Solvent Interaction Parameter Research Articles

Sorption and diffusion of normal alkanes through poly(ethylene‐co‐vinyl acetate) membranes

AbstractThe transport behavior of uncrosslinked and crosslinked poly(ethylene‐co‐vinyl acetate) membranes has been investigated using normal alkanes as probe molecules, in the temperature range of 30–60 °C. Benzoyl peroxide was used for crosslinking the matrix. It has been observed that, a critical concentration of crosslinker is necessary for maximum solvent uptake, followed by a decrease at higher concentration. The effect of free volume on liquid transport was investigated by positron annihilation lifetime spectroscopy. The mechanism of transport has been found to deviate from the regular Fickian behavior. The dependence of the transport coefficients on crosslink density, nature of penetrants, and temperature was studied. The polymer–solvent interaction parameter, enthalpy, and entropy of sorption have also been estimated from the transport data. The affine and phantom models for chemical crosslinks were used to predict the nature of crosslinks. Finally, the experimental sorption data were compared with theoretical predictions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2470–2480, 2007

Molecular transport characteristics of poly (ethylene‐co‐vinyl acetate) membranes in the presence of normal alkanes

AbstractSolvent transport into poly (ethylene‐co‐vinyl acetate) membranes exposed to n‐alkanes has been studied in the temperature interval of 30–60°C. Pure and cross‐linked membranes were prepared. Membranes with different loading of cross‐linking agent were also prepared. It was found that for all liquids, the equilibrium penetrant uptake was influenced by the introduction of cross links. The mechanism of transport has been found to deviate from the regular Fickian behaviour. Transport parameters such as diffusion coefficient, sorption constant and permeability coefficient have been calculated. The influence of temperature on transport was analysed. Transport parameters and activation parameters for the process of diffusion have been calculated. The transport coefficients and the activation parameters showed a dependence on cross‐link density. The Van't Hoff's relationship was used to compute the entropy change. The values of polymer–solvent interaction parameters have been used to calculate the molar mass between cross links of the network polymer. The phantom and affine models were used to analyse the deformation of the networks during swelling. A correlation between theoretical and experimental results was also done. Copyright © 2007 John Wiley & Sons, Ltd.

A study of some equation‐of‐state parameters of poly(methylhydrosiloxane‐co‐dimethylsiloxane) with some solvents by gas chromatography

AbstractTrace amount of methyl acetate, ethyl acetate, tert‐butyl acetate, pentane, hexane, and heptane were passed through the chromatographic column loaded with poly(methylhydrosiloxane‐co‐dimethylsiloxane) coated on Chromosorb W. The retention diagrams of the solvents on the copolymer were plotted by means of specific retention volumes at temperatures between 40 and 80°C by inverse gas chromatography technique. In this study, some thermodynamic interaction parameters such as Flory–Huggins polymer–solvent interaction parameter, equation‐of‐state polymer–solvent interaction parameter, effective exchange energy parameter, and weight fraction activity coefficients at infinite dilution of the solvent were determined. Then, the exchange enthalpy parameter and entropy parameter were determined by using a relation for the enthalpy interaction parameter of the equation‐of‐state theory, which is arranged for the inverse gas chromatography conditions. Later, the partial molar heat of sorption and the partial molar heat of mixing were obtained. The solubility parameter of this copolymer was determined as 6.64 (cal/cm3)1/2 at room temperature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1627–1631, 2007

Swelling behavior, mechanical properties and network parameters of pH- and temperature-sensitive hydrogels of poly((2-dimethyl amino) ethyl methacrylate- co-butyl methacrylate)

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 DMAEMA 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 ( ν e ∗ ) . It was found that ν e ∗ exceeds the theoretical crosslinking density ( ν 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 pH- and 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.

Thermodynamic study of functionally modified chitosan and its blends in aqueous media at 298.15 K

Linear polysaccharides, negatively charged N-methylene phosphonic chitosan (NMPC) and positively charged quaternized chitosan (QC) were synthesized. Thermodynamic study of the synthesized materials NMPC, QC and the mixture of equal concentrations of NMPC + QC was carried out in aqueous media at 298.15 K. Solute–solvent interactions have been studied through ultrasonic, viscometry and densitometric techniques. Isentropic compressibilities of the polymer solutions and hydration number of monomeric units in the functionalized chitosan and their blends or different systems were derived using experimental values of speed of sound and densities. Viscometric studies were used in estimating average molecular weight M v of polymers and their blends and solute–solvent interaction parameter χ for quantitative estimation of solute–solvent interactions.

