Modified chitosan III,superabsorbency, salt‐ and pH‐sensitivity of smart ampholytic hydrogels from chitosan‐g‐PAN

Polyacrylonitrile (PAN) grafted chitosan was prepared by ceric‐initiated graft polymerization of acrylonitrile onto chitosan in a homogenous medium. The copolymer chitosan‐g‐PAN product was then hydrolyzed to yield a novel smart hydrogel (H‐chitoPAN) with superabsorbing properties. The influence of add‐on values as well as temperature and time of hydrolysis of the initial chitosan‐g‐PAN on swelling behavior of the hydrogel was evaluated in water and various salt solutions. The swelling kinetics of the superabsorbing hydrogel was studied as well. The hydrogels exhibited ampholytic and pH‐sensitivity characteristics. Several sharp swelling changes were observed in lieu of pH variations in a wide range (pH 2–13). The swelling variations were explained according to swelling theory based on the hydrogel chemical structure. Superabsorbency, pH‐ and salt‐sensitivity of the chitosan‐based hydrogel was briefly compared with the classical starch‐based superabsorbent, H‐SPAN. The pH‐reversibility and on–off switching behavior of the intelligent H‐chitoPAN hydrogels makes them good candidates for considering as potential drug carries. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Synthesis,characterization, and swelling behaviors of chitosan‐g‐poly(acrylic acid)/poly(vinyl alcohol) semi‐IPN superabsorbent hydrogels

A series of granulated semi‐interpenetrating polymer network (semi‐IPN) superabsorbent hydrogels composed of chitosan‐g‐poly(acrylic acid) (CTS‐g‐PAA) and poly(vinyl alcohol) (PVA) were prepared by solution polymerization using ammonium persulfate (APS) as an initiator and N,N′‐methylenebisacrylamide (MBA) as a crosslinker. The effects of reaction conditions such as the concentration of MBA, the weight ratio of AA to CTS, and the content of PVA on water absorbency were investigated. Infrared (IR) spectra and differential scanning calorimetry (DSC) analyses confirmed that AA had been grafted onto CTS backbone, and PVA semi‐interpenetrating into CTS‐g‐PAA networks. SEM analyses indicated that CTS‐g‐PAA/PVA has improved porous surface and PVA was uniformly dispersed in CTS‐g‐PAA network. The semi‐IPN hydrogel containing 10 wt% PVA shows the highest water absorbency of 353 and 53 g g^?1 in distilled water and 0.9 wt% NaCl solution, respectively. Swelling behaviors revealed that the introduction of PVA could improve the swelling rate and enhance the pH stability of the superabsorbent hydrogel. Copyright © 2009 John Wiley & Sons, Ltd.     

pH‐responsive ionic poly(N,N‐diethylaminoethyl methacrylate‐co‐N‐vinyl‐2‐pyrrolidone) hydrogels: Synthesis and swelling properties

A series of an ionic hydrogels composed of N,N‐diethylaminoethyl methacrylamide (DEAEMA), N‐vinyl‐2‐pyrrolidone (VP), and itaconic acid were synthesized by free‐radical cross‐linking copolymerization in water–ethanol mixture by using N,N‐methylenebis(acrylamide) as the cross‐linker, ammonium persulfate as the initiator, and N,N,N′,N′‐tetramethylenediamine as the activator. The swelling behaviors of these hydrogels were analyzed in buffer solutions at various pH. It was observed that the swelling behavior of cross‐linked ionic poly(N,N‐diethylaminoethyl methacrylamide‐co‐N‐vinyl‐2‐pyrrolidone) [P(DEAEMA/VP)] hydrogels at different pH agreed with the modified Flory–Rehner equation based on the affine network model and the ideal Donnan theory. The swelling process in buffer solutions at various pH was found to be Fickian‐type diffusion. The pH‐reversibility and on–off switching properties of the P(DEAEMA/VP) hydrogels may be considered as good candidate to design novel drug‐delivery system. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2819–2828, 2005     

Preparation and characterization of poly(acrylic acid)‐iron rich smectite superabsorbent composites

