R. D. Jenkins

Solution rheology of a hydrophobically modified alkali-soluble associative polymer

Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependence of the zero shear viscosity, with η0∼c8. Evidence for shear-induced structuring in steady shear, large amplitude oscillatory shear, and parallel superposed steady and dynamic shear is presented. Such structuring is more pronounced at lower polymer concentrations, consistent with the formation of intermolecular associations at the expense of intramolecular. In contrast to the simple linear telechelic associative polymers considered in a number of previous studies, the network dynamics of the polymer are no longer represented by a single characteristic time. This deviation from a classical Maxwellian response in oscillatory shear is interpreted as a broadening of the relaxation spectrum, arising from the coexistence of both hydrophobic associations and topological entanglements. Mechanistically, stress relaxation is better envisaged in terms of “hindered reptation” [Liebler et al. (1991)] of the chains, rather than Rouse-like behavior moderated purely by the hydrophobe disengagement rate [Annable et al. (1993)].Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependen...

Rheological properties of methacrylic acid/ethyl acrylate co-polymer: comparison between an unmodified and hydrophobically modified system

Earlier experimental studies on the emulsion polymerized methacrylic acid/ethyl acrylate co-polymer indicated that the ethyl acrylate (EA) segments are sufficiently blocky to induce hydrophobic association between the EA blocks once the methacrylic acid groups are neutralized by a base. Detailed rheological characterization on semi-dilute solutions suggests that the viscoelastic property is caused by the transient network assembled through hydrophobic associations, rather than by physical chain entanglements. In the semi-dilute solution regime, the hydrophobically modified associative polymer exhibits higher viscosities when compared to the unmodified analogue. This is due to the formation of higher proportion of intermolecular association between the polymer clusters, which enhances the hydrophobic interaction between the hydrophobic macromonomers of different polymer chains. A mechanistic model is proposed to describe the nature of associations between the blocky EA and the hydrophobic segments of the polymer.

Rheological properties of hydrophobically modified alkali‐soluble polymers—effects of ethylene–oxide chain length

The rheological properties of hydrophobic alkali-soluble associative polymers (HASE) were studied using controlled rate (Mettler LS40) and controlled stress (TA CSL 500) rheometers. The effects of pH and polymer concentrations on the rheological properties of three HASE model polymer systems (i.e., HASE 5141, 5134, and 5142, with a degree of ethoxylation of 2.5, 10, and 40 mol, respectively) and a reference polymer without associative hydrophobes (MAAEA) were examined. As the pH is increased by addition of ammonia to greater than 5–6, the carboxyl groups ionize to carboxylate ions and the polymers become water soluble. The HASE polymers thicken mainly by hydrophobic association. Viscosity can increase by two to three orders of magnitude as pH is raised to 9. The degree of ethoxylation in the macromonomer controls the nature of the hydrophobic association junctions by altering the flexibility and hydrophobicity of the macromonomer. Optimum thickening efficiency is observed in the system with approximately 10 mol of an ethylene–oxide spacer between the polymer backbone and the macromonomer. Viscoelastic study shows that the maximum thickening efficiency also corresponds to the dominant elastic property observed in the system with 10 mol of EO. All the model systems except the control system without hydrophobe exhibit strain thickening of the viscous and elastic components.

Associative polymers bearing n-alkyl hydrophobes: Rheological evidence for microgel-like behavior

Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable shear thinning and, a characteristic power-law behavior (G′, G″∼ω0.4) under oscillatory shear persisting over a broad range of time scales. Concentration-independent master curves are obtained for the storage modulus, G′, with the level of G′ increasing with hydrophobe size. The similarity in the dynamic response suggests that there exists a qualitative equivalence in microstructure over the range of systems, the only difference being the “softness” or compressibility of the particles. Data from this study are also contrasted with those for a similar HASE polymer bearing a smaller number of alkylaryl hydrophobes [J. Rheol. 41, 427–444 (1997)]. In the latter case, the rheology can be interpreted in terms of hydrophobic associations and chain entanglements occurring in solution. Thus, subtle variations in molecular architecture are shown to cause significant differences in morphology and microstructure for these polymer systems.Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable shear thinning and, a cha...

Effects of salt on the intrinsic viscosity of model alkali-soluble associative polymers

The effects of salt on four model hydrophobically modified alkali-soluble emulsion (HASE) associative polymers were studied. Their reduced viscosities in aqueous solution at different salt concentrations (10 -4 to 10 -1 M) were measured and the corresponding intrinsic viscosities were determined. The results show that the intrinsic viscosity at any given salt concentration decreases as the hydrophobicity of the hydrophobes (i.e., n in the side chain C n H 2n+4l ) increases. The interpretation is that stronger intra-molecular association of the more hydrophobic macromonomer leads to smaller polymer molecular coils. An increase of salt content in the solution causes the reduced viscosities of all the model polymers to decrease. This is believed to be the result of excess positive ions in solution which have a shielding effect and reduce the mutual repulsion of the charges along the polymer backbone. In dilute solution the hydrophobic associations of the model associative polymers are predominantly intra-molecular. The addition of salt to the polymer solutions hinders the inter-molecular interaction of the model polymers. The increase in salt content also leads to an increase of intra-molecular aggregations of the hydrophobes of the model associative polymers. Stiffness indices such as the stiffness parameter B and the persistence chain length were used to correlate the polymer conformation of the model associative polymers in NaCI solutions.

