Temperature/pH dual responsive microgels of crosslinked poly(N‐vinylcaprolactam‐co‐undecenoic acid) as biocompatible materials for controlled release of doxorubicin

Undecenoic acid functionalized thermo/pH responsive microgels, poly(N‐vinylcaprolactam‐co‐undecenoic acid) [poly(VCL‐co‐UA)], were synthesized by precipitation emulsion copolymerization. The microgels exhibit reversible thermo/pH responsive phase transition behavior, which can be tuned by varying the monomer feed ratio. The lower critical solution temperatures (LCSTs) of the materials are close to body temperature. As a result, when temperatures rise above ca. 37°C, a rapid thermal gelation process occurs, accompanied by a phase transition, resulting in expulsion of encapsulated compound. In vitro experiment evaluated its applicability as a drug carrier for controlled release of an anticancer agent (doxorubicin) and showed that the drug encapsulation efficiency (EE), releasing rate, and kinetics are dependent on the temperature and pH value as expected. Minimal cytotoxicity of the microgels was observed by a cytotoxicity assay using 3T3 fibroblast cells. Our finding suggests that the poly(VCL‐co‐UA) based microgels may be considered a promising candidate for temperature or pH‐controlled delivery of anticancer drugs. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41146.     

Alternative procedure for the incorporation of quantum dots into poly(N‐isopropyl acrylamide‐co‐acrylic acid) microgels based on multiple interactions

Monodisperse fluorescent poly(N‐isopropyl acrylamide‐co‐acrylic acid) microgels doped with quantum dots (QDs) were fabricated as follows. First, cysteamine‐capped cadmium telluride (CA–CdTe) QDs were introduced into the microgels at pH 7 by electrostatic interactions. Afterward, the CA–CdTe QDs were further immobilized in the microgels by the collapse of the polymer network when the pH of solution was adjusted to 4. In this system, there existed multiple interactions between the CA–CdTe QDs and the microgels, including hydrogen bonds, electrostatic interactions, and coordination bonds. The photoluminescence intensity and maximum emission wavelength of the resulting microgels could be easily adjusted by changes in the content of the CA–CdTe QDs in the hybrid microgels (HMs) and with differently sized QDs, respectively. We found that the lower the addition of CA–CdTe QDs was, the bigger the blueshift of the photoluminescence spectra of the HMs was and the weaker the photoluminescence intensity was. Finally, temperature‐responsive emission of the HMs was examined. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43227.     

Synthesis and solution properties of single‐chain microgels from poly(N‐isopropylacrylamide) copolymer

“Single‐chain” microgels were synthesized successfully from the cross‐linkable poly(N‐isopropylacrylamide) (PNIPAM) copolymer. This type of microgel has the exact chemical structure, molecular weight and molecular weight distribution of its precursor. It provides a direct way to compare the properties of linear polymers with those of their networks. The viscosity properties show that the microgels have lower critical solution temperatures (LCST) that are even higher than those of the corresponding linear copolymers. This can be attributed to the crosslinking points, which retard the change of the conformation of the network chains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2179–2183, 2003     

Effects of end groups on the thermal response of poly(N‐isopropylacrylamide) microgels

Poly(N‐isopropylacrylamide) (PNIPAM) microgels were prepared through soap‐free emulsion polymerization using 2, 2′‐ azobisobutyronitrile and potassium persulfate as initiator respectively. The thermal response of microgels was researched by measuring the transmittance and the hydrodynamic diameter of the microgels at different temperatures. The result shows that the different structure of the end groups of polymer that come from residues of initiator result in the different thermal response of PNIPAM microgels. The LCST (lower critical solution temperature) of AIBN‐initiator microgels is 5°C lower than that of the KPS‐initiator microgels, whereas the AIBN‐initiated PNIPAM microgels have better thermal response sensitivity. The scanning electron microscope characterization shows that the morphology of AIBN‐initiated PNIPAM microgels is more regular than that of KPS‐initiated. Furthermore, the T_g of the microgels was measured by differential scanning calorimeter and the result indicates that the end groups influences the T_g of microgels severely. This work demonstrated that the hydrophobic end group coming from initiators can decreases the LCST of PNIPAM microgels and increases the thermal response sensitivity, which providing a newly simple but effective method to regulate the thermal response of PNIPAM microgels. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1164‐1171, 2013     

