Polyampholyte Gels Research Articles

Linear Viscoelasticity and Swelling of Polyelectrolyte Complex Coacervates

Mixing oppositely charged hydrophilic polyelectrolytes is the simplest path to constructing a polyampholyte gel that is useful as a soft tissue scaffold for binding enzymes in their native state. The swelling and viscoelastic properties of such a synthetic polyampholyte gel coacervate, constructed from polyions of different charge density, are reported in water with various amounts of NaCl salt. When constructed, this coacervate is roughly 70% water and 15% of each polyion, nearly charge balanced. If salt is removed from the surrounding supernatant, the gel swells owing to the weak charge imbalance because small amounts of salt screen electrostatic repulsions. If instead more salt is added to this coacervate, the gel behaves as any polyampholyte gel, swelling as salt is added because the excess salt screens the electrostatic attractions and eventually this leads to redissolving the coacervate. The amount of salt needed to redissolve the coacervate increases with polyion molecular weight. To our surprise, ...

(Invited) Flexible and Self-Healing Aqueous Supercapacitors By Polyampholyte Gel Electrolytes with Biochar Electrodes and Their Unique Low Temperature Properties

Quenched polyampholytes provide a novel class of tough hydrogel that has self-healing ability, strong adhesion, and mechanical flexibility. Understanding the structure of polymer chains in the hydrogel and the phase behavior of water therein has broad impact on various applications, such as lubrication, adhesion, and electrical conductivity, as well as the hydrogel’s low temperature properties. In this paper, the structure of polymer chains of hydrogels made of a model charge-balanced polyampholyte, a random copolymer of poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride), was investigated by small- and wide-angle x-ray scattering (SAXS and WAXS). The SAXS results suggested a networked globule structure in the charge-balanced polyampholyte hydrogels prevented freezing of water in the hydrogel, while the evidence of non-frozen water at low temperatures, such as –45 °C was monitored by solid-state 2H NMR. Correspondingly, we observed high ionic conductivity at low temperatures using electrochemical impedance spectroscopy (EIS). Interestingly, multiple freezing-thawing cycles did not impact the phase behavior of water in the hydrogel. We also found evidence that the crosslinked network structure of the polyampholyte chains disrupts the crystalline growth of ice, resulting in ‘slush-like’ ice formation. Utilizing the scientific investigations, a flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a polyampholyte hydrogel electrolyte. The electrode material was a biochar (produced from the low-temperature pyrolysis of biological wastes) bound by self-assembled reduced graphene oxide. Our novel sample preparation technique to enable the self-reinforcing wrapping of reduced graphene oxide sheets is turned out to have universal efficacy. The procedures of this technique will be introduced in the presentation. At the room temperature, the fabricated supercapacitor showed high energy density of 30 Wh/kg with 90% capacitance retention after 5000 charge-discharge cycles at room temperature at a power density of 50 W/kg. At –30 °C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg, which is threefold increase compared to control sample that do not incorporate polyampholyte hydrogel electrolyte. We also harnessed the tunable optical property of the polyampholyte hydrogel to fabricate a smart window. Specifically, we modulated the overall hydrophilicity/phobicity of polyampholyte chains when synthesizing the random copolymer and adjusted the upper critical solution temperature (UCST) at high precision, thus achieved a fine-tuning of UCST between 15 and 65 °C. Finally, we developed a stretchable, high-contrast, optically tunable stretchable window which consists of the PA hydrogel and a printed stretchable electric heater by our own ink recipe. In summary, we performed fundamental studies on the phase behavior of polymer chains and water molecules in quenched polyampholyte hydrogels by using synchrotron SAXS/WAXS, solid-state NMR, EIS, and DSC. We utilized the understanding in energy storage and smart window applications, both of which are unconventional for the application of tough hydrogels. In addition, we utilized renewable resource of biochar to fabricate low-cost, high-performance electrode material.

