Properties of interpenetrating polymer network hydrogels composed of poly(vinyl alcohol) and poly(N‐isopropylacrylamide)

Interpenetrating polymer network (IPN) hydrogels based on poly(vinyl alcohol) and poly(N‐isopropylacrylamide) were prepared by the sequential‐IPN method. The IPN hydrogels were analyzed for sorption behavior of water at 35°C and at a relative humidity of 95% using a dynamic vapor sorption system, and water diffusion coefficients were calculated. Differential scanning calorimetry was used for the quantitative determination of the amounts of freezing and nonfreezing water. Free water contents in the IPN hydrogel of IPN1, IPN2, and IPN3 were 45.8, 37.9 and 33.1% in pure water, respectively. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2041–2045, 2003     

Thermal characterizations of semi‐interpenetrating polymer networks composed of poly(ethylene oxide) and poly(N‐isopropylacrylamide)

Poly(N‐isopropylacrylamide) (PNIPAAm)/poly(ethylene oxide) (PEO) semi‐interpenetrating polymer networks (semi‐IPNs) synthesized by radical polymerization of N‐isopropylacrylamide (NIPAAm) in the presence of PEO. The thermal characterizations of the semi‐IPNs were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dielectric analysis (DEA). The melting temperature (T_m) of semi‐IPNs appeared at around 60°C using DSC. DEA was employed to ascertain the glass transition temperature (T_g) and determine the activation energy (E_a) of semi‐IPNs. From the results of DEA, semi‐IPNs exhibited one T_g indicating the presence of phase separation in the semi‐IPN, and T_gs of semi‐IPNs were observed with increasing PNIPAAm content. The thermal decomposition of semi‐IPNa was investigated using TGA and appeared at around 370°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3922–3927, 2003     

Thermosensitive soy protein/poly(n‐isopropylacrylamide) interpenetrating polymer network hydrogels for drug controlled release

A series of the thermosensitive interpenetrating polymer network hydrogels composed of soy protein and poly(N‐isopropylacrylamide) were successfully prepared. The structure and properties were systematically characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and differential scanning calorimetry. It was found that the hydrogels had good miscibility and high porosity, and the volume phase transition temperatures of the hydrogels were around 32°C. The release behavior and the release mechanism of a model protein, bovine serum albumin (BSA), were also investigated in detail. The results indicated that the release behavior of BSA had strong temperature dependence and the release percentage of BSA could be controlled by modulating the amount of soy protein or crosslinking agent. The analysis of the release mechanism revealed that the Fickian diffusion controlled release was dominant under the experimental conditions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

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     

Preparation and properties of temperature‐sensitive soy protein/poly(N‐isopropylacrylamide) interpenetrating polymer network hydrogels

Temperature‐sensitive interpenetrating polymer network (IPN) hydrogels based on soy protein and poly(N‐isopropylacrylamide) were successfully prepared. The structure and properties were systematically characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry and thermogravimetric analysis, and the swelling and deswelling behavior was also investigated. It was found that the hydrogels had good miscibility, thermal stability and temperature sensitivity, and the lower critical solution temperature was ca 32 °C. Changing the content of soy protein or crosslinker could be used to control the swelling behavior, water retention and network structure of the IPN hydrogels. The results show that the novel IPN hydrogels may be of potential interest in drug delivery systems. Copyright © 2011 Society of Chemical Industry     

Preparation and characterization of thermosensitive poly(N‐isopropylacrylamide)/poly(ethylene oxide) semi‐interpenetrating polymer networks

Temperature‐sensitive poly(N‐isopropylacrylamide) hydrogels were successfully synthesized by using poly(ethylene oxide) as the interpenetrating agent. The newly prepared semi‐interpenetrating polymer network (semi‐IPN) hydrogels exhibited much better properties as temperature‐sensitive polymers than they did in the past. Characterizations of the IPN hydrogels were investigated using a swelling experiment, FTIR spectroscopy, and differential scanning calorimetry (DSC). Semi‐IPN hydrogels exhibited a relatively high temperature dependent swelling ratio in the range of 23–28 at room temperature. DSC was used for the determination of the lower critical solution temperature of the semi‐IPN hydrogel. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3032–3036, 2003     

Thermal characteristics of interpenetrating polymer networks composed of poly(vinyl alcohol) and poly(N‐isopropylacrylamide)

Interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly(N‐isopropylacrylamide) (PNIPAAm) were prepared by the sequential‐IPN method. The thermal characterization of the IPNs was investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dielectric analysis (DEA). Depression of the melting temperature (T_m) of the PVA segment in IPNs was observed with increasing PNIPAAm content using DSC. DEA was employed to ascertain the glass‐transition temperature (T_g) of IPNs. From the result of DEA, IPNs exhibited two T_g values, indicating the presence of phase separation in the IPNs. The thermal decomposition of IPNs was investigated using TGA and appeared at near 200°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 881–885, 2003     

