Intermolecular Hydrophobic Associations Research Articles

Solution behavior of hydrophobically associating water-soluble block copolymers of acrylamide and N-benzylacrylamide

Hydrophobically modified water-soluble block copolymers have been prepared by an aqueous micellar copolymerization technique from acrylamide and small amounts of N-benzylacrylamide (2 and 3 mol%) with the objective of investigating the copolymers' rheological behavior and surface and interfacial activities under various conditions such as polymer concentration, shear rate, temperature and salinity. As expected, the block copolymers exhibit improved thickening properties due to intermolecular hydrophobic associations as the solution viscosity of the copolymers increases sharply with increasing polymer concentration. Additional evidence for intermolecular association is provided by the effect of NaCl salt on the solution viscosity, and as expected, the addition of NaCl to the polymer solution gives rise to copolymers with enhanced thickening properties. An almost shear rate independent viscosity (Newtonian plateau) is also exhibited at high shear rate and a typical non-Newtonian shear thinning behavior appears at low shear rates and high temperatures. Furthermore, the block copolymers exhibit high air–liquid surface and liquid–liquid interfacial activities as the surface and interfacial tensions (IFTs) decrease with increasing polymer concentration. This behavior is yet another evidence of polymolecular micelles formation of the copolymers in aqueous solution, and thus the high tendency to adsorb at an interface. The surface and IFTs exhibited by the copolymers were found to be relatively insensitive to the concentration of salt (NaCl).

Association Behavior of Singly Chain-End Hydrophobically-Modified Poly(vinyl alcohol) in Aqueous Solution.

The association behavior of hydrophobically-modified poly(vinyl alcohols), HMPVAs, bearing a single hydrophobic modifier (dodecyl group) located at one end of the polymer chain, was investigated in aqueous solution using the techniques of pulsed gradient spin echo nuclear magnetic resonance (PGSE NMR) spectroscopy and viscometry. In comparison to control polymers lacking hydrophobic modification, at concentrations above 2.5 wt% HMPVAs exhibited slower diffusion, as measured using PGSE NMR, and higher viscosity, as measured using viscometry. The results are consistent with the formation of intermolecular association structures consisting of approximately 9 polymer chains. The finding that singly chainend HMPVAs form intermolecular hydrophobic associations suggests that such polymers may prove useful as latex rheology modifiers.

Hydrophobically modified polyelectrolytes: I. Dilute solution properties of fluorocarbon‐containing poly (acrylic acid)

AbstractDilute solution viscosity of fluorocarbon‐containing hydrophobically modified poly (acrylic add) was measured in aqueous solutions of various NaCl concentrations. The intrinsic viscosity ([η]) and Huggins coefficient (kH) were evaluated using Huggins equations. It is found that, at low Nacl concentration, the modified polymers exhibit values of intrinsic viscosity ([η]) and Huggins coefficient (kH) similar to those of unmodified polymers. For both of the modified and unmodified polymers, the intrinsic viscosity decreases with increase of NaCl concentration, while the Huggins coefficient increases upon addition of NaCl. But the variation of [η] and kH is more significant for the modified polymers, which reflects the enhanced intra‐ and intermolecular hydrophobic association at higher Nacl concentration.

Synthesis and aqueous solution properties of hydrophobically modified graft copolymer of sodium carboxymethylcellulose with acrylamide and dimethyloctyl(2-methacryloxyethyl)ammonium bromide

The hydrophobically modified water-soluble graft copolymer of sodium carboxymethylcellulose with acrylamide and dimethyloctyl(2-methacryloxyethyl)ammonium bromide has been synthesized using potassium persulfate and dimethylaminoethyl methacrylate as initiators in aqueous solution. The structure and composition of the graft copolymer were characterized by infrared spectrum and elementary analysis. Molecular weight was determined by gel permeation chromatography. The solubility and viscosity of the graft copolymers in aqueous solution were investigated, as a function of copolymer concentration, added salt, temperature, shear rate, and surfactant. In addition, the intermolecular hydrophobic association in aqueous solutions in the presence of added salt and surfactant was demonstrated through the retention time from gel permeation chromatography measurement. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 537–542, 2000