Lifetime and network relaxation time of a HEUR-C20 associative polymer system

The end-capped telechelic HEUR associative polymer was reported to exhibit a single mode Maxwell typed relaxation behavior. suggested that the single mode Maxwell relaxation time corresponded to the lifetime of the hydrophobe in the micellar junction. However, results from some recent publications suggested that a more complex relaxation behavior existed. In order to verify this, a hydrophobically modified urethane–ethoxylate (HEUR) polymer with Mn of 26 000 and a C20H41 hydrophobe was synthesized for the present study. Results from oscillatory measurements of a 2 wt % sample in distilled water indicates that two different relaxation modes are present. This is confirmed with a two-mode Maxwell model and the determination of the relaxation time spectrum from the dynamic moduli data. Further analysis of the data at different temperatures confirmed that one of the modes is possibly related to the lifetime (∼0.01 s at 25 °C) of the hydrophobe in the micellar junction and the other to the network relaxation (∼...

Evaluation of intrinsic viscosity measurements of hydrophobically modified polyelectrolyte solutions

The dilute solution properties of the hydrophobically modified alkali-soluble emulsion (HASE) were examined using the intrinsic viscosity method. The HASE model systems with essentially the same chemical structure and molecular weight but different numbers of carbon in hydrophobe were synthesized for this study. The effects of ionic strength on the intrinsic viscosity measurement of HASE systems were studied. The ionic strength of the dilute polymer solution was found to influence the degree of hydrophobic association and the size of the micellar clusters during dilution. The variations of the above factors were experimentally found to have significant effects on the accuracy, reliability and repeatability of the intrinsic viscosity and Huggins coefficient.

Viscoelastic properties of hydrophobically modified alkali-soluble emulsion in salt solutions

Abstract An experimental study was carried out to examine the effects of added salt on the rheology of four model hydrophobic alkali-soluble associative polymer (HASE) solutions. With increasing salt concentrations, the solution viscosities of all the model polymers decreased continuously and the viscoelastic properties of the hydrophobe-containing polymer solutions changed from predominantly elastic to viscous behavior. Model polymers with stronger hydrophobic associations were more susceptible to the presence of added salt, however such effects were weakened at high shear stresses. At a certain level of added salt, the model polymer with hydrophobes consisting of C20 alkyl chain demonstrated a unique shear-thickening behavior at high shear stresses. The critical shear stress at which the shear-thickened viscosity reached the maximum value was independent of the salt concentration, but was proportional to the polymer concentration. The shear super-imposed oscillation technique was employed to probe the shear-induced structural changes of the model polymer that contributed to the shear-thickening behavior. With increasing applied shear stresses, a corresponding increase in the storage modulus ( G ′) was observed at the shear-thickening region where the viscosity increased with applied stresses. This observation indicated that the shear-thickening behavior is caused by the formation of a larger number of network junctions. The increase in the network junction densities was attributed to the conversion of intra- to inter-molecular associations.

Rheological properties of model alkali-soluble associative (HASE) polymer in ionic and non-ionic surfactant solutions

Abstract The rheological properties of 0.5 wt.% aqueous solution of hydrophobically alkali-soluble emulsions (HASE) at around pH 9 in the presence of cationic, anionic and non-ionic surfactants were examined. The results revealed that the presence of surfactant below a critical concentration c* (around cmc of surfactant) strengthened the associating network through the formation of mixed polymer hydrophobic group/surfactant aggregates. The increased strength was due to an increase in the number of mechanically active inter-molecular hydrophobic junctions and lifetime of the average junctions. Above c*, the results showed that the presence of cationic or anionic surfactant disrupted and weakened the associating network caused by a decrease in number of these junctions and strength of the overall network structure. At the same time, free cations of surfactant molecules screened the carboxyl-anion charges on the polymer backbone, which decreased the electrostatic repulsion between polymer chains. In the presence of non-ionic surfactant, the results showed a further reinforcement of associating network attributed to the formation of bilayers, which were stabilized as large vesicles.

Solution rheology of hydrophobically modified associative polymers: Effects of backbone composition and hydrophobe concentration

We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different exponents. In all cases, three power-law regimes, regardless of the MAA content, are observed that can be attributed to the presence of different modes of hydrophobic interaction. However, the transitions shift to lower concentrations as the MAA content increases. With regards to the effects of the macromonomer side-chain concentration, we observe a substantial increase in viscosity at intermediate macromonomer content (1 mol %), possibly due to an increase in the number of intermolecular junctions as the number of hydrophobes per chain increases. This is in contrast to (i) low macromonomer concentration (0.3 mol %) behavior that reveals low viscosity due to weak hydrophobic associations, and (ii) high macromonomer concentration (1.9 mol %) behavior that favors more intramolecular association resulting in lower viscoelastic properties compared to intermediate macromonomer concentrations.We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different expone...