Remarkable expansion of the poly(acrylic acid) chain by acid-base complexation with low molecular weight amines

The intrinsic viscosity [eta] of a poly(acrylic acid) (PAA) sample with a molecular weight of 4 x 10(6) in aqueous salt solution drastically increased by adding isopropyl amine (A-1). At the infinitely high concentration of the hydrochloric salt of A-1, where the electrostatic interaction is completely screened out, [eta] was enlarged by 40 times as the degree of complexation f(c) increases from ca. zero to 0.6. Here f(c) was determined by potentiometric and NMR titrations. Furthermore, [eta] at infinite ionic strength for PAA complexed with a bulkier and more hydrophobic amine, (R)-(+)- 1-(phenyl)ethylamine (A-2), was higher than that for the complex of PAA and A-1 at the same f(c). The drastic expansion of the PAA chain by the complexation with the amines A-1 and A-2 may arise from the chain stiffening and enhanced excluded volume effect.