Effect of amino acid surfactants on phase transition of poly(N-isopropylacrylamide) gel

The volume phase transition behavior of a poly(N-isopropylacrylamide) gel (NIPA gel) in solutions of N-acyl amino acid surfactants were studied as a function of surfactant concentration. The addition of a surfactant beyond the critical micelle concentration (cmc) produced elevation in the transition temperature of the NIPA gel and its swelling. The changes in the volume phase transition temperature and in the swelling of the NIPA gel became more significant with the decreasing size of the amino acid side chain. This result could almost be explained only by the binding amount of surfactant onto the NIPA gel regardless of molecular structure of the amino acid. The binding amount increased in the order of sodium N-lauroyl-glycinate>-alaninate>-valinate>-leucinate>or=-phenylalaninate. For an N-acyl amino acid surfactant to bind onto the NIPA gel, to increase the transition temperature, and to facilitate swelling of the gel, the steric hindrance of the amino acid side chain was more effective than its hydrophobicity.     

Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

Summary: Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems, depending on monomer concentration. In addition to pure hydrogels, the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. N,N′-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization, thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent β obeyed the percolation result.     

Potential application of poly(N-isopropylacrylamide) gel containing polymeric micelles to drug delivery systems

We have investigated rapidly thermo-responsive NIPA gel containing polymer surfactant PMDP (NIPA-PMDP gel) as a potential drug carrier using (+)-l-ascorbic acid as a model drug. In the NIPA-PMDP gel system micelles of polymer surfactant PMDP are trapped by the entanglement of polymer chains inside the gel networks. Therefore, in principle the gel system tightly stores targeted drug in the micelles and rapidly releases controlled amount of the drug by switching on-off of external stimuli such as temperature or infrared laser beam. In our investigation on release profile, the NIPA-PMDP gel system showed completely different releasing behavior from that of the conventional NIPA gel. The NIPA-PMDP gel released rapidly all loaded (+)-l-ascorbic acid above the phase transition temperature (ca. 34 degrees C), while slowly released the corresponding amount of the drug below the temperature. In contrast, the conventional NIPA gel released more slowly limited amount of the drug above the phase transition temperature while similarly did to the NIPA-PMDP gel below the temperature. The release profile of the NIPA-PMDP gel seems to be governed by only kinetics of volume phase transition of the gel network but not by the hydrophobic domains of the micelles probably because of too hydrophilic nature of (+)-l-ascorbic acid.     

Macromolecule–metal complexes exhibiting phase transition

A copolymer gel prepared from N-isopropyl-acrylamide (NIPA) and vinylferrocene (VF) exhibits volume phase transition. The phase transition can be controlled electrochemically, and electrochemical behavior of the gel can be controlled thermally. A copolymer of NIPA and VF, which is not crosslinked, also possesses similar characteristics. Those polymer and gel can be applied to enzyme electrodes. © 1997 John Wiley & Sons, Ltd.     

Small-angle neutron-scattering study on a structure of microemulsion mixed with polymer networks

The structure of a microemulsion mixed with polymer networks was investigated by means of small-angle neutron scattering (SANS). The system consists of nonionic surfactant, polymer network, oil, and water. The microemulsion and the polymer network employed in this work are known to undergo temperature-induced structural transition and volume phase transition, respectively. Polymer solutions and gels were made by polymerizing monomer solutions in the presence of microemulsion droplets. In the case of a mixture of an N-isopropylacrylamide (NIPA) monomer solution and a microemulsion, the NIPA monomer was found to behave as a cosurfactant. However, polymerization resulted in a phase separation to polymer-rich and -poor phases. Interestingly, SANS results indicated that a well-developed ordered structure of oil domains was formed in polymer network and the structure was very different from its parent systems. Furthermore, the system underwent two different types of structural transitions with respect to temperature. One was originated from the structural transition of microemulsion due to the change of the spontaneous curvature and the other from the volume phase transition of the NIPA gel.     

