Polyampholyte Gels Research Articles

Swelling−Shrinking Behavior of a Polyampholyte Gel Composed of Positively Charged Networks with Immobilized Polyanions

Polyampholyte gels were prepared by free radical polymerization of aqueous monomer solutions with the following composition: 69% N-isopropylacrylamide (thermosensitive neutral monomer), 1% N,N'-methylenebisacrylamide (cross-linker), 15% 1-vinylimidazole (cationic monomer), and either 15% acrylic acid (AAc, anionic monomer) or poly(acrylic acid) (PAAc, polyanion). We thus obtained two sorts of polyampholyte gels; that is, G1 with immobilized PAAc and G2 with randomly copolymerized AAc. The equilibrium swelling ratio (Qe) was studied as a function of the pH, NaCl concentration, and temperature. Also studied was the kinetics of swelling and shrinking in response to a sudden pH change. The significant results obtained were as follows: (i) A fully collapsed state was observed at pH 4.5-9.0 for G1 and at pH 4.5-7.0 for G2. (ii) Below and above these pH ranges, both gels were in a swollen state; therefore, an isoelectric point (pI) appeared in a wide pH range. (iii) At alkaline pH regions where a hysteresis was observed in the Qe versus pH curves of G1 and G2 as the pH was first increased then decreased, G1 exhibited very slow swelling-shrinking kinetics. (iv) An increase in the NaCl concentration allowed the gel to swell at pH approximately pI (antipolyelectrolyte behavior) but to shrink at pHs below and above the pI range (polyelectrolyte behavior). (v) The magnitude of the salt-induced shrinking of G1 is smaller than that of G2 at pH 10 and at NaCl concentrations > 0.01 M. (vi) At pH 10, an increase in the temperature from 35 to 50 degrees C led to a shrinking change of G1 but not of G2. These results were found to be explicable in terms of a different distribution of negative charges within the polyampholyte gel network.

Synthesis, characterization and stimuli‐sensitive properties of novel linear and crosslinked polybetaines based on acrylic acid and ethyl 3‐aminocrotonate

AbstractNovel linear and crosslinked polybetaines based on acrylic acid (AA) and ethyl 3‐aminocrotonate (CRO) have been synthesized by a Michael addition reaction followed by radical polymerization. The polymerization of AA and CRO was carried out in bulk, water and organic solvents. The dependence of polymer yield on the molar ratio of monomers and water content was found. Primary attention was paid to linear and crosslinked polybetaines synthesized at equimolar ratio of monomers in the feed. The composition and structure of linear polybetaines was determined by elemental analysis, potentiometric titration, FTIR and NMR spectroscopy. The isoelectric points of linear and crosslinked polybetaines determined by electrophoresis, viscometry and swelling experiments corresponded to pH 2.0‐2.2. The stimuli‐sensitive properties of amphoteric gels were studied as a function of pH, ionic strength, water‐organic solvent mixture, electric, and combined electric and magnetic fields. Appearance of pH gradient within the polyampholyte gel matrix under the externally imposed DC electric field was observed.

Electrophoretic Behavior of Ampholytic Polymers and Nanogels

Polyampholyte gel particles with hydrodynamic radii (Rh) 132 to 87 nm at an isoelectric point were synthesized by aqueous redox polymerization in the presence of sodium dodecylbenzene sulfonate as ...