Synthesis and characterization of fluorocarbon-modified poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide) copolymers (PNIPAMs) containing pendent perfluoroalkyl (R F ) or dodecyl groups have been synthesized by copolymerization of NIPAM with small amounts of R R -acrylates or -methacrylates containing a sulfonamido moiety between the acrylate and R F groups or with dodecyl acrylate. Evidence for strong intermolecular hydrophobic association of the fluorocarbon groups is provided by large viscosity increases with copolymer concentration and upon addition of NaCl and surfactants. These interactions appear to be much stronger than that of the corresponding copolymers of poly(N,N-dimethylacrylamide) with similar comonomer contents. Hydrophobic association between the R F groups is found to be much stronger than that of the corresponding dodecyl groups. The viscosity of some of the copolymer solutions, particularly in the presence of perfluorocarbon surfactants, was unusually temperature sensitive, decreasing by a factor of at least 1000 upon increasing the temperature from 10 to 20 °C. This large decrease is most probably related to the collapse of the copolymer coils near the lower critical solution temperature. This is in sharp contrast to the corresponding polyacrylamide or poly(N,N-dimethylacrylamide) R F -acrylate copolymers that show viscosity increases with increasing temperature in the 40-60 °C range. The NIPIAM copolymers were also found to be different from the acrylamide or N,N-dimethylacrylamide perfluorocarbon acrylate copolymers in that they were found to be Newtonian at a low RF content but dilatant at a higher comonomer content. # 2000 Society of Chemical Industry

Complex Formation of β-Cyclodextrin- and Perfluorocarbon-Modified Water-Soluble Polymers

The complex formation has been studied by 19F NMR and viscometry of β-cyclodextrin (β-CD) and water-soluble telechelic and one-ended perfluorocarbon derivatives (C6F13 and C8F17) of poly(ethylene glycol)s (PEGs) of molecular weights of 5000 and 10000 and poly(dimethylacrylamide)s containing C8F17 pendent groups. Fluorine-19 NMR of the PEG derivatives shows CF3 resonances at about −80 and −82 ppm corresponding to monomeric and associated structures, respectively. For the case of the one-ended C6F13 PEG (M = 5000) derivatives, a formation constant on the order of 104 was determined. Addition of β-CD to this polymer leads to the formation of 1:1 β-CD−RF complexes that gives CF3 resonances that are very close to that of the monomeric RF groups. Essentially quantitative complex formation with β-CD was observed in the presence of free β-CD concentrations larger than about 10-2−10-3 M. The reduced viscosity of 1.0 wt % aqueous solutions of the telechelic PEG of MW of 10 000 containing C8F17 end groups is greatly reduced as a result of intermolecular hydrophobic association. Addition of stoichiometric β-CD completely suppresses hydrophobic association in this solution, as demonstrated by a virtually complete loss of viscosity due to association. Similar reductions in viscosity were observed for the case of poly(dimethylacrylamide) containing pendent groups. In this case a larger excess of β-CD was required to completely suppress RF association.

Solvent‐polymer interactions in associative polymer/aqueous media systems

AbstractLight scattering measurements in the dilute regime (C<C*) of the amphiphilic hydrophobically modified hydroxyethyl cellulose (HMHEC) have shown a metastable equilibrium of solubility in pure water. As a consequence such ageing solutions present a great instability and in severe conditions of added NaCl or temperature, a demixion has been evidenced. In a more concentrated range of concentration, aqueous solutions of HMHEC possess higher viscosity than HEC precursor due to intermolecular hydrophobic associations. But in a 1M NaCl/ethanol mixture this effect is greatly weakened.

