Poly(methacrylic acid)‐grafted clay–thermoplastic elastomer composites with water‐induced shape‐memory effects

Composites with excellent water‐induced shape‐memory effects (SMEs) were successfully synthesized by first using clay as the SME‐activating phase and thermoplastic polyurethane (TPU) as the matrix. Naturally abundant clay was grafted with poly(methacrylic acid) (PMAA) to improve particle interactions, which allowed for the formation of strong percolation networks in the composites, determined by swelling tests and dynamic mechanical analysis in combination with theoretical modeling. This led to significant improvements of the polymer modulus and high water absorptions, causing reversible modulus changes of up to 30 times from the wet to the dry condition. The results from cyclic wetting‐drying‐stretching tests showed the TPU–clay composite containing 10.4 vol % PMAA‐grafted clay exhibited the best SMEs among the composites investigated, with the shape fixity and shape recovery ratios being 82% and 91%, respectively. Besides SMEs, these new polymer–clay composites were also pH‐sensitive and mechanically adaptive upon exposure to water. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1513–1522     

Mechanically Adaptive and Shape‐Memory Behaviour of Chitosan‐Modified Cellulose Whisker/Elastomer Composites in Different pH Environments

Biomimetic polymer composites with water‐active mechanically adaptive and shape‐memory behaviour in different pH environments are synthesised by using chitosan‐modified cellulose whiskers (CS‐CWs) as the stimulus‐responsive phase and thermoplastic polyurethane (TPU) as the resilient matrix. The effect of surface modification on the mechanically adaptive behaviour of CS‐CW/TPU composites is investigated by using three representative solutions with various pH values. The results show that surface modification significantly enhances the modulus contrast under wet and dry conditions with the acidic solution as the stimulus, while maintaining the high modulus contrast with the basic solution as the stimulus. CS‐CW/TPU composites also exhibit excellent shape‐memory effects in all three solutions that are comparable to those pristine CW/TPU composites. Furthermore, activation of force generation in the stretched CS‐CW/TPU composites by water absorption/desorption was observed.     

pH‐Sensitive Mechanical Properties of Elastin‐Based Hydrogels

Ionizable amino acids in protein‐based hydrogels can confer pH‐responsive behavior. Because elastin‐like polypeptides (ELPs) have an established sequence and can crosslink to form hydrogels, they are an ideal system for creating pH‐sensitive materials. This study examines different parameters that might affect pH‐sensitive behavior and characterizes the mechanical and physical properties between pH 3 and 11 of three ELP‐based crosslinked hydrogels. The first finding is that varying the amount of crosslinker affects the overall stiffness and resilience of the hydrogels but does not strongly affect water content, swelling ratio, or pH sensitivity. Second, the choice of two popular tag sequences, which vary in histidine and aspartic acid content, does not have a strong effect on pH‐sensitive properties. Last, selectively blocking lysine and tyrosine residues through acetylation significantly decreases the pH‐sensitive zeta potential. Acetylated hydrogels also demonstrate different behavior at low pH values with reduced swelling, reduced water content, and higher stiffness. Overall, this work demonstrates that ELP hydrogels with ionizable groups are promising materials for environmentally‐responsive applications such as drug delivery, tissue engineering, and microfluidics.     

Synthesis,swelling properties,and network structure of new stimuli‐responsive poly(N‐acryloyl‐N′‐ethyl piperazine‐co‐N‐isopropylacrylamide) hydrogels

Poly(N‐acryloyl‐N′‐ethyl piperazine‐co‐N‐isopropylacrylamide) hydrogels were prepared by thermal free‐radical copolymerization of N‐acryloyl‐N′‐ethyl piperazine (AcrNEP) and N‐isopropylacrylamide (NIPAM) in solution using N, N′‐methylene bisacrylamide as the crosslinking agent. The gels were responsive to changes in external stimuli such as pH and temperature. The pH and temperature responsive character of the gels was greatly dependent on the monomer content, namely AcrNEP and NIPAM, respectively. The gels swelled in acidic (pH 2) and de‐swelled in basic (pH 10) solutions with a response time of 60 min. With increase in temperature from 23 to 80 °C the swelling of the gels decreased continuously and this effect was different in acidic and basic solutions. The temperature dependence of equilibrium water content of the gels was evaluated by the Gibbs–Helmholtz equation. Detailed analysis of the swelling properties of these new gels in relation to molecular heterogeneity in acidic (pH 2) and basic (pH 10) solutions were performed. Water transport property of the gels was studied gravimetrically. In acidic solution, the diffusion process was non‐Fickian (anomalous) while in basic solution, the diffusion was quasi‐Fickian. The effect was more evident in solution of pH 2 than in pH 10. Various structural parameters of the gels such as number‐average molar mass between crosslink (M_c), the crosslink density (ρ_c), and the mesh size (ξ) were evaluated. The mesh sizes of the hydrogels were between 64 and 783 Å in the swollen state in acidic solution and 20 and 195 Å in the collapsed state in basic solution. The mesh size increased between three to four times during the pH‐dependent swelling process. The amount of unbound water (free water) and bound water of the gels was also evaluated using differential scanning calorimetry. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

