Molecular-mass characteristics and association behavior of weakly charged hydrophobically modified poly(acrylamides)

The evolution of compositional heterogeneity and molecular-mass distribution in the course of micellar polymerization of weakly charged hydrophobically modified poly(acrylamides) has been studied. The relationship between the molecular characteristics, association behavior, and rheological properties of aqueous solutions and gels of these copolymers has been examined. The molecular-mass distribution broadens with conversion owing to accumulation of a low-molecular-mass fraction depleted of the hydrophobic monomer. A change in the molecular-mass distribution has a strong effect on the viscoelastic behavior of the polymers. The properties of the copolymers are dependent on the pH of the reaction mixture: the copolymers prepared in the alkaline medium show a more pronounced ability to experience hydrophobic aggregation than the copolymers prepared in the acidic medium. The aqueous solutions and the gels of these polymers are characterized by higher viscosities and dynamic moduli. The rheological characteristics of the hydrophobically modified poly(acrylamides) are improved with an increase in both their molecular mass and the length of hydrophobic sequences with a constant molecular mass.     

Local mobility and rheological characteristics of gels based on hydrophobically modified poly(acrylamides)

For hydrophobically modified poly(acryl amide), we analyze the effect of various parameters of the macromolecular structure (number and length of side hydrophobic groups, content of charged groups, type of bonding between side chains and the polymer backbone, and the degree of blocking of hydrophobic groups distributed along the chain) on the local mobility of physical network junctions and rheological characteristics of gels. We have found that the local mobility measured by the method of spin-probe EPR spectroscopy is either independent or it slightly depends on the above parameters. At the same time, these parameters exert a strong effect on the rheological characteristics of gels. This disagreement can be explained by the fact that local mobility of junctions is primarily controlled by the intermolecular interactions of hydrophobic groups and by the covalent bonding between these groups and a macromolecule. However, the rheological characteristics depend on the number of junctions, their dimensions, and other parameters.     

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.     

Stabilisation of emulsions using hydrophobically modified inulin (polyfructose)

Oil-in-water (O/W) emulsions were prepared using a hydrophobically modified inulin surfactant, INUTEC^®SP1. The quality of the emulsions was evaluated using optical microscopy. Emulsions, prepared using INUTEC^®SP1 alone had large droplets, but this could be significantly reduced by addition of a cosurfactant to the oil phase, namely Span 20. The stability of the emulsions was investigated in water, in 0.5, 1.0 and 2 mol dm⁻³ NaCl as well as 0.5, 1.0, 1.5 and 2 mol dm⁻³ MgSO₄. All emulsions containing NaCl did not show any strong flocculation or coalescence up to 50 °C for almost 1 year storage. With MgSO₄ they were stable up to 50 °C and 1 mol dm⁻³. The stability of the emulsions against strong flocculation and coalescence could be attributed to the conformation of the polymeric surfactant at the O/W interface (multipoint attachment with several loops) and the strong hydration of the polyfructose chain in such high electrolyte concentrations. This was confirmed using cloud point measurements, which showed absence of any cloudiness up to 100 °C and at NaCl concentrations reaching 4 mol dm⁻³ and MgSO₄ reaching 1 mol dm⁻³. These high cloud points in electrolyte solutions could not be reached with polyethylene glycol. This clearly demonstrated the superiority of INUTEC^®SP1 surfactant as an emulsion stabiliser when compared with surfactants based on polyethylene glycol. Viscoelastic measurements showed a gradual increase in the storage modulus G′ with storage time both at room temperature and 50 °C. This was indicative of weak flocculation and absence of coalescence. The weak flocculation of the emulsions could be attributed to the presence of an energy minimum, G_min, in the energy–distance curve.     