Stimuli‐sensitive behaviour of novel betaine‐type polyampholytes

AbstractNovel linear and crosslinked polyampholytes of betaine structure based on acrylic acid and ethyl 3‐aminocrotonate (ethyl ester of 3‐amino‐2‐butenoic acid) have been synthesized by Michael addition reaction followed by radical copolymerization. The mechanism of formation of monomer and polymer betaines is discussed. The linear polyampholyte has been characterized by potentiometric titration, IR, NMR and GPC. Crosslinked polymeric betaines were synthesized in the presence of N,N′‐methylenebisacrylamide. The stimuli‐sensitive properties of amphoteric gels have been studied as a function of pH, ionic strength, water–organic mixture composition, electric, and combined electric and magnetic fields. The isoelectric points of linear and crosslinked polymeric betaines correspond to pH 2.0–2.1. The effect of ionic strength on the solution and gel properties of polybetaine has been interpreted on the basis of destruction of inter‐chain, intra‐chain and intra‐group salt bonds. Water–acetone, water–ethanol or water–DMF mixtures cause the shrinking of amphoteric gel due to change of the dielectric constant of the medium and decrease of the osmotic pressure. Electrocollapse is observed under the action of DC electric field. Simultaneously cross action of electric and magnetic fields enhances the collapsing rate. Appearance of pH gradient within the volume of polyampholyte gel under the externally imposed DC electric field has been observed. Copyright © 2003 Society of Chemical Industry

Preparation and Characterization of Thermosensitive Polyampholyte Nanogels

Nanosized polyampholyte gel particles were synthesized by aqueous redox polymerization in the presence of sodium dodecylbenzene sulfonate as a surfactant. Acrylic acid (AAc) and 1-vinylimidazole (VI) were respectively used as an anionic and a cationic monomer, both of which were incorporated into the network of N-isopropylacrylamide (NIPA) cross-linked with N,N‘-methylenebisacrylamide. Potentiometric titration (0.05−0.5 M KCl solutions as a solvent) gave the following contents (in mol %) of anions (A) and cations (C) in the nanogels: A = 3 and C = 13 for nanogel G(1/4); A = 14 and C = 12 for nanogel G(1/1). It is worthy of notice that G(1/1) is insoluble in pure water but soluble in KCl solutions over the concentrations (CS) > 0.01 M; this phenomenon is the so-called “antipolyelectrolyte” behavior. Both nanogels were characterized in detail by a combination of static and dynamic light scattering, electrophoretic light scattering, and turbidity measurements. The pH dependence of the gel size exhibited a characteristic pattern, from which an isoelectric point (pI) was observed: pH 6.5 for G(1/4) and pH 5.3 for G(1/1). At pI, at which the negative and positive charges are fully balanced, the electrophoretic mobilities became zero. From the light scattering of nanogels in 0.1 M KCl solution at pH = pI, the following data were obtained: radius of gyration (Rg) = 107 nm for G(1/4) and 81 nm for G(1/1); hydrodynamic radius (Rh) = 132 nm for G(1/4) and 87 nm for G(1/1). Thus, the Rg/Rh ratio was within 0.85 ± 0.05, suggesting a spherical conformation of the nanogel particles. From the study of the effects of CS and temperature, the following characteristics were obtained: (i) Rh of G(1/4) varies little depending on KCl concentration, while G(1/1) shows a slight increase in Rh with CS from 0.01 to 0.5 M and a marked decrease at CS ∼ 2 M without accompanying aggregation. (ii) G(1/1) undergoes a shape phase separation at CS ∼ 0.008 when decreasing CS, whereas G(1/4) is dispersible both in pure water and in KCl solutions at CS < 1 M, the concentration at which aggregation takes place, as observed in the pure NIPA nanogel system. (iii) Both G(1/4) and G(1/1) shrink up to 41 and 51 °C, respectively, and around these temperatures the gel suspensions undergo a shape phase separation. The results obtained were compared with those of terpolymers as a control sample and discussed in terms of the intra- and interparticle interactions, in which hydrogen bonding and/or hydrophobic association, other than the usual electrostatic attraction, were found to play an important role.

Swelling behavior of radiation-polymerized polyampholytic two-component gels: Dynamic and equilibrium swelling kinetics

