Swelling behavior of poly(sodium acrylate) gels crosslinked by aluminum ions

The cylindrical poly(sodium acrylate) gel (SA gel) was synthesized in the glass capillary using aluminum ions as the crosslinker. The swelling ratio of the gel was measured after the repeated exchange of solvent (distilled deionized water, about pH 5.8). The gel exhibited two relaxation processes; at first the gel swells rapidly as exchange of water (the swelling process), then shrinks very slowly (the shrinking process). In order to reveal the microscopic structural change (especially, the formation of hydrogen bonding) by water exchange, attenuated total refraction (ATR) Fourier transform infrared (FT-IR) spectroscopy was applied to the gels with different swelling ratio. The IR absorption peaks of the gel were assigned based on those of poly(sodium acrylate) aqueous solutions at different pH. On the swelling process, the carboxyl groups were gradually protonated, and the intermolecular hydrogen bonding started to form in the gel with maximum swelling ratio. On the shrinking process, the formation of hydrogen bonding gradually increased with long-time repeated water exchange which resulted in the shrinkage of the gel. Effects of the repeated water exchange on the swelling behavior were discussed in terms of the exchange of counter ions and the formation of hydrogen bonding.     

Effects of repeated water exchange on the swelling behavior of poly(sodium acrylate) gels crosslinked by aluminum ions

Polyelectrolyte hydrogels, physically crosslinked by metal ions, were synthesized using poly(sodium acrylate) as the main constituent and Al ions as the crosslinker. The swelling ratio of the gel was measured whenever the solvent water was repeatedly exchanged in a constant interval. The as‐synthesized gel exhibited two relaxation processes; the gel swelled at the first stage, then shrunk very slowly at the second stage, and recovered to the initial size just after the gelation (ultimately, the gel became smaller than that). The relaxation times of both processes were found longer (exceptionally longer for the shrinking process) than the conventional collective diffusion of polymer networks. The diffused amounts of Al ions and Na counter ions in the solvent were also measured at each water exchange. The diffusion of Al ions into the solvent was found to finish when the swelling ratio took the maximum (at the end of the first stage), while Na ions continued to diffuse until the diameter became the final one (at the end of the second stage). The microscopic structural changes by the repeated water exchange were obtained by the measurements of ATR FT‐IR spectroscopy on the gels with different swelling ratios. The carboxyl groups were gradually protonated on both stages, and the formation of hydrogen bonding was accelerated on the second stage. Effects of the repeated water exchange on the swelling behavior are discussed in terms of the diffusion of Al ions into the solvent, the exchange of Na counter ions by protons, and the formation of hydrogen bonding. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 753–763, 2005     

Microphase separation of PEG-modified urethane acrylate and the swelling behavior of its gels

The viscosity of PEG-modified urethane acrylate (PMUA) showed peculiar behavior in the course of soap-free emulsification. Moreover, the viscosity change with added amounts of water was influenced by the reaction molar ratio of polyethylene glycol (PEG). The rate of increase in viscosity slowed and the ratio of increase in viscosity increased as the reaction molar ratio of PEG increased. This peculiar viscosity behavior was due to the microphase separation between hydrophilic and hydrophobic segments of PMUA, and the orientation of polyoxyethylene groups at O/W interface which influenced droplet size of the soap-free PMUA emulsion. The location of polyoxyethylene groups of this resin at O/W interface was confirmed using the adsorption isotherm measurement of PMUA molecules containing polyoxyethylene groups at water/benzene interface. The microphase separation behavior of PMUA between hydrophilic and hydrophobic segments could apply to the preparation of the PMUA gels containing peculiar structure. PMUA gels were prepared using dioxane (UAG) and the swelling behavior of these gels were compared to that of gels prepared using water (UAHG) in the same medium. In the same medium, the swelling behavior of UAHG gels differed from that of UAG gels because of the difference in the microstructure of gel due to the microphase separation between hydrophilic and hydrophobic segments. This phase separation in the course of gelation in water could be confirmed using contact angle measurement.     