H NMR studies of poly(N-isopropylacrylamide) gels near the phase transition

The phase transition and critical phenomenon of equilibrium swollen poly(N-isopropylacrylamide) (NIPA) hydrogels were studied by ¹H NMR spectroscopy in liquid solution mode. The quantitative NMR observation shows that the peak height and line width of polymer proton and of the HOD proton, and relaxation times of HOD proton all transitionally change as the temperature approaches the transition temperature. The relaxation times of water protons are also measured quantitatively, which shows that the temperature dependence of relaxation times of HOD on temperature before the transition is not consistent with relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion. To explain the surprising relaxation behavior of HOD, we suggest that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition. The pulsed-gradient spin-echo NMR experiments of NIPA gel confirm this suggestion. We believe that these results have important implications concerning the mechanism of the phase transition of NIPA hydrogels.     

温敏凝胶体积相转变温度的压敏性

温敏凝胶体积相转变温度的压敏性钟兴，王宇新，王世昌（天津大学化学工程研究所，天津，３０００７２）金曼蓉（天津第二医学院药学系，天津，３００２０３）关键词Ｎ－烷基丙烯酰胺凝胶、体积相转变、温度敏感性、压力敏感性环境敏感性凝胶（交联聚合物）在生化分离、固...     

Formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide and sodium acrylate observed by ATR-FTIR spectroscopy

In this paper, the formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) were studied using Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR). Aqueous solutions of NIPA and SA monomers with different pHs were prepared, and the ATR-FTIR spectra were obtained immediately after preparing the solution and after having been stored at room temperature for 6 months. It was found that the IR spectra evidently changed after 6 months due to polymerization, and the viscosity of a solution in the lowest pH system increased with time and finally the solution became a gel. The detailed analysis of the IR spectra indicated that the network of the gel was formed by the formation of hydrogen bonds (without crosslinker). Moreover, this physical gel exhibited the re-swelling transition by increasing the pH of solvent. The transition was caused by the destruction of hydrogen bonds due to the dissociation of carboxyl groups, which was also confirmed by the IR spectroscopy.     

Preparation of poly(N-isopropylacrylamide) emulsion gels and their drug release behaviors

Stimuli-sensitive drug delivery systems (DDSs) have attracted considerable attention in medical and pharmaceutical fields; thermosensitive DDS dealing with poly(N-isopropylacrylamide) (poly(NIPA)) have been widely studied. Novel NIPA emulsion gels, i.e., NIPA hydrogels containing distributed oil (oleyl alcohol) microdroplets, were synthesized by means of an emulsion-gelation method in which the polymerization of hydrogels in an aqueous phase in an oil-in-water (O/W) emulsion and the loading of a lipophilic drug (indomethacin) dissolved in an oil phase were accomplished simultaneously. The pulsatile (on-off) drug release from the NIPA emulsion gel loading indomethacin to a phosphate buffered saline (PBS) solution was successfully controlled by a temperature swing between 25 degrees C (release off) and 40 degrees C (release on). The mechanism of the pulsatile drug release was discussed in relation to the diffusion rate, distribution ratio, solvent exchange of NIPA hydrogels, and drug release from an NIPA organogel. The mechanism was as follows: the solvent exchange occurred within the NIPA emulsion gel (the NIPA gel-network absorbed oleyl alcohol with indomethacin) at temperatures above the LCST, and the diffusion rate of indomethacin through the solvent-exchanged gel was higher at 40 degrees C than at 25 degrees C.     

Nanostructural characterization of the dehydrated (NIPA/SA+additive ion) gels

In the present study, the authors have investigated ion-absorption effects on a nanoscopic structure of a dehydrated N-isopropylacrylamide/sodium acrylate (NIPA/SA) gel.