Swelling characteristics of thermo‐ sensitive poly[(2‐diethylaminoethyl methacrylate)‐co‐(N,N‐dimethylacrylamide)] porous hydrogels

AbstractTemperature‐sensitive poly[(2‐diethylaminoethyl methacrylate)‐co‐(N,N‐dimethylacrylamide)] [P(DEAEMA‐co‐DMAAm)] hydrogels with five different DMAAm contents were synthesized with and without the addition of sodium carbonate as porosity generator. The synthesized hydrogels were characterized with dry gel density measurements, scanning electron microscopy observation and the determination of swelling ratio. The influence of the pore‐forming agent and content of DMAAm on swelling ratio and network parameters such as polymer–solvent interaction parameter (χ), average molecular mass between crosslinks (M̄c) and mesh size (ζ) of the cryogels are reported and discussed. The swelling and deswelling rates of the porous hydrogels are much faster than for the same type of hydrogels prepared via conventional methods. At a temperature below the volume phase transition temperature, the macroporous hydrogels also absorbed larger amounts water compared to that of conventional hydrogels and showed obviously higher equilibrated swelling ratios in aqueous medium. In particular, the unique macroporous structure provided numerous water channels for water diffusion in or out of the matrix and, therefore, an improved response rate to the external temperature changes during the deswelling and swelling processes. These properties are attributed to the macroporous and regularly arranged network of the porous hydrogels. Scanning electron micrographs reveal that the macroporous network structure of the hydrogels can be adjusted by applying porosity generation methods during the polymerization reaction. Copyright © 2007 Society of Chemical Industry

Reentrant phase transition and network parameters of hydrophobically modified poly[2-(diethylamino)ethylmethacrylate-co- N-vinyl-2-pyrrolidone/octadecyl acrylate] hydrogels

Hydrophobically modified poly[2-(diethylamino)ethylmethacrylate-co- N-vinyl-2-pyrrolidone/octadecyl acrylate] [P(DEAEMA-NVP/OA)] hydrogels with different OA content were prepared by free-radical crosslinking copolymerization of corresponding monomers in tert-butanol. The swelling equilibrium of the hydrogels was investigated as a function of temperature and hydrophobic comonomer content in pure water. An interesting feature of the swelling behavior of the P(DEAEMA-NVP/OA) hydrogels was the reentrant phase transition where the hydrogels collapse once and reswell as temperature increased. The average molecular mass between crosslinks ( M ¯ c ) and polymer–solvent interaction parameter ( χ) of the hydrogels were calculated from equilibrium swelling values. The enthalpy (Δ H) and entropy (Δ S) changes appearing in the χ parameter for the hydrogels were determined by using the Flory–Rehner theory. It was observed that the experimental swelling data of the hydrophobic hydrogels at different temperature agreed with the Flory–Rehner theory, that provided that the sensitive dependence of χ parameter on both temperature and polymer concentration is taken into account.

Studies on swelling behaviors, mechanical properties, network parameters and thermodynamic interaction of water sorption of 2-hydroxyethyl methacrylate/novolac epoxy vinyl ester resin copolymeric hydrogels

A series of the highly cross-linked hydrogels has been developed by the syntheses of bulk copolymers based on 2-hydroxyethyl methacrylate (HEMA) and novolac epoxy vinyl ester resin (NEVER). The resulting hydrogels were investigated on their equilibrium water content (EWC), swelling behaviors and tensile properties. Dynamic swelling behaviors of copolymeric hydrogels indicate that the swelling process of these polymers follows Fickian behavior. The EWC decreased and volume fraction of polymer in hydrogel ( ϕ 2) increased with NEVER content increasing due to its hydrophobicity. The increase of ionic strength of swelling medium or temperature results in a decrease in EWC and an increase in values of ϕ 2. Young’s modulus and tensile strength of hydrogels, as well as effective cross-link density ( v e), increased as NEVER content increased or ionic strength of swelling medium increased, attributing to increasing interaction between hydrophobic groups and polymer–polymer interaction with an increase in NEVER content or in ionic strength. The polymer–solvent interaction parameter χ reflecting thermodynamic interaction was also studied. As NEVER content, ionic strength of swelling medium or temperature increased, the values of χ increased. The values of χ and its two components χ H and χ S varied with increasing T. The negative values and trend of the enthalpy and entropy of dilution derived from values of χ H and χ S, could be explained on the basis of structuring of water through improved hydrogen bonding and hydrophobic interaction.

