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     

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     

Preparation of temperature sensitive poly(vinylidene fluoride) hollow fiber membranes grafted with N‐isopropylacrylamide by a novel approach

In the present study, the temperature sensitive PVDF‐g‐NIPAAm HFM was prepared by grafting N‐isopropylacrylamide (NIPAAm) on poly(vinylidene fluoride) (PVDF) hollow fiber membrane (HFM) using a novel approach, alkaline treatment method. The structures of PVDF‐g‐NIPAAm HFM were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The effects of alkaline treatment time and grafting yield on the mechanical properties of PVDF HFM were measured and analyzed. In addition, the temperature sensitive behavior of PVDF‐g‐NIPAAm HFM and the effect of grafting yield on the temperature sensitive behavior were investigated by the flux of pure water and the rejection of ovalbumin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 833–837, 2006     

pH‐ and temperature‐sensitive microfiltration membranes from blends of poly(vinylidene fluoride)‐graft‐poly(4‐vinylpyridine) and poly(N‐isopropylacrylamide)

The copolymer poly(vinylidene fluoride)‐graft‐poly(4‐vinylpyridine) (PVDF‐g‐P4VP) was prepared through the graft copolymerization of poly(vinylidene fluoride) with 4‐vinylpyridine. Through the blending of the PVDF‐g‐P4VP copolymer with poly(N‐isopropylacrylamide) (PNIPAm) in an N‐methyl‐2‐pyrrolidone solution, PVDF‐g‐P4VP/PNIPAm membranes were fabricated by phase inversion in aqueous media. Elemental analyses indicated that the blend concentration of PNIPAm in the blend membranes increased with an increase in the blend ratio used in the casting solution. Scanning electron microscopy revealed that the membrane surface tended to corrugate at a low PNIPAm concentration and transformed into a smooth morphology at a high PNIPAm concentration. The surface morphology and pore size distribution of the microfiltration membranes could be regulated by the blend concentration of the casting solution, temperature, pH, and ionic strength of the coagulation bath. X‐ray photoelectron spectroscopy revealed a significant enrichment of PNIPAm on the membrane surface. The flux of aqueous solutions through the blend membranes exhibited a pH‐ and temperature‐dependent behavior. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4089–4097, 2006     

Temperature and pH‐Dependent Swelling Behavior of Poly(N‐isopropylacrylamide) Copolymer Hydrogels and Their Use in Flow Control

Poly(N‐isopropylacrylamide) (PNIPAAm) copolymer gels with acidic and basic comonomers of various molar ratios were prepared by radical polymerization. The relationship between the swelling behavior of the gels and the copolymerization ratio was studied experimentally at different temperatures and in various pH value buffer solutions. The results of the experiments revealed that the transition temperatures of the PNIPAAm copolymer gels were changed in proportion to the monomer ratio used in copolymerization. The pH value of the buffer solution strongly affected the swelling ratio and some of the transition temperatures of the gels. The PNIPAAm copolymer gels were used in a chemo‐mechanical valve. The liquid flows directly through a gel actuator, which consists of a cylindrical actuator chamber filled with small particles of the sensitive cross‐linked polymer. The flow rate as well as the pressure drop was measured in dependence on the solvent properties. With the presented experimental arrangement it could be shown that sensitive polymers can be used for controlling the flow in dependence on temperature and pH.

pH‐Dependence of the pressure in a chemo‐mechanical valve.     

Thermosensitive poly(N‐isopropylacrylamide) hydrogel for refolding of recombinant bovine prethrombin‐2 from E. coli inclusion bodies

The poly(N‐isopropylacrylamide) (PNIPA) hydrogel, which is a kind of temperature‐sensitive polymer, was synthesized by inverse suspension polymerization. The microscopy and scan electron microscopy (SEM) of PNIPA hydrogel were studied. The microscope photograph showed that the particles were in the range of 0.2–0.5mm in diameter, with numerous conjoint pores about 1–2μm spreading all over the surface of the beads. The swelling properties of PNIPA gel beads indicated that the lower critical solution temperature (LCST) of the gel was 33°C. The PNIPA prepared was applied to the renaturation of bovine prethrombin‐2 (pThr‐2) from inclusion bodies produced in E. coli. It was observed that PNIPA was quite efficient in assisting protein renaturation at high protein concentration. When mixing with 105mg/mL PNIPA hydrogel during the refolding, the total activity of the thrombin was about 6222U/mL, compared with only 2800U/mL by simple dilution refolding. The kinetics of pThr‐2 refolding with the absence or the presence of PNIPA was also studied respectively. The time required for the refolding with PNIPA gel was a little bit longer than that by the dilution method owing to the diffusion resistance of the protein into the network of the gel and the hydrophobic interaction between the protein and the polymer. The mechanism of the enhancement for the PNIPA gel to the refolding was further discussed. The porosity of the PNIPA hydrogel allows penetration of the unfolded protein into the inside of the polymer with a hydrophobic side chain, which can facilitate the formation of intermediate via hydrophobic interaction with the unfolded protein and the folding intermediate that are liable to re‐aggregation. About 1.2mg of purified active thrombin could be recovered from 1 L of cells, which greatly facilitated the scale‐up to the quantities of protein necessary for further functional and structural studies. A novel protein renaturation method mediated by PNIPA hydrogel beads, which highly increases the refolding efficiency with easy handling, recycling, and low cost, was proposed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1734–1740, 2005     