Flexible and Self-Healing Aqueous Supercapacitors for Low Temperature Applications: Polyampholyte Gel Electrolytes with Biochar Electrodes

A flexible and self-healing supercapacitor with high energy density in low temperature operation was fabricated using a combination of biochar-based composite electrodes and a polyampholyte hydrogel electrolyte. Polyampholytes, a novel class of tough hydrogel, provide self-healing ability and mechanical flexibility, as well as low temperature operation for the aqueous electrolyte. Biochar is a carbon material produced from the low-temperature pyrolysis of biological wastes; the incorporation of reduced graphene oxide conferred mechanical integrity and electrical conductivity and hence the electrodes are called biochar-reduced-graphene-oxide (BC-RGO) electrodes. The fabricated supercapacitor showed high energy density of 30 Wh/kg with ~90% capacitance retention after 5000 charge–discharge cycles at room temperature at a power density of 50 W/kg. At −30 °C, the supercapacitor exhibited an energy density of 10.5 Wh/kg at a power density of 500 W/kg. The mechanism of the low-temperature performance excellence is likely to be associated with the concept of non-freezable water near the hydrophilic polymer chains, which can motivate future researches on the phase behaviour of water near polyampholyte chains. We conclude that the combination of the BC-RGO electrode and the polyampholyte hydrogel electrolyte is promising for supercapacitors for flexible electronics and for low temperature environments.

Generalization of the sacrificial bond principle for gel and elastomer toughening

Although conventional polymer gels are known as mechanically weak materials, their fracture toughness can be effectively improved by introducing weak and brittle bonds into soft and stretchy polymer networks. This toughening method, denoted as the ‘sacrificial bond principle’, has been recently proposed. In this focus review, I describe some extremely tough gels prepared using this principle, e.g., double- or multiple-network gels with high water content featuring covalent sacrificial bonds, self-healing polyampholyte gels containing ionic sacrificial bonds and PDGI/PAAm gels based on hydrophobic sacrificial bonds exhibiting stress-responsive structural colors.

The effects of pH and ionic strength on equilibrium swelling of polyampholyte gels

A model is developed for the elastic response of a charge-balanced polyampholyte gel subjected to swelling in an aqueous solution of a monovalent salt with arbitrary pH. A gel is treated as a three-phase continuum composed of a solid phase (polymer network formed by chains with acidic and basic functional groups), solvent (water), and solute (mobile ions). Transport of solvent and solute is thought of as their diffusion through the network accelerated by an electric field formed by mobile ions and bound charges and accompanied by chemical reactions (self-ionization of water, ionization of functional groups, formation of ion pairs between bound charges and mobile counter-ions, and formation of physical cross-links between fixed ions with opposite charges). Constitutive equations are derived by means of the free energy imbalance inequality for three-dimensional deformation with finite strains. The governing equations are applied to study equilibrium water uptake by polyampholyte and polyelectrolyte gels. Numerical analysis demonstrates that the model reproduces all characteristic features of swelling diagrams qualitatively. The ability of the model to describe swelling curves quantitatively is confirmed by comparison of results of simulation with experimental data on three polyampholyte gels.

Polyelectrolyte and antipolyelectrolyte effects in swelling of polyampholyte and polyzwitterionic charge balanced and charge offset hydrogels

The swelling properties of polyampholytic hydrogels with different charge stoichiometric ratio were characterized by continuous monitoring changes in optical length of 60μm sized hydrogels with 2nm resolution using an interferometric readout platform. The hydrogels were synthesized by copolymerizing acrylamide, crosslinker Bis and charged monomers. Anionic–cationic polyampholyte gels consisted of different molar ratio between anionic monomer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) and cationic monomer (3-acrylamidopropyl) trimethylammonium chloride (APTAC). The polyzwitterionic hydrogels consisted of zwitterionic monomer [2-(methacryloyloxy)ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBA) with a certain molar of AMPSA or APTAC in addition to the AAM. The observed swelling properties versus the ionic strength in the aqueous immersion solution revealed apparent anti-electrolyte signatures for both types of polyampholytic hydrogels. The swelling kinetics was not affected by charge imbalance within the polyampholyte or polyzwitterionic hydrogels. Polyampholyte hydrogels with balanced charge ratio slightly shrinked and then swelled with the increase of ionic strength, which is at variance with the anionic AMPSA offset hydrogels displaying shrinking in low ionic strength and relatively small swelling at high ionic strength. The swelling properties of imbalanced polyampholyte hydrogels were not equal to simply linear overlap of those of polyampholyte hydrogels with balanced charges, and corresponding polyelectrolyte hydrogels.

