Session S25: Gels and Elastomers

Large strain deformation of hydrophobically modified polyelectrolyte hydrogels

Hydrogels made from charged polyelectrolytes have been widely studied for their ability to absorb large amounts of water. However this occurs usually at the expense of mechanical properties. Because of recent reports describing very tough charged hydrogels, we have investigated the large strain and fracture resistance of novel chemically and physically crosslinked hydrogels. The backbone was polyacrylic acid modified with hydrophobic side groups and subsequently chemically crosslinked with thiol-ene chemistry. We performed compression/decompression experiments up to large strains and found that at polymer concentrations of 5-8 wt\% these gels depart significantly from Gaussian behaviour at strains above 150\% showing a pronounced strain hardening. We argue that this hardening leading also to a significant hysteresis in the unloading cycle, is due to the formation of clusters of same charge segments of the polyelectrolyte chains. An increase in ionic strength or the substitution of water with an organic solvent reduces or eliminates the effect implying that it is the charges that cause the hardening and the hysteresis.   

Creasing instability of surface-attached hydrogels

Surface-attached hydrogels provide a convenient means to tune interfacial material properties such as biocompatibility and tribology. When the gel undergoes hydration, however, the substrate provides a constraint against lateral expansion, thereby generating an in-plane compressive stress within the gel. For sufficiently large degrees of compression a creasing instability takes place, in which the gel surface locally buckles and forms sharp folds. While this instability has been known in practice for well over a century, it remains poorly understood. Using model polyacrylamide hydrogel systems, we have studied the onset of creasing as a function of material properties and gel thickness, and addressed basic questions regarding crease morphologies and growth mechanisms. Using the understanding gained from these studies, we are developing this instability as a route to create active surfaces, where both surface topography and chemical patterns can be controllably modulated.   

Swelling-Induced Deformation of Nanopatterned Polymer Lines

The deformation behavior of isolated poly ($N$-isopropylacrylamide) nanolines due to swelling is studied to quantitatively establish the parameters governing their mechanical stability. The features are patterned using electron-beam lithography and are supported on a rigid substrate. In the range of 50-250 nm linewidth, we show that the swollen lines undergo Euler buckling with stable higher order modes due to the constraint for polymer chain expansion at the substrate interface. The critical wavelength for buckling surprisingly increases with decreasing line length. The linear swelling strain thus becomes a function of the initial length and decreases dramatically as the line length approaches twice the buckling wavelength for infinitely long lines. A critical length, \textit{larger} than the buckling wavelength for infinitely long features, exists below which the lines remain mechanically stable regardless of their crosslink density. For sufficiently long lines with height-to-width aspect-ratio in the range 0.5-1.7, the scaling relationship for buckling wavelength vs. linewidth suggests that swelling is anisotropic and is more dominant through height than width. The results established in this study are more generally applicable to nanopatterned polymer lines since buckling instabilities involve simultaneous bending and compression, which deform the material with equal force but in opposite directions.   

Drop spreading and resorbtion on gel surfaces

We have studied the dynamics of liquid drops on agar gels, using a visualisation method which captures the evolution of the free surface. A first remarquable observation is that drops of water deposited on the surface do not spread, although the gel consists of up to 99.7\% water and as low as 0.3\% agarose. Instead, the drop slowly de-wets and resorbs into the gel which swells locally. If the deposited drop contains surfactants, the dynamics is very different. A sharp circular swelling front develops and progressively invades the whole surface. We study the propagation of this front as a function of surfactant and agarose concentration, and compare its typical properties to similar fronts appearing during mass swarming events of bacterial colonies under the same conditions. The observations reveal the complex nature of gel surface physico-chemistry and its aging, and may be related to recent friction measurements at gel interfaces.   

Anomalous Composition-Dependent Swelling Behavior of Photocrosslinked VP/AA Copolymeric Hydrogels

We are investigating the swelling behavior and network structure of vinylpyrrolidone (VP)/acrylic acid (AA) copolymeric hydrogels synthesized using ultraviolet (UV) initiated polymerization. The goal of this work is to develop hydrogels with large expansion volumes but low expansion rates for use in tissue expander applications. In a number of reports, hydrogels composed of VP and AA units have shown that with increasing AA content higher equilibrium swelling ratios ($q_e )$ are produced. This behavior is due to higher osmotic pressure induced by the dissociation of carboxyl groups in the AA units into carboxylate anions and hydrogen ions during the swelling. In contrast, in our UV-cured system we found that the value of $q_e $ showed a distinct maximum at approximately 50 wt{\%} AA. This anomalous swelling behavior has been studied by measuring the modulus and investigating swelling kinetics as a function of AA content, in addition to the composition and thermal analysis of the dried VP/AA gels. These measurements provide detail of the effective crosslink density as well as weight between crosslinks for these hydrogels. From these results we propose a model which describes this anomalous behaviour based on the molecular chain structure of the hydrogels.   