Temperature‐responsive characteristics of poly(N‐isopropylacrylamide) hydrogels with macroporous structure

AbstractMacroporous poly(N‐isopropylacrylamide) (PNIPA) hydrogels were synthesized by free‐radical crosslinking polymerization in aqueous solution from N‐isopropylacrylamide monomer and N,N‐methylenebis (acrylamide) crosslinker using poly(ethylene glycol) (PEG) with three different number‐average molecular weights of 300, 600 and 1000 g mol−1 as the pore‐forming agent. The influence of the molecular weight and amount of PEG pore‐forming agent on the swelling ratio and network parameters such as polymer–solvent interaction parameter (χ) and crosslinking density (νE) of the hydrogels is reported and discussed. Scanning electron micrographs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights and compositions of PEG during polymerization. At a temperature below the volume phase transition temperature, the macroporous hydrogels absorbed larger amounts of water compared to that of conventional PNIPA hydrogels, and showed higher equilibrated swelling ratios in aqueous medium. Particularly, the unique macroporous structure provides numerous water channels for water diffusion in or out of the matrix and, therefore, an improved response rate to external temperature changes during the swelling and deswelling process. These macroporous PNIPA hydrogels may be useful for potential applications in controlled release of macromolecular active agents. Copyright © 2006 Society of Chemical Industry

Correlation of swelling and crosslinking density with the composition of the reacting mixture employed in radical crosslinking copolymerization

AbstractThis article reports the scaling laws relating the synthesis conditions with the crosslinking density (νe) and swelling degree (S) of poly(N‐vinylimidazole) hydrogels (PVI) prepared by radical crosslinking copolymerization in aqueous solution, with N,N′‐methylene bisacrylamide (BA) as crosslinker. Multiple linear regression of νe versus BA concentration ([BA]) and total comonomers concentration (CT) in double log scale render the scaling law νe ∼ C × [BA]1.04 as comparable to that predicted by the model of polymer network with pendant vinyl groups (νe ∼ CT × [BA]), and showing inverse dependence on CT to that expected, following from stoichiometry, for an ideal network (νe ∼ 2[BA]/CT). S scales with νe through a solvent‐dependent exponent ranging from −0.46 to −0.54, only slightly over the value predicted by the Flory–Rehner theory (−0.6) or the blob's model by de Gennes (−0.5 to −0.8). Finally, the scaling law of S with the composition of the reacting mixture is also solvent‐dependent and it seems to result not only from the dependence of νe on CT and [BA] but also from that of v2r, the polymer volume fraction in the reference state, and χ, the polymer–solvent interaction parameter. Models used seem to overestimate the contribution of entanglements to the effective crosslinking density of PVI. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 263–269, 2007

Surface-textured PEG-based hydrogels with adjustable elasticity: Synthesis and characterization

Poly(ethylene glycol)–dimethacrylate (PEGDMA)-based hydrogels with adjustable shear modulus within the range of 10 kPa to 1 MPa and precisely predefinable surface textures on a micro-scale were made. It was observed that the volume of all hydrogels after preparation almost exactly matched the volume of the precursor solution and that there were only slight volume changes upon equilibration in excess solvent. This characteristic swelling behavior enables the preparation of textures on the hydrogel's surface with precisely predefinable dimensions. The behavior can be modeled with the Flory–Huggins theory assuming a concentration-dependent polymer–solvent interaction parameter. Additionally, activation of the hydrogels by electrophilic oxirane groups creates reactive sites that will enable the later grafting of the hydrogel's surface with various specific nucleophiles, e.g. biomolecules. Thus, these hydrogels are particularly suitable as biomaterials for systematic investigations of cellular response to surface topography and elasticity of the substrate, both in vivo and in vitro.

