The influence of pH on a hydrogen-bonded assembly film

A detailed study of the influence of pH on the hydrogen-bonding layer-by-layer (LBL) assembly film constructed from poly(vinylpyrrolidone) (PVPON) and poly(acrylic acid) (PAA) was carried out. It was found that the LBL assembly process of a PVPON-PAA film is very sensitive to pH. pH 4.0 is the critical value for film build-up: at pH values above 4.0, the assembly of PVPON and PAA is almost halted, whereas for pH < 4.0, PVPON and PAA can be successfully deposited. For pH < 4.0, there are two transitions in the plot of film thickness as a function of pH value, and pH can affect the film thickness greatly, from several angstroms to hundreds of nanometres per dipping cycle. Moreover, pH also affects the morphology of the films and the composition of PVPON and PAA in the film. The influence of pH upon the PVPON-PAA film can be considered as a consequence of the pH-induced charge density and the conformation change of PAA. In addition, the pH discrepancy between the build-up and disintegration of the hydrogen-bonded film is discussed, and pH 5.5 can be considered to be the onset value of disintegration of the PVPON-PAA film, which is higher than the critical pH value of build-up (pH 4.0).