EFFECTS OF GELATION RETARDATION ON THERMAL RESPONSE BEHAVIOR OF PNIAPM GELS： A METHOD TO IMPROVE THEIR DESWELLING KINETICS

P(N-isopropylacrylamide)(PNIPAM)prepared by reversible addition fragmentation chain transfer(RAFT) polymerization exhibited gelation retardation.The intermediate before gelation was characterized and indicated the presence of branched or hyperbranched chains.The swelling behavior was investigated,and the gel by RAFT polymerization(RAFT gel)showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional gels(CG).The study was extended to gels prepared by using 2-hydroxy-1-ethanethiol as chain transfer agent and by using low concentration solutions. The two systems also exhibited retardation effects and improved deswelling kinetics.The different swelling behaviors of these gels and CG could be attributed to the presence of dangling chains caused by gelation retardation.     

Differences in binding of a cationic surfactant to cross-linked sodium poly(acrylate) and sodium poly(styrene sulfonate) studied by Raman spectroscopy

Raman spectroscopy has been used to investigate the structure of gel-surfactant complexes. Cross-linked sodium poly(acrylate) and sodium poly(styrene sulfonate) were immersed in solutions of the cationic surfactant dodecyl trimethylammonium bromide. During the deswelling process, two distinct regions could be observed for both types of gels. Looking at the Raman spectra, however, for the poly(styrene sulfonate), the surfactant could be found throughout the gel particle, whereas for poly(acrylate), essentially all the surfactant was bound in a surface layer.     

Effects of guest microparticles on the swelling behavior of polyacrylamide gels

We report the effects of guest particles on the swelling properties of bulk polyacrylamide gels. The guest particles were the spheres of poly(N‐isopropylacrylamide) gel with submicrometer diameter, which were synthesized by an emulsion‐polymerized reaction in water. Polyacrylamide gels were prepared by a free radical polymerization reaction, immobilizing the gel microparticles with different concentrations at gelation. The macroscopic swelling ratio of this hybrid gel in a cylindrical shape was measured as functions of temperature and acetone concentration. The presence of guest particles was found to strongly affect the swelling behavior in the bulk gels when the concentration of incorporated particles exceeded a threshold. The experimental results indicated that the macroscopic volume in response to the temperature change should be determined by the guest particles above the threshold. On the other hand, the hybrid gel could not evidently shrink by adding acetone when the concentration of guest particles exceeded the threshold. To make clear the distribution of guest particles in the bulk networks, the fractured surfaces of dried gels were imaged by tapping mode atomic force microscopy. The guest particles were found to aggregate in the bulk homogeneous networks to form microdomains with densely connected structure, which became larger with increasing particle concentration. The roles of bulk networks as well as guest particles on the swelling behavior of hybrid gels were qualitatively discussed on the basis of the incorporated structure of guest particles, depending on the concentration of guest particles. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1696–1704, 2005     

Effect of montmorillonite on gelation and swelling behavior of sulfonated polyacrylamide nanocomposite hydrogels in electrolyte solutions

Nanocomposite hydrogels were prepared by cross-linking of aqueous solutions of sulfonated polyacrylamide/sodium montmorillonite with chromium triacetate. The gelation process and effects of clay content and ionic strength on swelling behavior were investigated. X-ray diffraction patterns indicated that exfoliated type of microstructure was formed. Study of the gelation behavior using dynamic rheometery showed that the limiting storage modulus of the nanocomposite (NC) gels decreased with increasing clay content up to 1000 ppm, but it increased by further increase of Na⁺-montmorillonite concentration. It was also found that with increasing the clay content, the viscous energy dissipation properties of the nanocomposite gels increased. The swelling ratio of nanocomposite gels in tap water decreased as the concentration of the clay increased. However, nanocomposite gels showed higher resistance against syneresis in electrolyte solutions as compared with unfilled gels. Therefore, they are potentially good candidates for enhanced oil recovery applications.     

Swelling of sodium acrylate gels

The swelling process of sodium acrylate gel is experimentally investigated. It is found that sodium acrylate gels weakly crosslinked with N,N-methylene-bis-acrylamide may undergo volume phase transition and that different kinds of mechanical instabilities occur in sequence at the transition. Peculiar wrinkle patterns appear on the free surface of an unstable gel and are changed in geometry as swelling proceeds. Cellular patterns seen at various instances in the late period of swelling are ascertained to be geometrically similar to each other and different only in size. The radii of spherical acrylate gels allowed to swell in water are measured as functions of time. The results are discussed and compared with the kinetic theories of swelling. As a result, these theories are proved to be unsatisfactory to fully describe the experimental facts.     