Synthesis and colloidal stability of poly(N‐isopropylacrylamide) microgels with different ionic groups on their surfaces

Temperature‐sensitive poly(N‐isopropylacry‐lamide) (PNIPAM) microgels with sulfate, amidino, or carboxylic groups on their surfaces, were synthesized by precipitation polymerization based on ammonium persulfate (APS), 2,2′‐azobis(amidinopropane) dihydrochloride (V50), or 4,4′‐azobis(4‐cyanovaleric acid) (ACVA) as initiator, respectively. Their particle sizes and swelling ratios depended on the reaction pH due to the pH dependence of the ionization degree of the decomposed fragments originating from the initiators and their hydrophilicity–hydrophobicity. The more hydrophobic initiators partitioned into the interior of the PNIPAM microgels under certain pH conditions initiated the crosslinking reaction between the PNIPAM chains, leading to higher crosslinking density of the microgels resulting in their smaller swelling ratio. pH dependence of surface charge density of the microgels with amidino groups or carboxylic acid groups on their surfaces was evidenced by the variation of their ζ‐potentials as a function of pH. Correspondingly, due to their pH dependence of electrostatic repulsive effect, the colloidal stability of the microgels with amidine groups or carboxylic acid groups on their surfaces was dependent on the pH value of dispersion medium. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3893–3898, 2007     

Synthesis of poly(N,N‐diethylacrylamide‐co‐acrylic acid) hydrogels with fast response rate in NaCl medium

A series of thermo‐ and pH‐sensitive poly (N,N‐diethylacrylamide‐co‐acrylic acid) (P(DEA‐co‐AA)) hydrogels were prepared in NaCl aqueous solutions with different concentrations. Swelling and deswelling studies showed that in comparison with conventional P(DEA‐co‐AA) hydrogels (prepared in distilled water), the P(DEA‐co‐AA) hydrogels thus prepared had almost the same volume phase transition temperature (VPTT), but exhibited much faster response rates as the temperature was raised above their VPTT. Besides, the hydrogels prepared by this method had faster response rates in low pH buffer solutions, and the response rates increased with the increased concentration of the NaCl solutions used during the polymerization. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Temperature‐induced volume change and glucose sensitivity of poly[(N‐isopropylacry‐ lamide)‐co‐acrylamide‐co‐(phenylboronic acid)] microgels

Nearly monodisperse glucose‐sensitive poly[(N‐isopropylacrylamide)‐co‐acrylamide‐co‐(phenylboronic acid)] microgels were synthesized in aqueous media by the functionalization of poly[(N‐isopropylacrylamide)‐co‐acrylamide‐co‐(acrylic acid)] microgels with 3‐aminophenylboronic acid via carbodiimide coupling. The glucose‐sensitive and thermosensitive behaviour of the microgels was investigated using a dynamic light scattering technique. The introduction of the hydrophobic phenylboronic acid (PBA) group significantly decreases the temperature at which maximum volume change of the resultant microgel particles is observed. The glucose sensitivity of the PBA‐containing microgels relies on the stabilization of the charged phenylborate ions by binding with glucose, which can convert more hydrophobic PBA groups to the hydrophilic phenylborate ions. The effect of pH, ionic strength and PBA content on temperature‐induced volume change and glucose sensitivity was systemically studied. The effect of NaCl on the glucose sensitivity was also investigated at physiological pH and ionic strength. Copyright © 2011 Society of Chemical Industry     

Fast responsive poly(acrylic acid‐co‐N‐isopropyl acrylamide) hydrogels based on new crosslinker