Rheological properties of semi-dilute aqueous solutions of hydrophobically modified propylene glycol alginate derivatives

Abstract New hydrophobically associating water-soluble polymers have been prepared by covalent fixation of dodecyl and tetradecyl hydrocarbon side chains onto propylene glycol alginate (PGA). The viscosity of aqueous solutions of such deriatives is highly dependent not only on the shear rate but also on the shear time. The obtention of reliable data in this domain therefore requires that measurements are carefully carried out under steady state conditions. In the semi-dilute regime, intermolecular hydrophobic associations can be evidenced at low shear rates. These interactions result in considerably enhanced viscosities and the formation of physically cross-linked gel-like structures may ultimately be observed.

Influence of a nonionic surfactant on the rheology of a hydrophobically associating water soluble polymer

The rheological behavior of a 1% w/w commercial hydrophobically modified (hydroxypropyl)guar (HMHPG) in water solution is examined in the presence of a low molecular weight nonionic surfactant. The linear and nonlinear response to steady and oscillatory shear flow, at different surfactant concentrations around the cmc, is mainly compared with the response of a 1% w/w aqueous solution of HMHPG alone, following a rheological approach previously used by the authors. Below the cmc of the free surfactant in water, the rheological results show a reinforcement of the reversible associating network mainly due to an increase of the number of intermolecular hydrophobic associations, with hardly no modification of the average ‘‘strength’’ of the hydrophobic junctions. Above the cmc, the rheological data show that there is a progressive weakening leading to the disruption of the intermolecular hydrophobic junction network. Those results are interpreted in terms of interactions of surfactant molecules with the regions of intermolecular hydrophobic association.

Water-Soluble Copolymers. 64. Effects of pH and Composition on Associative Properties of Amphiphilic Acrylamide/Acrylic Acid Terpolymers

Acrylamide (AM)/acrylic acid (AA) copolymers and AM/AA/N-(4-hexylphenyl)acrylamide (HPAM, 0.5 mol %) terpolymers with varied AA content have been synthesized by micellar copolymerization. Co- and terpolymers with 5−37 mol % AA and similar hydrodynamic volumes were prepared. Solution viscosity studies of the copolymers as a function of pH and copolymer concentration in deionized water and NaCl solution indicate normal polyelectrolyte behavior. In aqueous solutions, intra- or intermolecular hydrophobic associations may be favored by variation of terpolymer composition. Three-dimensional plots of both I3/I1 (from pyrene probe fluorescence) and viscosity as functions of pH and terpolymer concentration demonstrate the responsive nature of associations in water and in 0.5 M NaCl. Terpolymers containing approximately 9 and 21 mol % AA exhibit intermolecular (open) associations at ≤pH 5 and in the presence of NaCl. The terpolymer containing approximately 37 mol % AA, however, exhibits no intermolecular associations in deionized water even at low pH. Pyrene fluorescence studies of this terpolymer indicate a significantly apolar environment sensed by the probe at pH 4, suggesting that intramolecular (closed) microdomains are formed. Conceptual models based on hydrophobic, ionic, and hydrogen-bonding interactions in these systems are presented.

Shear and extensional viscosity characteristics of a series of hydrophobically associating polyelectrolytes

The shear and extensional viscosity characteristics of aqueous solutions of a series of hydrophobically associating polyelectrolytes have been investigated and compared. The polymers studied consisted of a methacrylic acid/ethyl acrylate copolymer backbone containing varying levels of incorporation (0–2 mol%) of a pendant C18 hydrophobe. The extensional viscosities of all solutions were found to be higher than their corresponding shear viscosities, with the calculated Trouton ratio tending towards the predicted value of three at low strain rates but increasing with increasing extensional strain rate thereafter. In both flow modes, the viscosities of the solutions increased with increasing concentration and hydrophobe content of the polymers. In contrast to the purely shear-thinning characteristics of the polymer solutions observed in shear flow, the extensional viscosity–strain rate profiles of a number of systems were found to contain an initial region of strain-thickening, although strain-thinning behaviour was observed at higher strain rates. For any given system, the strain rate at which the evident maxima in extensional viscosity occurred decreased with increasing polymer concentration. In the presence of additives such as excess surfactant or a miscible alcoholic non-solvent, which are known to disrupt regions of intermolecular hydrophobic associations, the extensional viscosity of a typical solution of an associative polyelectrolyte was found to be significantly reduced and no longer to exhibit a maximum in the viscosity–strain rate profile.

