Elastic Modulus and Equilibrium Swelling of Polyelectrolyte Gels (original) (raw)
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Swelling and Elastic Properties of Polyelectrolyte Gels
Macromolecules, 1995
Ionized poly(acry1ic acid) gels were studied both at concentrations close to the concentration of preparation and at swelling equilibrium. In the first experimental condition, the introduction of electrostatic interactions decreases the shear modulus. The addition of salt screens these interactions and allows one to recover the shear modulus of unneutralized gels. The correlation of these effects with light scattering results suggests that they are related to a change of the gel microstructure with electrostatic interactions. The swelling equilibrium of these gels if found to scale like the ratio of the ionization degree to the Debye-Huckel screening parameter with an exponent 6 6. The shear modulus at swelling equilibrium is given by the simple affine deformation law for not too high swelling degrees (<200). For larger swelling ratios, the shear modulus increases with swelling ratio due to deviations from Gaussian elasticity. These results can be partly explained by a recently proposed model. Finally, the cooperative diffusion coefficient can be measured by kinetics of swelling experiments and its behavior does not follow the predictions of the same model, possibly due to the coupling of cooperative diffusion with the establishment of a Donnan equilibrium.
Stress relaxation of a polyelectrolyte network as affected by ionic strength
Macromolecules, 1982
Stress relaxation behavior of polyelectrolyte films under simple tension is affected by the ionic strength (NaC1) of the swelling medium. In this study, the cationic polyelectrolyte chitosan [ (1+4)-2amino-2-deoxy-P-~-glucan] was used to prepare films. Swelling a t 24 O C showed a salting-in effect up to ionic strength 0.4, reaching an equilibrium volume of approximately 180% (w/w). Linear stress relaxation behavior indicated a predominant first elastic component in a two-component Maxwell model, with a first relaxation time of approximately lo4 s. The apparent Young's modulus increased, with an inflection point at ionic strength between 0.4 and 1.0. Internal stress of the polycationic films, calculated from linear stress relaxation experiments, decreased as the ionic strength increased. The internal stress changed from positive to negative a t ionic strengths between 0.4 and 1.0. Nonlinear stress relaxation measurements at extension ratios between 10 and 40% showed that the instantaneous stress-train relationships can be represented by the Mooney-Rivlin equation, where both C1 and lCzl decrease with time, showing a marked dependence on ionic strength. The results indicate a rearrangement of the molecular chains in the network under tensile stress and also the electrostatic nature of the interactions responsible for the network integrity in the polyelectrolyte films.
Large deformation and electrochemistry of polyelectrolyte gels
2010
Immersed in an ionic solution, a network of polyelectrolytes imbibes the solution and swells, resulting in a polyelectrolyte gel. The swelling is reversible, and the amount of swelling is regulated by ionic concentrations, mechanical forces, and electric potentials. This paper develops a field theory to couple large deformation and electrochemistry. A specific material model is described, including the effects of stretching the network, mixing the polymers with the solvent and ions, and polarizing the gel.
Swelling of neutralized polyelectrolyte gels
Polymer, 2001
Measurements are reported of the swelling pressure v and of the collective diffusion coefficient D c in two series of salt-free neutralized polyelectrolyte gels, one a poly(acrylamide-acrylic acid) copolymer, the other cross-linked potassium polyacrylic acid. The values of D c measured by macroscopic deswelling are in reasonable agreement with those measured by dynamic light scattering. D c increases as the degree of swelling of the gel increases, with similar behaviour being found for the elastic modulus G. The osmotic pressure, P v ϩ G; displays a power law behaviour over the concentration range explored, the exponent s depending on the counter-ion concentration: s Ϸ 1 for the sample of lower charge density, while for the sample with a high charge density, s Ϸ 1:5: The latter behaviour, unexpected at low polymer concentrations, is interpreted in terms of the inequivalence between static and dynamic concentration fluctuations.
Polyelectrolyte Gels: A Unique Class of Soft Materials
Gels, 2021
The objective of this article is to introduce the readers to the field of polyelectrolyte gels. These materials are common in living systems and have great importance in many biomedical and industrial applications. In the first part of this paper, we briefly review some characteristic properties of polymer gels with an emphasis on the unique features of this type of soft material. Unsolved problems and possible future research directions are highlighted. In the second part, we focus on the typical behavior of polyelectrolyte gels. Many biological materials (e.g., tissues) are charged (mainly anionic) polyelectrolyte gels. Examples are shown to illustrate the effect of counter-ions on the osmotic swelling behavior and the kinetics of the swelling of model polyelectrolyte gels. These systems exhibit a volume transition as the concentration of higher valence counter-ions is gradually increased in the equilibrium bath. A hierarchy is established in the interaction strength between the c...
