Multi‐responsive hydrogels based on N‐isopropylacrylamide and sodium alginate

Hydrogels consisting of sodium alginate and N‐isopropylacrylamide covalently crosslinked with N,N′‐methylenebisacrylamide were prepared. The mixed‐interpenetrated networks obtained were characterized using elemental analysis, Fourier transform infrared and Raman spectroscopy, swelling measurements and environmental scanning electron microscopy. The thermo‐ and pH‐responsive properties of these hydrogels were evidenced by their swelling behaviour, which depended also on the amount of crosslinking agent and hydrogel composition. Copyright © 2010 Society of Chemical Industry     

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     

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

Thermosensitive 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     

Analysis of the swelling dynamics of crosslinked P(N‐iPAAm‐co‐MAA) copolymers,the corresponding homopolymers and their interpenetrating networks. Kinetics of water penetration into the gel above the pKaof MAA comonomeric units

The effect of composition on the swelling kinetics of a series of crosslinked poly(N‐isopropylacrylamide)s [P(N‐iPAAm)] and poly(methacrylic acid)s [P(MAA)], their statistical copolymers P(N‐iPAAm‐co‐MAA), and some sequential interpenetrating networks (IPNs) has been studied at pHs above the pK_a of MAA comonomeric units. The swelling process of the hydrogels upon immersion in buffer solutions was monitored by the weight change as a function of time. First‐order kinetics apply better than second‐order kinetics which fail, especially for predicting swelling equilibrium values. However, hydrogels presenting hydrogen bonding between MAA and N‐iPAAm units do not obey either first‐order or second‐order kinetics. The high n values found indicate that the swelling process is mostly controlled by polymer relaxation. Copyright © 2003 Society of Chemical Industry     

RAFT synthesis of poly(N‐isopropylacrylamide) and poly(methacrylic acid) homopolymers and block copolymers: Kinetics and characterization

Reversible addition‐fragmentation chain transfer (RAFT) polymerization was used successfully to synthesize temperature‐responsive poly(N‐isopropylacrylamide) (PNIPAAm), poly(methacrylic acid) (PMAA), and their temperature‐responsive block copolymers. Detailed RAFT polymerization kinetics of the homopolymers was studied. PNIPAAm and PMAA homopolymerization showed living characteristics that include a linear relationship between M _n and conversion, controlled molecular weights, and relatively narrow molecular weight distribution (PDI < 1.3). Furthermore, the homopolymers can be reactivated to produce block copolymers. The RAFT agent, carboxymethyl dithiobenzoate (CMDB), proved to control molecular weight and PDI. As the RAFT agent concentration increases, molecular weight and PDI decreased. However, CMDB showed evidence of having a relatively low chain transfer constant as well as degradation during polymerization. Solution of the block copolymers in phosphate buffered saline displayed temperature reversible characteristics at a lower critical solution temperature (LCST) transition of 31°C. A 5 wt % solution of the block copolymers form thermoreversible gels by a self‐assembly mechanism above the LCST. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1191–1201, 2006     

Preparation and performance of thermosensitive poly(N‐isopropylacrylamide) hydrogels by frontal photopolymerization

In this study, frontal photopolymerization was applied to the fabrication of thermosensitive poly(N‐isopropylacrylamide) (PNIPAm) hydrogels. The influence of experimental conditions and reactant components on the feature of the polymerization front was investigated. The morphology of the samples indicated the successful preparation of PNIPAm hydrogels. The mechanical properties and thermal stability of the obtained hydrogels are discussed. The results indicated that the performance of the hydrogels is related to their microstructure and the type of crosslinkers. The swelling behavior and drug delivery ability were determined under different temperature conditions. The hydrogels exhibit a classical thermoresponsive behavior, which was also demonstrated by the DSC results. Therefore, frontal photopolymerization can be an alternative method for the preparation of PNIPAm hydrogels under mild conditions. © 2019 Society of Chemical Industry     

Swelling behaviour of hydrogels from methacrylic acid and poly(ethylene glycol) side chains by magnetic resonance imaging

