Reentrant behavior of poly(vinyl alcohol)-borax semidilute aqueous solutions

The reentrant behavior of Poly(vinyl alcohol) (PVA)-borax aqueous semidilut e solutions with a PVA concentration of 20 g/l and borax concentrations var ies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS ) and dynamic viscoelastic measurements. Two (fast and slow modes) and thre e (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solut ions without and with the presence of borax, respectively, were observed fr om DLS measurements. The fast and middle relaxation modes were q(2)-depende nt (q is the scattering vector) characteristic of diffusive behavior; howev er, the slow modes were q(3)-dependent, characteristic of intraparticle dyn amics. The experimental results showed that the slow relaxation mode domina tes the DLS relaxation. The DLS slow mode relaxation time, tau(s), and the viscoelastic modulus G'(omega) and G "(omega) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased fr om 0.0 to 0.20 M, i.e. tau(s), G'(omega), and G '(omega) fluctuated with in creasing borax concentration. The excluded-volume effect of polymers, charg e repulsion among berate ions bound on PVA molecules, and intermolecular cr oss-linking didiol-borate complexation caused an expansion of the polymer c hain; however, the screening effect of free Na+ ions on the negative charge of the berate ions bound on PVA and intramolecular cross-linking didiol-bo rate complexation led to a shrinkage of the polymer chain. The reentrant be havior was the consequence of the balance between expansion and shrinkage o f the PVA-borate complex.