Swelling behavior of chemically crosslinked PVA gels in mixed solvents

We report the swelling behavior of chemically crosslinked polyvinyl alcohol (PVA) gels with different degrees of hydrolysis in water, several organic solvents, and their mixed solvents. The gels were dried after gelation and were put into their respective solvents. The gel volume in pure water decreased with increasing temperatures, and the total changes increased with decreasing degrees of hydrolysis. The swelling ratio depends on the solvent and its concentration. In the cases of mixed solvents of methanol–water, ethanol–water, and acetone–water, the gels shrank continuously with increasing concentrations of solvents and reached the collapsed state in the pure organic solvent. In the case of dimethyl sulfoxide (DMSO), on the other hand, the gels shrunk, swelled, and finally reached the swollen state in pure DMSO. Results of measurements using Fourier Transform infrared spectroscopy (FTIR) and X‐ray diffraction (XRD) suggested that crosslinks and microcrystallites were formed due to hydrogen bonds during the drying process after gelation. The hydrogen bonds were partly destroyed in a rich solvent, but the residual hydrogen bonds had an essential role in determining the swelling behavior in a poor solvent. The swelling behavior and the possible phase transition of the present system are discussed in terms of the solubility of polymers with different degrees of hydrolysis in given mixed solvents and in terms of the formation and destruction of physical crosslinks in the chemical PVA gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1978–1986, 2010     

Synthesis of Hydrophobically Modified Poly（acrylic acid） gels and Interaction of the gels with Cationic／Anionic Surfactants

Poly(acrylic acid)(PAA) gel network with only chemical crosslinking and hydorophobically modified PAA(HM-PAA)gels with both chemical and physical crosslinking were synthesized by radical polymerization in tert-butanol,using ethylene glycol dimethacrylate (EGDMA) as crossliker,and 2-(N-ethylperfluorooctanesulfoamido)ethyl methacrylate (FMA),stearyl acrylate (SA) or lauryl acrylate (LA) as Hydrophobic comonomer respectively.The effcet of the fractions and the species of the hydrophobes on swelling properties of HM-PAA gels and the interaction of gels and surfactants were studied.The results showed that the swelling ratio of HM-PAA gels exhibited a sharp decrease with increasing hydrophobic comomomer comcentration,Which Could be sacribed to the formation of strong hydrophobic association among hydrophobic groups.It was proved that two kinds of binding mechanisms of surfactan/gel and different kinds of hydrophobic clusters existed in gels containing both physical and chemical networks.     

The microfibrillar network in gels of poly(γ-benzyl-L-glutamate) in benzyl alcohol

The gelation and gel-melting phenomena in semidilute isotropic solutions of poly(γ-benzyl-L -glutamate) (PBLG) in benzyl alcohol were studied by small-angle neutron scattering measurements, using a deuterated solvent, and by cryotransmission electron microscopy. The reversible gels are formed when the solution is cooled below the gelation temperature, and the gels melt upon heating. Hysteresis, of about 15°C, is observed between gelation and melting temperatures. In the isotropic solution, PBLG exists as isolated helices. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying the heterogeneous microstructure of the gel. Direct visualization by electron microscopy of vitrified gel samples shows the formation of a microfibrillar network. The dimension of the observed microfibrils is about 10 nm. Upon melting, microstructural changes appear in a temperature range of about 10°C. The unique feature of the gel melting is that initially only the intensity in the mid-angle range decreases. This is interpreted as thickening of the microstructure due to melting of the thinner microfibrils. The final stage marks the melting of the thicker microfibrils. © 1996 John Wiley & Sons, Inc.     

Solvent effects on the kinetics of gelation and the crosslink density of polysiloxane gels

A wide range of hydrocarbons were rapidly gelled by adding a polysiloxane copolymer in the presence of divinylbenzene and a platinum catalyst. The gel point was measured over a range of concentrations for hydrocarbons/solvents and organogels, using three separate methods: rheology, visual (tilt-tube) and FTIR. As the fraction of solvent was increased, the rate of reaction decreased, leading to an increase in the gelation time. The absolute value of the gel point depends upon the techniques used to measure it. For any particular system the gel point values always followed the order: rheology > visual > FTIR. The crosslink densities of the gel systems were determined using both swelling and thermomechanical analysis. The swelling measurements confirmed that the addition of large quantities of solvent markedly reduced the crosslink density of the obtained chemical gel networks, which helped in designing gels with suitable critical strength for effective field work. Also the effectiveness of gelation with the proposed gelling system for different hydrocarbons/solvents was evaluated, and discussed in relation to their dielectric properties.This paper is dedicated to Mike Owen on occasion of his winning the DeBruyn medal, the first silicon chemist to do so.     

