Hydrogen-bonded multilayers of a neutral polymer and a polyphenol

We report on association of tannic acid (TA) with neutral or charged polymers in solution and at surfaces and contrast hydrogen-bonded and electrostatically associated polymer/TA complexes and TA/polymer layer-by-layer (LbL) films as per their stability in the pH scale. The neutral polymers used for hydrogen bonding with TA were poly(N-vinylcaprolactam) (PVCL), poly(N-vinylpyrrolidone) (PVPON), poly(ethylene oxide) (PEO), or poly(N-isopropylacrylamide) (PNIPAM), and the polymer used to explore electrostatic binding with TA was 90% quaternized poly (4-vinylpyridine) (Q90). Association of TA with polymers in solution was explored by measuring the turbidity of solutions. At surfaces, LbL film deposition and pH stability were followed by phase-modulated ellipsometry and in-situ Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR). While electrostatically stabilized films of TA with Q90 could not be deposited at low pH values (pH = 2), hydrogen-bonded films of TA with PVCL, PVPON, PEO, and PNIPAM could be constructed at pH 2 and did not dissolve until a critical dissolution pH of 9.5, 9, 8.5, and 8 (measured in 0.01 M buffer solutions) for PVCL/TA, PVPON/TA, PEO/TA, and PNIPAM/TA, respectively. In addition, all multilayers could be also constructed at pH 7.5 in solutions with low ionic strength. The high pH stability of these systems as compared to multilayers of the same neutral polymers with polyacrylic (PAA) or polymethacrylic (PMAA) acids is due to higher pK(a) value of TA of similar to 8.5 as estimated in this paper. We also show that multilayers of TA with a copolymer of N-vinylpyrrolidone containing 20 mol % of primary amino groups, PVPON-NH2-20, were highly stable in a wide pH range from 1.3 to 11.7 because of combined stabilization through both. electrostatic and hydrogen-bonding interactions. For all systems, pH windows for deposition and stability of LbL films at surfaces correlated with the phase behavior of TA complexes in solution. High pH stability of hydrogen-bonded films of TA as well as the capability of tuning the critical pH value for film dissolution in the range close to physiological pH values makes such multilayer systems promising candidates for biomedical applications.