pH-induced swelling kinetics of polyelectrolyte hydrogels (original) (raw)
Journal of Applied Polymer Science, 2009
The isothermal kinetics curves of the swelling of a poly(acrylic acid) hydrogel in buffer solutions of different pH values (5, 7, and 9) at temperatures ranging from 30 to 40 C were determined. The possibilities of applying the Fick kinetics model and the Peppas equation were examined. It was found that the applicability of these models were limited. The kinetics model of a first-order chemical reaction was found to describe the swelling kinetics of the PAA hydrogel in all the investigated buffer solution at all the investigated temperatures. Swelling kinetics is deter-mined by the rate of expansion of the network. The kinetic parameters (E a , ln A) of the swelling of the PAA hydrogel in buffer solutions of different pH values were determined. The activation energy and the pre-exponential factor of the swelling of the PAA hydrogel in buffer medium decreased with increasing pH value of the swelling medium.
International Journal of Pharmaceutics, 2020
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Journal of the Serbian Chemical Society, 2007
The isothermal kinetics curves of the swelling of a poly(acrylic acid) hydrogel in distilled water and physiological solution at temperatures ranging from 20 to 40 ºC were determined. The possibility of applying both the Fick's kinetics model and kinetics model of the first order chemical reaction to the swelling kinetics of the PAA hydrogel in distilled water and physiological solution were examined. It was found that the possibilities of applying these models were limited. The new model of the kinetics of swelling in distilled water and physiological solution was established. The kinetic parameters (E a , ln A) for the swelling in distilled water and physiological solution were determined. The decrease of the equilibrium degree of swelling and the saturation swelling rate of the swelling of the PAA hydrogel in physiological solution compared to swelling in distilled water could be explained by the decreased differences in the ionic osmotic pressures between the hydrogel and the swelling medium. The increase of the initial swelling rate in the physiological solution might be caused by an increased density of charges at the network and by an increased affinity of the network towards the water molecules. The increase of the activation energy of the swelling of the PAA hydrogel in the physiological solution is a consequence of its additional "ionic crosslinking".
European Polymer Journal, 2007
In this study, swelling behavior and mechanical properties of polyelectrolyte cationic hydrogels of poly((2-dimethylamino) ethyl methacrylate) (PDMAEMA), and poly((2-dimethylamino) ethyl methacrylate-co-butyl methacrylate) (P(DMAEMA-co-BMA)), were investigated. Hydrogels were prepared by free-radical solution copolymerization of DMA-EMA and BMA using ethylene glycol dimethacrylate (EGDMA) as the crosslinking agent. Compression-strain measurements were used to analyze the mechanical properties of the hydrogels. It was found that increasing the amount of BMA comonomer in the gel structure increases the compression modulus of the material. The results of mechanical measurements were used to characterize the network structure of the hydrogels, namely the effective crosslinking density (m à e Þ. It was found that m à e exceeds the theoretical crosslinking density (m t) calculated from the initial amount of EGDMA used for hydrogel synthesis. These hydrogels demonstrated dual sensitivity to both pH and temperature. It was shown that the pH-sensitive or temperature-sensitive phase transition behavior of the gels can be changed by changing the temperature or pH of the swelling medium at constant hydrogel composition. Increasing the temperature decreased the transition pH of the pH-sensitive phase transition. On the other hand, increasing the pH of the surrounding medium decreased the transition temperature of the temperature-sensitive phase transition. Incorporation of BMA in the gel structure has a significant effect on the transition point of the gel. Increasing the BMA content reduced the transition pH and temperature of the pHand temperature-sensitive phase transition, respectively. The similar effect of increasing temperature or BMA content can be explained by the role of hydrophobicity in the phase transition behavior of hydrogels. Finally, the results of equilibrium swelling and compression-strain measurements were used to calculate the polymer-solvent interaction parameters of these hydrogels using the Flory-Rehner equation of equilibrium swelling.