A novel poly(acrylic acid)‐iron rich smectite (IRS) superabsorbent composite was synthesized by graft copolymerization reaction of acrylic acid (AA) in the presence of N,N‐methylenebisacrylamide (MBA) as a crosslinker. IRS was used to strengthen the hydrogel products in the polymerization process. Water absorbencies for these superabsorbent composites in water and saline solutions were investigated. IRS caused a reduced equilibrium swelling as low as 8–26%. However, grafted IRS particles resulted in improved gel strength as high as 66% compared to the IRS‐free sample. IRS modified superabsorbent hydrogel composites exhibited higher thermal stability compared to the IRS‐free sample. The pH dependent reversible swelling behavior of hydrogels was also investigated. It is found that the swelling process is pH dependent and reversible for synthesized superabsorbent. Superabsorbent hydrogel composites were characterized by thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). FTIR spectroscopy was confirmed grafting of acrylic chains onto the surface of IRS particles. From the standpoint of these results, these strengthened and thermostabilized hydrogels may be considered as good candidates for a controlled release study and agricultural applications. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis,characterization, and drug release behaviors of a novel thermo‐sensitive poly(N‐acryloylglycinates)

In this study, a novel thermo‐sensitive poly(N‐acryloylglycinates) was prepared in order to get a potential drug release carrier. The corresponding monomers and the polymers were characterized with Fourier‐transform infrared (FTIR) and ¹H NMR. The thermo‐sensitivity of the poly(N‐acryloylglycinates) was evaluated by measuring their lower critical solution temperatures (LCST) in water, inorganic salt solution, and different pH solutions. The results indicated that poly(N‐acryloylglycine methyl ester) (NAGME) and poly(N‐acryloylglycine ethyl ester) (NAGEE) exhibit a reversible thermo‐sensibility in their aqueous solutions at 61.5 and 12.5°C, respectively. However, no thermo‐sensitive behavior of poly(N‐acryloylglycine propyl ester) (NAGPE) was found due to its over hydrophobicity. The swelling studies on hydrogels were carried out at different temperatures, in different pH, and inorganic salt solutions. The hydrogels showed a remarkable phase transition at about 35°C with changing temperature. The release rate of caffeine from the thermo‐sensitive hydrogel was apparently decreased as the crosslinker content increased and temperature decreased. Seventy five percent caffeine from the polymeric hydrogel with 5% NMBA (N, N‐methylenebis(acrylamide)) was released at room temperature within 240 min, whereas 95.4% caffeine diffused into the medium at 37°C. Copyright © 2009 John Wiley & Sons, Ltd.     

A novel pH‐ and thermo‐sensitive PVP/CMC semi‐IPN hydrogel: Swelling,phase behavior,and drug release study

Poly(N‐vinyl‐pyrrolidone) (PVP) hydrogel has been considered as a very interesting and promising thermosensitive material. The most vital shortcoming of PVP hydrogel as thermosensitive material is that it does not exhibit thermosensitivity under usual conditions. In this work, semi‐interpenetrating polymer network (semi‐IPN) hydrogels based on PVP and carboxymethylcellulose (CMC) were prepared. The volume phase transition temperature (VPTT) of the hydrogels was determined by swelling behavior and differential scanning calorimetry (DSC). The results showed that the VPTT was significantly dependent on CMC content and the pH of the swelling medium. The amount of CMC in the semi‐IPN hydrogels was 0.050, 0.075, and 0.100 g, the VPTT in buffer solution of pH 1.2 was 29.9 °C, 27.5 °C and 24.5 °C, respectively. In addition, the VPTT occurred in buffer solution of pH 1.2, but did not appear in alkaline medium. Bovine serum albumin (BSA) as a model drug was loaded and the in vitro release studies were carried out in different buffer solutions and at different temperatures. The results of this study suggest that PVP/CMC semi‐IPN hydrogels could serve as potential candidates for protein drug delivery in the intestine. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1749–1756, 2010     

Synthesis and characterization of κ‐carrageenan/poly(N,N‐diethylacrylamide) semi‐interpenetrating polymer network hydrogels with rapid response to temperature

In this study, a novel classical thermo‐ and salt‐sensitive semi‐interpenetrating polymer network (semi‐IPN) hydrogel composed of poly(N,N‐diethylacrylamide) (PDEAm) and κ‐carrageenan (KC) was synthesized by free radical polymerization. The structure of the hydrogels was studied by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). FTIR and SEM revealed that the semi‐IPN hydrogels possessed the structure of H‐bonds and larger number of pores in the network. Compared to the PDEAm hydrogel, the prepared semi‐IPN hydrogels exhibited a much faster response rate to temperature changes and had larger equilibrium swelling ratios at temperatures below the lower critical solution temperature (LCST). The salt‐sensitive behavior of the semi‐IPN hydrogels was dependent on the content of KC. In addition, during the reswelling process, semi‐IPN hydrogels showed a non‐sigmoidal swelling pattern. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis,characterization and evaluation of semi‐IPN hydrogels consisted of poly(methacrylic acid) and guar gum for colon‐specific drug delivery

