Session K18: Focus Session: Dillon Medal Symposium

Stimuli Responses of Topology-Controlled Polymer Networks and Liquid Crystalline Gels

In this talk I will present and discuss the stimulus-response relationships of topology-controlled polymer networks and liquid crystalline gels. I will assess several modern entanglement theories of rubber elasticity on the basis of the multi-axial stress-strain data of end-linked polydimethylsiloxane (PDMS) networks with well-characterized structures. The dynamics of guest linear PDMS in host PDMS networks will also been discussed as a function of mesh size and molecular mass of guest chains. I will also demonstrate the highly extensible or damping elastomers of PDMS by simply controlling the topological characteristics such as the conformation of network chains, the amount of trapped entanglement and pendant chain. Furthermore, I will present the volume transition accompanying the shape variation induced by nematic-isotropic transition in liquid crystalline gels. I will also reveal the electrically-driven deformation coupled to director rotation in nematic gels.   

Self-Assembly of Magnetic Particles into Polymer Chains and Networks

The increasing demand to manufacture structures at the nanoscale has made it necessary to pursue new fabrication strategies based on self-assembly. Although it is generally appreciated that this type of ordering process relies on the interplay of directional and isotropic interactions to guide the organization process, the theoretical principles governing this process remain uncertain. There is clearly a need for the development of \textit{real model systems} exhibiting self-assembly and for their intensive investigation by experimental and simulation studies, in conjunction with analytic modeling. Since particle size is not an intrinsic limitation in this type of study, we investigate the self-assembly of vertically vibrated magnetic `beads' (spheres with embedded magnets) into dynamic polymer chains where the effective temperature of the fluid is determined by the shaking velocity. Self-assembly is assumed to be described by equilibrium polymerization and we perform Monte Carlo simulations for this model fluid at thermodynamic equilibrium. The experiments, simulations and analytic calculations lead to a self-consistent description of the self-assembly process in this fluid. We regulate polymer branching by adding beads having relatively short embedded magnets that are characterized by appreciable \textit{multipole }interactions in addition to the dipole interaction.   

Electrical Properties of Poly(ethylene oxide)-based Ionomers as Single Ion Conductors

Polyethers, such as poly(ethylene oxide) (PEO) are of interest for development of advanced lithium batteries because Li$^{+}$ ions have facile transport in this media. We make ionomers based on PEO by reacting poly(ethylene glycol) (PEG) oligomers with the sodium salt of dimethyl 5-sulfoisophthalate. Since the sulfonate group is covalently bonded to the chain, it is essentially immobile and hence these materials are single-ion conductors. The charge spacing on the chain can be directly controlled by the molar mass of the PEG oligomers (we use M = 400, 600 and 900) used in the synthesis. Conductivity depends strongly on temperature, with nearly identical conductivities in all of our samples at the same $T - T_{g}$, suggesting that Li$^{+}$ ion transport is controlled by segmental motion of the PEO. Using the onset of electrode polarization (usually considered a nuisance in dielectric spectroscopy) we quantitatively estimate the free ion concentration and mobility, based on work of MacDonald (1952 {\&} 1974) and Coelho (1983 {\&} 1991). The temperature dependence of the free ion concentration is described by a simple pairing energy, which decreases in going from Li$^{+}$ to Na$^{+}$ to Cs$^{+}$, consistent with larger ions being less strongly bound to the sulfonate groups. The ion mobility shows a Vogel-Fulcher temperature dependence, as anticipated by the polymer's segmental motion controlling ion mobility.   

Rheological Properties of Nanotube -- Polymer Nanocomposites

Single walled carbon nanotubes represent the next generation of nanoparticles for the development of polymer nanocomposite materials with potential in multifunctional applications. We have successfully dispersed such SWNTs in various polymer nanocomposites and have recently examined the linear and non-linear viscoelastic measurements. The nanocomposites in the melt state of the polymer demonstrate solid -- like behavior beyond a percolation threshold, which in many of these nanocomposites is below 0.1 wt {\%} SWNT. The plateau modulus (corresponding to the stress supported by the percolated nanoparticle network structure) scales as the volume fraction of the SWNTs to the third power and inconsistent with current fractal models. The onset of the non-linear behavior occurs at progressively lower strain values with increasing SWNT concentration and is similar to other filled polymers. Interestingly the recovery of the polymer network following large strain is extremely slow and is similar to materials that are classified as soft- glassy materials.   