Swelling Behavior of Physically Cross-Linked Gelatin Gels in Varied Salt Solutions

A series of physically cross-linked gelatin networks were prepared and the effects of salt concentration and chemical valence of the salt ions on the swelling properties of the gelatin gels were studied in detail. It was found that the swelling behaviors as polyelectrolytic or polyampholytic gels depended on the charge distribution on the gelatin chains. The swelling capacity of polyelectrolytic gels with excess positive charges decreased with an increase in salt concentrations. However, the swelling capacity of polyelectrolytic gels with excess negative charges showed a nonmonotonic change with salt concentration. For polyampholytic gels, the equilibrium swelling ratio increased monotonously with the concentration of NaCl and CaCl2 solutions. On the other hand, the swelling capacities of the polymer networks increased first and were then followed by a decrease with an increase in the concentration of AlCl3 solution. Moreover, the swelling kinetics of the gelatin gels in varied salt solutions with different concentrations was also investigated.

The pH-sensitive polyampholyte nanogels: Inclusion of carbon nanotubes for improved drug loading

We report a simple and facile method to prepare a novel pH sensitive polyampholyte nanogel by copolymerizing vinylimidazole (VIM) with acrylic acid (AA) using functionalized single-walled carbon nanotubes (f-SWCNTs) (as reinforcing material) and cyanuric chloride via an intermolecular quaternization reaction. The polyampholyte nanogels have been characterized by various microscopic and spectroscopic methods. These studies reveal that incorporation of nanotubes in cross-linked copolymer of poly(vinylimidazole-co-acrylic acid) (PVI-co-AA) form polyampholyte nanogel with enhanced physical properties. The thermal experiments show that the introduction of f-SWCNTs into PVI-co-AA has significant impact on the thermal stability of nanogels. The rheological study showed that the nanogel is more viscoelastic than native gel. MTT assay indicates that the prepared polyampholyte gels possess biocompatibility and cell viability. The nanogel is also useful material to load water-soluble drug such as promethazine hydrochloride. The releasing profile of the drug from the nanogel clearly shows the pH-sensitive property of the material.

Study on novel exfoliated polyampholyte nanocomposite hydrogels based on acrylic monomers and Mg–Al–Cl layered double hydroxide: Synthesis and characterization

Novel exfoliated polyampholyte nanocomposite hydrogels (NH-LDHs) based on acrylic acid (AA), 2-(diethylamino) ethyl methacrylate (2-DEAEMA), N,N-methylenebisacrylamide (NMBA) and layered double hydroxide (Mg–Al–Cl LDH) were synthesized using an in situ free radical addition polymerization method in aqueous media. First, Mg–Al–Cl LDH with a highly crystalline structure was prepared and characterized using FTIR, XRD, TGA and SEM analyses. The formation of NH-LDHs was confirmed by FTIR and XRD analyses. The morphological structure and select properties, such as the thermal, swelling and mechanical properties, of the NH-LDHs were also investigated. NH-LDHs have an exfoliated structure and smooth and nonporous surface morphologies. They can also be used as thermally stable nanocomposite hydrogels. The results revealed that adding a small amount of exfoliated Mg–Al–Cl LDH could effectively increase the compressive modulus of the polyampholyte gels. When the Mg–Al–Cl LDH was incorporated into the polyampholyte hydrogel matrix, two synergic effects, additional co-crosslinking and additional hydrophilicity, occur in the swelling behaviors of the gels. All NH-LDHs are sensitive to pH, and they have zwitterion characteristics at pH 5.5. It was concluded that the NH-LDHs synthesized in this study might be used as alternative water absorbents in solutions with different pH. The diffusion of water into the NH-LDHs gel systems in deionized water at room temperature is assumed to be a non-Fickian behavior.