pH and thermo‐responsive poly(N‐isopropylacrylamide) copolymer grafted to poly(ethylene glycol)

pH and thermo‐responsive graft copolymers are reported where thermo‐responsive poly(N‐isopropylacrylamide) [poly(NIPAAm), poly A ], poly(N‐isopropylacrylamide‐co‐2‐(diethylamino) ethyl methacrylate) [poly(NIPAAm‐co‐DEA), poly B ], and poly(N‐isopropylacrylamide‐co‐methacrylic acid) [poly(NIPAAm‐co‐MAA), poly C ] have been installed to benzaldehyde grafted polyethylene glycol (PEG) back bone following introducing a pH responsive benzoic‐imine bond. All the prepared graft copolymers for PEG‐g‐poly(NIPAAm) [ P‐N1 ], PEG‐g‐poly(NIPAAm‐co‐DEA) [ P‐N2 ], and PEG‐g‐poly(NIPAAm‐co‐MAA) [ P‐N3 ] were characterized by ¹H‐NMR to assure the successful synthesis of the expected polymers. Molecular weight of all synthesized polymers was evaluated following gel permeation chromatography. The lower critical solution temperature of graft copolymers varied significantly when grafted to benzaldehyde containing PEG and after further functionalization of copolymer based poly(NIPAAm). The contact angle experiment showed the changes in hydrophilic/hydrophobic behavior when the polymers were exposed to different pH and temperature. Particle size measurement investigation by dynamic light scattering was performed to rectify thermo and pH responsiveness of all prepared polymers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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     

Preparation of composite‐crosslinked poly(N‐isopropylacrylamide) gel layer and characteristics of reverse hydrophilic–hydrophobic surface

The composite‐crosslinked poly(N‐isopropylacrylamide) (PNIPAAm) gels were prepared by grafting N‐isopropylacrylamide on the surface of glass plates modified by organosilanes. The glass plates as the substrate increase the mechanical strength of composite PNIPAAm gel layers. We investigated the effects of a series of organosilanes and the reaction time of organosilanes on surface characteristics, such as the static contact angle and the layer thickness. We discuss the equilibrium swelling ratio and the water release behavior of the gel layers in terms of the crosslinking density of the composite gels. The composite gels exhibit not only the characteristics of remarkable water release but also the reversed hydrophilic–hydrophobic surface properties. The gel layers are hydrophilic under 25°C and change to hydrophobic above 40°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1–11, 1999     

pH/temperature‐responsive semi‐IPN hydrogels composed of alginate and poly(N‐isopropylacrylamide)

Amino semitelechelic poly(N‐isopropylacrylamide) (PNIPAAm) was prepared by radical polymerization with aminoethanethiol hydrochloride as a chain‐transfer agent. Semi‐interpenetrating polymer network (semi‐IPN) hydrogels, composed of alginate and amine‐terminated PNIPAAm, were prepared by crosslinking with calcium chloride. From the swelling behaviors of semi‐IPNs at various pH's and Fourier transform infrared spectra at high temperatures, the formation of a polyelectrolyte complex was confirmed from the reaction between carboxyl groups in alginate and amino groups in modified PNIPAAm. Semi‐IPN hydrogels reached an equilibrium swelling state within 24 h. The water state in hydrogels, investigated by differential scanning calorimetry, showed that sample CAN55 [alginate/PNIPAAm (w/w) = 50/50] exhibited the lowest equilibrium water content and free water content among the hydrogels tested, which was attributed to its more compact structure compared to other samples and the high content of interchain bonding within the hydrogels. Alginate/PNIPAAm semi‐IPN hydrogels exhibited a reasonable sensitivity to the temperature, pH, and ionic strength of swelling medium. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1128–1139, 2002     

Blends composed of poly(N‐isopropylacrylamide) and an ethylene/vinyl alcohol copolymer: Thermal and morphological studies

Blends of poly(N‐isopropylacrylamide) (PNIPAM) and an ethylene/vinyl alcohol copolymer (EVAL) were obtained through casting from dimethyl sulfoxide (DMSO) solutions and phase inversion in 50/50 DMSO/H₂O solutions. The miscibility and morphology of the PNIPAM/EVAL blends were investigated with thermal and morphological analysis. Differential scanning calorimetry indicated that the crystallinity of EVAL decreased with increasing PNIPAM content and that the blends cast from DMSO/H₂O solutions were miscible in the melt state. Measurements of the melting point depression allowed the determination of the interaction energy density (B) and Flory–Huggins interaction parameter (χ₁₂) with the Nishi–Wang equation. The negative B and χ₁₂ values obtained were examined in terms of the specific intermolecular interactions between the polymers. Scanning electron micrographs revealed that blends obtained by the casting method led to dense membranes, whereas the phase‐inversion method rendered typical macroporous membranes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 501–505, 2004     

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.     