Fluorocarbon- containing hydrophobically modified poly(N-isopropylacrylamide)

A fluorocarbon-modified poly(N-isopropylacrylamide) has been synthesized by copolymerization of N-isopropyl acrylamide with a small amount of acrylate or methacrylate containing a perfluoroalkyl group. It was found that the hydrophilicity of macromolecular backbone is an important factor to the solution properties of the copolymers and that hydrophobic association between fluorocarbon groups is stronger than that between the corresponding hydrocarbon analogies. The viscosity of some of the copolymer solutions was very sensitive to temperature. It was dilatant at higher fluorocarbon comonomer content ( > 0.20-1.0 mol%) and was Newtonian at very low fluorocarbon comonomer content (0.03-0.2 mol% ) . Evidence for hydrophobic association of the fluorocarbon groups was obtained from the effects of adding Nad and surfactants on the solution viscosity. The LC-ST properties of these copolymers were studied by DSC method and this was also found to be consistent with hydrophobic association between the fluorocarbo     

Aggregation behaviour of hydrophobically modified poly(allylammonium) chloride

 The behaviour of hydrophobically modified poly(allylammonium) chloride having octyl, decyl, dodecyl and hexadecyl side chains has been studied in aqueous solution using fluorescence emission techniques. Micropolarity studies using the I ₁/I ₃ ratio of the vibronic bands of pyrene show that the formation of hydrophobic microdomains depends on both the length of the side chain and the polymer concentration. The I ₁/I ₃ ratio of the polymers with low hydrophobe content (less than 5% mol) changes substantially when reaching a certain concentration. These changes are assigned to aggregation originating from interchain interactions. This behaviour is also confirmed by the behaviour of the monomer/excimer emission intensities of pyrenedodecanoic acid used as a probe. For polymers having dodecyl side chains and hydrophobe contents higher than 10%, aggregates are formed independently of the polymer concentration. Anisotropy measurements show that microdomains resulting from the inter- and/or intramolecular interactions are similar to those observed for cationic surfactants. Viscosity measurements show that the coil dimensions are substantially decreased for the polymers having high hydrophobe contents, indicating intramolecular associations. Received: 10 November 1999/Accepted: 7 April 2000     

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide)

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.     

Interactions of hydrophobically modified poly(sodium acrylate) with globular proteins

The association of a series of hydrophobically modified poly(sodium acrylate) (HMPA) with lysozyme, a cationic globular protein, or with bovine serum albumin (BSA), an anionic globular protein, was investigated at pH=9 by rheology and to a lesser extent by steady-state fluorescence spectroscopy. Under suitable concentration conditions, this association leads to a drastic viscosity enhancement which is improved when the polymer hydrophobicity is increased. A mechanism is proposed: the hydrophobic regions of the globular proteins interact strongly with the alkyl groups of one or more polymer chains. In the later case, the macromolecules are crosslinked via the proteins, which leads to viscosity enhancement and even gelation. Analogies and differences between these systems and surfactant/HMPA systems previously studied in our laboratory are emphasized and discussed.     

Self-aggregates of hydrophobically modified poly(2-hydroxyethyl aspartamide) in aqueous solution

Dehydrocholic acid (DHA) grafted poly(2-hydroxyethyl aspartamide) (PHEA)s were successfully synthesized and their self-aggregates in aqueous solution were characterized by fluorescence spectra and light scattering. PHEA was obtained by a simple reaction of ethanolamine with synthesized poly(succinimide) (PSI), and then PHEA-g-DHA was synthesized through an ester linkage between DHA and PHEA. The degree of substitution (DS) of DHA groups, defined by grafted mole%, was determined from both ¹H-NMR and elemental analysis. The grafting reaction of DHA was retarded up to almost 10 mole% feed ratio of DHA/PHEA, but increased linearly above the threshold ratio. Nano-size self-aggregates in aqueous solution were examined with four DSs less than 10. As DS increased, the critical aggregation concentrations (CAC) of polymers were continuously reduced and the size of primary aggregates reduced to as small as 40 nm in diameter. When stored, the sonicated aggregates of high DS were destabilized, apparently forming large aggregates with small curvatures. The formation of irreversible interfused secondary structures would be induced by the curvature change or aggregation of primary particles. A simple calculation indicates that a small change of separation between grafted DHA groups may induce the large curvature shift, in fact, sphere-to-planar surface transition.     