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     

Ultrafine hydrogel fibers with dual temperature‐ and pH‐responsive swelling behaviors

Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N‐isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 μm) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water‐insoluble by the addition of either Na₂HPO₄ or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na₂HPO₄ swelled to 30–120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA‐crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 °C. They were also more stable after 1 week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA‐crosslinked hydrogel fibers exhibited distinct temperature‐responsive phase‐transition behavior of PNIPAAm, whereas the Na₂HPO₄‐crosslinked hydrogel fibers showed altered two‐stage phase transitions that reflected side‐chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331–6339, 2004     

Morphology and elastic modulus of novel poly[oligo(ethylene glycol) diacrylate]‐montmorillonite nanocomposites

Novel thermosetting poly[oligo(ethylene glycol) diacrylate]‐sodium montmorillonite nanocomposites containing a range of clay volume fractions were prepared by an in situ polymerization method. X‐ray diffraction showed that the basal plane spacing of the clay was increased to approximately 1.7 nm regardless of clay volume fraction. Transmission electron microscopy confirmed the basal spacing and intercalated structure. The elastic moduli of the composites were measured using ultrasonic pulse‐echo equipment. The results show that the Young's modulus and shear modulus increase with nominal clay volume fraction up to 0.22, and are in good agreement with the well‐established Christensen method and derived Hashin–Shtrikman bounds for conventional composites provided that the true volume fraction of clay reinforcement filler is calculated. At low clay volume fractions, the composites were transparent. When the nominal clay volume fraction was further increased, cracks and porous surfaces appeared, as observed by scanning electron microscopy. These defects decreased the elastic modulus, indicating an upper limit for clay additions in this preparation route. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1785–1793, 2005     

Polyamide‐6/natural clay mineral nanocomposites prepared by solid‐state shear milling using pan‐mill equipment

The polyamide‐6 (PA6)/natural clay mineral nanocomposites were successfully prepared by solid‐state shear milling method without any treatment of clay mineral and additives. PA6/clay mixture was pan‐milled to produce PA6/clay compounding powder, using pan‐mill equipment. The obtained powder as master batch was diluted with neat PA6 to prepare composites by a twin‐screw extruder. The clay silicate layers were found to be partially exfoliated and dispersed homogeneously at nanometer level in PA6 matrix. The rheological measurements and mechanical properties of nanocomposites were characterized. The shear viscosities of nanocomposites were higher than that of pure PA6, and tensile strength and tensile modulus increased, but Izod impact strength decreased, with increasing concentration of clay. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 249–255, 2006     

Synthesis of backbone thermo and pH dual‐responsive hyperbranched poly(amine‐ether)s through proton‐transfer polymerization

Stimuli‐responsive hyperbranched polymers have attracted great attention in recent years because of their wide applications in biomedicine. Through proton‐transfer polymerization of triethanolamine and 1,2,7,8‐diepoxyoctane with the help of potassium hydride, a series of novel backbone thermo and pH dual‐responsive hyperbranched poly(amine‐ether)s were prepared successfully in one‐pot. The degrees of branching of the resulting polymers were at 0.40–0.49. Turbidity measurements revealed that hyperbranched poly(amine‐ether)s exhibited thermo and pH dual‐responsive properties in water. Importantly, these responsivities could be readily adjusted by changing the polymer composition as well as the polymer concentration in aqueous solution. Moreover, in vitro evaluation demonstrated that hyperbranched poly(amine‐ether)s showed low cytotoxicity and efficient cell internalization against NIH 3T3 cell lines. These results suggest that these backbone thermo and pH dual‐responsive hyperbranched poly(amine‐ether)s are promising materials for biomedicine. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Enhanced swelling and adsorption properties of AAm‐AMPSNa/clay hydrogel nanocomposites for heavy metal ion removal