J044024 高粘弾性ハイドロゲルの高靱性・自己修復機能([J044-02]ソフトマター・イノベーション(2))

Biological soft tissues, such as tendons, ligaments, muscles, cartilages, usually contain 50-85% of water and exist in a gel-like state, exhibit high toughness and excellent mechanical properties. It has been extensively discussed in the literature that load-bearing biological materials such as bone, teeth and nacre have acquired some interesting mechanical properties, one of which is the tolerance of crack-like flaws acquired during tissue function, growth, repair and remodeling. A structure is said to be flaw tolerant if pre-existing flaws do not propagate until it ultimately fails through a uniform rupture near the theoretical strength of material. Numerous studies in the literature have addressed flaw tolerance in elastic structures, so far there has been little investigation on time-dependent, viscoelastic systems, in spite of its importance to biological materials. A central hypothesis adopted in these studies is that the load-bearing biological materials have been evolved to tolerate crack-like flaws at multiple size scales. The optimal state of a material which induces the maximum strength corresponds to a uniform distribution of stress at failure even in the presence of crack-like flaws. In this state, the material fails by uniform rupture, rather than by crack propagation. Here, We report on the roles of ionic association in polyampholyte gel, serves as reversible sacrificial bond that break and reform dynamically giving some extraordinary properties to this novel viscoelastic hydrogel in contrast to conventional elastic hydrogels, for instances, high toughness and resistance against crack propagation by forming huge blunting at the crack tip.

A phase diagram of neutral polyampholyte - from solution to tough hydrogel.

Our recent study has revealed that neutral polyampholytes form tough physical hydrogels above a critical concentration Cm,c by forming ionic bonds of wide strength distribution. In this work, we systematically investigate the behavior of a polyampholyte system, poly(NaSS-co-DMAEA-Q), randomly copolymerized from oppositely charged monomers, sodium p-styrenesulfonate (NaSS) and acryloyloxethyltrimethylammonium chloride (DMAEA-Q) without and with a slight chemical cross-linking. A phase diagram of formulation has been constructed in the space of monomer concentration Cm and cross-linker density CMBAA. Three phases are observed for the as-synthesized samples: homogeneous solution at dilute Cm, phase separation at semi-dilute Cm, and homogenous gel at concentrated Cm. Above a critical Cm,c, the polyampholyte forms a supramolecular hydrogel with high toughness by dialysis of the mobile counter-ions, which substantially stabilizes both the intra- and inter chain ionic bonds. The presence of the chemical cross-linker (CMBAA > 0) brings about a shift of the tough gel phase to lower Cm,c. The tough polyampholyte gel, containing ∼50 wt% water, is highly stretchable and tough, exhibits fracture stress of σb∼ 0.4 MPa, fracture strain of εb∼ 30, and the work of extension at fracture Wext∼ 4 MJ m-3. These values are at the level of most tough soft materials. Owing to the reversible ion bonds, the poly(NaSS-co-DMAEA-Q) gels also exhibit complete self-recovery (100%) and high fatigue resistance upon repeated large deformation.