Sorption and diffusion of arenes through poly (ethylene-co-vinyl acetate) membranes

The sorption and diffusion of aromatic hydrocarbons through poly (ethylene-co-vinyl acetate) membranes has been investigated in the temperature interval of 25–70 °C using the sorption gravimetric analysis. The carbon–carbon crosslinks are introduced by using benzoyl peroxide (BP). Diffusion through membranes containing different loading of BP was carried out. For all liquids, the equilibrium solvent uptake was influenced by the penetrant size, temperature and crosslinking density. Thermodynamic constants are estimated from the sorption measurements. The values of polymer–solvent interaction parameters obtained from the diffusion experiments have been used to calculate the molecular mass between crosslinks of the network polymer. The transport mechanism was found to follow close Fickian behavior at 28 °C, but at high temperature, mechanism deviates to anomalous mode of transport.

Synthesis and network parameters of hydrophobic poly(N‐[3‐(dimethylaminopropyl)]methacrylamide‐co‐lauryl acrylate) hydrogels

AbstractHydrophobic poly(N‐[3‐(dimethylaminopropyl)]methacrylamide‐co‐lauryl acrylate) [P(DMAPMA‐co‐LA)] hydrogels with different LA content were synthesized by free‐radical crosslinking copolymerization of corresponding monomers in water by using N,N‐methylenebis(acrylamide) as the crosslinker, ammonium persulfate as the initiator, and N,N,N′,N′‐tetramethylethylenediamine as the activator. The swelling equilibrium of the hydrogels was investigated as a function of temperature and hydrophobic comonomer content in pure water. An interesting feature of the swelling behavior of the P(DMAPMA‐co‐LA) hydrogels with low LA content was the reshrinking phase transition where the hydrogels swell once and collapse as temperature was varied in the range of 30–40°C. The average molecular mass between crosslinks (M̄c) and polymer–solvent interaction parameter (χ) of the hydrogels were calculated from equilibrium swelling values. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter for the hydrogels were determined by using the Flory–Rehner theory based on the phantom network model of swelling equilibrium. The positive values for ΔH and ΔS indicated that the hydrogels had a positive temperature‐sensitive property in water, that is, swelling at a higher temperature and shrinking at a lower temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4159–4166, 2006

Studies on Equation-of-State Parameters and Linear Polymer Dimensions of Poly(p-chlorostyrene) in Diethylene Glycol Monobutyl Ether by Intrinsic Viscosity Measurements

Intrinsic viscosities, [η], of poly(p-chlorostyrene) (PPCS) in diethylene glycol monobutyl ether (DGMBE) which exhibits an exothermic solubility behavior with the polymer were measured using an Ubbelohde type capillary viscometer between 25 and 85°C. Polymer solvent interaction parameters at infinite dilution (χ1), exchange energy parameter \((\bar{X}_{12})\), exchange enthalpy (X12), and entropy parameters (Q12), of the PPCS/DGMBE pair were found at studied temperature range according to equation-of-state theory.

Self-Assembled Polystyrene-block-poly(ethylene oxide) Micelle Morphologies in Solution

We have investigated the self-assembly behavior of an amphiphilic diblock copolymer, polystyrene-block-poly(ethylene oxide) (PS-b-PEO), in N,N-dimethylformamide (DMF)/water and DMF/acetonitrile. In both cases water and acetonitrile are selective solvents for the PEO block. The degrees of polymerization of the PS and PEO blocks were 962 and 227 (PS962-b-PEO227), respectively. Micelle morphologies of the block copolymer in both systems could be controlled by varying copolymer and selective solvent concentrations. With increasing the water concentration in the DMF/water or the acetonitrile concentration in the DMF/acetonitrile system, the micelle morphology observed in transmission electron microscopy changed from spheres to wormlike cylinders and then to vesicles. The morphological diagrams were constructed from the study of the micelle morphology changes in different copolymer concentrations and the critical micellization concentrations for both systems at different copolymer concentrations as determined by static light scattering experiments. In between the concentration regions of two neighboring pure micelle morphologies, mixed morphologies such as spheres with short cylinders or wormlike cylinders with vesicles could be found. Although the trend in morphological changes was identical in these two systems, there were remarkable differences in the morphological diagrams of PS962-b-PEO227 with respect to the percentage of selective solvent added. This is due to the large difference between the polymer-selective solvent interaction parameters. On the basis of the observations of morphological reversibility and annealing experiments, these two morphological diagrams were proven to be in thermodynamic equilibrium. The driving force for these morphological changes was understood to approach micelle free energy minimization. Approximate micelle free energy calculations confirmed that the free energy decreases as the morphology changes from spheres to wormlike cylinders and then to vesicles with an increase in the selective solvent concentrations. Possible change mechanisms are also discussed.