The semi‐IPN hydrogels consisting of poly(methacrylic acid) and guar gum (GG) are prepared at room temperature using water as solvent. 5‐aminosalicylic acid (5‐ASA) is entrapped in the hydrogel in the synthesis of hydrogel and all entrapment efficiencies are found above 85%. The hydrogel shows excellent pH‐sensitivity. It exhibited minimum swelling in an acidic pH medium through the formation of a complex hydrogen‐bonded structure and maximal swelling due to the electrostatic repulsion due to the ionization of the carboxylic groups in pH 7.4 medium. The degradation in vitro shows that the degree of degradation (R%) depended on the concentration of cross‐linking agent and content of GG. The hydrogel shows a minimum release of 5‐ASA due to the complex hydrogen bonded structure of the hydrogels in the medium of pH 2.2. The enzymatic degradation of hydrogels by cecal bacteria can accelerate the release of 5‐ASA entrapped in the hydrogel in pH 7.4 medium. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis,characterization, and evaluation of enzymatically degradable poly(N‐isopropylacrylamide‐co‐acrylic acid) hydrogels for colon‐specific drug delivery

The enzymatically degradable poly(N‐isopropylacrylamide‐co‐acrylic acid) hydrogels were prepared using 4,4^′‐bis(methacryloylamino)azobenzene (BMAAB) as the crosslinker. It was found that the incorporated N‐isopropylacrylamide (NIPAAm) monomer did not change the enzymatic degradation of hydrogel, but remarkably enhanced the loading of protein drug. The hydrogels exhibited a phase transition temperature between 4°C (refrigerator temperature) and 37°C (human body temperature). Bovine serum albumin (BSA) as a model drug was loaded into the hydrogels by soaking the gels in a pH 7.4 buffer solution at 4°C, where the hydrogel was in a swollen status. The high swelling of hydrogels at 4°C enhanced the loading of BSA (loading capability, ca. 144.5 mg BSA/g gel). The drug was released gradually in the pH 7.4 buffer solution at 37°C, where the hydrogel was in a shrunken state. In contrast, the enzymatic degradation of hydrogels resulted in complete release of BSA in pH 7.4 buffer solution containing the cecal suspension at 37°C (cumulative release: ca. 100 mg BSA/g gel after 4 days). Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrogels of sodium alginate based copolymers grafted with sodium-2-acrylamido-2-methyl-1-propane sulfonate and methacrylic acid for controlled drug delivery applications

Novel sodium alginate based hydrogels were prepared by grafting the binary mixture of methacrylic acid and sodium-2-acrylamido-2-methyl-1-propane sulfonate on to sodium alginate (SA) with N,N'-methylenebisacrylamide (MBA) and ammonium persulfate (APS) as crosslinker and initiator respectively. The resultant SA-g-poly(AMPSNa-co-MAA) hydrogels were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Thermo Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD). The results indicated that an increase in the amount of sodium-2-acrylamido-2-methyl-1-propane sulfonate, swelling capacity was found to increase. The effects of temperature, pH and inorganic salt on the swelling behavior of the hydrogels were investigated. The pH reversibility behavior at pH levels of 1.0 and 9.0 indicated great potential of this hydrogel as a candidate system for controlled drug delivery.     

P(AA-DAC)两性聚电解质水凝胶的合成及性质

以丙烯酸(AA)和丙烯酰氧乙基三甲基氯化铵(DAC)为单体, 采用水溶液聚合法制备了P(AA-DAC)聚电解质水凝胶. 采用红外光谱和核磁共振等方法对其结构进行了表征. 研究了不同组成比的聚电解质水凝胶在去离子水、不同pH值溶液以及不同离子强度盐溶液中的溶胀行为. 研究结果表明, 摩尔比为1∶1的聚电解质水凝胶表现出典型的两性聚电解质凝胶的溶胀行为. 离子强度对其溶胀行为有着显著影响, 在溶液离子强度较高时, 凝胶网络的溶胀主要受溶剂向凝胶内部扩散所控制, 满足Fick型扩散规律n≤0.5, 随着溶液离子强度的增加, 凝胶网络平衡含水量增加, 扩散系数增大.     

