Preparation and modification of poly(methacrylic acid) and poly(acrylic acid) multilayers

By employment of a strategy of post-transfer modification, precursor Langmuir-Blodgett (LB) films of poly(tert-butyl methacrylate) (PtBMA) and poly(tert-butylacrylate) (PtBA) can be converted to poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA) through acid-catalyzed hydrolysis in the gas phase. X-ray reflectivity studies show that these films possess surface roughnesses and controllable thicknesses, which are consistent with the retention of the "two-dimensional" configuration of the precursor polymers at the air/water interface. On this basis, the PMAA and PAA films with presumable layered architecture can be obtained, even though PMAA and PAA are too hydrophilic to undergo direct LB-multilayer formation. A combination of infrared spectroscopy, contact angle measurements, and sorption experiments confirms the chemical transformation and increased hydrophilicity of the films. Using the same approach, gas-phase reactions with organic amines convert the reactive cartoxylic acid groups to their corresponding ammonium salts, thereby leading to the formation of polyelectrolyte LB films. When these films are heated to elevated temperatures, amide bonds are formed. The use of difunctional amines opens up the possibility to cross-link the PMAA or PAA films efficiently. Fourier transform infrared measurements and X-ray reflectivity studies clearly indicate the conversion from PtBMA or PtBA LB films to ultrathin, highly swellable network films.