First of all, the authors compared small-angle X-ray scattering (SAXS) profile of a dehydrated NIPA/SA gel with that dehydrated after absorbing Cu²⁺ ion. Then, in order to examine copper-ion distribution structure in the NIPA/SA gel dehydrated after absorption of copper-ion, an incident-X-ray energy-dependence of a small-angle X-ray scattering profile was observed, in which the anomalous dispersion effect was clearly perceived especially around a distinct SAXS-peak. Because the SAXS-peak is thought to come from a dehydration-induced microphase separation between hydrophilic and hydrophobic network-polymers in the NIPA/SA gel, such a feature indicates that the copper-ions gather in the dehydration-induced hydrophilic domains.

In addition to this interesting copper-ion nanostructure in the dehydrated NIPA/SA gel, a difference in the SAXS-peak position between the dehydrated NIPA/SA gels with and without absorbing the copper-ion has shown a possibility of a controlling method of the nanostructure in relatively gentle conditions without special instruments. Along this line, in order to get further information on the ion-absorption effects on the nanostructure, the authors have compared the SAXS profiles of the several NIPA/SA gels which were dehydrated after absorbing respectively different kinds of ions. In the observation, the SAXS-peak positions have shown characteristic features which are different with the kind of the absorbed ions and found to be classified into several kinds according to the periodic-table group of the absorbed ion.     

Cyclic voltammetric study of redox-active surfactant by hydrogel-modified electrode

The dissolved states of redox-active non-ionic surfactant (FPEG) in the swollen state of N-isopropyl acrylamide (NIPA) hydrogel have been studied by using a gel-modified electrode. The pronounced decrease in the peak current and the negative shift in the formal potential of CV at the gel-modified electrode, as compared with the normal GC electrode, was observed in the micelle-solution; this indicates that the diffusive FPEG molecules which form the micelle hardly penetrate into the NIPA gel. This result suggests that there exists an interaction between FPEG molecules and the NIPA gel in the vicinity of the surface of the NIPA gel in the micelle-solution. However, this also indicates that a small amount of FPEG molecules which can form micelles exist in the NIPA gel.     

A Novel Thermosensitive Gel Adsorbent for Phosphate Ions

Summary: A novel thermosensitive gel adsorbent for phosphate ions was developed and its adsorption/desorption properties were investigated. The gel adsorbent was made by the copolymerization of N-isopropylacrylamide (NIPA) and N-[3-(dimethylamino)propyl]acrylamide (DMAPAA). The adsorbent has a volume phase transition temperature (VPTT), below which it becomes hydrophilic. The tertiary amino groups of DMAPAA were ionized and showed a cationic state when the gel swelled. Phosphate ions were adsorbed onto the ionized tertiary amino groups in the gel network below the VPTT and were desorbed above the VPTT because of the suppression of the ionization of the tertiary amino groups of gel and the shrinkage of the gel.     

Roles of hydrophobic interaction in a volume phase transition of alkylacrylamide gel induced by the hydrogen-bond-driving alkylphenol binding

It was found that a degree of the binding of alkylphenols to N-alkylacrylamide gel increased transitionally to induce the volume phase transition of gel. Binding isotherms of nonylphenol (n-Ph), propylphenol (p-Ph), ethylphenol (e-Ph), methylphenol (m-Ph), and phenol (Ph) to N-isopropylacrylamide (NIPA), N,N-diethylacrylamide (DEA), N,N-dimethylacrylamide (DMA), and acrylamide (AM) gels were examined. Two types of binding, the site binding at beta < 1 and the multimolecular binding at beta > 1, were observed, where beta was a degree of binding to a monomeric unit of the chain. The former binding was analyzed with the Hill equation applicable to the cooperative binding and the latter binding with the Brunauer-Emmett-Teller (BET) equation applicable to the multilayer adsorption. The binding constant, K, and the Hill coefficient, N, decreased and increased, respectively, in the order of DEA, NIPA, and DMA gels in the case where the binding alkylphenol was the same. The K value increased in the order of Ph, m-Ph, e-Ph, p-Ph, and n-Ph that bound to the same type of gel. The N value was found to change little with the type of binding alkylphenol. The complexes of N-alkylamide with alkylphenol were condensed to form the ordered nanostructures that were observed as broad scattering peaks in small-angle X-ray scattering experiments. The fluorescence excimer emission was observed for the phenol-binding DMA gel, which corresponded to the condensed state of phenol.     