The swelling behaviour of hydrogels of methacrylic acid and poly(ethylene glycol) monomethyl ether monomethacrylate macromonomer, P[(MAA)‐co‐(PEGMEMA)], copolymers was investigated. Under pH 7, characteristic sigmoidal swelling appears depending on composition. This anomalous swelling phenomenon is related to the ability of moieties in the comonomeric units ? COOH and ? [? O? CH₂? CH₂? ]? to form complexes by hydrogen bonding, stabilized by hydrophobic interactions. The swelling was followed using magnetic resonance imaging (MRI) providing spatial and temporal resolution. Data from MRI are compared with gravimetric results and photographs during swelling. In the MRI images two processes were distinguished, corresponding to a swollen external region in contrast to a rigid core which starts to swell after a lag time depending on sample composition. Microscopic and macroscopic results are in good agreement. Copyright © 2007 Society of Chemical Industry     

Swelling behavior of poly(acrylamide‐co‐N‐vinylimidazole) hydrogels under different environment conditions

A series of random copolymers of acrylamide and N‐vinylimidazole, poly(AAm‐co‐NVI), with various compositions were prepared using redox copolymerization. The influence of environmental conditions such as pH, temperature, and ionic strength on the swelling behavior of the copolymeric hydrogels was investigated. The hydrogels exhibited the highest equilibrium swelling in basic medium at high temperature. Equilibrium swelling decreased with rising ionic strength at pH 5.0. As pH increased, equilibrium swelling of the hydrogels increased at pH 11.0 and I = 0.20 M. Swelling kinetics of the hydrogels was found to be non‐Fickian at 25°C. The process tended to be Fickian at higher pH and temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1783–1788, 2005     

Poly(N‐isopropylacrylamide‐co‐sodium acrylate) hydrogels: Interactions with surfactants

Poly(N‐isopropylacrylamide‐co‐sodium acrylate) [poly(NIPAM‐co‐SA)] hydrogels were modified with three different kind of surfactants (cationic, anionic, and nonionic) to study the effect on the swelling properties. The structural variation of the surfactant‐modified hydrogels was investigated in detail. The interaction between the surfactants and the hydrogel varies and strictly depends on the surfactant type. The variation in thermal stability of the modified surfactant hydrogels was investigated and compared with unmodified hydrogel. Further, the hydrogel swelling/diffusion kinetic parameters were investigated and diffusion of water into hydrogel was found to be of the non‐Fickian transport mechanism. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3423–3430, 2007     

Thermoreversible hydrogels. IX. Swelling behaviors of thermosensitive hydrogels copolymerized by N‐isopropylacrylamide with 1‐vinyl‐3‐(3‐sulfopropyl) imidazolium betaine

A series of thermosensitive hydrogels were prepared from the various molar ratios of N‐isopropylacrylamide, 1‐vinyl‐3‐(3‐sulfopropyl) imidazolium betaine (VSIB), and N,N′‐methylene‐bis‐acrylamide. The influence of the amount of VSIB in the copolymeric gels on the swelling behaviors in water, in various saline solutions, and at various temperatures was investigated. The results indicated that the higher the VSIB content in the hydrogel system, the higher the swelling ratio and the gel transition temperature. In the saline solution the results showed that when the concentration of salt is higher than the minimum salt concentration (MSC) of poly(VSIB), the deswelling behavior of the copolymeric gel was more effectively suppressed as more VSIB was added to the copolymeric gels. In addition, only the sample containing 12 mol % VSIB (V4) exhibited an antipolyelectrolyte's swelling behavior when the concentration of salt was higher than the MSC of poly(VSIB). This means that the swelling ratio of the hydrogel can be improved with a higher concentration salt solution. In addition, the anion effects were larger than the cation effects in the presence of a common anion (Cl⁻) with different cations and a common cation (K⁺) with different anions for the hydrogel. Finally, the more VSIB in the hydrogel, the higher the diffusion coefficient in dynamic swelling. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 14–23, 2000     

pH‐thermoreversible hydrogels. I. Synthesis and swelling behaviors of the (N‐isopropylacrylamide‐co‐acrylamide‐co‐2‐hydroxyethyl methacrylate) copolymeric hydrogels

