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.