Spontaneous formation of gel emulsions in organic solvents and commercial fuels induced by a novel class of amino acid derivatized surfactants

A novel class of amphiphiles, sodium N-(n-dodecyl-2-aminoethanoyl)-l-amino acidate, have been synthesized. These amphiphiles have been shown to form oil-in-water-type gel emulsions with a high internal-phase ratio in organic solvents as well as in commercial fuels simply by agitation. No heating and cooling cycle was required for the formation of gels. The amphiphiles also showed efficient phase-selective gelation in the presence of excess water. The minimum gelator concentrations for the amphiphiles in the solvents employed have been determined. The effects of the chain length of the hydrocarbon tail and the chirality of a representative amphiphile on its ability to promote gelation in a given organic solvent have been investigated. Also, the effect of acid and alkali on the gelation has been examined. The optical microscopic picture of the gel emulsion showed foamlike structures with oil compartments separated by the continuous aqueous phase. The mechanism of the formation of gel emulsions has been discussed.     

Adhesion properties of physically crosslinked elastic gels of poly(sodium acrylate)–poly(acrylic acid) mixtures evaluated by a point contact method

Adhesion properties of physically crosslinked hydrogels consisting of partially hydrated poly(sodium acrylate) (PSA) and poly(acrylic acid) (PAA) were investigated. The adhesion force and separation energy between the gels in swollen states were measured using a simple tack-evaluation technique by a point contact. As a result, the adhesion properties were significantly affected by the PSA/PAA ratio, which resulted from the fact that the physical and chemical properties of gels were changed through the replacement of PSA by PAA at gelation. The measurements of attenuated total reflectance Fourier transform infrared spectroscopy and the dynamic shear viscoelasticity suggested that the number of nonionized carboxyl groups increased and the stiffness increased with increasing the PAA ratio, respectively. In order to understand the adhesion properties further, the apparent contact area was evaluated by a stamp experiment using vermilion ink. The relationships between the adhesion properties and the physical and chemical features of gels are discussed in terms of the surface molecular interaction, the elastic and viscous properties of the bulk, and the apparent and true contact areas of the present system. From these results, the factors that determine the adhesion curve and the uniqueness and advantages of the present evaluation method are clarified.     

Formation of hydrogen bonding in ionized poly(N‐isopropylacrylamide) gels by continuous water exchange

The effects of continuous water exchange on the swelling behavior of poly(N‐isopropylacrylamide‐co‐sodium acrylate) gel were studied. The diameter of gels and the Na⁺ concentration in the solvent were measured at several constant intervals after the solvent (the distilled deionized water) was exchanged. The diameter decreased at room temperature as the solvent was exchanged with water, and it continued to decrease even after more than about 97% of the initial Na⁺ diffused into the water. Thus, the final swelling ratio of the gel was only slightly larger than that of the neutral poly(N‐isopropylacrylamide) gel. To reveal the structural change in molecular level, solid‐state ¹³C cross‐polarization/magic‐angle spinning, solid‐state ¹H combined rotation and multiple‐pulse spectroscopy, and swollen‐state ¹³C dipolar decoupled/magic‐angle spinning NMR experiments were carried out for several dried and swollen samples by varying the times of water exchange. As a result, the intensity and position of the carboxyl peak changed, and the relative intensity of the nonionized carboxyl groups of gels increased with an increasing number of water exchanges. These results indicated that hydrogen bonding was formed between the two, nonionized carboxyl groups (? COOH) and/or between the ? COOH and ? CONH? groups. The macroscopic polymer network shrinkage is discussed in terms of the replacement of counterions Na⁺ by H⁺ and of the formation of intermolecular hydrogen bonding. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1090–1098, 2004     

STUDY ON ACRYLAMIDE-SODIUM ACRYLATE COPOLYMER GELS

Acrylamide-sodium acrylate copolymer hydrogels have been obtained by radiation techniques. Two different methods have been used to introduce—COONa groups into polymer chains of the gels: (1) by partial hydrolysis of acrylamide homopolymer gel; (2) by direct copolymerization and crosslinking of acrylamide and sodium acrylate in aqueous solutions. It was found that the gels obtained in different ways had different properties, the swelling character of the gels obtained by partial hydrolysis were more sensitive to pH of swelling aqueous media, in order to explain these differences, ~(13)C-NMR techniques were used to investigate the sequence distribution of monomer units of both gels.     