Stress Generation by Solvent Absorption and Wrinkling of a Cross-Linked Coating atop a Viscous or Elastic Base

An in-plane constrained cross-linked gel layer absorbs an equilibrium amount of solvent and experiences in-plane compressive stress. A stability analysis of such an elastic gel layer that is attached to either a viscous or an elastic bottom layer atop a rigid substrate is considered. The effects of the top and bottom layer moduli (E(t) and E(b)), the bottom-to-top layer thickness ratio (H/h), and the polymer solvent interaction parameter (chi) on the critical condition of wrinkling, wrinkle wavelength, and amplitude are examined. When the bottom layer is viscous, the compressed top layer is always unstable, and wrinkling is rate-controlled. The viscous flow of the bottom layer governs the rate and determines the fastest growing wavelength. As E(t) rises, the bending stiffness of the elastic layer does as well, and so the fastest growing wavelength (lambda(m)) rises and the equilibrium amplitude (A(e)) falls. As H/h rises, the constraint of the rigid substrate diminishes, and so lambda(m) and A(e) rise. As chi falls or as the solvent has higher affinity for the polymeric gel, lambda(m) falls and A(e) rises because better solvents create higher compressive strain that promote low-wavelength, high-amplitude wrinkles. When the bottom layer is elastic, a critical compressive stress exists. If the generated compressive stress by solvent absorption is greater than the critical stress, the top layer wrinkles. It was found that wrinkling is most likely at intermediate E(t), low E(b), high H/h, and low chi. Further, lower chi, higher H/h, and lower E(b) were found to promote higher equilibrium amplitude and higher wavelength wrinkles.

Network parameters and volume phase transition behavior of poly(N‐isopropylacrylamide) hydrogels

AbstractThermosensitive hydrogels were prepared by free radical polymerization in aqueous solution from N‐isopropylacrylamide (NIPA) monomer and N,N‐methylenebis(acrylamide) (MBAAm) crosslinker. The swelling equilibrium of the hydrogels in deionized water was investigated as a function of temperature and MBAAm content. The results indicated that the swelling behavior and temperature sensitivity of the hydrogels were affected by the amount of MBAAm content. The average molecular mass between crosslinks and polymer–solvent interaction parameter (χ) of the hydrogels were determined from equilibrium swelling values. The swelling variations were explained according to swelling theory based on the hydrogel chemical structure. The swelling equilibrium of the hydrogels was also investigated as a function of temperature in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). In deionized water, the hydrogels showed a discontinuous volume phase transition at 32°C. In SDS and DTAB solutions, the equilibrium swelling ratio and the volume phase transition temperature (lower critical solution temperature) of the hydrogels increased, which is ascribed to the conversion of nonionic PNIPA hydrogel into polyelectrolyte hydrogels because of binding of surfactant molecules through the hydrophobic interaction. Additionally, the amount of free SDS and DTAB ions was measured at different temperatures by a conductometric method; it was found that the electric conductivity of the PNIPA–surfactant systems depended strongly on both the type and concentration of surfactant solutions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1756–1762, 2006

New gasoline and gas barrier from plasticized carbon black reinforced butyl rubber/high‐density polyethylene blends