Polyampholyte gels of a cross-linked polyanion or polycation network into which an oppositely charged polyion was immobilized

Three polyampholyte gels (G1 to G3) composed of acrylic acid (AA), 1-vinylimidazole (VI) and N-isopropylacrylamide (NIPA) were prepared: G1 with a pre-gel solution (pH ≈ 12) containing NIPA (700 mM), AA (150 mM), VI (150 mM) and N,N′-methylenebis(acrylamide) (10 mM); G2 with the same pre-gel, except for containing poly(acrylic acid) (PAA) (150 unit mM) instead of AA; and G3 with the same pre-gel, except for containing poly(1-vinylimidazole) (PVI) (150 unit mM) instead of VI. The immobilization of PAA ( $$ {\overline M_{\text{w}}} = {2}0{5},000 $$ ) and PVI ( $$ {\overline M_{\text{w}}} = 35,900 $$ ) resulted in transparent G2 and G3, respectively, while G3 with another PVI ( $$ {\overline M_{\text{w}}} = {193},000 $$ ) was opaque. Potentiometric titrations and swelling measurements of an anionic and a cationic copolymer gel (as a control sample) suggested that each polyampholyte gel exhibits an isoelectric point (pI) at pH ≈ 5.6, at which the electrostatic attraction between the carboxylate and imidazole ions leads to gel collapse. Indeed, all the ampholyte gels underwent a collapse transition at pH around the pI upon cyclic pH changes (first increased and then decreased). The pH region where gel collapse was observed, however, was broader for G2 and G3 (with immobilized polyions) than for G1 (random terpolymer gel). The swelling/deswelling characteristics of transparent and opaque G3 gels were different from each other, and also from those of G1 and G2. These results were discussed in terms of the role of hydrogen bonding of the amide with the carboxyl or imidazole groups in the collapse transition of the ampholyte gels.

Preparation of dual temperature/pH-sensitive polyampholyte gels and investigation of their protein adsorption behaviors

Dual temperature/pH-sensitive polyampholyte gels were developed for use as adsorbents and carriers that deal with organic electrolytes such as proteins. NIPA-co-DMAPAA-co-AA gels were prepared by copolymerizing N-isopropylacrylamide (NIPA) as a thermosensitive component with N,N-dimethylaminopropylacrylamide (DMAPAA) as a weakly-basic cation-comonomer, acrylic acid (AA) as a weakly-acidic anion-comonomer, and N,N′-methylenebisacrylamide (MBAA) as a cross-linker, and their thermosensitive and pH-sensitive swelling properties and protein adsorption properties were investigated. The gel adsorbed bovine serum albumin (BSA) applied as a model protein at pH values in the range of 4.5–10 and 25°C as a result of the electrostatic interactions. In addition, the pre-adsorbed BSA was well desorbed from the gel at pH values less than 4 or greater than 11. The pH-sensitivity of the adsorption is found to be based on the protonation of amino groups in DMAPAA and dissociation of carboxyl groups of AA. The BSA adsorption was investigated as a function of MBAA concentration and temperature, and was discussed in terms of the difference in the sizes of the BSA molecule and gel network.

Investigation of temperature-swing adsorption of aromatic compounds in water using polyampholyte gels with hydrophilic nonionic groups

The adsorption and desorption behaviors of bisphenol-A in water have been studied using the temperature-responsive polyampholyte gels composed of sodium styrene sulfate (SSS), vinylbenzyl trimethylammonium chloride (VBTA), and N,N′-dimethylacrylamide (DMAA). The resultant behaviors of the SSS-VBTA-DMAA gels of three different monomer compositions are compared with those of the SSS-VBTA gel that can adsorb bisphenol-A at room temperature and desorb the pre-adsorbed bisphenol-A at higher temperature as in our previous report (Morisada et al., Adsorption 2008, 14, 621), to elucidate the effect of DMAA on the adsorption property of the polyampholyte gel. The adsorption isotherms, which were described by Henry's law, indicated that the adsorption amounts of bisphenol-A onto the SSS-VBTA-DMAA gels at 298 K are much larger than those at 353 K. At 298 K, the Henry's adsorption constant for the SSS-VBTA-DMAA gel with the lowest mole fraction of DMAA was larger than that for the SSS-VBTA gel, whereas a further increase in the DMAA mole fraction resulted in a decrease in the adsorption constant. Moreover, the temperature-swing adsorption experiments demonstrated that the SSS-VBTA-DMAA gel can repeatedly adsorb bisphenol-A at 298 K and desorb some of the pre-adsorbed bisphenol-A at 353 K, where the desorption amount of bisphenol-A is larger than that from the SSS-VBTA gel. The results obtained here show that the addition of DMAA to the SSS-VBTA gel slightly improves the performance of the gel as a temperature-responsive adsorbent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Temperature-swing adsorption of aromatic compounds in water using polyampholyte gel

