Hydrogel composites of neutral and slightly charged poly (acrylamide) gels with incorporated bentonite. Interaction with salt,linear polyelectrolytes and ionic surfactants

The swelling behavior in the solutions of sodium chloride, linear polyelectrolytes and ionic surfactants of the composites based on clay mineral bentonite (BENT) embedded in neutral and slightly charged poly(acrylamide) (PAAm) gels is studied. Negatively charged flat clay particles incorporated into polymer gel adsorb oppositely charged surfactant and linear polyelectrolyte and attract the charged chains of cationic polymer matrix. The results of SAXS study manifest the formation of lamella structure of the cationic surfactant adsorbed by the clay plates. The gels loaded with the clay show a strong response to changes in the nature and the composition of the ionic environment.     

Kinetic aspects of flocculation of bentonite clay in the presence of anionic and cationic acrylamide copolymers

The flocculation kinetics in aqueous-salt medium in the presence of anionic and cationic high-molecular-weight random acrylamide copolymers was studied in the hindered sedimentation mode, with a suspension of bentonite clay as example. The influence exerted on the flocculation parameters by the concentration and order of addition of ionic acrylamide copolymers taken in various combinations was analyzed.     

Swelling of hydrophobically modified poly(acrylamide) and poly(acrylamide)-co-(acrylic acid) gels in surfactant solutions

A series of hydrophobically modified polyacrylamide and polyacrylamide-co-poly(acrylic acid) gels with systematically varying hydrophobicity were prepared by free-radical polymerization of acrylamide, n-alkylacrylamides (n = 10, 12, and 14), and acrylic acid. The swelling of these gels was examined in water and in both anionic and cationic surfactant solutions. It was found that the gels which incorporated acrylic acid showed extremely high swelling in water. Maximum swelling was observed in gels which incorporated 10 mol% acrylic acid. The swelling of these gels was much less in solutions of both anionic and cationic surfactants than in water. The gels which did not incorporate acrylic acid demonstrated little swelling in water, but showed increased swelling in both anionic and cationic surfactant solutions with increased hydrophobicity of the gel. Received: 1 February 1999 Accepted in revised form: 5 March 1999     

Fluorescence studies on volume phase transition in poly(acrylamide) and poly(N-isopropylacrylamide) gels

The changes in microenvironments during the volume phase transition of poly(acrylamide) and poly(N-isopropylacrylamide) gels induced by pH change or the change in solvent composition were studied by using dansyl or pyrenyl fluorescent probes.     

Preparation of poly(oxybutyleneoxymaleoyl) catalyzed by a proton exchanged montmorillonite clay

The polycondensation of tetrahydrofuran with maleic anhydride catalyzed by Maghnite-H+ (Mag-H) was investigated. Maghnite is a montmorillonite sheet silicate clay that is exchanged with protons to produce Maghnite-H [1]. It was found that the polymerization in bulk is initiated by Mag-H in the presence of acetic anhydride at 40 degrees C.The effects of the amounts of Mag-H and acetic anhydride were studied. The polymerization yield increased as the proportions of catalyst and acetic anhydride were increased.     

Rheology of aqueous poly(ethylene oxide) solutions reinforced with bentonite clay

The rheological properties of bentonite clay-filled aqueous solutions of high-molecular-mass poly(ethylene oxide) (PEO) have been studied. The PEO solution is a typical polymer solution characterized by the highest Newtonian viscosity and the range of non-Newtonian flow. The addition of small amounts of bentonite to the PEO solution causes passage to a viscoplastic behavior that manifests itself as the appearance of the yield stress. Therewith, the flow at the highest Newtonian viscosity in the region of low shear stresses (rather than rates) remains possible. After passing through the yield stress, the effect of antithixotropy, i.e., an increase in the viscosity with the deformation rate in a certain shear rate region, has been observed for the multicomponent systems. The data obtained have been interpreted assuming that the addition of the solid filler to the polymer solution destroys the random network of entanglements between macromolecules, while the presence of the polymer in the clay suspension reduces the strength of the coagulation structure of bentonite.     