Tripolyphosphate-sensitive egg phosphatidylcholine liposomes incorporating hydrophobically modified poly(ethylene imine)

Liposomes, which release their contents in answer to tripolyphosphate (TPP, a penta-anion), were prepared by immobilizing hydrophobically modified poly(ethylene imine) (HmPEI) on the surface of egg phosphatidylcholine (egg PC) liposome. HmPEI was prepared by covalently attaching decanoyl chloride to PEI through a condensation reaction. According to the ¹H NMR spectrum, the number of decanoyl chloride per one molecule of PEI was about 21, and HmPEI was air/water interface-active. HmPEI could readily complex with TPP in HEPES buffer (30 mM, pH 7.0), confirmed by Fourier transformed infrared spectrophotometer spectroscopy. The complexation increased with increasing the concentration of HmPEI and TPP, investigated through the measurement of optical density and light scattering intensity. Liposomes incorporating HmPEI were prepared by a film hydration and sonication method. The liposomes were multi-lamellar vesicles, observed on transmission electron microscope. Liposomes free of HmPEI did not release calcien when they were mixed with TPP. Liposomes whose egg PC/HmPEI was relatively low (e.g., 20:1 and 20:2) released calcein but not extensively (less than 10%) when mixed with TPP. Liposomes whose egg PC/HmPEI was relatively high (e.g., 20:4 and 20:20) released calcein extensively. For example, when the liposomes with lager amount of HmPEI were mixed with TPP so that HmPEI/TPP weight ratio was 8:1, the release degree in 60 sec was more than 70%. HmPEI can complex with TPP through electrostatic interaction and the complexation was thought to cause perturbation in the liposomal membranes and trigger the release.     

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     

Photo-responsive microspheres prepared using hydrophobically modified poly(vinyl alcohol)-coumarin conjugate

Photo-responsive microspheres were prepared by dispersing hydrophobically modified poly(vinyl alcohol)-epoxypropoxy coumarin conjugates (HmPVA-EPC) in an aqueous phase. These conjugates were prepared by subsequent covalent attachment of decanoyl chloride (DC) to PVA, and then EPC to HmPVA. The air/water interfacial activities of HmPVA-EPC were higher than unmodified PVA, due to the hydrophobicity of DC moiety. Under a cyclic irradiation by 365 and 254 nm light, the degree of photo-dimerization of EPC residues of HmPVA-EPC was as high as that of free EPC. The obtained HmPVA and HmPVA-EPC were immobilized into submicron-sized microspheres. The degree of Nile Red release from HmPVA-EPC microspheres was more than 75% within 24 h that was higher than the value of 40% for HmPVA microspheres. This difference is most likely due to the less compact packing of HmPVA-EPC in the microspheres due to the smaller number of hydrophobic DC residues. The dye release from HmPVA-EPC microspherses was inhibited by the irradiation with 365 nm light, possibly due to the photo-induced cross-linking of the polymer.     

疏水改性聚丙烯酸溶液的流变特性研究

本文通过沉淀聚合制备了具有不同主链结构的疏水改性聚丙烯酸(HMPA),由Huggins方程确定了HMPA在溶液(水,盐/水,醇/水)中的特性粘数[η]和Huggins常数KH,采用流变学法研究了HMPA溶液的疏水缔合行为和流变特性.研究表明,HMPA溶液具有典型的剪切变稀行为,主链结构对HMPA溶液的缔合行为和流变特性有显著影响.主链含有疏水链段的HMPA在乙二醇/水溶液中形成类似弹性体的凝胶网络结构,具有较大的活化能,其表观粘度具有明显的温度敏感性.     

Demulsification of water-in-oil emulsions by permeation through Shirasu-porous-glass (SPG) membranes

To investigate the effect of the droplet/pore size ratio on membrane demulsification, water-in-oil (W/O) emulsions with uniform-sized droplets was demulsified by permeation through Shirasu-porous-glass (SPG) membranes with a narrow pore size distribution at mean droplet/pore diameter ratios of 0.52–5.75. At transmembrane pressures above a critical pressure, the water droplets larger than the membrane pore size were demulsified, where the SPG membrane acted as a coalescer because the hydrophilic membrane surface had a high affinity for the water droplets. By contrast, at transmembrane pressures below the critical pressure, the larger water droplets were all retained by the membrane due to the sieving effect of the uniform-sized pores. When a W/O emulsion with a mean droplet diameter of 2.30 μm was allowed to permeate through a membrane with a mean pore diameter of 0.86 μm, the demulsification efficiency increased with increasing transmembrane pressure, to a maximum value of 91% at a transmembrane pressure of 392 kPa, and then decreased, while the transmembrane flux increased almost linearly with increasing transmembrane pressure. The demulsification efficiency was higher for higher water phase content and lower concentration of the surfactant, tetraglycerin condensed ricinoleic acid ester, in the emulsions due to the reduction of the emulsion stability.     