The adsorption property of the polyampholyte gel composed of sodium styrene sulfate (SSS) and vinylbenzyl trimethylammonium chloride (VBTA) has been investigated with several hydrophobic aromatic compounds as adsorbate. Using the N-isopropylacrylamide (NIPA) gel, the corresponding experiments were also performed for comparison. At room temperature, the NIPA gel hardly adsorbed the aromatic compounds, while it adsorbed them at higher temperatures. As for the SSS-VBTA gel, the adsorption amounts of the polyaromatic compounds decreased with increasing temperature, while the adsorption amounts of the monoaromatic compounds were almost independent of temperature and smaller than those of the polyaromatic compounds. These results indicate that the aromatic rings in the SSS-VBTA gel may play an important role in the adsorption of the aromatic compounds. Also, it has been demonstrated that the SSS-VBTA gel can repeatedly adsorb bisphenol-A at room temperature and desorb it at higher temperature by the temperature-swing operation: this behavior is diametrically opposite to that of the NIPA gel. This shows that the SSS-VBTA gel is much more suitable for the adsorption removal of the hydrophobic aromatic compounds from very dilute aqueous solutions, because a vast amount of energy is required for heating a large amount of water when using the NIPA gel.

Swelling behavior of ionically cross-linked polyampholytic hydrogels in varied salt solutions

Ionically cross-linked polyampholytic hydrogels were synthesized by redox copolymerization of acrylamide and an ionic complex of (N,N-diethylamino)ethyl methacrylate and acrylic acid (designated as PADA hydrogel). The swelling behavior of the hydrogels in water indicated that a minimal equilibrium swelling ratio is found when the molar ratio of anionic/cationic monomers was 1.55. In NaCl solution, the hydrogels exhibited the typical swelling behavior of conventional polyampholytic gels. Their equilibrium swelling ratios increased with an increase in the NaCl concentration. In solutions of multivalent ions (CaCl2 and trisodium citrate solutions), the equilibrium swelling ratios of the hydrogels increased first and were then followed by a decrease with an increase in salt concentration. Interestingly, an unexpected abrupt swelling phenomenon was observed when the fully swollen hydrogels in salt solution were transmitted to pure water. The unique swelling behavior of PADA hydrogels depends not only on the molar ratio of the anionic/cationic monomers but also on the valency of the ions.

Phase Transition of Acrylamide‐Based Polyampholyte Gels in Water

The swelling behavior of acrylamide (AAm)–based polyampholyte hydrogels in water and in aqueous salt (NaCl) solutions was investigated. [(Methacrylamido)propyl]trimethyl‐ammonium chloride (MAPTAC) and acrylic acid (AAc) were used as the ionic comonomer in the hydrogel preparation. Three sets of hydrogels containing 70 mol% AAm and 30 mol% ionic comonomers of varying mole ratios were prepared. The variations of the hydrogel volume in response to changes in pH, and salt concentration were measured. As pH increases from 1, the hydrogel volume V eq in water first increases and reaches a maximum value at a certain pH. Then, it decreases again with a further increase in pH and attains a minimum value around the isoelectric point (IEP). After passing the collapsed plateau region, the gel reswells again up to pH=7.1. The reswelling of the collapsed gels containing 10 and 4% MAPTAC occurs as a first‐order phase transition at pH=5.85 and 4.35, respectively, while the hydrogel with 1% MAPTAC reswells continuously beyond its IEP. Depending on pH of the solution, the hydrogels immersed in salt solutions exhibit typical polyelectrolyte or antipolyelectrolye behavior. The experimental swelling data were compared with the predictions of the Flory‐Rehner theory of swelling equilibrium including the ideal Donnan equilibria. It was shown that the equilibrium swelling theory qualitatively predicts the experimental behavior of polyampholyte hydrogels.