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     

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     

Effects of cononsolvency on preferential adsorption phenomenon in poly(N‐isopropylacrylamide) ternary solutions

In this work, the effect of cononsolvency on the phase transition and preferential adsorption phenomenon behaviors of poly(N‐isopropylacrylamide)/methanol/water ternary solutions was studied. In this cononsolvent system, the , , and χ₁₂ values show a nonlinear behavior and the minimum values of and , while the maximum value of χ₁₂ at ϕ₂ is around 0.7. These facts indicated that one water molecule could directly bond with one methanol molecule to form the H₂O MeOH complex. The H₂O MeOH complex structure was found to remarkably affect the phase transition of poly(N‐isopropylacrylamide) (PNIPAM) in ternary solution. However, at the composition of mixed cononsolvent, ϕ₂ < 0.2, the PNIPAM molecules may preferentially adsorbed pure water molecules; therefore, the LCST decreases slightly with composition of mixed cosolvent and this may be because of the small amount of H₂O MeOH complexes in the mixed cononsolvent. While, at ϕ₂ > 0.7, the PNIPAM molecules may preferentially adsorbed pure methanol molecules. PNIPAM ternary solutions were transparent and no transition occurred in this region. This indicates that the PNIPAM coils exhibited a much‐extended conformation in solutions. In contrast, at 0.2 < ϕ₂ < 0.4 and 0.4 < ϕ₂ < 0.7, PNIPAM molecules preferentially adsorbed water and methanol molecules, respectively, and also adsorbed large amount of H₂O MeOH complexes. In these regions, the clathrate‐like structure around the side chain of PNIPAM molecule became more defected with adsorbing H₂O MeOH complex. Therefore, we considered that the various thermodynamic behaviors between PNIPAM and mixture solvents must be related different preferential adsorption phenomena, which were mainly related to different degrees of polymer–solvent interaction and structures of solvent used. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

Concentration effect on aggregation and dissolution behavior of poly(N‐isopropylacrylamide) in water

The concentration effect on aggregation and dissolution behavior of poly(N‐isopropylacrylamide) (PNIPAM) in water was studied. Three concentration regimes with different phase behavior were identified by differential scanning calorimetry (DSC). Further optical, light‐scattering, and rheological studies indicated that the appearance of different regimes arose from their corresponding solution structures below lower critical solution temperature (LCST): free chains and small clusters in regime I, large clusters in regime II, and a gel‐like network in regime III. Different solution structures below LCST led to different phase‐separated patterns formed above LCST: colloidal particles in regime I, large precipitate in regime II, and the sponge‐like solid in regime III, which was well understood based on the overlapping parameter P. Different phase‐separated patterns therefore resulted in different remixing behavior as observed by DSC. This work suggests that the swelling and collapse behavior of PNIPAM based hydrogels was controlled through the design of their phase‐separated patterns, and therefore provided a way to develop high performance thermo‐sensitive materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41669.     

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     

Swelling–shrinking hysteresis of poly(N‐isopropylacrylamide) gels in sodium dodecylbenzenesulfonate solutions

The swelling and shrinking behaviors of a series of poly(N‐isopropylacrylamide) (PNIPA) hydrogels are studied in aqueous solutions of sodium dodecylbenzenesulfonate (SDBS). Between 0 and 3 mol % 2‐acrylamido‐2‐methylpropanesulfonic acid sodium salt (AMPS) is used as an ionic comonomer in the hydrogel synthesis. It is shown that the collapsed PNIPA gels in water at 52°C start to swell above a critical SDBS concentration in the external solution. This critical concentration decreases as the ionic group content of PNIPA gel increases. A comparison of the swelling and shrinking experiments in SDBS solutions indicates strong hysteresis behavior of PNIPA gels. A more diluted solution is required to make a swollen gel start to reshrink than to cause gel swelling. The results show strong attractive forces between the isopropyl groups of the PNIPA network and the DB groups of SDBS molecules. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1228–1232, 2002     

Characteristic role of crosslinker on thermally induced volumetric contraction–expansion processes in poly(N‐isopropylacrylamide) networks and water systems

In this work, the degree of crosslinking on the volumetric contraction–expansion processes of hydrogels made of poly(N‐isopropylacrylamide) (NIPA) (initial amount: C_m) with varied amount (z) of crosslinking agent methylene‐bis‐acrylamide (BIS) in reference to most commonly used NIPA gel that was synthesized with C_m = 700 mM and z = 8.6 mM was investigated by applying our recently developed pycnometry. We focused on characteristic role of four polymeric NIPA residues directly bonded to a single BIS molecule by evaluating the total volume of gels per four NIPA residues directly bonded to a BIS molecule, plus associated water [ν_sp(gel)(NIPA)_bonded)(T)], and the corresponding number of water molecules per four NIPA residues [N_s(gel)(NIPA)_bonded)(T)]. We elucidated how these quantities characteristically contribute to changes in the volumetric contraction–expansion processes of hydrogels. A comparison of these quantities with the corresponding quantities for (NIPA)_unbonded residues clearly revealed a significant structural difference between (NIPA)_bonded and (NIPA)_unbonded. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009