Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X18: Elastomers and Gels I |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Alfred Crosby, University of Massachusetts Room: 319 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X18.00001: Phase behavior of poly(ethylene oxide) in ethyl alcohol / water mixtures S. H. Shin, R. M. Briber, B. Hammouda, D. L-T Ho PEO in ethanol forms an opaque gel-like mixture with a partial crystalline structure. Addition of a small amount of water disrupts the gel: 5{\%} PEO in ethanol with the addition of 4{\%} water becomes a transparent solution. We have studied the phase behavior of PEO in mixed solvents using small angle neutron scattering (SANS). PEO solutions (5{\%} PEO) which have more than 4-10 {\%} water behave as an athermal polymer solution and the phase behavior changes from UCST to LCST rapidly as the fraction of water is increased. The proposed origin of this unusual phase behavior comes from the formation of a hydration layer around the PEO chain. Two water molecules can hydrate one PEO monomer which is consistent with the suppression in the crystallization and change in the phase behavior observed by SANS. We measured the spinodal temperature and phase behavior of PEO solutions with different concentrations of PEO (2{\%} PEO and 10{\%} PEO) in the mixed water/ethanol solvent system to assess the role of hydration. The observed phase behavior is consistent with a hydration layer forming upon the addition of water and the system shifting from UCST to LCST behavior. The amount of water necessary to form a hydration layer around PEO chains varies in a self-consistent manner as the PEO concentration increases from 2 to10{\%}. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X18.00002: pH-Responsive Swelling of PAMAM Dendrimer-Gels. Ronald Hedden, Burcu Unal End-linked hydrogels containing high mass fractions of amine-terminated poly(amidoamine) (PAMAM) dendrimers are prepared by reaction of dendrimers with monodisperse, epoxide-terminated linear poly(ethylene glycol) chains. PAMAM dendrimers impart pH-dependent swelling characteristics to the gels, which absorb large amounts of water due to protonation of the dendrimers' amine groups under neutral or weakly acidic conditions. The equilibrium swelling of the gels passes through a maximum at pH of approximately 4.5, due to extensive protonation of the amine groups. Interestingly, the equilibrium swelling ratio is markedly lower at both high external pH and low external pH. We model the swelling behavior by invoking the Donnan equilibrium theory, treating the gels as phantom networks that contain a high concentration of Lewis bases having pKb=3.5 The model captures the maximum in swelling near pH=4.5, though equilibrium swelling ratio is overpredicted in some cases. The collapse of the gels at both high and low external pH is explained in terms of the differential between the concentrations of mobile ions inside and outside the gel. We will discuss recent attempts to prepare stimuli-responsive gels based upon the remarkable swelling characteristics of PAMAM dendrimer-gels. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X18.00003: Studies on shear-thinning and recovery properties of beta-hairpin peptide hydrogel Congqi Yan, Radhika Nagarkar, Joel Schneider, Darrin Pochan In solution, freely soluble, unfolded MAX1 peptide ((VK)$_{4}$-V$^{D}$PPT-(VK)$_{4}$-CONH$_{2})$ can undergo a conformation change into a folded$\beta $-hairpin by exposure to a folding stimulus, e.g. pH change, salt addition, or temperature rise. The consequent self-assembly leads to a stiff hydrogel stabilized by physical crosslinks between fibrillar nanostructures. When a proper shear stress is applied, the hydrogel shear-thins and flows. Moreover, as soon as the stress is ceased, the gel immediately reheals into a stiff solid and recovers its original mechanical strength quickly. This shear-thinning and rehealing property makes possible hydrogel delivery via syringe injection. In this work, Rheo-SANS was adopted to monitor the gel network morphology under shear flow. Also, rheological experiments were performed to measure the gel recovery after shear-thinning under various shear treatment conditions. Laser scanning confocal microscopy was used to observe the flow and velocity profile of the hydrogel through a channel. The results explain morphology changes of the gel network during shear-thinning and subsequent rehealing process. The fundamental gel shear-thinning and rehealing mechanisms will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X18.00004: Poly(Z-Lysine)-based Block Copolymer Organogels Sandeep S. Naik, Adam D. Richardson, Daniel