Intermolecular associations in hydrophobically modified derivatives of propyleneglycol alginate

A partially esterified derivative of sodium alginate was substituted by various long-chain alkylamines. The resulting amphiphilic polymers exhibit intermolecular hydrophobic associations in semidilute aqueous solution, evidenced by rheological measurements and fluorescence spectroscopy experiments.

Amphiphilic derivatives of alginate: evidence for intra- and intermolecular hydrophobic associations in aqueous solution

Amphiphilic derivatives of alginate: evidence for intra- and intermolecular hydrophobic associations in aqueous solution

Fluorocarbon-modified water-soluble cellulose derivatives

1,1-Dihydroperfluorobutyl,1,1-dihydroperfluorooctyl, and octyl derivatives of (hydroxyethyl)-cellulose (HEC) were prepared by reaction of the Na salt of HEC in aqueous isopropyl alcohol with 1,1-dihydroperfluorobutyl or octyl p-toluenesulfonate (4 or 5), with 1,1-dihydroperfluorobutyl or octyl glycidyl ether or (8 or 9), and with octyl glycidyl ether (12). The degrees of substitution of HEC were determined using [sup 19]FNMR and varied between 0.023 and 1.20 mol % of anhydroglucose units (AGU). The HEC derivatives modified with 4, 5, and 8 were sparingly soluble or insoluble, and their aqueous solutions were generally low in viscosity. In contrast solutions of HEC modified with 9 did have significantly enhanced viscosities with respect to unmodified HEC or compared to HEC modified with the octyl glycidyl ether, having the same chain length as 9. The Brookfield viscosity plotted against perfluorocarbon content of aqueous solutions of HEC modified with 9 was shown to go through a maximum at about 0.42 mol % (0.92 wt %) with respect to AGU's. The solutions of this and similar polymers were shown to be highly pseudoplastic. Furthermore, increases in viscosity were observed upon addition to NaCl. These observations are consistent with the very strong intermolecular hydrophobic association of the perfluorocarbon groups.

ツユクサの青色色素コンメリニンの立体構造決定とその発色機構

Flower colors, from red through purple to blue, are mostly from anthocyanins. Although there are many colors, only a few anthocyanins are found. We have succeeded to solve the structure of real anthocyanin and a sugar containing flavonoid, Cd-commelinin, using the Weissenberg camera for macromolecular crystallography with imaging plates at the Photon Factory. The blue flower-color development and the stability of the color can be explained by metal complexation of anthocyanin and intermolecular hydrophobic association. Commelinin is a new type of stoichiometric supramolecule, which consists of low molecular mass components.

Structural basis of blue-colour development in flower petals from Commelina communis

FLOWER colours, from red through purple to blue, are mostly from anthocyanins, a type of flavonoid1–5. Although there are many colours, only a few anthocyanidins, chromophores of the pigments, have been found. The colour of the pigments is stable in plants for a few days to one month but the extracted anthocyanins are, nevertheless, unstable and quickly lose colour by hydration in a neutral aqueous solution1–5. In 1915 Willstatter proposed that flower-colours vary because anthocyanins change their colour with the pH6. Shibata and Shibata questioned the theory because most plant cell sap was weakly acidic or neutral. Their alternative, based on metal-complex theory7 was refuted by Everest8. In 1958 Hayashi isolated in crystal form a blue pigment9, commelinin, a metal-complex anthocyanin (named metalloanthocyanin)10,11, from the blue petals of Commelina communis. But the existence of a blue-coloured magnesium complex was denied by Bayer et al.12. We obtained the same blue pigment as intact commelinin by reconstruction from its components13. We also prepared Cd-commelinin in which the complexation metal Mg2+ was replaced with Cd2+. We report here the X-ray crystal structure of a real anthocyanin and a sugar-containing flavonoid, using Cd–commelinin. The blue flower-colour development and the stability of the colour can be explained by metal complexation of antho-cyanin and intermolecular hydrophobic association.