Novel dual temperature‐ and pH‐sensitive poly(acrylic acid‐co‐N‐isopropylacrylamide), AA/NIPAAm, hydrogels were successfully prepared by chemical crosslinking with crosslinkers. Copolymers of AA/NIPAAm were crosslinked in the presence of different mol % of N,N‐methylene bisacrylamide (MBA) and melamine triacrylamide (MAAm) as crosslinkers by bulk radical polymerization. The resultant xerogels were characterized by extracting the soluble fractions and measuring the equilibrium water content. Lower critical solution transition temperatures (LCST) were measured by DSC. The properties of crosslinked AA/NIPAAm series are evaluated in terms of compositional drift of polymerization, heterogeneous crosslinking, and chemical structure of the relevant components. Soluble fractions of the crosslinked networks were reduced by varying the MAAm and MBA concentrations. The influence of environmental conditions such as temperature and pH on the swelling behavior of these polymeric gels was investigated. The swelling behaviors of the resulting gels show pH sensitivity. The prepared MAAm type AA/NIPAAm hydrogels exhibited a more rapid deswelling rate than MBA type AA/NIPAAm hydrogels in ultra pure water in response to abrupt changes from 20°C to 50°C. The results of this study provide valuable information regarding the development of dual stimuli‐sensitive hydrogels with fast responsiveness. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and characterization of fast responsive thermo‐ and pH‐sensitive poly[(N,N‐diethylacrylamide)‐co‐(acrylic acid)] hydrogels

BACKGROUND: A considerable amount of research has been focused on smart hydrogels that can respond to external environmental stimuli, especially temperature and pH. In this study, fast responsive thermo‐ and pH‐sensitive poly[(N,N‐diethylacrylamide)‐co‐(acrylic acid)] hydrogels were prepared by free radical copolymerization in aqueous solution using poly(ethylene glycol) (PEG) as a pore‐forming agent. RESULTS: Swelling studies showed that the hydrogels produced had both temperature and pH sensitivity. The deswelling kinetics at high temperature demonstrated that the shrinking rates were influenced by the addition of the pore‐forming agent and the amount of acrylic acid in the initial total monomers. The deswelling curves in low‐buffer solutions had two stages. Pulsatile swelling studies indicated that the PEG‐modified hydrogels were superior to the normal ones. These different swelling properties were further confirmed by the results of scanning electron microscopy. CONCLUSION: Such fast responsive thermo‐ and pH‐sensitive hydrogels are expected to be useful in biomedical fields for stimuli‐responsive drug delivery systems. Copyright © 2008 Society of Chemical Industry     

Covalent immobilization of trypsin onto thermo‐sensitive poly(N‐isopropylacrylamide‐co‐acrylic acid) microspheres with high activity and stability

Poly(N‐isopropylacrylamide‐co‐acrylic acid) (P(NIPAM‐co‐AA)) microspheres with a high copolymerized AA content were fabricated using rapid membrane emulsification technique. The uniform size, good hydrophilicity, and thermo sensitivity of the microspheres were favorable for trypsin immobilization. Trypsin molecules were immobilized onto the microspheres surfaces by covalent attachment. The effects of various parameters such as immobilization pH value, enzyme concentration, concentration of buffer solution, and immobilization time on protein loading amount and enzyme activity were systematically investigated. Under the optimum conditions, the protein loading was 493 ± 20 mg g^?1 and the activity yield of immobilized trypsin was 155% ± 3%. The maximum activity (V_max) and Michaelis constant (K_m) of immobilized enzyme were found to be 0.74 μM s^?1 and 0.54 mM, respectively. The immobilized trypsin showed better thermal and storage stability than the free trypsin. The enzyme‐immobilized microspheres with high protein loading amount still can show a thermo reversible phase transition behavior. The research could provide a strategy to immobilize enzyme for application in proteomics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43343.     

Influence of ethyl methacrylate content on the volume‐phase transition of temperature‐sensitive poly[(N‐isopropylacrylamide)‐co‐(ethyl methacrylate)] microgels

A novel series of temperature‐sensitive poly[(N‐isopropylacrylamide)‐co‐(ethyl methacrylate)] (p(NIPAM‐co‐EMA)) microgels was prepared by the surfactant‐free radical polymerization of N‐isopropylacrylamide (NIPAM) with ethyl methacrylate (EMA). The shape, size dispersity and volume‐phase transition behavior of the microgels were investigated by transmission electron microscopy (TEM), ultraviolet–visible (UV–Vis) spectroscopy, dynamic light scattering (DLS) and differential scanning calorimetry (DSC). The transmission electron micrographs and DLS results showed that microgels with narrow distributions were prepared. It was shown from UV–Vis, DLS and DSC measurements that the volume‐phase transition temperature (VPTT) of the p(NIPAM‐co‐EMA) microgels decreased with increasing incorporation of EMA, but the temperature‐sensitivity was impaired when more EMA was incorporated, causing the volume‐phase transition of the microgels to become more continuous. It is noteworthy that incorporation of moderate amounts of EMA could not only lower the VPTT but also enhance the temperature‐sensitivity of the microgels. The reason for this phenomenon could be attributed to changes in the complicated interactions between the various molecules. Copyright © 2004 Society of Chemical Industry     

