Hydrophobically modified poly(vinyl alcohol) using alkoxy-substituted methyl gallate: Synthesis and rheology

Abstract

Hydrophobically modified poly(vinyl alcohol) (HMPVA) polymers were synthesized by potassium t-butoxide-catalyzed reaction of PVA with methyl 3,4,5-tris(n-octyloxy) benzoate (MGC₈)/3,4,5-tris(n-dodecyloxy) benzoate (MGC₁₂) and 1,3-propane sultone. The concentration of 1,3-propane sultone was kept constant at 10 mol % and that of MGC₈ (2, 3, and 4 mol %)/MGC₁₂ (2 and 3 mol %) was varied to obtain HMPVAs with different hydrophobic contents. The incorporation of MGC₈/MGC₁₂ and 1,3-propane sultone onto HMPVA was confirmed by NMR spectroscopy. Rheological properties of aqueous solutions also confirmed the presence of hydrophobic and charged functional groups on HMPVAs. In the semidilute regime, the specific viscosity of HMPVAs followed concentration scaling that is typical of polyelectrolytes. At higher concentrations, the HMPVA solutions with 3 and 4 mol % of MGC₈ exhibited large increase in specific viscosity. Oscillatory experiments on these solutions exhibited gel-like behavior at polymer concentrations of 40-50 g/L. Confocal microscopy images of HMPVA with 4 mol % of MGC₈ clearly indicated the existence of microgels. The tendency of formation of microgels further increased with increasing chain length of the hydrophobe, that is, with MGC₁₂. These samples exhibited rheological behavior that is typical of soft solids and was therefore probed by the strain-rate frequency superposition technique reported recently in the literature. HMPVAs with improved rheological properties show potential applications as thickeners in cosmetic creams, lotions and as drug carriers in pharmaceutical formulations.