Polymer, 2006
The elastic shear modulus G and swelling pressure ω are studied for a basic, pH-responsive hydrogel synthesized by crosslinking copolymerization of co-monomers hydroxypropyl methacrylate and N,N-dimethylaminoethyl methacrylate with crosslinker tetraethylene glycol dimethacrylate. Under normal conditions of use as a "smart" material, hydrogel swelling ratio Q and pH vary simultaneously, but here G and ω values are presented as a function of pH with Q held constant and vice-versa. At fixed pH, G decreases with increase in Q in a power law dependence, as predicted by the Flory-Rehner model. However, at fixed Q, G increases with decrease in pH (i.e, increase in degree of ionization). The pH effect is more pronounced than the volume effect, thus the hydrogel stiffens as it swells in response to pH change. At high pH, ω values of the uncharged hydrogel obey the Flory-Rehner model, whereas explicit ionic contributions can be identified at lower pH values.
A theory of constrained swelling of a pH-sensitive hydrogel
2010
Many engineering devices and natural phenomena involve gels that swell under the constraint of hard materials. The constraint causes a field of stress in a gel, and often makes the swelling inhomogeneous even when the gel reaches a state of equilibrium. This paper develops a theory of constrained swelling of a pH-sensitive hydrogel, a network of polymers bearing acidic groups, in equilibrium with an aqueous solution and mechanical forces.
Deformation kinetics of pH-sensitive hydrogels
Polymer International, 2013
Polymeric gels can undergo large deformation when subjected to external solutions of varying pH. It is imperative to understand the deformation process of pH-sensitive hydrogels for the effective application of these attractive materials in the biomedical and microfluidic fields. In the modeling of these multi-phase materials, finite element (FE) modeling is a useful tool for the development of future applications, and it allows developers to test a wide variety of material responses in a cost-effective and efficient manner, reducing the need to conduct extensive laboratory experiments. Although a FE user-defined material model is available for the equilibrium state, the transient response of pH-sensitive gels has not been effectively modeled. Based on our recent work using the heat transfer analogy to tap into the readily available coupled temperature-displacement elements available in the commercial FE software ABAQUS for simulation of the transient swelling process of neutral hydrogels, the transient swelling process of a pH-sensitive hydrogel is studied and a FE model is further developed to simulate the transient phenomena. Some benchmark examples are investigated to demonstrate the model's capabilities in the simulation of nonlinear deformation kinetics relevant to several applications of pH-sensitive hydrogels.
The Swelling Responsiveness of pH-Sensitive Hydrogels in 3 D Arbitrary Shaped Geometry
2013
The pH-sensitive hydrogels are physically responsive to a change in the pH of surrounding solution, which often resemble to biomaterials. These hydrogels can expand up to 4 times to its original dimensions under certain pH of surrounding buffer. We present the simulation of swelling characteristic of 3Darbitrary-geometry, pH-sensitive hydrogel in steady state conditions. Three nonlinear partialdifferential equations that are representing responsible physical phenomena namelychemical, electrical, and mechanical can describe the swelling responses to a chemical change in surrounding solution. Finite element analysis used for present study was carried out by full coupling of above three partial-differential equations with variable material properties in COMSOL Multiphysics®. Employing a moving mesh method for 3D geometry, the FEM simulation was performed to account for largeswelling of the pH-sensitive hydrogel.
Analysis of a model for pH-sensitive hydrogels
Polymer, 2012
A mathematical model comprising conservation of mass, momentum, and ions for a hydrogel subject to alterations of the solution pH is derived, analyzed, validated and presented. Good agreement between model predictions and their experimental counterpart are achieved. To gain a physical insight into the deformation behavior of the pH-sensitive hydrogel, a scaling analysis coupled with a parametric study is carried out for key physical and operational parameters. The results suggest the significance of initial fixed-charge density, solution ionic strength, Poisson ratio and Young modulus in determining swelling degree of the hydrogel. In addition, it is noted that changes in acid dissociation constant and temperature lead to a notable shift in equilibrium swelling curves. Permeability and size of the hydrogel were found to significantly affect the deformation kinetics: A hydrogel with higher permeability and/or smaller size exhibits faster deformation. These findings and the characteristic scales could provide important guidelines in designing systems utilizing pH-sensitive hydrogels.