Poly[(N‐isopropylacrylamide)‐co‐(itaconic acid)] hydrogels with poly(ethylene glycol)

The aim of the work reported here was to investigate temperature‐ and pH‐sensitive hydrogels of N‐isopropylacrylamide (NIPAM) and itaconic acid (IA) and their semi‐interpenetrating polymer networks (semi‐IPNs) with varying contents of poly(ethylene glycol) (PEG). The stimuli responsiveness, swelling behaviour and mechanical properties of the hydrogels and semi‐IPNs were studied in order to investigate the effect of various amounts of PEG. Pulsed‐gradient spin‐echo NMR experiments were carried out to investigate the diffusion process. The pH sensitivity increased with an increasing amount of PEG in the semi‐IPNs, while the overall rate of water uptake was diffusion‐controlled (n < 0.5). For certain PEG contents (5 and 10 wt%), the semi‐IPNs exhibited better mechanical properties than the poly(NIPAM‐co‐IA) copolymer. The calculated values of the self‐diffusion coefficients of water indicated facilitated diffusion of water through the system with increased amounts of PEG, while the self‐diffusion coefficients of a model compound, metoprolol tartrate, showed no significant dependence on the amount of PEG. According to the results obtained and compared to results reported in the literature, the investigated semi‐IPNs may have potential applications in the controlled release of macromolecular active agents such as proteins and peptides. Copyright © 2009 Society of Chemical Industry     

Biocompatible and biodegradable alginate/poly(N‐isopropylacrylamide) hydrogels for sustained theophylline release

Mixed‐interpenetrated polymeric networks based on sodium alginate (ALG) and poly(N‐isopropylacryl amide) (PNIPAAm) covalently cross‐linked with N,N'‐methylenebisacrylamide are studied for their biocompatibility, nontoxicity, and biodegradability aiming their application in drug delivery. The presence of drug‐polymeric matrix interactions and the distribution of the drug in the polymeric network for theophylline‐loaded ALG/PNIPAAm hydrogels are also investigated by spectroscopic and microscopic methods. The quantitative evaluation of theophylline loaded hydrogels performed by NIR‐CI technique shows a better drug entrapment and a higher homogeneity of the samples with increased alginate content. The thermal behavior of the hydrogels is significantly modified by theophylline presence. The application of the ALG/PNIPAAm hydrogels as carriers for sustained drug release formulations was assessed by the theophylline release tests performed both by in vitro and in vivo studies. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40733.     

Adsorption of ammonium and nitrate ions by poly(N‐isopropylacrylamide) gel and poly(N‐isopropylacrylamide‐co‐chlorophyllin) gel in different states

The adsorption of ammonium and nitrate by temperature‐stimulus‐responsive poly(N‐isopropylacrylamide) (NIPA) gel and poly(N‐isopropylacrylamide‐co‐chlorophyllin) (NIPA‐CH) gel in different states was investigated. Both the NIPA gel and NIPA‐CH gel could adsorb ammonium and nitrate in a swollen state (swollen gel) and a swelling state (swelling gel), and they adsorbed ammonium more than nitrate. When the gels were shrinking (shrinking gel), they could adsorb a little ammonium from solution, but when the gels were in a shrunken state (shrunken gel), they hardly adsorbed ammonium. The adsorption of both ammonium and nitrate increased for the swelling NIPA gel in comparison with the swollen gel. The NIPA‐CH gel was the opposite in this respect. The difference in the amounts of adsorption of ammonium and nitrate by the swollen and swelling NIPA‐CH gels was more significant than that of the NIPA gels. It was suggested that ions such as ammonium and nitrate could not diffuse into the gels freely. The adsorption of ammonium and nitrate was affected not only by the phase transitions of the gels but also by the electrical charges. The experimental results for the adsorption of ammonium and nitrate during the volume changes of the gels imply that if the gels are applied to the immobilization of microorganisms, they may improve mass transfer between the immobilization matrix and bulk liquid under cyclic temperature changes and promote reactions of the immobilized microorganisms, especially the nitrification of nitrifying bacteria immobilized by the NIPA‐CH gel. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2367–2372, 2005     