Novel poly(N‐isopropylacrylamide)/clay/poly(acrylamide) IPN hydrogels with the response rate and drug release controlled by clay content

Novel interpenetrating network (IPN) hydrogels (PNIPAAm/clay/PAAm hydrogels) based on poly(N‐isopropylacrylamide) (PNIPAAm) crosslinked by inorganic clay and poly(acrylamide) (PAAm) crosslinked by organic crosslinker were prepared in situ by ultraviolet (UV) irradiation polymerization. The effects of clay content on temperature dependence of equilibrium swelling ratio, deswelling behavior, thermal behavior, and the interior morphology of resultant IPN hydrogels were investigated with the help of Fourier transform infrared spectroscopy, differential scanning calorimeter (DSC), scanning electron microscope (SEM). Study on temperature dependence of equilibrium swelling ratio showed that all IPN hydrogels exhibited temperature‐sensitivity. DSC further revealed that the temperature‐sensitivity was weakened with increasing amount of clay. Study on deswelling behavior revealed that IPN hydrogels had much faster response rate when comparing with PNIPAAm/clay hydrogels, and the response rate of IPN hydrogels could be controlled by clay content. SEM revealed that there existed difference in the interior morphology of IPN hydrogels between 20 [below lower critical solution temperature (LCST)] and 50 °C (above LCST), and this difference would become obvious with a decrease in clay content. For the standpoint of applications, oscillating swelling/deswelling behavior was investigated to determine whether properties of IPN hydrogels would be stable for potential applications. Bovine serum albumin (BSA) was used as model drug for in vitro experiment, the release data suggested that the controlled drug release could be achieved by modulating clay content. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 96–106, 2009     

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.     

Interactions of cationic gemini surfactants with hydrophobically modified poly(acrylamides) studied by fluorescence and microcalorimetry

Steady-state fluorescence, time-resolved fluorescence quenching, and isothermal titration microcalorimetry have been used to study the interactions of cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(S)C(12)Br(2), S = 3, 6, and 12) with hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM). Without addition of gemini surfactant, 0.2 wt % HMPAMs except PAM have already self-aggregated into hydrophobic aggregates. Different from single-chain surfactants, C(12)C(S)C(12)Br(2) have stronger interactions with HMPAMs to form surfactant/polymer aggregates, even with PAM. Addition of C(12)C(S)C(12)Br(2) may cause the disruption of HMPAM hydrophobic aggregates and the formation of mixed micelles. It is found that HMPAMs generate lower micropolarity of mixed micelles, larger values of enthalpy of interaction (DeltaH(ps)), and nearly constant values of Gibbs free energy of interaction (DeltaG(ps)). On the other hand, C(12)C(S)C(12)Br(2) with longer spacer brings out slightly lower micropolarity of mixed micelles, owing to the lower electrostatic repulsion between surfactant headgroups. Especially for C(12)C(12)C(12)Br(2), the values of DeltaH(ps) are much more endothermic and the values of DeltaG(ps) are much less negative. The weaker interactions of C(12)C(12)C(12)Br(2) with HMPAMs arise from the marked reduction of attraction between surfactant headgroups and polymer hydrophilic groups induced by its longer spacer.     

Specific interactions of poly(4-vinyl phenol) gel with cationic and anionic surfactants

Binding behaviors of ionic surfactants (decyl- and dodecyltrimethylammonium bromide (C(10)TAB, C(12)TAB), sodium decane sulfonate (SDeSo), and sodium dodecyl sulfate (SDS)) to poly(4-vinyl phenol) (P4VPh) gel were investigated to elucidate a specific swelling behavior that has been found for P4VPh gel in aqueous solutions of tetraalkylammonium salts. With increasing cationic surfactant concentration, P4VPh gel significantly deswelled and then remarkably reswelled at a concentration somewhat below the respective cmc values. On the other hand, in the case of the anionic surfactants, the gel only showed a marked swelling at a concentration just below the respective cmc values. A similar charge-specific behavior of the surfactants was also found for the P4VPh dispersion system studied with a UV-vis spectroscopy; namely, in the cationic surfactant-P4VPh systems, the turbidity of the dispersion first increased with increasing the surfactant concentration and then decreased. This result suggests that aggregation of P4VPh particles first occurred and finally the particles were solubilized. A red shift followed by a blue shift observed for a pi-pi absorption of phenol at around 278 nm was also consistent with the aggregation-solubilization behavior. In the anionic surfactant-P4VPh system, however, only solubilization of the polymer particle was observed, and the UV peak only showed a blue shift. All these results in the gel and the dispersion systems strongly suggest that the cation-pi interaction is involved in the binding of the cationic surfactants to P4VPh.     