Erosion Behavior of Ionically Crosslinked Polyampholyte Gels Under DC Electric Fields

AbstractSummary: An ionic complex of anionic and cationic monomers was obtained when (N,N‐diethylamino)ethylmethacrylate (DEAEM) was protonated with acrylic acid (AAc). Through the free radical copolymerization of the ionic complex and acrylamide (AAm) in aqueous solution, a novel ionically crosslinked polyampholyte gel (poly(AAc‐DEAEM‐AAm), designated PADA) was prepared without chemical crosslinker. An investigation of swelling behavior indicated that PADA gels exhibited peculiar erosion phenomena under contact and non‐contact DC electric fields. Under contact DC electric fields, PADA gels of swollen equilibrium experienced a dual response to DC fields, i.e., swelling first and then erosion. This phenomenon has not been described elsewhere. In contrast to with the contact DC electric fields, PADA gels did not undergo a swelling process, but eroded completely within 20 min under non‐contact DC fields. The NaCl concentration in electrolyte solutions greatly influenced the swelling/eroding behavior of PADA gels because the ionic crosslinkages of PADA gels could be weakened and screened by salt ions. The combined effects of higher NaCl concentration and local pH change near the electrode under DC electric fields resulted in crosslinkages quickly dissociating into cationic and anionic groups. In this case, PADA gels did not experience the swelling process, but directly eroded. Moreover, the erosion rate of PADA gels was related to the voltage of the electric field. The swelling/eroding rate for the gels in contact with the cathode was faster than that for the gels in contact with the anode. This difference can be attributed to varying local pH near different electrodes.The erosion process of PADA gels under non‐contact DC fields.magnified imageThe erosion process of PADA gels under non‐contact DC fields.

Preparation and pH‐sensitive swelling behavior of physically crosslinked polyampholyte gels

AbstractA novel polyampholyte hydrogels were prepared by free radical aqueous copolymerization of ionic complex consisting of acrylic acid (AAc), (N,N‐diethylamino) ethyl methacrylate (DEAEM), and acrylamide (designated as PADA). Without any chemical crosslinker, the formation of PADA network structures was mainly attributed to the electrostatic interaction of AAc (anionic monomer) and DEAEM (cationic monomer). The PADA gels exhibited a typical swelling behavior of polyampholyte gels in buffer solutions. The hysteresis of isoelectric point (pI) for PADA gels was observed after preswollen in alkaline solutions. The swelling behavior of PADA gels in response to change of pH was investigated. It was found that the equilibrium swelling ratio orderly decreased in buffer solutions with alternating pH between 2.2 and 5. An abrupt swelling was observed for the hydrogels at the pH near isoelectric point in the earlier stage of swelling process. The study of swelling kinetics of the hydrogels showed that the swelling process was in agreement with the second‐order swelling kinetics. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3857–3861, 2006

Preparation and Characterization of Maleic Acid Grafted Chitosan

Modification of chitosan (CHI) with maleic acid (MA) via maleic anhydride (MAn) was investigated in acetic acid solution, in the presence of cerium ammonium nitrate (CAN) as the initiator. The effects of reaction time, temperature, and CAN and MA concentrations on the grafting percentage were studied. The products were characterized by Fourier-transform infrared, differential scanning calorimetry, and 13C NMR, in addition to gravimetric evidence to grafting. A maximum of 105% grafting was achieved under the reaction conditions of 0.12 M CHI, 2.0 M MA, and 0.091 M CAN by stirring for 3 h at 70°C. Dissolution and swelling properties of grafted products were tested in the pH range 3–11. Products up to 36.5% grafting were found to be soluble in aqueous solution, while those with higher grafting percentages (48–81%) were partly soluble in the pH range studied. The product with maximum grafting percentage (105%) swelled and acted as a polyampholyte gel.