SYNTHESIS AND CHARACTERIZATION OF MODEL ASSOCIATIVE POLYMERS

Model linear associative polymers with number average molecular weight s in the range of 16,600 to 100,400 were prepared by connecting blocks of commercially available poly(oxyethylene) with isophorone diisocyan ate, followed by capping with either hydroxyl, dodecyl, or hexadecyl l inear alkyl end groups. The molecular weight distributions measured by gel permeation chromatography are somewhat broad, as expected from th e synthetic method. In a 40/60 by weight solvent mixture of diethylene glycol monobutyl ether (Butyl Carbitol) and water, the relative visco sities of model associative polymers and poly(oxyethylene) standards c ollapsed to a single master curve; viscosity average molecular weights obtained from the intrinsic viscosities measured in this solvent mixt ure compare favorably to those obtained by size exclusion chromatograp hy. In water, the model polymers with alkyl end groups interact at ext remely dilute concentrations to produce a pronounced increase in reduc ed viscosity that increases as concentration and alkyl end group lengt h 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 d ecreases, and as temperature increases. The concentration at which the viscosity data deviate from the Huggins equation is less than the pol ymer coil overlap concentration, which is on the order of 1-3 g/dL, as estimated from the reciprocal of the intrinsic viscosity data. This s uggests that we can define a critical network concentration c(h) as t he concentration at which the associative polymers hydrophobic end gro ups first interact to form a theologically significant network. Howeve r, the transition occurs over a concentration range, rather than at; a particular critical micellar concentration, as is the case of ordinar y surfactants or hydrophobically modified hydroxyethylcellulose. The d imensions of the associative polymer coils in solution, and the signs and relative magnitudes of heat and entropy of dilution as estimated f rom classical molecular theories, are similar to those obtained by oth er authors for poly(oxyethylene) homopolymers. A physical model based on equilibrium kinetics for the association process correctly mimics t he dependence of viscosity on molecular weight and concentration, and indicates that the free energy of association must become larger as th e length of the alkyl end groups becomes larger relative to the hydrop hilic backbone. (C) 1995 John Wiley & Sons, Inc.