AbstractThe effects of the high‐density polyethylene volume fraction on the curing characteristics and network structure of rubber blends have been studied in terms of the torque, scorch time, optimum curing time, Mooney viscosity, number of elastically effective chains, viscosity, interfacial tension, glass‐transition temperature, scanning electron microscopy, internal friction, sound velocity, acoustic attenuation, polymer–solvent interaction parameter, swelling index, and gel fraction. The applicability of the blends for gasoline barriers has been examined through the changes in the electrical resistance and volumetric swelling in gasoline versus time at room temperature. The transport mechanism of the solvent through the crosslinked butyl rubber/high‐density polyethylene blends is governed by Fickian diffusion law. The transport coefficients, namely, the diffusion coefficient, intrinsic diffusion, and permeation coefficient, have been computed. The experimental data for the permeation coefficient are in good agreement with the values calculated by Maxwell's model and far from those of Robeson's model. In addition, some thermodynamics parameters, namely, the standard entropy, standard enthalpy, and standard Gibbs free energy, have been estimated as functions of the high‐density polyethylene concentration of the butyl rubber blends. Furthermore, the applicability of butyl rubber/high‐density polyethylene composites for Freon gas barriers and antistatic charge dissipation has been examined. Finally, the mechanical properties, such as the tensile strength, hardness, stiffness, and elongation at break, of butyl rubber composites with different high‐density polyethylene concentrations have been evaluated. The increase in the mechanical properties is due to the increase in the crosslinking density and the interfacial adhesion of the blend. This proves that these new blends have important technological applications as gasoline and Freon barriers and for antistatic charge dissipation with good mechanical properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1237–1247, 2006

Effect of the amount and type of the crosslinker on the swelling behavior of temperature-sensitive poly(N-tert-butylacrylamide-co-acrylamide) hydrogels

Temperature-sensitive poly(N-tert-butylacrylamide-co-acrylamide) [P(NTBA-co-AAm)] hydrogels were synthesized by free-radical copolymerization in a water–methanol mixture using three types of crosslinkers: 1,2-ethyleneglycol dimethacrylate, N,N-methylenebisacrylamide, and 1,3-butandiol dimethacrylate. These thermosensitive hydrogels were swollen to equilibrium in water at 20°C and examined by gravimetric measurements. The influence of type and content of crosslinkers on the swelling ratio, the polymer–solvent interaction parameter (χ), the average molecular mass between crosslinks \({\left( {\overline{M} _{c} } \right)},\) and the effective crosslinking density (νE) of the hydrogels were reported and discussed. The swelling process in water was found to be non-Fickian diffusion. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter for the hydrogels were determined by using the Flory–Rehner theory based on the phantom network model of swelling equilibrium. Negative values for ΔH and ΔS indicated that the hydrogels had a negative temperature-sensitive property in water; that is, swelling at a lower temperature and shrinking at a higher temperature. The temperature-reversibility and on–off switching properties of the P(NTBA-co-AAm) hydrogels may be considered as good candidates for designing novel drug-delivery systems.

PH-dependent swelling behavior and network parametes of ionic poly(N-t-butylacrylamide-co-acrylamide) hydrogels

Ionic poly(N-t-butylacrylamide-co-acrylamide) [P(TBA-co-AAm)] hydrogels were synthesized by the free-radical crosslinking copolymerization of N-t-butylacrylamide and acrylamide monomers in fixed amounts and the maleic acid (MA) comonomer in methanol in different amounts with N,N-methylene bis(acrylamide) as the crosslinker, ammonium persulfate as the initiator, and N,N,N′,N′-tetramethylenediamine as the activator. The swelling behavior of these hydrogels was analyzed in buffer solutions at various pHs. The polymer–solvent interaction parameter (χ) and the average molecular weight between crosslinks of the ionic P(TBA-co-AAm) hydrogels were calculated from swelling studies in buffer solutions at various pHs and were related to the MA content. The results indicated that the swelling behavior of the ionic P(TBA-co-AAm) hydrogels at different pHs agreed with the modified Flory–Rehner equation based on the affine network model and the ideal Donnan theory. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter for the hydrogels were also determined with the modified Flory–Rehner equation. The negative values for ΔH and ΔS indicated that the hydrogels had a negative temperature-sensitive property in water, that is, swelling at a lower temperature and shrinking at a higher temperature. The experimental swelling data of the hydrogels at different temperatures agreed with the modified Flory–Rehner approach based on the affine network model, which ensured that the sensitive dependence of the χ parameter on both the temperature and polymer concentration was taken into account. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1624–1630, 2006