similar at temperatures above the nominal value of T$_{g}$; however, the time scales diverge at temperatures below T$_{g}$ with volume and creep exhibiting longer relaxation timescales compared to enthalpy. However, when the timescales are re-analyzed using the cooling rate dependence of T$_{g}$ from capillary dilatometry and DSC, no divergence between the timescales of volume and enthalpy was observed, in contradiction with the general picture. It is hypothesized that the divergence in timescales observed in earlier work is due to the pronounced nonlinearity of volume relaxation compared to enthalpy relaxation. In this work, we use capillary dilatometry to test this hypothesis; in particular, we examine the effect of the magnitude of temperature down jumps on the volume relaxation timescale for polystyrene.   

Photothermal studies of polymers using polarized light

Visible light has been used as the pump beam in surface thermal lensing experiments involving nominally transparent polymers. A small portion of the pump beam is absorbed by the sample, producing local heating and a thermal bump. The nature of the bump depends on thermal, optical, and mechanical properties of the sample. The presence of the bump is detected by a weaker probe beam scattered off the surface. We have used a polarized probe beam and have observed the reflected beam as a function of polarization. The resulting time dependence is unlike anything observed in the absence of polarizers. These experiments suggest that photothermal techniques using polarized light can provide new insight into structural changes in polymers.   

Gradient Copolymers Yield Uniquely Broad Glass Transition Temperatures in Comparison with Block Copolymers and Polymer Blends

Gradient copolymers, which can be made by controlled radical polymerization or ring-opening metathesis polymerization, are distinct from random and block copolymer because of the gradient in comonomer composition along the copolymer backbone. As a result of this gradient along the chain, in the ordered lamellar state gradient copolymers are believed to exhibit a sinusoidal composition profile that is distinct from the ``square-wave'' composition profile observed in ordered lamellar block copolymers. This difference in the manner in which the local composition varies in the ordered state leads to dramatic differences in the glass transition behavior of gradient copolymers and block copolymers and similarly between gradient copolymers and polymer blends. Five gradient copolymer systems have been examined allowing for study of the effects of the enthalpic incompatibility of the comonomer units, the Tg differences among the homopolymers, and hydrogen bonding effects leading to random copolymers exhibiting higher Tgs than those found in block copolymers. We show via differential scanning calorimetry and dynamic mechanical analysis that single, continuous Tg breadths as large as 70-100 K are possible in gradient copolymers.   

Utilizing Nanoparticle Surface Plasmons for Surface-Initiated Polymerization and Conformational Switching of Polymers

Spherical gold nanoparticles on the order of 50nm in diameter experience a localized surface plasmon resonance peak at an incident light wavelength of around 550nm. This resonance is a result of an extremely efficient coupling of the incoming oscillating electric field with the free electrons in the gold. Some of the light is absorbed, while some is scattered. The absorbed portion of light is lost to phonons in the gold, which results in localized heating. Our research seeks to capitalize on this heating to switch the conformational state of surface grafted stimulus-responsive poly(N-isopropylacrylamide) (pNIPAAm). Furthermore, we seek to harness the strong amplification of scattered light at the plasmon resonance to induce near-field surface-initiated polymerization of pNIPAAm. We will report on the progress of our research, which aims to utilize these plasmonic effects as the basis for nanofabrication and sensing devices.