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.

Deuterium nuclear magnetic resonance (2H NMR) spectroscopy of mixed cationic plus zwitterionic surfactant micelles exposed to the anionic polyelectrolyte poly(acrylic acid) (PACA) has been used to detect a thermotropic transition undergone in the phase-separated stoichiometric cation:anion complexes so formed. The deuteron labels were located on the quaternary methyls of either the cationic surfactant cetyltrimethylammonium bromide (CTAB-γ-d 9) or the zwitterionic surfactant hexadecylphosphocholine (HDPC-γ-d 6). The 2H NMR spectra of CTAB-γ-d 9 consisted of a superposition of two Pake doublet spectral components, indicating the presence of two surfactant populations undergoing different types of anisotropic motional averaging. With decreasing temperature, and over a narrow temperature range, the Pake doublet with the larger quadrupolar splitting increased in intensity at the expense of that with the smaller quadrupolar splitting, demonstrating the occurrence of a highly cooperative thermotropic transition between two states represented by the two Pake doublets. This thermotropic transition was manifested by CTAB-γ-d 9 but not by HDPC-γ-d 6, attesting to its origin in the electrostatic interactions between the cationic and anionic species. The midpoint temperature of this transition decreased with increasing CTAB:HDPC ratio of the surfactant micelles. The 2H NMR T1 relaxation time of both Pake doublet spectral components increased with increasing temperature but was a factor of 2 shorter in the Pake doublet with the larger quadrupolar splitting, indicating that the molecular motions contributing to relaxation were fast on the 2H NMR time scale but hindered in the low-temperature versus high-temperature states. The overall results are consistent with a thermotropic transition within the PACA/CTAB/HDPC complexes from a high-temperature normal hexagonal (HI) liquid crystalline phase to a low-temperature lamellar (Lα) liquid crystalline phase.

No abstract

No abstract