SAXS study on complexes formed by anionic poly(sodium methacrylate‐co‐N‐isopropylacrylamide) gels with cationic surfactants

Small‐angle X‐ray scattering (SAXS) has been used to study the nanostructures of complexes formed by slightly crosslinked anionic copolymer gels of poly(sodium methacrylate‐co‐N‐isopropylacrylamide) [P(MAA/NIPAM)] interacting with cetylpyridinium bromide (CPB), and alkyltrimethylammonium bromide (C_nTAB, 10 ≤n ≤ 18), respectively. Both the charge density of polyelectrolyte gels and the surfactant alkyl tail length could induce the phase structure transition from Pm3n space group cubic to hexagonal close packing of spheres (HCP), while the different polar groups of pyridinium and trimethylammonium with the same hydrophobic cetyl chain in surfactants had no significant effects on the structures of complexes formed with the same gels. The highly ordered structures were shown to be formed by the self‐assembly of ionic surfactants inside the anionic gel network, driven by both electrostatic and hydrophobic interactions. Freeze drying the water‐equilibrated complexes could collapse the formed ordered structures. However, the highly ordered structures could be restored after the dried complexes were reswollen by water under the same conditions, indicating that the highly ordered water‐equilibrated complexes were thermodynamically stable. Copyright © 2000 John Wiley & Sons, Ltd.     

Soluble complexes of sodium poly(isoprene-b-methacrylate) micelles with cationic surfactants in aqueous media

Water-soluble complexes between sodium poly(isoprene-b-methacrylate) (NaIMA) amphiphilic block copolymer micelles and two cationic surfactants with different hydrophobic tail lengths, namely, dodecyltrimethylammonium bromide (DTMAB) and octyltrimethylammonium bromide (OTMAB), were prepared by mixing individual aqueous solutions of block copolymers and surfactants. The complexes were characterized in terms of size, overall charge, and micropolarity by dynamic light scattering, zeta-potential measurements, and fluorescence spectroscopy. Properties of the systems were investigated as a function of surfactant concentration and surfactant type and state in the initial solutions, as well as temperature. Experiments reveal surfactant complexation at the coronal sodium poly(methacrylate) (NaMA) chains, followed by an increase in mass and a decrease in size of the micelles. Complexation of individual surfactant micelles was observed when the DTMAB concentration in the starting solutions was higher than the surfactant cmc. The complexes show a temperature dependence of their dimension due to the hydrophobic effect.     

Interaction of poly(L-glutamic acid) with cationic surfactants carrying different head groups

Three cationic surfactants carrying a common hydrocarbon tail (dodecyl group) interact differently with fully ionized poly(L-glutamic acid)(PGA), depending on the type of their ionic head groups. Decrease of pH occurred in the order; dodecylammonium chloride (DAC) > dodecyldimethylammonium chloride (DDAC) > dodecyltrimethylammonium chloride (DTAC). The-helix of PGA was strongly induced by the addition of DAC and DDAC but weakly by DTAC. The induction was inhibited when NaCl concentration was greater than 0.05 M. In the solid state, proton transfer through hydrogen bonds from ionic heads of DDAC to carboxylate groups of the polypeptide was observed. Distortion of circular dichroism spectra occurred at high mixing ratios of surfactant to polymer, due to the aggregation of-helices, as confirmed by light scattering measurements and infrared absorption spectra.     

Fluorescence and viscometry study of complexation of poly(acrylic acid) with poly(acrylamide) and hydrolysed poly(acrylamide)

Interpolymer complexation of poly(acrylic acid) with poly(acrylamide) and hydrolysed poly(acrylamide) was studied by fluorescence spectroscopy and viscometry in dilute aqueous solutions. Changes in chain conformation and flexibility due to the interpolymer association are reflected in the intramolecular excimer fluorescence of pyrene groups covalently attached to the polymer chain. Both poly(acrylamide) and hydrolysed poly(acrylamide) form stable complexes with poly(acrylic acid) at low pH. The molecular weight of poly(acrylic acid) and solution properties such as pH and ionic strength were found to influence the stability and the structure of the complexes. In addition, the polymer solutions mixing time showed an effect on the mean stoichiometry of the complex. The intrinsic viscosity of the solutions of mixed polymers at low pH suggested a compact polymer structure for the complex.     

