Synthesis and characterization of model associative polymers

Abstract

AbstractModel linear associative polymers with number average molecular weights in the range of 16,600 to 100,400 were prepared by connecting blocks of commercially available poly(oxyethylene) with isophorone diisocyanate, followed by capping with either hydroxyl, dodecyl, or hexadecyl linear alkyl end groups. The molecular weight distributions measured by gel permeation chromatography are somewhat broad, as expected from the synthetic method. In a 40/60 by weight solvent mixture of diethylene glycol monobutyl ether (Butyl Carbitol) and water, the relative viscosities of model associative polymers and poly(oxyethylene) standards collapsed to a single master curve; viscosity average molecular weights obtained from the intrinsic viscosities measured in this solvent mixture compare favorably to those obtained by size exclusion chromatography. In water, the model polymers with alkyl end groups interact at extremely dilute concentrations to produce a pronounced increase in reduced viscosity that increases as concentration and alkyl end group length increase. The Huggins parameters for solutions of model associative polymers with the hexadecyl and dodecyl end groups vary between 1 and 16, and decrease as molecular weight increases, as hydrophobe length decreases, and as temperature increases. The concentration at which the viscosity data deviate from the Huggins equation is less than the polymer coil overlap concentration, which is on the order of 1–3 g/dL, as estimated from the reciprocal of the intrinsic viscosity data. This suggests that we can define a critical network concentration c*h as the concentration at which the associative polymers hydrophobic end groups first interact to form a rheologically significant network. However, the transition occurs over a concentration range, rather than at a particular critical micellar concentration, as is the case of ordinary surfactants or hydrophobically modified hydroxyethylcellulose. The dimensions of the associative polymer coils in solution, and the signs and relative magnitudes of heat and entropy of dilution as estimated from classical molecular theories, are similar to those obtained by other authors for poly(oxyethylene) homopolymers. A physical model based on equilibrium kinetics for the association process correctly mimics the dependence of viscosity on molecular weight and concentration, and indicates that the free energy of association must become larger as the length of the alkyl end groups becomes larger relative to the hydrophilic backbone. © 1995 John Wiley & Sons, Inc.