Swelling characteristics of acrylic acid polyelectrolyte hydrogel in a dc electric field
Smart Materials and Structures, 2007
A novel application of environmentally sensitive polyelectrolytes is in the fabrication of BioMEMS devices as sensors and actuators. Poly(acrylic acid) (PAA) gels are anionic polyelectrolyte networks that exhibit volume expansion in aqueous physiological environments. When an electric field is applied to PAA polyelectrolyte gels, the fixed anionic polyelectrolyte charges and the requirement of electro-neutrality in the network generate an osmotic pressure, above that in the absence of the electric field, to expand the network. The objective of this research was to investigate the effect of an externally applied dc electric field on the volume expansion of the PAA polyelectrolyte gel in a simulated physiological solution of phosphate buffer saline (PBS). For swelling studies in the electric field, two platinum-coated plates, as electrodes, were wrapped in a polyethylene sheet to protect the plates from corrosion and placed vertically in a vessel filled with PBS. The plates were placed on a rail such that the distance between the two plates could be adjusted. The PAA gel was synthesized by free radical crosslinking of acrylic acid monomer with ethylene glycol dimethacrylate (EGDMA) crosslinker. Our results demonstrate that volume expansion depends on the intensity of the electric field, the PAA network density, network homogeneity, and the position of the gel in the field relative to positive/negative electrodes. Our model predictions for PAA volume expansion, based on the dilute electrolyte concentration in the gel network, is in excellent agreement with the experimental findings in the high-electric-field regime (250-300 Newton/Coulomb). with biologically active peptides to fabricate smart environmentally responsive hydrogels . Hydrogels exhibit large volume changes to many environmental factors including solvent composition, pH, salt concentration, temperature, light intensity, glucose concentration, antigens, and the application of an external electric field . Polyelectrolyte gels are used extensively in medicine and pharmacy as drug delivery systems, contact lenses, catheters, wound dressings, and biosensors [9, 10].
Molecular simulation of the swelling of polyelectrolyte gels by monovalent and divalent counterions
The Journal of Chemical Physics, 2008
Permanently crosslinked polyelectrolyte gels are known to undergo discontinuous first-order volume phase transitions, the onset of which may be caused by a number of factors. In this study we examine the volumetric properties of such polyelectrolyte gels in relation to the progressive substitution of monovalent counterions by divalent counterions as the gels are equilibrated in solvents of different dielectric qualities. We compare the results of coarse-grained molecular dynamics simulations of polyelectrolyte gels with previous experimental measurements by others on polyacrylate gels. The simulations show that under equilibrium conditions there is an approximate cancellation between the electrostatic contribution and the counterion excluded-volume contribution to the osmotic pressure in the gel-solvent system; these two contributions to the osmotic pressure have, respectively, energetic and entropic origins. The finding of such a cancellation between the two contributions to the osmotic pressure of the gel-solvent system is consistent with experimental observations that the swelling behavior of polyelectrolyte gels can be described by equations of state for neutral gels. Based on these results, we show and explain that a modified form of the Flory-Huggins model for nonionic polymer solutions, which accounts for neither electrostatic effects nor counterion excluded-volume effects, fits both experimental and simulated data for polyelectrolyte gels. The Flory-Huggins interaction parameters obtained from regression to the simulation data are characteristic of ideal polymer solutions, whereas the experimentally obtained interaction parameters, particularly that associated with the third virial coefficient, exhibit a significant departure from ideality, leading us to conclude that further enhancements to the simulation model, such as the inclusion of excess salt, the allowance for size asymmetric electrolytes, or the use of a distance-dependent solvent dielectricity model, may be required. Molecular simulations also reveal that the condensation of divalent counterions onto the polyelectrolyte network backbone occurs preferentially over that of monovalent counterions.
Polymer Journal, 2001
Networks of statistical copolymers of 1-vinyl-2-pyrrolidone and N-vinylcaprolactam (molar ratios of VPNCL= 1/0, 0.8/0.2, 0.6/0.4, and 0.5/0.5) with ionic comonomer, N,N-dimethyl-N,N-diallylammonium chloride (mole fractions x8 =0, 0.03, and 0.05) and crosslinker, 1-1'-divinyl-3,3'-(ethane-1,1-diy])di(1-vinyl-2-pyrrolidone), were prepared by radiation polymerization in a water/ethanol mixture (H20/EtOH = 0.5/0.5 by vol.). Swelling and mechanical behavior was investigated in water/acetone (w/a) mixtures. For charged copolymers, a first-order phase transition (collapse) was found. The extent of collapse (stepwise change in the gel volume), 1' >, and critical acetone concentration in the mixture at collapse, a" slightly increase with content ofVCL and ammonium salt in gels. Shift of transition to higher a, is caused by increasing hydrophobicity of network chains with VCL content. Decrease in swelling with increasing acetone concentration in w/a mixtures is accompanied by increase in equilibrium modulus, so that mechanical behavior is predominantly determined by swelling. The theory of swelling equilibria of polyelectrolyte networks, in which the effects of electrostatic interactions of the charges on the chain and finite chain extensibility are included, semiquantitatively describes the swelling data provided an effective concentration of charges (lower than x,) was introduced.
Volume-phase transitions of cationic polyelectrolyte gels
Polymer, 1992
The swelling equilibrium behaviour of N-n-butyl-N,N-dimethyl-4-vinyl benzylammonium bromide (BBABr) gel was measured in aqueous solutions of various inorganic electrolytes: HBr, LiBr, NaBr, KBr, NH4Br, NaC1, NaI and NaC104. The equilibrium swelling volume of BBABr gel does not depend strongly on the cationic species but on the anionic species. In aqueous solutions containing I-and C10~ ions, the volume-phase transitions of the BBABr gel were induced by a change in the concentration of the electrolytes. It was verified experimentally with 79Br n.m.r, and an ion electrode that the volume-phase transition is controlled by the change in the association state of the charged groups in the gel.