Novel type hydrogel‐clay nanocomposites based on the acrylamide (AAm)‐ 2‐acrylamido‐ 2‐methylpropane sulfonic acid (AMPS) sodium salt and clay were synthesized via in situ copolymerization in aqueous solution. Samples were characterized by determining total basic group (TGB) content and swelling degree, XRD analysis, and FTIR spectroscopy. Effects of monomer ratio and clay amount on the swelling properties of the samples were investigated. It was found that the hydrogel/clay nanocomposites exhibited improved swelling capacity compared with the hydrogels. Samples were used to remove heavy metal ions (Cu (II), Cd (II), and Pb (II)) from aqueous solution in competitive and non‐competitive conditions for the first time. The effects of time and pH of the initial metal ion solution on the adsorption capacity were investigated and selectivity properties of the samples were evaluated. It was found that incorporation of a low amount of clay (10% (wt)) into the polymer structure increased the heavy metal ion adsorption capacity of the sample. It was concluded that the AAm‐AMPS/clay nanocomposites could be used as novel type, fast‐responsive, and high capacity sorbent materials in heavy metal removing processes. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis of pH‐ and thermoresponsive poly(2‐n‐propyl‐2‐oxazoline) based copolymers

Polymers that possess lower critical solution temperature behavior such as poly(2‐alkyl‐2‐oxazoline)s (PAOx) are interesting for their application as stimulus‐responsive materials, for example in the biomedical field. In this work, we discuss the scalable and controlled synthesis of a library of pH‐ and temperature‐sensitive 2‐n‐propyl‐2‐oxazoline P(nPropOx) based copolymers containing amine and carboxylic acid functionalized side chains by cationic ring opening polymerization and postpolymerization functionalization strategies. Using turbidimetry, we found that the cloud point temperature (CP) is strongly dependent on both the polymer concentration and the polymer charge (as a function of pH). Furthermore, we observed that the CP decreased with increasing salt concentration, whereas the CP increased linearly with increasing amount of carboxylic acid groups. Finally, turbidimetry studies in PBS‐buffer indicate that CPs of these polymers are close to body temperature at biologically relevant polymer concentrations, which demonstrates the potential of P(nPropOx) as stimulus‐responsive polymeric systems in, for example, drug delivery applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1573–1582     

Nanostructure and properties of polysiloxane‐layered silicate nanocomposites

The relationship between nanostructure and properties in polysiloxane layered silicate nanocomposites is presented. Solvent uptake (swelling) in dispersed nanocomposites was dramatically decreased as compared to conventional composites, though intercalated nanocomposites and immiscible hybrids exhibited more conventional behavior. The swelling behavior is correlated to the amount of bound polymer (bound rubber) in the nanocomposites. Thermal analysis of the bound polymer chains showed an increase and broadening of the glass‐transition temperature and loss of the crystallization transition. Both modulus and solvent uptake could be related to the amount of bound polymer formed in the system. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1595–1604, 2000     

High water content clay–nanocomposite hydrogels incorporating guanidinium‐pendant methacrylamide: Tuning of mechanical and swelling properties by supramolecular approach

Novel clay–polymer composite hydrogels with high water content (up to 98 wt %) are developed, in which mechanical properties are reinforced by the formation of multiple ion‐pairs between the polymer chains and clay nanosheets (CNS). When a small amount of guanidinium‐pendant methacrylamide (0.1–0.2 wt %) is copolymerized with a neutral monomer (0.5–2.0 wt %) in an aqueous dispersion of CNS (1.0–3.0 wt %), a self‐standing hydrogel with satisfactory mechanical toughness and elasticity results, despite its high water content (95–98 wt %). The mechanical properties and swelling behaviors of the hydrogels can be tuned by the amount of the guanidinium‐pendant acrylamide. A systematic study indicates that the ion pairs, formed between the guanidinium groups in the polymer chains and the oxyanions on the surfaces of the CNS, serve as crosslinking points in the three‐dimensional network developed in these hydrogels. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 839–847     

Exploring preferential association of laponite and cloisite with soft and hard segments in TPU‐clay nanocomposite prepared by solution mixing technique