Synthesis of Poly(2‐oxazoline)‐Based Hydrogels with Tailor‐Made Swelling Degrees Capable of Stimuli‐Triggered Compound Release

A 32‐membered library of poly(2‐oxazoline)‐based hydrogels of the composition p EtOx _m‐p PhOx _n‐p PBO _q (m/n = 150/0, 100/50, 50/100, and 0/150; q = 1.5–30) was prepared from 2‐ethyl‐ ( EtOx ), 2‐phenyl‐2‐oxazoline ( PhOx ), and phenylene‐1,3‐bis‐(2‐oxazoline) ( PBO ). The polymerizations were performed from ground monomer mixtures at 140 °C in a single‐mode microwave reactor in reaction times as short as 1 h. Purified hydrogels, containing no residual monomers, were obtained in yields of 95% or higher. Acid‐mediated hydrolysis rates as well as swelling degrees of the hydrogels were adjustable over a broad range; swelling degrees in water/ethanol/dichloromethane ranged from 0 to 13.8/11.7/20.0. The hydrogels could incorporate organic molecules according to in situ or post‐synthetic routines. Post‐synthetic routines enabled for the preparation of hydrogels from which the incorporated compounds were only released through diffusion processes if the solvent was changed or through hydrogel degradation if the pH was lowered.     

Synthesis of poly(ethylene glycol)‐based hydrogels and their swelling/shrinking response to molecular recognition

To synthesize the novel molecular‐ and pH‐stimulus‐responsive hydrogel, we prepared poly(ethylene glycol)‐based hydrogel containing ionic groups. We evaluated the fundamental swelling/shrinking properties of the hydrogels synthesized by various conditions. Decreasing the molecular weight of a crosslinker provided the increasing of the equilibrium swelling ratio. Also, the equilibrium swelling ratio was changed by the introduction of functional ionic monomers and its compositions. Furthermore, the swelling/shrinking behaviors of the hydrogels were affected by the environmental condition of aqueous solution, in fact the hydrogels were considerably shrunk (to one‐fifth volume) using a di‐ionic solute in the aqueous solution through the ionic interactions between the hydrogel and the solutes. Additionally, the specific shrinking to diamine compounds was also observed in response to pH change. These results clearly show the swelling/shrinking responsibility of the hydrogels toward the molecular recognitions and its pH conditions. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3153–3158     

Thermal behavior and network structure of poly(N‐vinyl‐2‐pyrrolidone‐crotonic acid) hydrogels prepared by radiation‐induced polymerization

Poly(N‐vinyl‐2‐pyrrolidone‐crotonic acid) [P(VP/CrA)] hydrogels were prepared by irradiating the ternary mixture of VP/CrA and crosslinking agent ethylene glycol dimethacrylate (EGDMA) in water by γ rays at ambient temperature. Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal properties of ionized networks and to establish if they showed thermal differences that could be related to the CrA content in the gel system. The volume swelling ratio of P(VP/CrA) hydrogels were investigated as a function of the pH in the immersing solution. The volume swelling ratio of these hydrogels increased with an increase in pH and a decrease CrA content in the hydrogel. The volume swelling ratio of the hydrogels was also evaluated using an equation, based on the Flory—Huggins thermodynamic theory, the phantom network theory of James–Guth and Donnan theory of swelling of weakly charged ionic gels for determination of the molecular weight between crosslinks and the polymer–solvent interaction parameter (χ). Copyright © 2004 John Wiley & Sons, Ltd.     

pH and temperature responsive hydrogels of poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-methacrylic acid): Synthesis and swelling characteristics

Temperature and pH sensitive 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and methacrylic acid (MAA) homopolymer and copolymer hydrogels have been prepared using N,N′-methylenebisacrylamide (MBA) and ethylene glycol dimethacrylate (EGDMA) as crosslinkers. Swelling of these hydrogels has been studied in terms of monomer ratio, type and concentration of the crosslinkers and in various concentrations of mono, di and trivalent salt solutions. Though swelling of the EGDMA crosslinked poly(AMPS-co-MAA) hydrogels is found to be higher than those based on MBA crosslinker, strength of the latter system has been found to be better. Swelling behavior of these hydrogels in different salt solutions at different concentrations has shown a drop in swelling from monovalent to trivalent cations and also at higher salt concentrations. The results have indicated the possibilities of developing tailor made hydrogels combining optimum swelling with better strength characteristics that will suit different physiological and biological environments.     