Non-Newtonian Behavior of Diblock and Triblock Copolymer Solutions

Non-Newtonian flow behavior was examined for butadiene-styrene (BS) diblock and BSB triblock copolymers dissolved in a S-selective solvent, dibutyl phthalate (DBP). Spherical domains of the non-solvated B blocks were arranged on a bcc lattice in both solutions at equilibrium, as revealed from SANS. The solutions exhibited significant thinning under steady flow, which was well correlated with the disruption of the bcc lattice detected with SANS. The lattice disruption was most prominent at a shear rate comparable to the frequency of B/S concentration fluctuation. For the BS/DBP solution, the recovery of the lattice structure after cessation of flow was the slowest for the most heavily disrupted lattice, as naturally expected. In contrast, for the BSB/DBP solution, the recovery rate was insensitive to the magnitude of lattice disruption. This peculiar behavior of the BSB solution suggests that the rate-determining step of the recovery in this solution is the transient B/S mixing required for reformation of the S bridges connecting the B domains.   

Arm Retraction of Star and Dangling Polymers in the Absence of Dynamic Dilution

The dynamic response of model polymer networks containing low contents of star shaped and linear dangling polymers was studied through stress relaxation experiments. As compared with their melts, the behavior of star and dangling polymers leads to a dynamic response with unprecedented large relaxation times. By comparing data of star melts with those corresponding to stars and dangling chains residing in polymer networks, the effects of dynamic dilution were clearly identified. Since in polymer networks the dynamic dilution effect is suppressed, we were able to experimentally test the validity of the potential for arm retraction proposed by Pearson and Helfand.   

Consequences of Switchable Solvent Quality on the Self-Assembly of Block Copolymers in a Nematic Liquid Crystal Solvent

The microstructure of a diblock copolymer having a coil and liquid crystalline blocks dissolved in liquid crystal (LC) solvent is exceptionally sensitive to changes in the LC order; the quality of the solvent changes discontinuously across the LC phase transitions. This ``switchable solvent quality'' manifests itself in the nanostructure and rheology of solutions of AB diblocks with a polystyrene (PS) block (``coil'' type) and a side-group liquid crystalline polymer block (SGLCP) dissolved in the LC solvent 5CB. The nematic order of the solvent presents a large entropic penalty to solvation of the PS block, but in the isotropic phase it is a good solvent for PS. The SGLCP is soluble in both the nematic and isotropic phases. In these block copolymers, the LC solvent switches from being strongly selective toward the SGLCP block to being a good solvent for both blocks at the isotropization point . Pairwise thermodynamic interactions between the LC solvent, PS, and the SGLCP are inferred from the ternary phase diagram of homopolymer solutions in 5CB. In the nematic phase, unfavorable 5CB-PS interactions dominate at all concentrations. In the isotropic phase, segregation occurs even though 5CB is a good solvent for both blocks driven by either the slight preference of isotropic 5CB for SGLCP at c$<$c* or SGLCP-PS interactions at c$>$c*.   

Swelling and Elasticity of Entangled Polymer Networks

We develop and solve a molecular model for nonlinear elasticity of entangled polymer networks, called non-affine slip-tube model. Each chain passes through a sequence of slip-links. The topological constraints imposed by neighboring network chains on a given one are represented by the confining potential acting on the slip-links. This topological potential restricts fluctuations of the network chains to the non-affinely deformed confining tube and changes upon network deformation. The non-affine tube model puts softer restriction on swelling gels, leading to larger equilibrium swelling ratio. It also predicts that weakly-entangled networks initially swell following the dependencies of entangled gels, but as soon as affine length reaches the size of network strand, further swelling is described by unentangled gel relations. The non-affine tube model predicts stronger concentration dependence of elastic modulus, $G$, than affine model, as well as weaker dependence of $G$ on preparation concentration. The non-affine tube model also predicts a cross-over at the preparation condition to a much stronger concentration dependence of elastic modulus, $G$, upon de-swelling and weaker dependence of $G$ on preparation concentration.   

Electric Field Response of Electroclinic Liquid Crystal Elastomers

Supramolecular ordered assemblies such as liquid crystal elastomers provide an excellent framework for incorporating anisotropy as well as functionalities in materials that respond to external stimuli. Electroclinic Liquid Crystal Elastomers (ELCEs) are of particular interest due to their ability to exhibit linear actuation under an applied voltage. The crosslinked elastomer network consists of chiral liquid crystal mesogens attached to a polymer backbone and exhibiting a chiral smectic A phase with large field induced tilt angle. The response of these materials to electric field and their mechanical properties will be discussed.   