Thermoplastic polyurethane (TPU) is a versatile polymer exhibiting many engineering applications. In this article, two varieties of clay (Cloisite and Laponite RD) have been used to prepare TPU‐based nanocomposites. They differ in, chemical composition, hydrophobicity, aggregation tendency, and dispersibility in a particular solvent. A detailed investigation of the thermal, morphological, and rheological behavior reflects the affinity of Cloisite towards the soft segment, whereas it is the hard segment for modified Laponite. The maximum improvement in onset degradation temperature has been observed to be 17.5 and 8.3 °C for Cloisite and Laponite, respectively. Five percent Cloisite‐filled sample shows optimum storage modulus in the glassy region where as it is the 10% filled sample at the rubbery region. However, the trend remains indifferent both in rubbery and glassy regions for Laponite, and properties have been found optimum for 3% filled sample. To explore the behavior in the terminal and flow regions, dynamic rheological experiments were performed in low shear rate. Variation in dynamic rheological properties can be explained well on the basis of the combination of partly exfoliated, intercalated, and aggregated structures of the nano clay inside the TPU matrix, depending on their nature and preferential association with different segments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2341–2354, 2008     

Polypeptide‐based nanocomposite: Structure and properties of poly(L‐lysine)/Na+‐montmorillonite

The feasibility of constructing polymer/clay nanocomposites with polypeptides as the matrix material is shown. Cationic poly‐L‐lysine · HBr (PLL) was reinforced by sodium montmorillonite clay. The PLL/clay nanocomposites were made via the solution‐intercalation film‐casting technique. X‐ray diffraction and transmission electron microscopy data indicated that montmorillonite layers intercalated with PLL chains coexist with exfoliated layers over a wide range of relative PLL/clay compositions. Differential scanning calorimetry suggests that the presence of clay suppresses crystal formation in PLL relative to the neat polypeptide and slightly decreases the PLL melting temperature. Despite lower crystallinity, dynamic mechanical analysis revealed a significant increase in the storage modulus of PLL with an increase in clay loading producing storage modulus magnitudes on par with traditional engineering thermoplastics. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2579–2586, 2002     

Shear‐induced clay dispersion in HDPE/PEgMA/organoclay composites as studied via real‐time rheological method

In current study, a real‐time rheological method was used to investigate the intercalation and exfoliation process of clay in high‐density polyethylene/organoclay (HDPE/OMMT) nanocomposites using maleic anhydride grafted polyethylene (PEgMA) as compatibilizer. To do this, a steady shear was applied to the original nonintercalated or slightly intercalated composites prepared via simple mixing. The moduli of the composites were recorded as a function of time. The effect of matrix molecular weight and the content of compatibilizer on the modulus were studied. The role of the compatibilizer is to enhance the interaction between OMMT and polymer matrix, which facilitates the dispersion, intercalation, and exfoliation of OMMT. The matrix molecular weight determines the melt viscosity and affects the shear stress applied to OMMT platelets. Based on the experimental results, different exfoliation processes of OMMT in composites with different matrix molecular weight were demonstrated. The slippage of OMMT layers is suggested in low‐molecular weight matrix, whereas a gradual intercalation process under shear is suggested in high‐molecular weight matrix. Current study demonstrates that real‐time rheological measurement is an effective way to investigate the dispersion, intercalation, and exfoliation of OMMT as well as the structural change of the matrix. Moreover, it also provides a deep understanding for the role of polymer matrix and compatibilizer in the clay intercalation process. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 302–312, 2010     

Reduction and pH dual‐responsive block copolymers containing pendent p‐nitrobenzyl carbamate functionalities: Synthesis and self‐assembly behavior

We report on the preparation of reduction‐responsive amphiphilic block copolymers containing pendent p‐nitrobenzyl carbamate (pNBC)‐caged primary amine moieties by reversible addition–fragmentation chain transfer (RAFT) radical polymerization using a poly(ethylene glycol)‐based macro‐RAFT agent. The block copolymers self‐assembled to form micelles or vesicles in water, depending on the length of hydrophobic block. Triggered by a chemical reductant, sodium dithionite, the pNBC moieties decomposed through a cascade 1,6‐elimination and decarboxylation reactions to liberate primary amine groups of the linkages, resulting in the disruption of the assemblies. The reduction sensitivity of assemblies was affected by the length of hydrophobic block and the structure of amino acid‐derived linkers. Using hydrophobic dye Nile red (NR) as a model drug, the polymeric assemblies were used as nanocarriers to evaluate the potential for drug delivery. The NR‐loaded nanoparticles demonstrated a reduction‐triggered release profile. Moreover, the liberation of amine groups converted the reduction‐responsive polymer into a pH‐sensitive polymer with which an accelerated release of NR was observed by simultaneous application of reduction and pH triggers. It is expected that these reduction‐responsive block copolymers can offer a new platform for intracellular drug delivery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1333–1343     