A. Savin A series of AB diblock copolymers consisting of poly(Lysine(Z)) (A = P(Lys(Z))) and poly(propylene oxide) or polyhedral oligomeric silsesquioxane (B = PPO, POSS) were synthesized and found to form stable, rigid organogels in THF and chloroform 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 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 350 K exhibited a gel-sol transition. The role of solvent polarity and molecular weight of the P(Lys(Z)) chains on the mechanical strength will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X18.00005: Anisotropic Poly(Vinyl Alcohol) Hydrogel: Connection Between Structure and Bulk Mechanical Properties Stephen Hudson, Jeffrey Hutter, Leonardo Millon, Wankei Wan, Mu-Ping Nieh Poly(vinyl alcohol) (PVA) hydrogels are formed from PVA solution by creation of physical cross-links during freeze/thaw cycling. By choosing a suitable freeze/thaw protocol and applying a strain during thermal processing, gels with permanent, anisotropic bulk mechanical properties matching those of cardiovascular tissues can be made, making them useful for applications ranging from artificial heart valves to vascular grafts. We have performed small- and ultra small-angle neutron scattering (SANS and USANS) measurements covering length scales from 2 nm to 10 $\mu$m, and modeled the structure as interconnected PVA blobs of size 20 to 50 nm arranged in fractal aggregates extending to at least 10 $\mu$m. Here, we discuss the relationship between the microstructure and bulk mechanical properties. Strength increases with the number of thermal cycles due to reinforcement of the small-scale gel phase, while anisotropy is due to elongation of the much larger fractal aggregates. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X18.00006: Kinetics of phase separation of thermoreversible gels Francisco J. Solis, Christine Leon, Brent Vernon We study the kinetics of phase separation and thermoreversible gel formation of LCST-type polymers. A large number of NIPAM-based polymers exhibit transitions near room temperature from a liquid phase to a two-phase state. In the two phase-region of the phase diagram, a polymer dilute phase coexists with a gel. The corresponding shrinking transition for chemically-linked gels has been extensively studied in both its thermodynamic and kinetic aspects. We show that, in the thermoreversible case, the formation of the gel phase proceeds in a similar way. Upon entrance to the two-phase region, the gel volume follows a double exponential decay. The gel undergoes a fast shrinking associated with water ejection, followed by a slower reorganization regime. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X18.00007: Structure and Rheology of Leucine Zipper Protein Hydrogels B.D. Olsen, J.A. Kornfield, D.A. Tirrell Protein hydrogels from telechelic polymers physically crosslinked by the specific association of leucine zipper domains provide fundamental insight into polymer network structures due to the unparalleled control over molecular weight and network junction multiplicity. Two different leucine zippers are used to confer either tetrameric or pentameric end block association. By varying the length of the polyelectrolyte midblock, we show that the structure and rheological properties of the hydrogels depend on both the polymer molecular weight and the aggregation state of the leucine zipper junctions. Cryo-TEM and negative staining are used together to visualize the gels, revealing heterogeneous structures. The gels are strongly shear thinning, and examination of Lissajous figures of stress vs. strain suggest a yielding mechanism. Under many conditions the gels can recover nearly their full strength less than a minute after the cessation of shear. These properties combined with the ease of biofunctionalization and pH and temperature responsive gelation transitions make the materials attractive for tissue engineering. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X18.00008: Morphology of biaxially stretched triblock copolymer gels using SAXS Arjun Krishnan, Tushar Ghosh, Richard Spontak Gels of styrenic triblock copolymers swollen by a low-volatility, midblock-selective oil behave as high-strain, low-field dielectric elastomers in the design of electroactive polymeric actuators. A standard configuration of such devices involves stretching, or ``prestraining,'' the elastomer film biaxially. However, little is known about the effect of biaxial prestrain on copolymer morphology. In this study, small-angle X-ray scattering (SAXS) is used to probe the nanostructure of gels composed of poly[styrene-$b$-(ethylene-\textit{co-}butylene)-$b$-styrene] and mineral oil by systematically changing the concentration of polymer from 5 to 30 wt{\%} and the biaxial prestrain from 0 to 300{\%}. In the azimuthally integrated intensity profiles, the form factor due to scattering from polystyrene microdomains correlates strongly with polymer concentration and does not change with the applied prestrain, indicating that the polystyrene crosslinks remain as polydisperse spheres. The structure factor data correlates with prestrain, and is fitted using the Percus-Yevick approximation for interacting spheres. While a hard sphere interaction model is sufficient for unstrained gels, we resort to a square shoulder hard sphere potential for strained samples. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X18.00009: Computer Simulation of a Switchable Metallo-Supramolecular Gel Shihu Wang, Elena Dormidontova Using Monte Carlo simulation, we studied reversible metallo- supramolecular gel comprised of linear oligomers end- functionalized with ligands and metal ions that can form trans- or cis- ligand-metal complexes with a ratio up to 3:1. We found that cis- isomers exhibit a larger overall degree of association and higher average molecular weight compared to trans- isomers due to a larger fraction of 3:1 or 2:1 ligand- metal complexes. Furthermore the metallo-supramolecular gel formed by cis-isomers occurs within a wider range of metal-to- oligomer ratios at a lower oligomer concentration and exhibits a larger elastic modulus and a smaller mesh-size compared to gel formed by trans-isomers. We found that exchanging cis- to trans- isomers leads to a monotonic change of the materials properties for most cases except for the 2:1 ligand:metal ratio at which the mesh size exhibits a minimum due to the favorable formation of intra-molecular bonds by cis- isomers. These switchable properties suggest interesting application opportunities. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X18.00010: Cavitation Rheology and Fracture Behavior of Polyacrylamide Hydrogels Santanu Kundu, Alfred Crosby Cavitation rheology is a new characterization technique for the measurement of mechanical properties on small length scales, e.g. 10 -1000 $\mu $m, at any arbitrary location within a soft material. The technique involves growing a cavity at the tip of a syringe needle and monitoring the pressure of the cavity at the onset of instability. This critical pressure is directly related to the local modulus of the material. We used this technique to characterize the network mechanics of polyacrylamide hydrogel materials, a common material used in many biological applications. Gels with different moduli, which were obtained by varying initial monomer to water ratio, were investigated. As monomer concentration increased, a transition from stable cavity to fracture was observed. Applying scaling theory for gels, we modify the Lake-Thomas Theory for the fracture of crosslinked networks to relate the transition from cavitation to fracture in terms of molecular parameters. We anticipate this fundamental understanding of cavitation and fracture mechanism will be applicable to biological tissues, as well as the development of advanced soft materials [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X18.00011: Dynamic display of biomolecular patterns through an elastic creasing instability of stimuli-responsive hydrogel surfaces Jungwook Kim, Ryan Hayward Swelling a soft hydrogel film attached to a rigid substrate generates a lateral biaxial compressive stress within the gel. For sufficiently large stresses, the free surface of the gel undergoes a mechanical instability to form sharp creases on its surface. We have taken advantage of this process using temperature-responsive hydrogels to fabricate dynamic scaffolds that reversibly hide and display biomolecular patterns. Desired bioactive ligands are grafted to polyelectrolytes, which are then selectively deposited to pattern the hydrogel surface. The shapes of the patterns are directed by topographic features of the underlying substrate. At room temperature, the functionalized areas of the surface are hidden within creases, but as the temperature is raised, dehydration of the gel leads to unfolding of creases and exposure of the biomolecular patterns. By switching on and off the patterned functionalities, we could engineer dynamic interactions between our scaffolds and target objects such as microscopic beads or cells. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X18.00012: Photo-induced locomotion of chemo-responsive polymer gels Pratyush Dayal, Olga Kuksenok, Anna C. Balazs The need to translate chemical energy into a mechanical response, a characteristic of many biological processes, has motivated the study of stimuli-responsive polymer gels. Recently, it has been shown experimentally that by coupling the mechanical properties of the gel with the Belousov-Zhabotinsky (BZ) reaction it is possible to induce self-sustained oscillations in the gel. One of the means for controlling these chemical oscillations is using light as an external stimulus. To study the effect of light on the mechanical behavior of the gel, we use our recently developed a 3D gel lattice spring model (gLSM) which couples the BZ reaction kinetics to the gel dynamics. In this model, the polymer-solvent interactions were taken into account by adding a coupling term to the Flory-Huggins free energy. By virtue of this coupling term, the swelling---de-swelling behavior of the gel was captured in 3D. In order to include the effect of the polymer on the reaction kinetics, the Oregonator model for the photo-sensitive BZ reaction was also modified. Using gLSM model, we probed the effect of non-uniform light irradiation on the gel dynamics. We were able to manipulate the direction and velocity of locomotion of the gel using light as a control parameter. This ability to control the movement of the gel can be utilized in a variety of applications, ranging from bio-actuators to controlled drug release systems. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X18.00013: Harnessing Labile Bonds between Nanogels Particles to Create Self-Healing Materials German Kolmakov, Krzysztof Matyjaszewski, Anna Balazs Using computational modeling, we demonstrate the self-healing behavior of novel materials composed of nanoscopic gel particles that are interconnected into a macroscopic network by both stable and labile bonds. Under mechanical stress, the labile bonds between the nanogels can break and readily reform with reactive groups on neighboring units. This breaking and reforming allows the units in the network to undergo a structural rearrangement that preserves the mechanical integrity of the sample. The stable bonds between the nanogels play an essential role by forming a backbone that provides a mechanical strength to the material. The simulations show that just a relatively small fraction of such labile bonds (roughly 15\%) are needed to prevent the catastrophic failure of the sample. The findings provide guidelines for creating high-strength, self-healing materials. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X18.00014: Effect of Confinement on the Dynamics of Three-Dimensional Chemo-responsive Gels Olga Kuksenok, Victor V. Yashin, Anna C. Balazs Chemo-responsive gels undergoing the Belousov-Zhabotinsky(BZ) reaction could be ideal candidates for creating materials that can perform sustained mechanical work. We use theory and simulation to investigate the behavior of three-dimensional samples of BZ gels that are spatially confined in various geometric arrangements and show that the spatial confinement has a dramatic effect on the samples' dynamics. We first perform a linear stability analysis in two limiting cases, where a small sample is either completely free or attached at all the boundaries to fixed, hard walls. We find the critical reaction parameters at which the gels undergo a transition from a stationary steady state to an oscillatory regime in each of these cases. We then carry out corresponding computer simulations using our 3D gel lattice spring model and find an excellent agreement between the theory and simulations. Furthermore, we illustrate that the above analysis allows us to predict the behavior of larger samples that are confined in more complex spatial arrangements. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X18.00015: Diffusion of molecular probes and proteins in hydrogels Riccardo Raccis, Robert Roskamp, Annette Brunsen, Bernhard Menges, Ulrich Jonas, Wolfgang Knoll, George Fytas We employ fluorescence correlation spectroscopy to study the diffusion of molecular probes (Cy5) and dye-tagged proteins (Cy5-AntiMouse, hydrodynamic radius 10nm and Alexa488-Streptavidin, 4nm) in surface-attached poly-N-isopropylacrylamide (PNIPAAm) and dextran based hydrogel layers. The diffusion process depends on the crosslinking density and the presence of electrostatic and steric interactions. The protein penetration into the hydrogel layer occurs close to the isoelectric point but the local probe concentration and diffusion rate diminish with increasing penetration depth. Mesh size characterization of the hydrogels is inferred from the diffusivity and the concentration profile of fluorescent probes with different size, with the molecular free dye diffusing deeper into the gel. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700