Self-assembly of ionogenic surfactants during their interaction with poly(propylenimine) dendrimers

The structure of complexes formed by poly(propylenimine) dendrimers of five generations and anionic micelle-forming surfactants is studied by X-ray diffraction. It is shown that, in complexes of lower generation dendrimers, the lamellar packing of surfactants is dominant. In complexes formed by dendrimers of the fourth and fifth generations, packing typical of compact dendrimer molecules prevails. This packing can be attributed to the distorted dense packing of ball-like complex species. Structural models of complexes that allow for penetration of surfactants into the dendrimer molecule and the size ratio of the aliphatic radical of a surfactant and a dendrimer are advanced.     

Solvatochromic studies of interactions between surfactants and poly(N-substituted acrylamide)s

Interactions between poly(N-substituted acrylamide)s and surfactants, such as sodium dodecyl sulfate (SDoS) and sodium decyl sulfate (SDeS), in aqueous solutions were investigated using a solvatochromic probe. The polymers used were poly(N,N-dimethylacrylamide) (PDMA), poly(N-isopropylacrylamide) (PIPA), poly(N-acryloylpyrrolidine) (PAPR), and poly(vinylpyrrolidone) (PVPy) for comparison. They were labeled with pyridinium dicyanomethylide chromophore as a solvatochromic probe, and the changes in the microenvironment polarity of the polymer upon association with surfactant micelles were investigated by monitoring the λ_max in the absorption spectra of the probe molecule. It was found that the Gibbs free energy of micelle stabilization by polymer complexation for SDoS is 7.6, 4.1, and 2.2 kJ mol⁻¹, and for SDeS 5.1, 2.9, and 0.8 kJ mol⁻¹ with PIPA, PAPR, and PDMA, respectively. These results indicate that the complexation between polymer and surfactant is influenced not only by the alkyl-chain length of the surfactant, but also by the polymer side groups.     

Preparation of poly(propylene)/clay layered nanocomposites by melt intercalation from pristine montmorillonite (MMT)

Poly(propylene)/clay nanocomposites were prepared by melt intercalation, using pristine montmorillonite (MMT), hexadecyl trimethyl ammonium bromide (C16), poly(propylene) (PP) and maleic acid (MA) modified PP (MAPP), The nanocomposites structure is demonstrated using X‐ray diffraction (XRD) and high resolution electronic microscopy (HREM). Our purpose is to provide a general concept for manufacturing polymer nanocomposites by melt intercalation starting from the pristine MMT. We found different kneaders (twin‐screw extruder or twin‐roll mill) have influence on the morphology of the PP/clay nanocomposites. Thermogravimetric analysis (TGA) shows that the thermal stability of PP/clay nanocomposites has been improved compared with that of pure PP. Copyright © 2003 John Wiley & Sons, Ltd.     

Flame retardancy of poly(styrene-co-acrylonitrile) by the synergistic interaction between clay and phosphomolybdate hydrates

A combination of montmorillonite (MT) clay and catalysts was used to explore possible synergistic effects in reducing the flammability of poly(styrene-co-acrylonitrile) (SAN). Several catalysts, including ammonium phosphomolybdate hydrate (NHPMo), melamine phosphomolybdate hydrate (MEPMo), zinc phosphomolybdate hydrate (ZnPMo) and sodium phosphomolybdate hydrate (NaPMo), were used. The combination of MT (Cloisite 20A) with NHPMo showed synergistic enhancement in improving the char formation and reducing the peak mass loss rate of SAN40 as compared with SAN40 composites containing MT or NHPMo alone, while similar synergistic performance was not obtained between MT and NaPMo or ZnPMo or MEPMo. The better flame retardancy with this combination is probably due to the two primary aspects. One is the increased catalytic activity of the combination of Cloisite 20A and NHPMo probably due to the overlapping degradation temperature range of the combination of 20A/NHPMo and SAN40, which probably results in more char formation; the other is more NHPMo being around or on the clay stacks while less is in the galleries of the clay; this probably bridges the clay stacks and strengthens the clay network with char formed during the degradation process. High temperature rheological data confirmed the formation of a stronger network structure in SAN40/20/NHPMo; this benefits flame retardancy by allowing fewer cracks to form in the outer char layer on the polymer mass. The more continuous char layer suppresses bubbles transport of fuel vapors and heat transfer through the char layer, thereby reducing the mass loss rate.