Brownian dynamic simulations of electrophoresis and electro-stretching of DNA molecules in polymer gels.

~We derive a model for the motion of long DNA chains entangled in a concentrated gel matrix in the presence of a strong electric field. The model is adapted from a tube-based slip-link approach, which was originally intended to model the rheology of entangled polymer fluids, and is suitable for solution by Brownian dynamic simulation. We account for the constraining effect of the surrounding matrix, motion due to the electric field and finite extensibility of the DNA chain. We are able investigate the effect of molecular weight and field strength on the DNA drift velocity in a constant electric field, along with molecular stretching in an oscillating field. Both examples have applications in DNA separation and sequencing. Our approach includes a detailed treatment of the chain end motion through the matrix, which our simulations demonstrate has a significant role in the DNA dynamics, particularly in oscillating fields. The model provides a convenient formalism for further refinements. For example, large fields may tend to cause hernia-like chain loops to protrude from the main tube. Furthermore, to model matrices comprised of linear polymers we can include the effect of constraint release, in which the confinement experienced by the DNA is diminished by the motion of the matrix chains.   

Diffusion and Equilibration in Surfactant-Bearing Interfaces

The efficacy of surfactants for stabilizing interfaces between immiscible fluids depends on both thermodynamic and dynamic effects. Dynamic effects are especially important when the fluids are immiscible high molecular weight polymers. Block copolymers are used to stabilize interfaces between immiscible polymers. The chain-like character of polymers leads to molecular entanglement, which leads to extremely slow dynamics. We have prepared two surfactant-bearing polymeric interfaces that are initially out of equilibrium. The distance between the interfaces was varied from 50-600 nm, and the transport of the surfactant molecules was measured by dynamic secondary-ion mass spectroscopy. This transport depends on diffusion coefficients and the depth of the thermodynamic potential wells that trap the surfactant molecules at the interfaces. The diffusion coefficients were measured in independent experiments. The depth of the thermodynamic potential well was calculated using self-consistent field theory (SCFT), which required Flory-Huggins interaction parameters and statistical segment lengths measured by small angle neutron scattering. This enables a comparison of our experimental interfacial transport measurement and theoretical predictions with no adjustable parameters.   

Thermodynamically Self-Consistent Theory of Crystalline Polymer Blends

Thermodynamically self-consistent theory has been developed to determine phase diagrams of binary crystalline polymer blends. The original Flory diluent theory, although captures the liquidus line, is unable to account for the solidus line due to the inherent assumption of complete immiscibility of solvent in the solid crystal. This over-simplification has led to the $\chi $ parameter obtained from the melting point depression curve to be at variance with the $\chi $ parameter by small-angle neutron scattering. The present theory takes into account all possible interactions such as amorphous-amorphous, crystal-amorphous, amorphous-crystal, and crystal-crystal interactions to predict various phase diagrams involving liquid-liquid, liquid-solid, and solid-solid coexistence regions bound by liquidus and solidus lines. It was found that the crystal-amorphous interaction is the major contributor to the melting point depression rather than the conventional Flory-Huggins $\chi $ parameter. However, in the limit of the complete insolubility of the solvent in the crystal phase, the original Flory diluent theory is recovered. In collaboration with Rushikesh Matkar, University of Akron.   

Polymer nano-adhesion promoted by surface mobility

We here propose a novel nano-adhesion technique on the basis of enhanced surface mobility in polymer films. As materials, monodisperse polystyrene and deuterated polystyrene (PS and dPS) with number average molecular weight of 29k were used. The surface and bulk glass transition temperatures were 294 and 373 K, respectively. PS bilayers were prepared, and were annealed at a temperature between the surface and bulk Tgs for a given time. Then, interfacial adhesion strength (G) was measured. In addition, adhesion measurement by a scanning force microscope using a probe tip covered with the PS layer was made. In this case, the adhesion area was also on nanometer level. Hence, this experiment is denoted as nano-adhesion hereafter. For both experiments, G value first increased with increasing time and then reached a constant. This implies that segments moved across the interface even at a temperature below the bulk Tg and thus adhesion took place at the interface. To confirm this, the interfacial evolution for the (PS/dPS) bilayers was examined by dynamic secondary ion mass spectroscopy. Since G value was linearly proportional to interfacial thickness, it was claimed that the adhesion at the bilayer interface was mainly governed by the interfacial thickening. Interestingly, G value by nano-adhesion measurement was much larger than that for a bilayer at a given time. The difference can be explained in terms of the completeness of the interfacial formation.