MACROMOLECULES, vol.41, no.22, pp.8737-8744, 2008 (SCI-Expanded)
We report on the construction of purely hydrogen-bonded hybrid polymer multilayers, which are composed of polymer pairs with low- and high- pH stability, and show that the critical pH value of film deconstruction, fraction of components released, and the rate of film dissolution can be tuned in a wide pH range from 3 to 9.5 by varying film composition and architecture. The film building blocks were poly(N-vinylcaprolactam) (PVCL)/poly(L-aspartic acid) (PLAA) bilayers as pairs of hydrogen-bonded polymers with low pH stability (critical disintegration pH of similar to 3.3), and poly(N-vinylcaprolactam) (PVCL)/tannic acid (TA) bilayers as hydrogen-bonded polymers with a higher critical disintegration pH of similar to 9.5. Hybrid TA/PVCL/PLAA multilayers were prepared at low pH using a layer-by-layer technique. Film deposition and pH-induced deconstruction were followed by in situ Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR) and phase-modulated ellipsometry. PVCL/TA and PVCL/PLAA pairs were deposited in either alternating or stacked manner. Films with various layer arrangements had drastically different pH dissolution profiles. In all cases, the presence of PVCL/TA layers shifted pH values for release of PLAA from the film to more basic values. In the films with stacked architecture [(PVCL/PLAA)(6) (PVCL/TA)(n)], the mode of film destruction was dependent on both the amount of consecutively deposited PVCL/PLAA pairs and the number of PVCL/TA layer pairs in the surface stack. For the films composed of six bilayers of PVCL/PLAA in the base stack, the critical pH for film disintegration and PLAA release varied with the thickness of the top (PVCL/TA),, stack in a range from pH 3.5 to 5 with n ranging from 0 to 12. In hybrid alternating films, [(PVCL/TA)(1) (PVCL/PLAA)(1)](n) (1:1), release of PLAA and TA was more interdependent. The proximity of PVCL/TA pairs has further delayed PLAA release up to near-neutral pH. In addition, PLAA chains diffusing through the film triggered disruption of PVCL/TA interactions resulting in release of similar to 15-20% of TA. These results demonstrate the effects of proximity and intermixing of hydrogen-bonded polymer pairs of greatly different pH stability on film decomposition modes. The possibility of releasing active molecules and/or polymers combined with the biocompatibility of film components makes such systems attractive candidates for future biomedical applications.