Temperature-reversible ultrathin films of N-isopropylacrylamide terpolymer adsorbed at the solid-liquid interface

This article describes the stability and reversibility of ultrathin films of N-isopropylacrylamide (NIPA)-vinylimidazole (VI)-poly(ethylene glycol) (PEG) graft terpolymer adsorbed at the solid-liquid interface upon temperature cycling from below to above its phase transition temperature. The coil-to-globule and globule-to-coil phase transitions were captured by in situ fluid tapping atomic force microscopy (AFM). The film thickness of 1 nm was determined by AFM, X-ray photoelectron spectroscopy, and X-ray reflectivity. The concentration required to reach full coverage was found to be higher when adsorption occurred below the phase transition temperature. From 23 to 42 degrees C, the adsorbed NIPA terpolymer film was observed to be molecularly smooth, corresponding to the close-packed structure of flexible polymer coils. Particles containing between one and a few globules appeared abruptly at the interface at 42-43 degrees C, the same temperature as the solution phase transition temperature, which was determined by dynamic light scattering. The size of the particles did not change with temperature, whereas the number of particles increased with increasing temperature up to 60 degrees C. The particles correspond to the collapsed and associated state of the globules. The film morphological changes were found to be reversible upon temperature cycling. Subtle differences were observed between dip-coated and spin-coated films that are consistent with a higher degree of molecular freedom for spin-coated films. The study contributes to the fundamental understanding and applications of smart ultrathin films and coatings.     

Solvent concentrations of dimethylsulfoxide-water and 1-propanol-water solutions inside and outside poly (N-isopropylacrylamide) gel

The volumes of poly (N-isopropylacrylamide) (NIPA) gel in both dimethylsulfoxide (DMSO)-water and 1-propanol-water solutions were measured at 25°C. The solvent concentrations inside and outside the NIPA gel were also measured. The gel was swollen in water, shrunk according to increase in concentration of organic solvent, and reswollen in pure organic solvent. This phenomenon is typical reentrant swelling behavior. The DMSO concentrations inside the gel were almost equal to those outside the gel in the whole concentration range. On the other hand, the 1-propanol concentrations between inside and outside the gel were much different from each other in the shrunk state, though they were almost the same in the swollen state.     

Correlation of chemical structure and swelling behavior in N-alkylacrylamide hydrogels

The thermoshrinking properties have been studied for the series of N-alkyl-acrylamide hydrogels (alkyl = methyl, ethyl, isopropyl, and n-propyl), which were prepared by free-radical copolymerization of the alkylacrylamide, sodium acrylate, and N,N′-methylenebis(acrylamide) (BIS) in aqueous solution. The reaction mixtures were prepared using the same nominal compositions in an effort to study the effect of the chemical structure of the alkyl substituent on the gel swelling behavior as a function of temperature. The alkyl group was found to have a pronounced effect on the features of gel swelling. Generally, larger alkyl chains produced dramatic decreases in gel transition temperature. In addition, a change in the nature of the swelling behavior from continuous to discontinuous was noted upon changing the alkyl group from ethyl to the two propyl derivatives. Discontinuous transitions were accompanied by hysteresis. The transition temperatures of the isomeric propyl derivatives were found to differ by 12°C, with n-propyl exhibiting the lower value. Additionally, a quantitative correlation was found between the gel transition temperatures and the water/octanol partition coefficients for appropriately chosen small molecule model compounds. The transition temperatures of other gels in the series, including the cyclopropyl derivative and the n-propyl/isopropyl copolymer gels (NIPA/NNPA), also fit this correlation. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2095–2102, 1998     

