pH-stability and pH-annealing of H-bonded multilayer films prepared by layer-by-layer spin-assembly

Abstract Spin-assisted assembly and pH-induced disintegration of H-bonded multilayers of poly(N-vinylpyrrolidone) and tannic acid (PVPON/TA) have been investigated. Multilayers showed significantly different pH-induced disintegration profiles depending on how long the multilayers were kept in the dry state between exposures to buffer solutions indicating relaxations of the molecules. To understand the effect of kinetic trapping and the consequent relaxations towards equilibrium on the pH-stability of H-bonded PVPON/TA spin-LbL multilayers, spin-coated multilayers were prepared at different nonequilibrium states by varying the spinning rate and the solution-film static contact time. The pH-stability of multilayers increased with increasing spinning rate and decreasing static contact time. The gradual decrease of bilayer thickness with increasing spinning rate indicated the flattening of the molecules. The enhancement of the pH-stability of multilayers was attributed to both the flattening and the relaxations of molecules having nonequilibrium conformations. A transition from linear to exponential growth profiles was observed with increasing static contact times indicating in and out diffusion in PVPON/TA system. pH-treatment of multilayers just below the critical disintegration pH, a process we call pH-annealing, enhanced their pH-stability significantly. Multilayers annealed longer at pH 7.0 showed better pH-stability (up to 1 h) at pH 9.0 whereas nonannealed films disintegrated within 10 min. The introduced pH-annealing process is promising for practical applications of H-bonded systems such as controlled drug delivery.