Semi‐interpenetrating polymer networks composed of poly(N‐isopropyl acrylamide) and polyacrylamide hydrogels

Three series of semi‐interpenetrating polymer networks, based on crosslinked poly(N‐isopropyl acrylamide) (PNIPA) and 1 wt % nonionic or ionic (cationic and anionic) linear polyacrylamide (PAAm), were synthesized to improve the mechanical properties of PNIPA gels. The effect of the incorporation of linear polymers into responsive networks on the temperature‐induced transition, swelling behavior, and mechanical properties was studied. Polymer networks with four different crosslinking densities were prepared with various molar ratios (25:1 to 100:1) of the monomer (N‐isopropyl acrylamide) to the crosslinker (methylenebisacrylamide). The hydrogels were characterized by the determination of the equilibrium degree of swelling at 25 °C, the compression modulus, and the effective crosslinking density, as well as the ultimate hydrogel properties, such as the tensile strength and elongation at break. The introduction of cationic and anionic linear hydrophilic PAAm into PNIPA networks increased the rate of swelling, whereas the presence of nonionic PAAm diminished it. Transition temperatures were significantly affected by both the crosslinking density and the presence of linear PAAm in the hydrogel networks. Although anionic PAAm had the greatest influence on increasing the transition temperature, the presence of nonionic PAAm caused the highest dimensional change. Semi‐interpenetrating polymer networks reinforced with cationic and nonionic PAAm exhibited higher tensile strengths and elongations at break than PNIPA hydrogels, whereas the presence of anionic PAAm caused a reduction in the mechanical properties. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3987–3999, 2004     

A novel sodium carboxymethylcellulose/poly(N‐isopropylacrylamide)/Clay semi‐IPN nanocomposite hydrogel with improved response rate and mechanical properties

A novel semi‐IPN nanocomposite hydrogel (CMC/PNIPA/Clay hydrogel) based on linear sodium carboxymethylcellulose (CMC) and poly(N‐isopropylacrylamide) (PNIPA) crosslinked by inorganic clay was prepared. The structure and morphology of these hydrogels were investigated and their swelling and deswelling kinetics were studied in detail. TEM images showed that the clay was substantially exfoliated to form nano‐dimension platelets dispersed homogeneously in the hydrogels and acted as a multifunctional crosslinker. The CMC/PNIPA/Clay hydrogels swell faster than the corresponding PNIPA/Clay hydrogels at pH 7.4, whereas they swell slower than the PNIPA/Clay hydrogels at pH 1.2. The CMC/PNIPA/Clay nanocomposite hydrogels showed much higher deswelling rates, which was ascribed to more passway formed in these hydrogels for water to diffuse in and out. The deswelling process of the hydrogels could be approximately described by the first‐order kinetic equation and the deswelling rate decreased with increasing clay content. The mechanical properties of the CMC/PNIPA/Clay nanocomposite hydrogels were analyzed based on the theory of rubber elasticity. It was found that with increasing clay content, the effective crosslink chain density, v_e, increased whereas the molecular weight of the chains between crosslinks M_c decreased. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1546–1555, 2008     

Dual temperature‐ and pH‐sensitive hydrogels from interpenetrating networks and copolymerization of N‐isopropylacrylamide and sodium acrylate

Hydrogels responsive to both temperature and pH have been synthesized in the forms of sequential interpenetrating networks (IPNs) of N‐isopropylacrylamide (NIPAAm) and sodium acrylate (SA) and compared with the crosslinked random copolymers of N‐isopropylacrylamide and SA. Whereas the stimuli‐sensitive behaviors of copolymer hydrogels were strongly dependent on the ionic SA contents, the IPN hydrogels exhibited independent swelling and thermal behaviors of each network component. The sequences and media in the synthesis of IPNs influenced the swelling capacities of the IPNs, but not the temperature or pH ranges at which the swelling changes occurred. In IPNs, a more expanded primary gel network during the synthesis of the secondary network contributed to the better swelling of the final IPNs. Both the swelling and thermal behaviors of the IPNs suggest that poly(N‐isopropylacrylamide) and poly(sodium acrylate) are phase separated regardless of their synthesis conditions. The presence of the poly(sodium acrylate) network did not influence the temperature or the extent of phase transition of the poly(N‐isopropylacrylamide) network in the IPNs, but did improve the thermal stability of the IPNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3293–3301, 2004