The Nature of Crosslinking in N‐Vinylformamide Free‐Radical Polymerization

N‐Vinylformamide (NVF) free‐radical polymerization was found to form polymer gels at high conversions both in bulk and in solution. The polymerization was conducted at different temperatures, monomer and initiator concentrations to show the gelation conditions. Gel fractions and gel swelling ratios were also measured after separating the gel from the polymer samples. In order to confirm the crosslinking unit, a series of hydrolysis experiments were conducted on the gel samples. The hydrolysis results showed that the crosslinks in PNVF gels could be destroyed by alkaline hydrolysis. The most appropriate explanation to this fact is that crosslinking takes place via the amide group.     

Synthesis and swelling behavior of thermosensitive hydrogels based on N-substituted acrylamides and sodium acrylate

A series of hydrogels based on N-isopropylacrylamide, sodium acrylate, and N-tert-butylacrylamide were synthesized by free radical polymerization in a mixture of dioxane and water with tetra(ethylene glycol) diacrylate as the crosslinker and benzoyl peroxide as the initiator. The swelling behavior including the swelling rate of the crosslinked gels in water was studied with gravimetric method. The swelling ratio of the gel (0.1 mol% crosslinking) can reach 420 g/g at 20 °C and such a gel can release 96% of the water absorbed at 40 °C. The lower critical swelling temperature (LCST) of the copolymers can be adjusted by changing the chemical composition of the polymers. Such crosslinked gels can be potentially used as thermosensitive superabsorbent because of their high water uptake and thermal sensitivity.     

Swelling equilibrium of poly(acrylamide) gels in aqueous salt and polymer solutions

The influence of some single salts (NaCl, KCl, Na₂HPO₄ and K₂HPO₄) and poly(ethylene glycol) (PEG) on the swelling of aqueous poly(arcylamide)-gels was studied at 25°C in more than 600 experiments. The chlorides and phosphates cause a different behavior at high salt concentrations: The polyacrylamide gels swell in aqueous solutions of sodium and potassium chloride whereas they shrink when chloride ions are substituted by hydrogen phosphate ions. These differences are due to differences in the interactions of chloride and hydrogen phosphate ions with the network groups. In aqueous solutions of poly(ethylene glycol) the gels shrink continuously with increasing polymer concentration. At constant PEG mass fraction in the liquid phase, the swelling of the gel decreases with increasing molecular weight of PEG. The experimental results (degree of swelling, partitioning of solutes to the coexisting phases) are correlated by combining a model for the Gibbs excess energy for aqueous systems of polymers and electrolytes with a modification of the phantom-network theory. The correlation gives a good agreement with the experimental data for the degree of swelling, whereas in most cases, there is only a qualitative agreement for the partitioning of the solutes.     

聚偏氟乙烯凝胶电解质组成间相互作用及其凝胶化研究

通过冷却聚偏氟乙烯 (PVDF) 丙烯碳酸酯 (PC)或PVDF PC LiClO4的溶液 ,制备了数个聚合物凝胶 .实验表明 ,聚合物凝胶的凝胶化时间 (tgel)与凝胶温度、聚合物浓度有关 ,且强烈地依赖于体系中盐的浓度 ,因为盐会缩短体系的tgel.凝胶体系中LiClO4的存在提高了其凝胶熔融温度 (Tgm) ,LiClO4的含量越大 ,相应凝胶的Tgm 越高 .用DSC和落球法所测凝胶的Tgm 有较大的差别 .这说明凝胶中可能存在热稳定性好和热稳定性相对较差的两种不同结构部分 .FT IR的研究结果表明 ,凝胶电解质的各组成 (LiClO4,PC和PVDF)间存在较强的相互作用 .对含盐和不含盐的两类凝胶体系的对比研究表明 ,两者不同的凝胶化现象和Tgm 归因于盐与聚合物或溶剂间的络合作用