Polyampholytic hydrogels, with varying degrees of crosslinking and ionic content, were prepared by radiation polymerization of p-sodium styrene sulfonate (SSS) and vinyl benzyl trimethylammoniumchloride (VBT). These gels were investigated for their dynamic and equilibrium swelling kinetics. Dynamic swelling of these gels established that the gels containing equal amounts of SSS and VBT strictly follow Fickian diffusion. The hydrogels containing excess of SSS followed the case II type of diffusion, whereas those containing excess of VBT followed anomalous diffusion. Equilibrium swelling kinetics of these gels in aqueous system, ethanol–water mixture, at different pHs, and in the presence of solutions of biological interest was studied. It was seen that gels containing equal amounts of SSS and VBT show the lowest equilibrium swelling. Swelling of the polyampholytic gel decreased with an increase in the radiation dose imparted and the amount of crosslinking agent incorporated in the gel. The gels having an excess of VBT showed higher equilibrium swelling in comparison to those having an excess of SSS. Differential scanning calorimetry (DSC) studies showed that crosslinking of the gels decreases equilibrium swelling but increases the bonded nonfreezable water content of the gels. The organic solvents like ethanol cause abrupt collapse of the polyampholyte gels containing excess of SSS and those containing equal amounts of both the monomers at some critical ratio of water and ethanol in swelling medium. However, the deswelling in the water–ethanol mixture was gradual for gels containing an excess of VBT and the extent of deswelling was also low for these gels in comparison to other gels. The swelled gels of all compositions deswelled when they were transferred to solutions at pH in the range 2–12. Biologically important solutes like urea, glucose, and surfactants like Triton-X tend to further swell the polymer matrices, whereas NaCl causes their deswelling. The additive effect is more prominent for polyampholyte gels containing excess of either of the monomers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 730–742, 2003

Novel pH, ion sensitive polyampholyte gels based on carboxymethyl chitosan and gelatin

AbstractNatural pH and ion sensitive polyampholyte gels were successfully prepared by blending carboxymethyl chitosan (CM‐chitosan) and gelatin using glutaraldehyde as crosslinking agent. Their swelling behaviour under the influence of pH and ionic strength of the solution has been studied, and molecular interaction and, morphology of the gels were investigated by infrared spectroscopy and scanning electron microscopy. At the isoelectric point (IEP) the blend gels shrunk most, and when pH deviated from the IEP they behaved as polycations or polyanions. A decrease in swelling degree (Ds) with rising NaCl concentration up to 0.15 M was observed for all gels, while addition of Ca2+ made them shrink due to formation of Ca2+ crosslinked bridges. The blended gels showed increasing sensitivity to pH and ionic strength as the weight fraction of CM‐chitosan increased. Two essential components seem to regulate the swelling behaviour of blended gel: the first is related to osmotic pressure difference caused by the redistribution of mobile ions; the second is related to the possible formation of interactions corresponding to hydrogen bonding and hydrophobic interactions.© 2003 Society of Chemical Industry

Stimulus responsive drug release from polymer gel.: Controlled release of ionic drug from polyampholyte gel

2-(Dimethylamino)ethyl methacrylate or Methacryloyloxyethyltrimethylammonium chloride as a cationic monomer was copolymerized by UV with anionic monomer such as acrylic acid (AAc), into a polyampholyte. The pH responsive swelling behaviour and the pH and electro-responsive drug release functions of polyampholyte were investigated. The result showed that a copolymer of cation rich composition swelled at acidic condition, and shrank at alkaline condition. On the other hand, an anion rich copolymer showed a reverse phenomenon. Polyampholyte proved to interact with an ionic drug both by ionic binding and physical adsorption.

Simultaneous Multiple-Point Adsorption of Aluminum Ions and Charged Molecules by a Polyampholyte Thermosensitive Gel: Controlling Frustrations in a Heteropolymer Gel

We report the data on concurrent multiple point adsorption of two oppositely charged species of “target molecules” (aluminum ions and 1,3,6,8-pyrenetetrasulfonic sodium salt (Py-4)) by a polyampholyte gel of N-isopropylacrylamide (NIPA, 6 M) with methacrylic acid (MAA, 80 mM) and methacrylamidopropyl trimethylammonium chloride (MAPTAC, 80 mM). The goal of this study is to test the mutual frustrations created by adsorption of one species on the adsorption of the other. Understanding these frustrations is an important step toward elucidating the memory of conformations in heteropolymer systems. We found that in the absence of aluminum, the adsorption of Py-4 was suppressed by gel collapse, presumably because of an increase in MAA/MAPTAC ionic pairs that prevents the formation of potential adsorbing centers. Adding a moderate amount of aluminum significantly enhances adsorption of Py-4 by the collapsed gel, indicating that aluminum ions compete for bonds with MAA and help release vacant MAPTAC centers. Final...

