Synthesis of boronate-containing copolymers of N,N-dimethylacrylamide, their interaction with poly(vinyl alcohol) and rheological behaviour of the gels

Cross-linking of polyvinylalcohol (PVA) by boronate-containing copolymer of N,N-dimethylacrylamide (DMAA, 1) was studied and compared to cross-linking of PVA by borate buffers in weakly alkaline solutions. The copolymer of Mw=19,000 g mol−1 containing 9 mol% N-acryloyl-m-aminophenylboronic acid (NAAPBA, 2) was prepared by free radical polymerization of the monomers, exhibiting copolymerization constants r1=0.84 and r2=2.2. Due to multipoint interaction of the copolymer with PVA via monodiols, the intermolecular cross-linking required for seven-fold and 10-fold lower boron concentrations as compared to borate buffers of pH 8.6 and 7.5, respectively. In rheological measurements, PVA–copolymer gels exhibited storage moduli (G′max) comparable to those of PVA–borate gels prepared at 7.5-fold higher boron concentration and the same pH 8.6, what testified to the similar concentration of cross-links in the gels. Therefore, DMAA–NAAPBA copolymer is a more effective cross-linker of PVA than borate. The PVA–copolymer gel exhibited much higher relaxation time (97 s) compared to PVA–borate gels (≤20 s) indicating a longer lifetime of junction zones. The ‘shape stability’ of the gel is suggested to originate in the structure of junctions, containing several boronate–diol complexes, between the macromolecules of PVA and the copolymer.