Organogels from Polypeptide-based Block Copolymers

A series of AB diblock and ABA triblock copolymers consisting of poly(Lysine(Z)) (A = P(Lys(Z))) and poly(propylene oxide) (B = PPO) were synthesized and found to form stable, rigid organogels in THF (ca. 1 – 1.5 wt.\% solutions) at room temperature. In these systems, the protecting group on the P(Lys) side-chains remains intact. As such, the secondary structure of the polypeptide chains retains its helicity over a wide range of solution conditions. Gel formation in these systems results from the assembly of the solventphobic P(Lys(Z)) chains, which pack densely in an anti-parallel fashion, minimizing interfacial curvature. These gels all exhibited shear-thinning behavior, and as the temperature was heated to 77$^{\circ}$C exhibited a gel-sol transition. The gels formed over a time scale of about 10 minutes and had a modulus on the order of 55 Pa. The molecular weight dependence of the gel formation and rheological properties was studied in THF, dioxane and toluene.   

Soft random solids and their spatial elastic heterogeneity

As a consequence of the disorder that is quenched in at synthesis, random solids are spatially heterogeneous, not only in their structure but also in their response to external conditions. For the case of soft random solids, the heterogeneity in the elasticity is particularly intriguing, owing to its entropic origin and the interplay with incompressibility. We have examined the issue of heterogeneity in the elastic properties of soft random solids by applying replica statistical mechanics to a semi-microscopic model of a random network medium [1]. We have characterized the elastic heterogeneity by random residual stress and Lame coefficient fields, and have determined the statistics of these quantities. We have found that correlations involving the residual stress field are long ranged, and are governed by a universal parameter that also determines the mean shear modulus. Non-affine elastic deformations in soft random solids can also be also studied within this framework. [1] X. Mao, P. M. Goldbart, X. Xing and A. Zippelius, Europhys. Lett. 80, 26004 (2007).   

Effective removal of entanglement points by network dilution

A polymer system in which the chains are much longer than the entanglement molecular weight, $M_{+} \gg M_{e}$ is well entangled. When a thin polymer film is uniaxially strained below the glass transition temperature crazes are formed. Measurements of the volume fraction of the deformed versus the undeformed regions can give information on the entanglement density of the system\footnote{ACM Yang, EJ Kramer, CC Kuo, SL Phoenix, \emph{Macromolecules} \textbf{19} 2020 (1986)}. We present results of such deformation experiments, probed using atomic force microscopy, in which well entangled networks have been diluted with chemically identical species of molecular weight, $M_{-}< \sim M_{e}$ which results in a decrease in the entanglement density. Varying the length of the diluent chains provides molecular information on how the entanglement network is swelled.   

Advances in elastomer reinforcement: slow dynamics

Elastomers reinforced with nanometric solid particles or aggregates exhibit remarkable properties which are still far to be completely understood: reinforcement, non linear effects, irreversibility. Important progress has been achieved recently in modeling these properties, based on glassy layers around filler particles. In some cases, reinforcement as a function of temperature and filler volume fraction was explained quantitatively. We shall present an ensemble of experimental results obtained in various filled elastomers, which give some new insight on the microscopic mechanisms of reinforcement. We shall focus on two aspects: local strain measured by combining various techniques (NMR, mechanics, onset of strain-induced crystallization), indicate the level of stress heterogeneity in the elastomer matrix in the presence of reinforcing fillers; slow relaxation phenomena which occur in these systems: stress relaxation, memory effects and irreversibility effects -such as plasticity- indicate the importance of glassy regions within the matrix. Refs: J Berriot et al., Europhys. Lett. 2003, 64, 50-56; D. Long and P. Sotta, Macromolecules 2006, 39, 6282.   

Polydomain-Monodomain Transition of Randomly Disordered Nematic Elastomers with Different Crosslinking Histories

When the mesogenic molecules (polymers) are crosslinked without a special care for global alignment, the resultant networks have a polydomain structure with local orientational order (in the order of micron) but without macroscopic order. It is well known that such polydomain liquid crystal elastomers (PLCE) exhibit a transition to the monodomain state with global orientation under a finite stretching stress. In present study, we examine the polydomain-monodomain (PM) transition of the two types of PLCE (I-PLCE and N-PLCE) that are originally formed in the high-temperature isotropic or low-temperature polydomain nematic states. They show no appreciable difference in the equilibrium properties such as transition temperature and swelling degree. In contrast, their transition behaviors are significantly different: The transition in I-PLCE occurs sharply at a critical stress whereas that in N-PLCE proceeds gradually over a wide range of stress.   

