Structure for shape stable poly(vinyl alcohol) hydrogel under pH shock

Shape stable poly(vinyl alcohol) (PVA) hydrogels can be synthesized in acidic or basic medium. The PVA hydrogels yielded at acidic or basic pH can be applied to applications with aqueous solutions at opposite pH's. This study investigated the changes in linkage core and the shape stability of the PVA hydrogels subjected to opposite pH shock, with the sizes, weights and chemical composition changes for the yielded hydrogels, together with the FTIR, Raman and NMR spectroscopy, and the TGA analysis on the hydrogel samples being recorded and reported. With basic pH shock, the B(OH)₃ ([B3]) cores in the “acidic” PVA hydrogels were converted to B(OH)₄⁻ ([B4]) cores; while with acidic pH shock, part of the [B4] cores in the “basic” hydrogels were transformed into [B3] cores. The acidic pH shock would deteriorate the shape stability and moisture swelling capability of the basic hydrogels; conversely, the basic pH shock would not affect shape stability but even enhanced swelling capability of the acidic hydrogels. The [B3][B4] or [B4] cores in the PVA matrix tend to enhance water stability of the hydrogels and the conversion of [B4] and SO₄²⁻ ([S4]) to [B4][B4] or [B4][S4] dimers would destabilize the shape stability of the hydrogels. The findings proposed a way to manipulate the structure stability of PVA hydrogels by converting linkage cores in the matrix.