Photo-responsive properties of poly(NIPAM-co-AAc) microgel particles with absorbed, hydrophobically modified organic salts

The absorption of two hydrophobically modified organic salts (HMOSs), containing azobenzene units, into poly(N-isopropylacrylamide-co-acrylic acid) microgel particles has been studied at pH 8 and 20 °C. These dispersions were then irradiated with UV light (wavelength 365 nm) for 10 min to observe the effect on the microgel particle properties, such as the adsorbed amount of the HMOS, the particle size, and the electrophoretic mobility. We show that irradiation of these dispersions with UV light can lead to induced, partial desorption of the HMOS molecules, with concomitant changes in the size and electrophoretic mobility of the microgel particles. This is due to a conformational switch (trans-form to cis-form) in the HMOS molecules, which reduces the strength of the hydrophobic interaction between the HMOS molecules and the isopropyl moieties within the microgel network. Moreover, the original absorbed amounts, size, and electrophoretic mobility values can be largely restored after storage in the dark for extended periods.     

Langevin-dynamics simulation of flocculation in water-in-oil emulsions

Aggregation of internal phase droplets in water-in-oil emulsions has been simulated by the Langevin-dynamics method. At initial stages, the process rate obeys the Smoluchowski equations. The concentration ranges have been determined in which percolation clusters are formed. It has been shown that the examined emulsions may be divided into three groups, which enable one to predict their sedimentation stability. Percolation clusters are not formed in emulsions with internal-phase fractions of ≤0.1; therefore, their sedimentation rate is high. In the range of internal-phase fractions from 0.1 to 0.3–0.4, the percolation clusters are formed within a few fractions of a second; however, the rate of subsequent droplet addition to the clusters is low. The flocculation of internal-phase droplets results in the formation of a weak network structure followed by its densification and the separation of a dispersion medium with time. In emulsions with internalphase fractions of >0.3–0.4, almost all droplets unite into percolation clusters within a few fractions of a second. This structure is slowly densified; therefore, the rate of dispersion medium separation is low. The results obtained have been confirmed by experimental data on the sedimentation stability of the water-in-oil emulsions.     

Catalysis by hydrophobically modified poly(propylenimine) dendrimers having quaternary ammonium and tertiary amine functionality

Four different quaternary ammonium chloride-modified poly(propylenimine) (PPI) dendrimers were synthesized by alkylation of a PPI dendrimer having eight dimethylamino end groups with 1-bromooctane or 1-bromododecane. By varying the mole ratio of alkyl bromide to dendrimer, averages of 4-10 quaternary ammonium groups were formed. The new amphiphilic dendrimers are surface active and are micellar catalysts in water. The dendrimers have critical aggregation concentrations between 8.5 x 10(-4) and 9.0 x 10(-5) M. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate at 25 degrees C was 650 times faster than in water alone in the presence of a dendrimer quaternized with eight dodecyl chains at a concentration of 2.45 mM in quaternary ammonium groups. The order of the catalytic efficiency of the new dendrimers decreased with the length and number of hydrophobic alkyl groups in the order (C(12))(8) > (C(12))(4) > (C(8))(10) > (C(8))(5). The pseudo-first-order rate constants for basic hydrolysis of p-nitrophenyl hexanoate in pH 9.4 buffer at 30 degrees C using the (C(12))(8) and (C(12))(4) dendrimers were 26 and 13 times higher than those for hydrolysis with no dendrimer. The kinetic data were fit to a single-site binding model to evaluate the contributions of binding constants of reactants to the dendrimers and catalytic rate constants of the bound species to the overall catalytic activity.     

