Diffusion of Model Hydrophobic Alkali-Swellable Emulsion Associative Thickeners

Abstract

Diffusion coefficients of hydrophobically modified alkali-swellable (HASE) associating polymers (APs) in aqueous solution were measured using pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR). HASE APs consist of a copolymer of methacrylic acid and ethyl acrylate to which hydrophobic “sticker groups” are attached through a poly(ethylene oxide) (PEO) “spacer arm”. Two series of HASE APs were investigated: one in which the length of the hydrophobic alkyl chain was varied (C x E35) and another in which the length of the PEO spacer arm was varied (C20E y ). With increasing hydrophobe or PEO spacer arm length the average diffusion coefficient decreased, reflecting the viscosity enhancement produced by the associating network formed by HASE polymers. The effect of external variables such as concentration, pH, and temperature mirrored established rheological properties for such systems. The average diffusion behavior, however, was the result of a superposition of two contributions, one slow (D 0 < 1 × 10-11 m2 s-1) and one fast (D 0 ≈ 1 × 10-10 m2 s-1) diffusing. Changing the hydrophobe chain length or the PEO spacer arm length modulated the proportion of the fast and slow diffusing populations but had relatively little effect on the diffusion coefficients of the two populations. Precipitation of the HASE AP from THF−HClaq produced two fractions, one enriched with hydrophobe and the other correspondingly depleted relative to the global average. The fractions enriched and depleted with respect to hydrophobe exhibited diffusion coefficients similar to the fast and slow diffusing populations. We conclude that the hydrophobe is inhomogeneously distributed among the HASE AP chains, that the population enriched with hydrophobe forms more compact structures capable of faster diffusion, and that the effects of hydrophobe chain length and PEO spacer arm length on HASE AP solution viscosity are mediated by their influence on the coupling of the hydrophobe enriched population into the slow-diffusing hydrophobe-depleted population.

Dynamic Light Scattering of Semi‐Dilute Hydrophobically Modified Alkali‐Soluble Emulsion Solutions with Varying Length of Hydrophobic Alkyl Chains

“…By increasing the hydrophobicity of the macromonomer and the length of the PEO spacer chain of the macromonomers, the aggregation number of the hydrophobic junction increases. This was also confirmed by the self‐diffusion coefficients as determined from pulsed‐gradient NMR 9…”

“…After full neutralization, HASE polymer backbone possesses a polyelectrolyte character and solubilizes in water, while the hydrophobic groups associate to form aggregates in solution. Dynamic light scattering and pulsed‐gradient NMR of HASE solutions revealed that the apparent particle sizes or the diffusion coefficients are independent of pH, when the pH values of the solution exceed 7 8,9. After full neutralization of MAA segments, the polymer backbones behave as polyelectrolytes, where the ionic strength of the solution could strongly influence the association mechanisms.…”

“…The presence of salt in the solution shields the electrostatic repulsion between the charged polymer backbones, which decreases the solution viscosities and increases the diffusion coefficients. Dilute HASE polymers in salt solutions at pH ∼ 9 have been studied by fluorescence spectroscopy,12,13 laser light scattering,8,14–17 and pulsed‐gradient NMR 9. Fluorescence spectroscopic studies revealed two kinds of associations in dilute solution regime, which correspond to the aggregations of macromonomers and blocky EA segments.…”

Dynamic light scattering of semidilute hydrophobically modified alkali‐soluble emulsion solutions with different lengths of poly(ethylene oxide) spacer chain

“…For dilute HASE solution, dynamic light scattering (DLS), fluorescence spectroscopy, and pg‐NMR, potentiometric and conductometric titrations revealed that the solution behavior of HASE polymers is extremely sensitive to pH 3, 4, 21, 22. With the addition of a base, HASE latex first swells and then dissolves into water.…”

Thermal properties of hydrophobically modified methacrylic acid–ethyl acrylate copolymer solutions

“…Hydrophobically modified alkali‐soluble emulsion (HASE) polymers have received considerable attention in recent years 6–20. Some studies on the thermal behavior of associative polymers have been reported previously,21–28 but detailed studies on the HASE system are scare 19, 29.…”