Independent control of lower critical solution temperature and swelling behavior with pH for poly(N‐isopropylacrylamide‐co‐maleic acid)

Polymer solutions that gel in response to changes in temperature and pH are of interest for various forms of drug delivery, and it is desirable to increase swelling for diffusion‐controlled release without bringing the lower critical solution temperature (LCST) above 37°C. N‐isopropylacrylamide (NIP) was polymerized with maleic acid (MAc), a diprotic acid, and acrylic acid (AAc), a monoprotic acid, to compare swelling and temperature response with changes in pH. For samples with equal acid contents and almost identical LCST responses to pH, poly(N‐isopropylacrylamide‐co‐maleic acid) (pNIP MAc) demonstrated greater swelling than poly(N‐isopropylacrylamide‐co‐acrylic acid) (pNIP AAc). The LCST increase for MAc occurred at a pH corresponding to the deprotonation of almost all of the first acid groups. Further increases in pH led to the deprotonation of the second ? COOH and only served to increase the charge concentration at a given location. These results provide strong support for the theory that LCST results largely from uninterrupted chain lengths of NIP and that swelling results from the actual charge density of acid groups along the chain. Because the use of a diprotic acid copolymer allows swelling to be decoupled from LCST, pNIP MAc may be an appropriate candidate for pH‐sensitive drug‐delivery applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2110–2116, 2004     

Temperature‐responsive catalytic performance of Ag nanoparticles endowed by poly (N‐isopropylacrylamide‐co‐acrylic acid) microgels

In this study, we facilely introduce silver nanoparticles into Poly(N‐isopropylacrylamide‐co‐acrylic acid)(Poly(NIPAM‐co‐AA)) microgels and specially focus on the effect of hydrophilic acrylic acid segments on the responsive catalytic performance of silver nanoparticles. The obtained Poly(NIPAM‐co‐AA)/AgNPs composites are characterized by Fourier transform infrared spectra, X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy. The composites as catalysts are applied to the hydrogenation reaction of p‐nitrophenol and the related conditions such as reaction temperature, concentration of p‐nitrophenol, and the loadings of Ag nanoparticles are studied in detail. NIPAM segments of composites conveniently give silver nanoparticles a controllable characteristic for catalytic reaction by their conformation variation. AA segments of composites not only provide good stability and dispersibility for silver nanoparticles but also favor an easier diffusion of p‐nitrophenol to Ag NPs. POLYM. COMPOS., 38:708–718, 2017. © 2015 Society of Plastics Engineers     

Hollow,pH‐sensitive calcium–alginate/poly(acrylic acid) hydrogel beads as drug carriers for vancomycin release

In this study, hollow calcium–alginate/poly(acrylic acid) (PAA) hydrogel beads were prepared by UV polymerization for use as drug carriers. The hollow structure of the beads was fortified by the incorporation of PAA. The beads exhibited different swelling ratios when immersed in media at different pH values; this demonstrated that the prepared hydrogel beads were pH sensitive. A small amount (<9%) of vancomycin that had been incorporated into the beads was released in simulated gastric fluid, whereas a large amount (≤67%) was released in a sustained manner in simulated intestinal fluid. The observed drug‐release profiles demonstrated that the prepared hydrogel beads are ideal candidate carriers for vancomycin delivery into the gastrointestinal tract. Furthermore, the biological response of cells to these hydrogel beads indicated that they exhibited good biological safety and may have additional applications in tissue engineering. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

pH and thermo‐responsive poly(N‐isopropylacrylamide) copolymer grafted to poly(ethylene glycol)