A series of pH‐thermoreversible hydrogels that exhibited volume phase transition was synthesized by various molar ratios of N‐isopropylacrylamide (NIPAAm), acrylamide (AAm), and 2‐hydroxyethyl methacrylate (HEMA). The influence of environmental conditions such as temperature and pH value on the swelling behavior of these copolymeric gels was investigated. Results showed that the hydrogels exhibited different equilibrium swelling ratios in different pH solutions. Amide groups could be hydrolyzed to form negatively charged carboxylate ion groups in their hydrophilic polymeric network in response to an external pH variation. The pH sensitivities of these gels also depended on the AAm content in the copolymeric gels; thus the greater the AAm content, the higher the pH sensitivity. These hydrogels, based on a temperature‐sensitive hydrogel, demonstrated a significant change of equilibrium swelling in aqueous media between a highly solvated, swollen gel state and a dehydrated network response to small variations of temperature. pH‐thermoreversible hydrogels were used for a study of the release of a model drug, caffeine, with changes in temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 221–231, 1999     

Rheological and thermal behaviour of poly(N‐isopropylacrylamide)/alginate smart polymeric networks

Interpenetrating polymeric networks based on sodium alginate and poly(N‐isopropylacrylamide) (PNIPAAm) covalently crosslinked with N,N′‐methylenebisacrylamide have been investigated using rheology, thermogravimetry, differential scanning calorimetry, X‐ray diffraction measurements and scanning electron microscopy (SEM). An improved elastic response of the samples with a higher PNIPAAm content and increased amount of crosslinking agent was found. The temperature‐responsive behaviour of the hydrogel samples was evidenced by viscoelastic measurements performed at various temperatures. It is shown that the properties of these gels can be tuned according to composition, amount of crosslinking agent and temperature changes. X‐ray scattering analysis revealed that the hydrophobic groups are locally segregated even in the swollen state whilst cryo‐SEM showed the highly heterogeneous nature of the gels. Copyright © 2011 Society of Chemical Industry     

Fast responsive and strong swelling hydrogels based on N‐isopropylacrylamide with sodium acrylate

Poly(N‐isopropylacrylamide‐co‐sodium acrylate) gels with N,N‐methylene bisacrylamde (BIS) as crosslinker were prepared by free radical polymerization method at the temperature of 35°C, which was just around the lower critical solution temperature (LSCT) of the hydrogels. The gels synthesized at 35°C demonstrated strong swellability and fast responseability when compared with the gels synthesized at the temperature of 0 and 18°C (below the LCST) and 50 and 80°C (above the LSCT). The response rate and swelling behavior of poly(N‐isopropylacrylamide‐co‐sodium acrylate) gels was investigated and characterized by the temperature‐dependent swelling ratio and swelling and deswelling kinetics. The swelling behavior of the gels indicated that the synthesis temperature was the main factor when the swellability concerned and also had effect on the responseability of the resulting hydrogels. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

pH‐ and temperature‐responsive IPN hydrogels based on soy protein and poly(N‐isopropylacrylamide‐co‐sodium acrylate)

pH‐ and temperature‐responsive interpenetrating polymer network (IPN) hydrogels based on soy protein and poly(N‐isopropylacrylamide‐co‐sodium acrylate) were successfully prepared. The structure and properties of the hydrogels were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analyzer. The equilibrium and dynamic swelling/deswelling behaviors and the drug release properties of the hydrogels responding to pH and/or temperature were also studied in detail. The hydrogels have the porous honeycomb structures, good miscibility and thermal stability, and good pH‐ and temperature‐responsivity. The volume phase transition temperature of the hydrogels is ca. 40°C. Changing the soy protein or crosslinker content could be used to control the swelling behavior and water retention, and the hydrogels have the fastest deswelling rate in pH 1.2 buffer solutions at 45°C. Bovine serum albumin release from the hydrogels has the good pH and temperature dependence. The results show that the proposed IPN hydrogels may have potential applications in the field of biomedical materials such as in drug delivery systems. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39781.     