Polymer Bulletin, 2001
Influence of some simulated physiological body fluids on the dynamic swelling behaviour of polyelectrolytic hydroxamic acid hydrogels (PHA) was investigated at 37 °C in vitro. The simulated physiological body fluids are distilled water, human sera, physiological saline (0.89 % NaCl), isoosmotic phosphate buffer at pH 7.4, gastric fluid at pH 1. 1, (gylicine-HCl buffer), urea (0-3 mol L-1), and the aquatic solutions of K 2 HPO 4 and KNO 3 (the sources of K +). The values of equilibrium swelling of PHA hydrogels varied in the range of 130-4625%, while the values of equilibrium fluid content of the hydrogels varied in the range of 57-97%. The initial rate of swelling, diffusional exponent, and, diffusion coefficient were calculated using swelling kinetics data. Diffusion of the fluids into the hydrogel was found to be non-Fickian character. The diffusion coefficients of the hydrogel varied between 0.6x10-6-8.1x10-6 cm 2 s-1 .
Chemical Industry
Hydrogels are synthesized by the method of radical polymerization of monomers: N-isopropylacrylamide (NIPAM) and acrylic acid (AA). Characterization of poly(N-isopropylacrylamide- co-acrylic acid) hydrogels, p(NIPAM/AA), has been performed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and by determination of the swelling behaviour in aqueous solutions at different temperatures (25, 31 and 37?C) and pH values (2.2, 4.5, 6 and 6.8). After lyophilisation in the solution at pH 6 and temperature of 25?C, p(NIPAM/AA) hydrogels have rapidly reached equilibrium degree of swelling, ?e, in comparison to non-lyophilized samples. The mechanism of solvent transport within matrix in lyophilized samples corresponds to less Fickian diffusion, whereas Super case II diffusion is characteristic for non-lyophilized samples. p(NIPAM/AA) hydrogel with 1.5 mol% of ethylene glycol dimethacrylate (EGDM) at the temperature of 25?C and pH 6.8, has reached the highest swelling equi...
Modeling Approaches to the Dynamics of Hydrogel Swelling
Journal of Computational and Theoretical Nanoscience, 2010
We consider a gel as an immiscible mixture of polymer and solvent, and derive governing equations of the dynamics. They include the balance of mass and linear momentum of the individual components. The model allows to account for nonlinear elasticity, viscoelasticity, transport and diffusion. The total free energy of the system combines the elastic contribution of the polymer with the Flory-Huggins energy of mixing. The system is also formulated in terms of the center of mass velocity and the diffusive velocity, involving the total and the relative stresses. This allows for the identification of special regimes, such as the purely diffusive and the transport ones. We also obtain an equation for the rate of change of the total energy yielding decay for special choices of boundary conditions. The energy law motivates the Rayleghian variational approach discussed in the last part of the article. We consider the case of a gel in a one-dimensional strip domain in order to study special features of the dynamics, in particular, the early dynamics. We find that the monotonicity of the extensional stress is a necessary condition to guarantee the propagation of the swelling interface between the gel and its solvent. Such monotonicity 0-1 condition is satisfied for data corresponding to linear entangled polymers. However, for polyssacharide gels the monotonicity of the stress fails at a critical volume fraction, suggesting the onset of de-swelling. The weak elasticity is responsible for the loss of monotonicity of the stress. The analysis also suggests that type II diffusion is a hyperbolic phenomenon rather than a diffusive one. One goal is to compare the derivation method, assumptions and resulting equations with other models available in the literature, and determine their regimes of validity. The stress-diffusion coupling model by Yamaue and Doi [26] is one main benchmark. We assume that the gel is non-ionic, and neglect thermal effects.