pH and thermo‐responsive graft copolymers are reported where thermo‐responsive poly(N‐isopropylacrylamide) [poly(NIPAAm), poly A ], poly(N‐isopropylacrylamide‐co‐2‐(diethylamino) ethyl methacrylate) [poly(NIPAAm‐co‐DEA), poly B ], and poly(N‐isopropylacrylamide‐co‐methacrylic acid) [poly(NIPAAm‐co‐MAA), poly C ] have been installed to benzaldehyde grafted polyethylene glycol (PEG) back bone following introducing a pH responsive benzoic‐imine bond. All the prepared graft copolymers for PEG‐g‐poly(NIPAAm) [ P‐N1 ], PEG‐g‐poly(NIPAAm‐co‐DEA) [ P‐N2 ], and PEG‐g‐poly(NIPAAm‐co‐MAA) [ P‐N3 ] were characterized by ¹H‐NMR to assure the successful synthesis of the expected polymers. Molecular weight of all synthesized polymers was evaluated following gel permeation chromatography. The lower critical solution temperature of graft copolymers varied significantly when grafted to benzaldehyde containing PEG and after further functionalization of copolymer based poly(NIPAAm). The contact angle experiment showed the changes in hydrophilic/hydrophobic behavior when the polymers were exposed to different pH and temperature. Particle size measurement investigation by dynamic light scattering was performed to rectify thermo and pH responsiveness of all prepared polymers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Metal ion retention properties of poly(acrylic acid) and poly[N‐3‐(dimethylamino)propyl acrylamide‐co‐acrylic acid]

The crosslinked resins poly(acrylic acid) (PAA) and poly[N‐3‐(dimethylamino)propyl acrylamide‐co‐acrylic acid] [P(NDAPA‐co‐AA)] are obtained by radical polymerization and characterized by FTIR spectroscopy. PAA at basic pH exists basically as an acrylate anion that may contain end carboxylate groups or form bridges acting as mono‐ or bidentate ligands. P(NDAPA‐co‐AA) presents three potential ligand groups in its structure: carboxylic acid, amide, and amine. The trace metal ion retention properties of these two resins is compared by using the batch equilibrium procedure. The metal ions are contained in saline aqueous solutions and are found in natural seawater. The retention of Cu(II), Pb(II), Cd(II), and Ni(II) metal ions is studied under competitive and noncompetitive conditions. The effects on the pH, contact time, amount of adsorbent, temperature, and salinity are investigated. The PAA resin presents a high affinity (>80%) for Cu(II) and Cd(II) ions. The P(NDAPA‐co‐AA) resin shows a high affinity for Pb(II) and Cd(II) ions. With 4M HNO₃ it is possible to completely recover the PAA resin charged with Cu(II) ions and the P(NDAPA‐co‐AA) resin charged with Pb(II) ions. The two resins show a high affinity for Cd(II) ions from the seawater containing Cu(II) and Cd(II) ions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1385–1394, 2005     

Poly(methacrylamide‐co‐acrylic acid) hydrogels for gastrointestinal delivery of theophylline. I. Swelling characterization

pH sensitive copolymeric hydrogels have been synthesized by free‐radical polymerization of methacrylamide and acrylic acid in aqueous medium. The gels were characterized by FTIR spectroscopy, thermogravimetric analysis, and swelling measurements. To determine the suitability of theses hydrogels for gastrointestinal oral delivery of model drug theophylline, their swelling behavior was investigated as a function of pH and various structural parameters such as the average molecular weight between crosslinks, crosslink density, and mesh size were calculated. Likewise initial, average and late time diffusion coefficients were also evaluated in simulating intestinal fluid of pH 6.8 at 37°C. The gel underwent sharp volume phase transition in the vicinity of pH 5.8. The mesh sizes of the hydrogel were between 8.4 and 9.2 Å in the collapsed state (pH range 1–2; SGF) and between 514 and 524 Å in the swollen state (pH range 7–8; SIF). The experimental data was found to fit well to Beren‐Hopfenberg equation thus suggesting that later part of swelling was chain relaxation controlled. The activation energy, as determined from Arrhenius equation was found to be 13.71 kJ mol^?1. Likewise, enthalpy of mixing was also evaluated using Gibbs‐Helmholtz equation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2995–3008, 2006