A Biochemo-Mechanical System Consisting of Polyampholyte Gels with Coimmobilized Glucose Oxidase and Urease

An attempt was made to prepare a biochemo-mechanical system that is capable of converting biochemical energy created as a result of an enzyme reaction into mechanical work through the swelling and shrinking of a gel. The preparation was performed via the physical entrapment of glucose oxidase (GOD) plus urease within an amphoteric polymer network. We used N-isopropylacrylamide (NIPA) as a main constituent of the network into which acrylic acid (AAc) as an anionic monomer and 1-vinylimidazole (VI) as an cationic monomer were introduced via radical polymerization. The gelation was carried out in the presence and the absence of the enzyme(s) by cross-linking with N,N‘-methylenebisacrylamide of the monomer mixtures with molar ratios of NIPA/AAc/VI = 14:1:1 and 14:1:2. The effects of pH and temperature on the swelling degree were studied in 50 mM NaCl solution. The ampholyte gels exhibited an isoelectric point (pI) at pH 5−6 at which the swelling degree reached a minimum value. Therefore, two pH regions bringing about gel swelling lie on either side of pI, that is, pH < 5 and pH > 6. This characteristic allowed the gel to undergo a shrinking−swelling oscillation passing through pI while an increase and a decrease in pH took place via the immobilized enzyme reactions within the gel phase. When the hydrolysis of urea with the immobilized urease raises the pH in the gel, it continues shrinking at pH < pI but swelling at pH > pI after attainment of an extremely collapsed state at pI. Such volume changes take place during a decrease in the pH because of the oxidation of glucose with the immobilized GOD. As a result, the shrinking−swelling oscillation was observed by alternative addition of the two substrates into the outer medium surrounding the gel.

First Order Phase Transition and Evidence for Frustrations in Polyampholytic Gels

Polyampholyte gel, frustrated by the conflicting interactions between its positive and negative monomers, can undergo a sharp discontinuous phase transition driven by changing either gel composition or solvent quality and/or permeability. This finding completes the investigation of all four major interactions involved in the biomolecular machinery with respect to their role in the behavior of polymer gels, considered here as a model of biopolymer system. The result also sheds light on the possible role of frustrations in biomolecular systems, such as proteins.

Swelling, Shrinking, Deformation, and Oscillation of Polyampholyte Gels Based on Vinyl 2-Aminoethyl Ether and Sodium Acrylate

The behavior of amphoteric hydrogels based on vinyl 2-aminoethyl ether and sodium acrylate under the influence of pH, ionic and solvent composition, temperature, and dc electric field has been studied. The excess of positive or negative charges causes the swelling of polyampholyte networks. At the isoelectric point (IEP) the polyampholyte gel is in shrunken state due to the formation of intraionic contacts. An antipolyelectrolyte effect is observed at the IEP: the gel considerably swells in the presence of neutral salt. The condensation of bulky anions to positively charged groups of polyampholyte gels enhances the shrinking rate. The shrinking process can be described by apparent first-order kinetics. Polyampholyte gel shrinks with the increasing of temperature when the overall charge is neutral (IEP), while it shrinks effectively with addition of acetone when the overall charge is negative. In dependence of the network net charge, the ionic strength of the external solution, and the direction of dc electric field, the polyampholyte specimen can bend, shrink, swell, and oscillate. Under the same conditions, positively and negatively charged precursors of polyampholytes only swell or shrink. Shrinking and swelling of amphoteric gels are determined by the concentration of mobile ions inside and outside of gel. To interpret the oscillation phenomenon the Donnan equilibrium and water hydrolysis are utilized. The realization of antipolyelec-trolyte or polyampholyte and polyelectrolyte effects is probably the driving force of gel behavior. The oscillation-relaxation regimes are characterized by numerical value of 1/τ (where τ is the relaxation period). The phase portraits of all oscillation-relaxation curves are in good agreement with the Faraday law.