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     

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     

Microwave‐assisted preparation of temperature sensitive poly(N‐isopropylacrylamide) hydrogels in poly(ethylene oxide)‐600

In this article, a series of poly(N‐isopropylacrylamide) (PNIPAM)‐based hydrogels were prepared under microwave irradiation using poly(ethylene oxide)‐600 (PEO‐600) as reaction medium and microwave‐absorbing agent as well as pore‐forming agent. All of the temperature measurements, gel fractions, and FTIR analyses proved that the PNIPAM hydrogels were successfully synthesized. Within 1 min, the PNIPAM hydrogel with a 98% yield was obtained under microwave irradiation. The PNIPAM hydrogels thus prepared exhibited controllable properties such as pore size, equilibrium swelling ratios, and swelling/deswelling rates when changing the feed weight ratios of monomer (N‐isopropylacrylamide, NIPAM) to PEO‐600. These properties are well adapted to the different requirements for their potential application in many fields such as biomedicine. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:4177–4184, 2006     

Lower critical solution temperatures of thermo‐responsive poly(N‐isopropylacrylamide) copolymers with racemate or single enantiomer groups

BACKGROUND: Thermo‐responsive copolymers with racemate or single enantiomer groups are attracting increasing attention due to their fascinating functional properties and potential applications. However, there is a lack of systematic information about the lower critical solution temperature (LCST) of poly(N‐isopropylacrylamide)‐based thermo‐responsive chiral recognition systems. In this study, a series of thermo‐responsive chiral recognition copolymers, poly[(N‐isopropylacrylamide)‐co‐(N‐(S)‐sec‐butylacrylamide)] (PN‐S‐B) and poly[(N‐isopropylacrylamide)‐co‐(N‐(R,S)‐sec‐butylacrylamide)] (PN‐R,S‐B), with different molar compositions, were prepared. The effects of heating and cooling processes, optical activity and amount of chiral recognition groups in the copolymers on the LCSTs of the prepared copolymers were systematically studied. RESULTS: LCST hysteresis phenomena are found in the phase transition processes of PN‐S‐B and PN‐R,S‐B copolymers in a heating and cooling cycle. The LCSTs of PN‐S‐B and PN‐R,S‐B during the heating process are higher than those during the cooling process. With similar molar ratios of N‐isopropylacrylamide groups in the copolymers, the LCST of the copolymer containing a single enantiomer (PN‐S‐B) is lower than that of the copolymer containing racemate (PN‐R,S‐B) due to the steric structural difference. The LCSTs of PN‐R,S‐B copolymers are in inverse proportion to the molar contents of the hydrophobic R,S‐B moieties in these copolymers. CONCLUSION: The results provide valuable guidance for designing and fabricating thermo‐responsive chiral recognition systems with desired LCSTs. Copyright © 2008 Society of Chemical Industry     

Synthesis and characterization of pH‐ and temperature‐sensitive silk sericin/poly(N‐isopropylacrylamide) interpenetrating polymer networks

Interpenetrating polymer networks (IPNs) composed of silk sericin (SS) and poly(N‐isopropylacrylamide) (PNIPAAm) were prepared simultaneously. The properties of the resultant IPN hydrogels were characterized by differential scanning calorimetry and SEM as well as their swelling behavior at various temperatures and pH values. The single glass transition temperature (T_g) presented in the IPN thermograms indicated that SS and PNIPAAm form a miscible pair. The swollen morphology of the IPNs observed by SEM demonstrated that water channels (pores present in SEM micrographs) were distributed homogeneously through out the network membranes. The swelling ratio of the IPNs depended significantly on the composition, temperature and pH of the buffer solutions. The dynamic transport of water into the IPN membrane was analyzed based on the Fickian equation. Copyright © 2006 Society of Chemical Industry     

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     

Effect of adamantyl methacrylate on the thermal and mechanical properties of thermosensitive poly(N‐isopropylacrylamide) hydrogels

A series of thermosensitive hydrogels containing adamantyl groups were fabricated by copolymerization of N‐isopropylacrylamide and adamantyl methacrylate (AdMA). The thermal properties of such copolymeric hydrogels were studied by differential scanning calorimetry. The mechanical properties were emphasized through compression, tension, and dynamic mechanical analysis (DMA). Moreover, Rubber elasticity theory was used to evaluate the network parameters based on compressive stress–strain measurements. The results indicate that both the microstructure and physical properties strongly depend on the quantity of AdMA in the copolymeric gels. As the content of AdMA increases, the volume phase transition temperature of hydrogels decreases linearly, and the mechanical strength can be significantly improved, the effective crosslinking density (ν_e) increases monotonously, while the polymer‐water interaction parameter (χ) decreases first and then increases with AdMA content. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Binary blends based on poly(N‐isopropylacrylamide): Miscibility studies with PVA,PVP, and PAA