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

Macroporous 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 (ν_E) 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     

Swelling kinetics of poly(N‐vinylimidazole‐co‐sodium styrenesulfonate) hydrogels

Hydrogels of N‐vinylimidazole and sodium styrenesulfonate were synthesized by radical crosslinking copolymerization. Transient swelling measurements in water, at room temperature, reveal an unusual behavior. For some gel compositions, the swelling time dependency is about a δ function: dry hydrogel pellets swell up readily to reach a high degree of swelling and, spontaneously, they deswell to collapse. Such peculiarity was interpreted in terms of several competing events in the swelling mechanism: water diffusion toward the gel, chain disentanglement, sodium–proton interchange through the gel–external bath boundary, approaching of chains to allow interaction of sulfonate groups with neighboring protonated imidazole moieties and diffusion of water outside the gel. The results of cyclic swelling–deswelling following abrupt changes of composition in the external bath, from water to sodium hydroxide solution, support that mechanism. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 191–200, 2002     

Thermoreversible hydrogels. VII. Synthesis and swelling behavior of poly(N‐isopropylacrylamide‐co‐3‐methyl‐1‐vinylimidazolium iodide) hydrogels

A series of N‐isopropylacrylamide/3‐methyl‐1‐vinylimidazolium iodide (NIPAAm/MVI) copolymer gels were prepared from the various molar ratios of NIPAAm, cationic monomer MVI, and N,N′‐methylene bisacrylamide (NMBA) in this study. The influence of the amount of MVI in the copolymer gels on the swelling behaviors was investigated in various aqueous saline solutions. Results showed that the swelling ratios (SRs) of copolymer gels were significantly greater than those of NIPAAm homopolymer gels, and the higher the MVI content, the higher the volume phase transition temperature. The SRs for the NIPAAm/MVI copolymer gels decreased with an increase of the salt concentration. In various saline solutions, results showed that the effect of divalent ions on the SR was greater than that of monovalent ions for these hydrogels. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3242–3253, 1999     

Thermodynamic investigation of thermoresponsive xanthan‐poly (N‐isopropylacrylamide) hydrogels

Polysaccharide‐based hydrogels, such as xanthan maleate/poly(N‐isopropylacrylamide) (PNIPAAm) interpenetrated polymer networks, are thermostimulable materials of interest for the controlled release of biologically active components due to conformation changes at the low critical‐solution temperature (LCST) PNIPAAm phase transition. The phase transition of these interpenetrated polymer network hydrogels, where PNIPAAm is in a ‘confined’ environment, was examined by high resolution magic angle spinning nuclear magnetic resonance and differential scanning calorimetry. High resolution magic angle spinning nuclear magnetic resonance spectroscopy allows the accurate determination of LCST and an evaluation of the corresponding thermodynamic data. More particularly, the evolution of these data as a function of the composition of the hydrogel, and of the external parameters such as pH and ionic strength, was considered. LCST shows a minimal value with increasing xanthan content. Moreover, it was possible to calculate, as a function of temperature, the fraction of NIPAAm which remains uncollapsed. The data obtained for pure PNIPAAm hydrogels are in good agreement with recently published results. The phase transition of PNIPAAm in a diphasic hydrogel is broader when PNIPAAm is ‘confined’ within an interpenetrated polymer network than in a pure PNIPAAm crosslinked network. The widening of the transition with increasing xanthan content indicates a reduction of the PNIPAAm interchain aggregation in a network structure. Copyright © 2011 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     

Water‐sorption and metal‐uptake behavior of pH‐responsive poly (N‐acryloyl‐N′‐methylpiperazine) gels

Hydrogels based on N‐acryloyl‐N′‐methylpiperazine (AcrNMP) swelled extensively in solutions of low pH due to the protonation of the tertiary amine. The water transport in the gels under an acidic condition was non‐Fickian and nearly Fickian in neutral pH with the collective diffusion coefficients determined as 2.08 × 10⁻⁷ and 5.00 × 10⁻⁷ cm⁻² s⁻¹, respectively. These gels demonstrated good metal‐uptake behavior with various divalent metal ions, in particular, copper and nickel, with the uptake capacity increased with increasing pH. The swelling ratio of the gel in the presence of metal ions decreased with increasing metal ion uptake. The results suggest that high metal ion uptake can lead to physical crosslinking arising from the interchain metal complex formation. The metal‐loaded gels could be stripped easily with 1M H₂SO₄ without any loss in their uptake capacity. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 268–273, 2001