Hydrogen bonding and the conformations of poly(alkyl acrylamides)

The conformations of poly(alkyl acrylamide) oligomers in nonpolar solvents were studied using molecular dynamics techniques. Poly(methyl acrylamide) was found to collapse to a globule-like conformation at low temperatures; however, excluded volume effects inhibited the collapse of poly(octadecyl acrylamide). A high density of structured units, characterized by a trans-gauche-trans-trans-gauche-trans torsional sequence along the backbone, was noted in all simulations. Such units were found to create a particularly stable set of intramolecular hydrogen bonds. An oligomer constructed with these stable units was found to have significantly lower minimized energy than both the all-trans and the helical backbone conformations. The constructed conformation had lower Coulomb energy (more hydrogen bonds) than the all-trans conformation and lower dihedral energy (less backbone distortion) than the helical conformation. The propensity for poly(octadecyl acrylamide) to form hydrogen bonds introduced significant disorder into the orientation of the alkyl side chains. This disorder would inhibit crystallization and restrict the ability of such polymers to form epitaxial seeds for nucleating paraffin crystals.     

Effects of temperature on neutron scattering from aqueous solutions of hydrophobically modified poly(ethylene oxide)

The effect of temperature on the structure of aqueous dispersions of hydrophobically end-capped poly(ethylene oxide) (PEOM) was investigated by small angle neutron scattering (SANS). Polymers with hydrogenated or deuterated n-octadecyl end-groups were studied in heavy water or in a mixture heavy water / water, respectively. In the latter case the PEO chains were selectively matched. In all the cases, the scattering curves were characterised by a main peak which revealed organisation of polymers into micelles consisting of hydrophobic cores surrounded by repulsive PEO coronae. Measurements were performed in the semi-dilute regime where micelles coronae overlap. At constant polymer concentration, an increase in temperature leads to decreasing solvent strength of water for the PEO chains and decreasing repulsion between the PEO coronae. As a result, the intensity of the peak in a mixture of water /heavy water decreases with temperature On the contrary, in heavy water, the peak of the scattered intensity increases with increasing temperature. This scattering behaviour is interpreted on the basis of a scaling theory of the semi-dilute solutions of star-like polymer micelles.     

Formation of unimer nanoparticles by controlling the self-association of hydrophobically modified poly(amino acid)s

Amphiphilic block or graft copolymers have been demonstrated to form a variety of self-assembled nano/microstructures in selective solvents. In this study, the self-association behavior of biodegradable graft copolymers composed of poly(γ-glutamic acid) (γ-PGA) as the hydrophilic segment and L-phenylalanine (Phe) as the hydrophobic segment in aqueous solution was investigated. The association behavior and unimer nanoparticle formation of these γ-PGA-graft-Phe (γ-PGA-Phe) copolymers in aqueous solution were characterized with a focus on the effect of the Phe grafting degree on the intra- and interpolymer association of γ-PGA-Phe. The particle size and number of polymer aggregates (N(agg)) in one particle of the γ-PGA-Phe depended on the Phe grafting degree. The size of γ-PGA-Phe with 12, 27, 35, or 42% Phe grafting (γ-PGA-Phe-12, -27, -35, or -42) was about 8-14 nm and the N(agg) was about 1, supporting the presence of a unimolecular graft copolymer in PBS. The pyrene fluorescence data indicated that γ-PGA-Phe-35 and -42 have hydrophobic domains formed by the intrapolymer association of Phe attached to γ-PGA. These results suggest that the Phe grafting degree is critical to the association behavior of γ-PGA-Phe and that γ-PGA-Phe-35 and -42 could form unimer nanoparticles. Moreover, when γ-PGA-Phe-42 dissolved in DMSO was added to various concentrations of NaCl solution, the particle size and N(agg) could be easily controlled by changing the NaCl concentration during the formation of the particles. These results suggest that biodegradable γ-PGA-Phe is useful for the fabrication of very small nanoparticles. It is expected that γ-PGA-Phe nanoparticles, including unimer particles, will have great potential as multifunctional carriers for pharmaceutical and biomedical applications, such as drug and vaccine delivery systems.