New Insights Regarding the Polydomain-to-Monodomain Transition in Smectic Elastomers

Smectic elastomers are rubber-like networks characterized by lamellar mesophases. Crosslinking a smectic polymer in the absence of an aligning field produces a polydomain elastomer containing numerous randomly oriented microdomains. Under uniaxial tension, polydomain smectic elastomers undergo a transition to a globally oriented ``monodomain'' state, which may proceed by rotation and/or transient disordering of microdomains. New studies of smectic main-chain elastomers suggest that disordering of microdomains via unfolding of hairpin structures is the dominant mechanism for elongation at intermediate strains. A ``plateau stress'' is found in plots of the nominal stress vs. strain, which correlates with the average domain size. At very high strains, elastic chains approach the finite extensibility limit, and layer buckling becomes the predominant mechanism for elongation. The elongation mechanism differs significantly from that in mesomorphic poly(diethylsiloxane) elastomers, which also exhibit a ``plateau'' in the nominal stress vs. strain curve due to spontaneous random coil-to-helix transition. Because of chain-folding in main-chain smectics, the P-M transition may exhibit some similarities with the cold drawing of semicrystalline polymers.   

Nematic elastomers: From a microscopic model to macroscopic elasticity theory

A Landau theory is constructed for the gelation transition in cross-linked polymer systems possessing spontaneous nematic ordering, based on symmetry principles and the concept of an order parameter for the amorphous solid state. This theory is substantiated with help of a simple microscopic model of cross-linked dimers. Minimization of the Landau free energy in the presence of nematic order yields the neo-classical theory of the elasticity of nematic elastomers and, in the isotropic limit, the classical theory of isotropic elasticity. These phenomenological theories of elasticity are thereby derived from a microscopic model, and it is furthermore demonstrated that they are the universal mean-field descriptions of elasticity for all chemical gels and vulcanized media.   

Elasticity of a Chiral elastomer

We study the nonlinear elasticity of an elastomer made by crosslinking a chiral polymer melt in its isotropic state. It is shown that such an elastomer is unstable to uniaxial extension/compression and tends to develop a twist along the axis of deformation. ~Both phonon correlation functions and polarization dependent sound velocity are calculated. ~Furthermore, when such an elastomer is cooled below the isotropic-nematic transition of the underlying polymer melt, it develops inhomogeneous spontaneous deformations to accommodate the emergent chiral ordering of the polymers. We perform variational analysis of these spontaneous deformation in different regimes of system parameters.   

Determination of the refractive indices of liquid crystal elastomers

Liquid Crystal Elastomers (LCEs) are fascinating materials due to the coupling between orientational order and mechanical strain. We investigate this coupling by studying the optical properties of LCEs. We have measured the ordinary and extraordinary refractive indices of nematic LCEs as function of strain using two different techniques. In both cases, the strain is applied along the nematic director. The first technique is a Brewster's angle measurement which is based on reflection of the incident light and the second is a conoscopic Mach-Zehnder interferometer based on transmission. We present our experimental results and methods of analysis. We compare our observations with theoretical predictions.   

Magnetoactive Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) offer an interesting spectrum of properties, including temperature induced, fully reversible shape changes connected with considerable development of pulling force, and synthetic diversity. In order to take advantage of LCEs for an extended number of viable devices, it is desirable to trigger such shape changes with electromagnetic fields rather than temperature changes. Magnetoactive LCEs are accessible by the incorporation of superparamagnetic Fe$_{3}$O$_{4}$ nanoparticles into oriented nematic side-chain LCEs and offer a contactless activation pathway to activate the nematic-to-isotrope transition by local magnetic heating in external fields due to relaxational processes. In magnetomechanical measurements at 300 kHz and 43 kA$\cdot $m$^{-1}$, a sample contraction of up to 30 {\%} is observed under field influence, that is fully released when the field is switched off. The load evolved reaches 60 kPa and more. The materials' ability to respond to a contactless electromagnetic stimulus with a well-defined contraction can be of use for various actuator applications.   

Calamitic liquid crystal elastomers swollen with bent-core liquid crystals

Liquid crystal elastomers are composed of a chemically bonded liquid crystal mesogen and an elastomeric polymer network. They exhibit both the properties of rubber elasticity, liquid crystallinity and their interplay, giving rise to unique systems rich in physics. Additionally, bent-core (``banana'') liquid crystals are also of interest exhibiting rich phase behaviour and non-classical properties such as flexoelectricity. Here we examine the swelling of existing calamitic liquid crystal elastomers with various bent-core mesogens and the intriguing properties of the resulting system. The consequential swollen liquid crystalline elastomer systems, can imbibe many times there weight and volume in bent core mesogens until saturated. The homogenous system displays new properties (transition temperatures and phases) depending strongly on the interaction of the elastomer mesogen and bent-core mesogen. Strangely, for some bent-core mesogens this leads to higher temperature phase behaviour than both compounds originally posses. The authors would like to acknowledge support from ONR (N00014-07-1-0440) and NSF (DMR-0606160).