Viscometric and spectroscopic studies on the solution behaviour of hydrophobically modified cellulosic polymers

The solution properties of hydroxyethyl cellulose (HEC) and hydrophobically modified hydroxyethyl cellulose (HM-HEC) have been investigated by means of viscometric and spectroscopic techniques involving free radical and fluorescent probes. The greater viscosity of HM-HEC solutions above a critical polymer concentration (C p ) of approximately 0·2% has been interpreted in terms of the formation of a three-dimensional network structure in which the polymer chains are effectively crosslinked by the intermolecular association of neighbouring hydrophobic side chains. C p is considerably less than the predicted polymer coil overlap concentration (C∗) of approximately 1%. The interaction of the polymers with an anionic surfactant, sodium dodecyl sulphate (SDS) has also been investigated. A mechanism involving the interaction of free surfactant with the regions of intermolecular hydrophobic association is suggested to account for the considerable differences in the rheological behaviour of the polymers in the presence of SDS.

Study on coacervation of the repeat pentapeptide model of tropoelastin: effect of cations.

The effects of various cations on coacervation of the polypentapeptide (Val-Pro-Gly-Val-Gly)n, a sequential model of tropoelastin, were studied by following the turbidity at 300 nm and the fluorescence intensity due to the interaction between the polypentapeptide and a hydrophobic probe (4-benzoylamido-4'-aminostilbene-2,2'-disulfonic acid). In the absence of cations, as the polypentapeptide concentration was increased, the turbidity curves shifted to lower temperatures and became much steeper, while the fluorescence intensity increased markedly. This suggests that a conformational change is induced by the association of the polypentapeptide molecules and indicates that coacervation occurs predominantly by intermolecular hydrophobic association. In the presence of Mg2+, the temperature profile of the coacervation curve of the polypentapeptide obtained by turbidity measurement moved to higher temperature and the fluorescence intensity decreased significantly. This suggests that Mg2+ inhibits the hydrophobic interaction of the polypentapeptide molecules on coacervation but does not induce conformational change on association of the molecules. In the presence of Ca2+, Na+, and K+, the temperature profile of the coacervation curve of the polypentapeptide was not affected appreciably, but the fluorescence intensity was decreased by these cations in the order of Na+, K+, and Ca2+. Circular dichroism spectra of the polypentapeptide in 80% trifluoroethanol in the presence of cations showed a less ordered state of the polypentapeptide. This less ordered state implies inhibition by the cations of the hydrophobic interaction between the molecules.

Effect of salt on the inter- and intramolecular hydrophobic interactions of macromolecules

The effect of salt on the structure of a low density lipoprotein (LDL) and on the reversible polymerization of bovine serum albumin (BSA) reduced with 2-mercaptoethanol was investigated by means of ultracentrifugal analysis. The chaotropic anion, e.g., SCN− and I−, at 5M completely disrupted the LDL structure and effectively dissociated BSA oligomers at lower concentrations. The parallelism between the anion order of these effects and that of the chaotropic effect suggested that the observed salt effects are primarily based on the disruption of hydrophobic interactions. The cation effectiveness disrupting the LDL structure followed the order of their promoting effect on the water structure, i.e., Li+>Na+>K+>Cs+. However, Cs+ was most effective in dissociating BSA oligomers, and this was attributed to the π-complex formation with the aromatic amino acid side chains which otherwise contribute to the promotion of the intermolecular hydrophobic association.