Nonmonotonic swelling and compression dynamics of hydrogels in polymer solutions
Physical Review E, 2020
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Equilibrium swelling properties of polyampholytic hydrogels
Chemical Physics, 1996
The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects
The Swelling Behavior of Charged Hydrogels
Macromolecular Symposia, 2006
We describe recent advances obtained in understanding the microscopic interplay of a charged hydrogel with its confined mobile counterions. Results of extensive computer simulations are compared to simple scaling descriptions for various solvent conditions. We will briefly discuss our simulation package ESPResSo which was used to perform the simulations. We demonstrate why simple scaling theories suffice to adequately describe the swelling behavior of these systems despite their enormous complexity. We will conclude with a presentation of ongoing results for polyelectrolyte networks under poor solvent conditions.
The Journal of Physical Chemistry B, 2007
The number of variables controlling the behavior of ionic gels is large and very often some of them are unknown. The aim of this work is to interpret quantitatively the swelling behavior of pH sensitive gels, with the minimum number of simplifying assumptions. With this purpose, the equilibrium degree of swelling (S) and protonation (R) of chemically cross-linked poly(N-vinylimidazole) (PVI) immersed in aqueous salt solutions were measured as a function of the ionic strength (µ), in the whole range of pH. In acid solutions with pH in the range 0 to 4, imidazole moieties become protonated, and PVI behaves as a polyelectrolyte gel: S decreases upon increasing µ both for NaCl and for CaCl 2 , with HCl as protonating acid. In aqueous solutions with larger pH, between 4 and 12, the hydrogel is practically neutral, and S increases as µ rises, showing a salting-in effect. From the quantitative analysis of these results, the following facts emerged. Protonation induces chain stiffness (as measured by the non-Gaussian factor) and worsening of the solvent quality of the aqueous media (as measured by the polymer-solvent interaction parameter). For R below 33%, swelling seems to be governed by the excess of mobile counterions inside the gel with respect to the bath, with a minor but still significantly negative contribution of the osmotic swelling pressure due to polymer-solvent mixing. Above 33% protonation, it is necessary to consider Manning counterion condensation to get parameters with physical meaning. The crossover between polyelectrolyte and salting-in effects corresponds to R and µ values with the same ionic and mixing contributions to the osmotic swelling pressure. The formation of ionic nonpermanent cross-links, with H 2 SO 4 as the protonating acid, was discarded.
Journal of Applied Polymer Science, 1995
Incorporation of sodium polyacrylate (NaPAA) in poly(vinyl alcohol) (PVA) gels as small, uniformly distributed precipitates greatly accelerates their volume expansion during swelling in water to form hydrogels. In addition to the usual water absorption, the swelling process includes dissolution of the precipitates that leads to a locally high osmotic pressure that in turn causes a further increase in water penetration and volume expansion. During swelling, soluble NaPAA is released into the water phase with a high initial release rate that then decreases continuously. The release can be described by a n exponential decay function with a power dependent rate coefficient. Because the diffusion of NaPAA through the PVA walls is too slow to account for this release rate, a morphology of a closed cell foamlike structure with interconnecting channels is proposed. An aqueous solution of NaPAA seems to diffuse out of the hydrogel through these channels. 0 1995 John Wiley & Sons, Inc.
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.
Non-monotonic swelling of surface grafted hydrogels induced by pH and/or salt concentration
The Journal of chemical physics, 2014
We use a molecular theory to study the thermodynamics of a weak-polyacid hydrogel film that is chemically grafted to a solid surface. We investigate the response of the material to changes in the pH and salt concentration of the buffer solution. Our results show that the pH-triggered swelling of the hydrogel film has a non-monotonic dependence on the acidity of the bath solution. At most salt concentrations, the thickness of the hydrogel film presents a maximum when the pH of the solution is increased from acidic values. The quantitative details of such swelling behavior, which is not observed when the film is physically deposited on the surface, depend on the molecular architecture of the polymer network. This swelling-deswelling transition is the consequence of the complex interplay between the chemical free energy (acid-base equilibrium), the electrostatic repulsions between charged monomers, which are both modulated by the absorption of ions, and the ability of the polymer netwo...