Novel synthesis of macroporous poly(N-isopropylacrylamide) hydrogels using oil-in-water emulsions

Porous N-isopropylacrylamide (NIPA) hydrogels having a unique structure, that is, spherelike cavities distributed randomly and a homogeneous network in the gel phase, were successfully synthesized by means of an emulsion templating method; this method involves the synthesis of NIPA gels in an oil-in-water (O/W) emulsion by free radical copolymerization with a cross-linker, followed by washing (removal) of the dispersed oil as a pore template (porogen). The synthesis conditions, O/W volume ratio, amount of added surfactant, and monomer concentration affect the internal pore structure, equilibrium swelling, and swelling/shrinking kinetics. A porous hydrogel swollen at 10 degrees C has a pore diameter distribution in the range of 1-40 microm, which was observed with a scanning electron microscope. Scanning electron micrographs and swelling degree reveal that the pore size and porosity can be adjusted by varying the O/W volume ratios and surfactant amounts. The porous hydrogels show very rapid swelling/shrinking in accordance with the temperature swing. The fast response is attributed to the convection flow of water through the macropores. In addition to a faster response gel, the emulsion templating method can yield potentially intelligent gels in which the pores function as spaces for reaction, separation, and storage.     

Temperature-swing adsorption of aromatic compounds in water using polyampholyte gel

The adsorption property of the polyampholyte gel composed of sodium styrene sulfate (SSS) and vinylbenzyl trimethylammonium chloride (VBTA) has been investigated with several hydrophobic aromatic compounds as adsorbate. Using the N-isopropylacrylamide (NIPA) gel, the corresponding experiments were also performed for comparison. At room temperature, the NIPA gel hardly adsorbed the aromatic compounds, while it adsorbed them at higher temperatures. As for the SSS-VBTA gel, the adsorption amounts of the polyaromatic compounds decreased with increasing temperature, while the adsorption amounts of the monoaromatic compounds were almost independent of temperature and smaller than those of the polyaromatic compounds. These results indicate that the aromatic rings in the SSS-VBTA gel may play an important role in the adsorption of the aromatic compounds. Also, it has been demonstrated that the SSS-VBTA gel can repeatedly adsorb bisphenol-A at room temperature and desorb it at higher temperature by the temperature-swing operation: this behavior is diametrically opposite to that of the NIPA gel. This shows that the SSS-VBTA gel is much more suitable for the adsorption removal of the hydrophobic aromatic compounds from very dilute aqueous solutions, because a vast amount of energy is required for heating a large amount of water when using the NIPA gel.     

Non-radiative energy transfer in N-isopropylacrylamide gel

Non-radiative energy transfer from donor to acceptor probes attached to the network chains of an N-isopropylacrylamide (NIPA) gel was investigated in water at various temperatures through fluorescence spectrum measurements. Naphthalene or biphenyl was used as a donor, and pyrene as an acceptor. Two types of doubly labeled NIPA gels (NIPA/2-vinylnaphthalene/pyrenylacrylamide and NIPA/4-vinylbiphenyl/pyrenylacrylamide) were prepared by copolymerization. The non-radiative transfer was observed for both gels. The ratio of emission intensity from donor to that from acceptor was strongly dependent on temperature, which indicates that the non-radiative energy transfer in doubly labeled NIPA gels could be controlled by means of the volume change with temperature.     

Facile preparation of methylcellulose/poly(vinyl alcohol)physical complex hydrogels with tunable thermosensitivity

Novel complex hydrogels of methylcellulose(MC)and poly(vinyl alcohol)(PVA)with wide-spectrum thermoresponsivity were prepared via physical and mild process.Thermal phase transition of MC/PVA hydrogels exhibited two forms including sol/sol to gel/sol and sol/gel to gel/gel.The phase transition temperature of MC/PVA solution ranged from 38.7 to 60.6℃and was able to be adjusted by simply changing the feeding ratios of two components.The interior morphology of MC/PVA gels was examined with fluorescence analy...