Blends of poly(vinyl alcohol) (PVA), poly(acrylic acid), (PAA), and poly(vinyl pyrrolidone) (PVP), with poly(N‐isopropylacrylamide) (PNIPAM), were prepared by casting from aqueous solutions. Mechanical properties of PNIPAM/PVA blends were analyzed by stress–strain tests. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to analyze the miscibility between the polymeric pairs. The results revealed that PNIPAM is not miscible with PVA and PVP in the whole range of composition. On the other hand, PNIPAM interacts strongly with PAA forming interpolymer complex due to the formation of cooperative hydrogen bonds. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 743–748, 2004     

Thermoresponsive sodium alginate‐g‐poly(N‐isopropylacrylamide) copolymers III. Solution properties

Stimuli‐responsive biocompatible and biodegradable materials can be obtained by combining polysaccharides with polymers exhibiting lower critical solution temperature (LCST) phase behavior, such as poly(N‐isopropylacrylamide) (PNIPAAm). The behavior of aqueous solutions of sodium alginate (NaAl) grafted with PNIPAAm (NaAl‐g‐PNIPAAm) copolymers as a function of composition and temperature is presented. The products obtained exhibit a remarkable thermothickening behavior in aqueous solutions if the degree of grafting, the concentration, and the temperature are higher than some critical values. The sol–gel‐phase transition temperatures have been determined. It was found that at temperatures below LCST the systems behave like a solution, whereas at temperatures above LCST, the solutions behave like a stiff gel, because of PNIPAAm segregation. This behavior is reversible and could find applications in tissue engineering and drug delivery systems. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Physical properties of poly(vinyl chloride)–grafted N‐isopropylacrylamide graft copolymers and corresponding polyblends

The physical properties of poly(vinyl chloride) (PVC) and poly(N‐isopropylacrylamide) [poly(NIPAAm)] blend systems, and their corresponding graft copolymers such as PVC‐g‐NIPAAm, were investigated in this work. The compatible range for PVC–poly(NIPAAm) blend systems is less than 15 wt % poly(NIPAAm). The water absorbencies for the grafted films increase with increase in graft percentage. The water absorbencies for the blend systems increase with increase in poly(NIPAAm) content within the compatible range for the blends, but the absorbencies decrease when the amount of poly(NIPAAm) is more than the compatible range in the blend system. The tensile strengths for the graft copolymers are larger than the corresponding blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 170–178, 2000     

Temperature‐sensitive switch from composite poly(N‐isopropylacrylamide) sponge gels

Thermally sensitive polymers change their properties with a change in environmental temperature in a predictable and pronounced way. These changes can be expected in drug delivery systems, solute separation, enzyme immobilization, energy‐transducer processes, and photosensitive materials. We have demonstrated a thermal‐sensitive switch module, which is capable of converting thermal into mechanical energy. We employed this module in the control of liquid transfer. The thermally sensitive switch was prepared by crosslinking poly(N‐isopropylacrylamide) (PNIPAAm) gel inside the pores of a sponge to generate the composite PNIPAAm/sponge gel. This gel, contained in a polypropylene tube, was inserted into a thermoelectric module equipped with a fine temperature controller. As the water flux through the composite gel changes from 0 to 6.6 × 10² L m⁻² h, with a temperature change from 23 to 40°C, we can reversibly turn on and off the thermally sensitive switch. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75:1735–1739, 2000     

Synthesis and characteristics of semi‐interpenetrating polymer network hydrogels based on chitosan and poly(hydroxy ethyl methacrylate)

Semi‐interpenetrating polymer networks (semi‐IPNs), composed of chitosan and poly(hydroxy ethyl methacrylate) hydrogels, were prepared and the effects of various pH, temperatures, and an electric‐field on the swollen hydrogels were investigated. The swelling kinetics increased rapidly, reaching equilibrium within 60 min. Semi‐IPN hydrogels exhibited relatively high swelling ratios, 150～350%. The swelling ratio increased when the pH of the buffer was below pH 7 as a result of the dissociation of ionic bonds. Semi‐IPN hydrogels showed electroresponsiveness by shrinking when an electric field was applied. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 86–92, 2005     

Thermoresponsive behaviour in aqueous solution of poly(maleic acid‐alt‐vinyl acetate) grafted with poly(N‐isopropylacrylamide)

The synthesis of a thermoresponsive graft copolymer consisting of a maleic acid/vinyl acetate alternating copolymer backbone (MAc‐alt‐VA) and poly(N‐isopropylacrylamide) (PNIPAM) side chains is reported. Turbidimetric measurements in dilute aqueous solutions showed that no macroscopic phase separation takes place when the temperature is raised above the lower critical solution temperature (LCST) of PNIPAM, even at pH = 2. Moreover, in semi‐dilute aqueous solutions, a pronounced thermally induced viscosity increase (thermothickening) was observed. This thermoresponsive behaviour has been attributed to the interconnection of the hydrophilic MAc‐alt‐VA graft copolymer backbones by means of the hydrophobic PNIPAM side chain aggregates formed as the temperature increases above the LCST of this polymer. Copyright © 2004 Society of Chemical Industry