Collagen-based highly porous hydrogel without any porogen: Synthesis and characteristics

In this contribution we have developed a collagen-based highly porous hydrogel by neutralizing the grafted poly(acrylamide-co-acrylic acid) after gel formation. Preparation of the hydrogels involved free radical polymerization of a combination of hydrolyzed collagen, acrylic acid (AA), acrylamide (AAm) and distilled water, in appropriate amounts and contained a crosslinking agent called N,N′-methylene bisacrylamide (MBA). The chemical structure of the hydrogels was characterized by means of FTIR spectroscopy, DSC and TGA thermal methods. Morphology of the samples was examined by scanning electron microscopy (SEM). Systematically, the certain variables of the graft copolymerization were optimized to achieve maximum swelling capacity. The absorbency under load (AUL) and centrifuge retention capacity (CRC) were measured. The swelling ratio in various salt solutions was also determined and additionally, the swelling of hydrogels was measured in solutions with pH ranged 1-13. The synthesized hydrogel exhibited a pH-responsiveness character so that a swelling-collapsing pulsatile behavior was recorded at pH 2 and 8.     

Swelling Behavior and Metal-Ion Uptake Capacity of pH-Responsive Hydrogels of Poly(N-acryloyl-N′-ethylpiperazine)

New hydrogels based on N-acryloyl-N′-ethylpiperazine (AcrNEP) and N,N-methylene bisacrylamide (MBA) were prepared by thermal initiated solution polymerization. The hydrogels swelled extensively in buffer solutions of low pH due to protonation of the amine functions of the monomers, while the swelling was less significant in buffer solutions of high pH. The increased swelling of the gel in low pH is due to the development and interaction of fixed charges within the gel network. As a result of the electrostatic repulsion between the charges the elastic constraint of the gel is modified which leads to pronounced swelling and hence to high water uptake. Water transport in the hydrogel both in buffer solutions of pH 2.6 and pH 8.4 was non-Fickian due to polymer relaxation (anomalous process). The gels demonstrated good uptake of divalent metal ions such as Ni²⁺, Co²⁺, and Zn²⁺, with high selectivity for Ni²⁺ ions due to the formation of a more stable ligand-metal complex. The metal uptake capacity increased with increase in pH of the solution, while an increase in the crosslinker amount of the hydrogel reduced its metal uptake capacity. In the presence of metal ions the swelling of the hydrogel reduced considerably due to the formation of additional physical crosslinks within the hydrogel network. The metal ion loaded hydrogels could be stripped and regenerated with 1 M sulfuric acid without any loss in swelling or metal uptake capacities.     

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.     

Thermo‐ and pH‐induced phase transitions and network parameters of poly(N‐isopropylacrylamide‐ co‐2‐acrylamido‐2‐methyl‐propanosulfonic acid) hydrogels

Dual temperature‐ and pH‐sensitive hydrogels composed of N‐isopropylacrylamide (NIPAM) and 2‐acrylamido‐2‐methyl‐propanosulfonic acid (AMPS) were prepared by free‐radical crosslinking copolymerization in aqueous solution at 22 °C. The mole percent of AMPS in the comonomer feed was varied between 0.0 and 7.5, while the crosslinker ratio was fixed at 5.0/100. The effect of AMPS content on thermo‐ and pH‐ induced phase transitions as well as equilibrium swelling/deswelling, interior morphology and network structure was investigated. The volume phase transition temperature (VPT‐T) was determined by both swelling/deswelling measurements and differential scanning calorimetry (DSC) technique. In addition, the volume phase transition pH (VPT‐pH) was detected from the derivative of the curves of the swelling ratio (dQ_v/dpH) versus pH. The polymer‐solvent interaction parameter (χ) and the average molecular mass between crosslinks ( ) of hydrogels were calculated from swelling ratios in buffer solutions at various pHs. The enthalpy (ΔH) and entropy (ΔS) changes appearing in the χ parameter of hydrogels were also determined by using 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. It was observed that the experimental swelling data of hydrogels at different temperature agreed with the modified Flory‐Rehner approach based on the affine network model. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1713–1724, 2008