Swelling, Structure, and Elasticity of Polyampholyte Hydrogels

We studied the swelling and elastic behavior of chemically cross-linked [2-(methacryloyloxy)ethyltrimethylammonium chloride−2-(acrylamido)-2-ethylpropanesulfonate] polyampholyte gels in equilibrium with saline solutions. The net charge of the polymer network and the ionic strength of the swelling solvent are crucial parameters governing the properties of the gels. The swelling equilibrium behavior of gels with a strong charge imbalance was analogous to polyelectrolyte systems (monotonic decrease of swelling ratio with rising salt concentration). Globally neutral gels showed an antipolyelectrolyte behavior (collapse at low ionic strength and monotonic increase of swelling ratio with rising salt concentration). Weakly unbalanced polyampholyte gels showed a characteristic minimum in their swelling curves, the same gel reaching the same equilibrium volume in two different ionic conditions. The internal structure was imaged using differential interference contrast microscopy and revealed that morphological mod...

Multiple conformations in polyampholytes

We study the configurational statistics of a ring polyampholyte chain made of N randomly charged monomers with elementary charge .To a large extent, the overall structure of a polyampholyte is controlled by a total sum Q of all charges. When the total charge is smaller than , the polyampholyte has a compact globular structure. At charges larger than , the configuration has the form of a ring of small globules (beads) connected by strings. Between Q1 and Q2 we find a remarkable diversity of meta-stable configurations having the shapes of irregular clusters of small globules connected by the strings. We estimate the number of these configurations and the energy barriers between them. Between Q1 and Q2, the minimum energy configurations are completely controlled by randomness in the charge distribution along the chain. There are hysteresis effects in the shapes of the clusters. As the total charge increases, the linearly extended configurations become dynamically more preferable. When the charge decreases, the circular shapes are preferred. We remark on the probable connection with the multiple phase transitions observed in polyampholyte gels.

Polyampholyte gels: swelling, collapse and interaction with ionic surfactants

Abstract Two series of polyampholyte gels formed from sodium methacrylate and diallyldimethylammonium chloride with variable composition were synthesized in water and in aqueous salt solution. It is shown that the swelling properties of polyampholyte gels are directly related to their chemical structure, which is defined by the process of gel synthesis. The swelling ratio of the polyampholyte gels prepared in salt solution is large compared with the gels prepared in pure water i.e. the polyampholyte gels with balanced stoichiometry show minimal swelling. The interaction of the polyampholyte gels with ionic surfactants (cationic, cetylpyridinium chloride and anionic, sodium dodecylbenzenesulphonate) was studied. It was shown that for polyampholyte gels with an excess of the charges of one sign the addition of oppositely charged surfactant leads to the collapse of the gel. It was found that the efficiency of surfactants absorption is determined by the ratio of positive and negative charges in the chains of polyampholyte gels.

Synthesis and characterization of polyampholyte hydrogels

AbstractSeveral types of water‐swelling polyampholyte gels have been synthesized by gamma‐irradiation copolymerization and the behaviour of crosslinked amphoteric macromolecules was studied in dependence of pH, ionic strength and DC electric field. It has been shown that both polyampholyte gels and interpenetrating polyelectrolyte networks swell near of the isoelectric points. Under the action of DC electric field polyampholyte gels are able to produce an eel‐like motion.

Enzyme‐synthetic polyampholyte systems

AbstractPolyampholytes, prepared by copolymerization of 2‐(N,N‐dimethylamino)ethyl methacrylate and methacrylic acid, increase the urease activity over a large pH range, when the acid content is between 24 and 68 mol‐%, while the activity of the enzyme is inhibited by polyampholytes having more than 84 mol‐% of methacrylic acid groups. The enzyme reaction induces conformational changes of the polyampholyte chain resulting from a pH increase due to the formation of ammonium carbonate. The contraction of the macromolecule was followed by viscometry and by the volume change of a cross‐linked analogous polyampholyte gel.