Tuning the Properties of Layer-by-Layer Assembled Poly(acrylic acid) Click Films and Capsules

The combination of click chemistry and layer-by-layer (LbL) assembly provides a useful and convenient means of preparing functional, covalently stabilized films and capsules. Herein, we examine various parameters that affect the buildup of click-LbL assembled multilayers of azide- and alkyne-modified poly(aerylic acid) (PAA(AZ) and PAA(Alk), respectively). We demonstrate that film thickness and morphology can be tailored by varying the assembly conditions. The thicknesses of multilayers assembled from PAA(AZ) and PAA(Alk) [(PAA(AZ)/PAA(Alk))(5)] on planar substrates varied from 70 nm when assembled at pH 2.5 to 10 nm when prepared at pH 4. Increasing the ionic strength of the adsorption solution resulted in an increase in the (PAA(AZ)/PAA(Alk))(5) film thickness, with a maximum of similar to 30 nm observed for solutions with ionic strengths of 150 mM and greater. A deposition time of 5 min was found to give close to saturated adsorbed layer amounts. Additionally, the influence of the click moieties on multilayer assembly was investigated. By altering the azide content of PAA(AZ) and maintaining the alkyne content of PAA(Alk) at similar to 15%, the thicknesses of (PAA(AZ)/ PAA(Alk))s films were shown to increase exponentially from about 20 nm at 5% azide functionalization of PAA(AZ) to 90 nm at 30% azide functionalization of PAA(AZ) Furthermore, atomic force microscopy measurements showed distinct morphological changes (i.e., enhanced porosity and/or creases and folds) for (PAA(Az)/PAA(AIk))(5) films prepared from PAA(AZ) with different azide contents at pH 3.5 when subjected to basic conditions (pH 10). This was attributed to the different cross-linking degree between the multilayers. The current study was extended to the assembly of hollow polymer capsules to determine an optimum range of 10-20% azide functionalization of PAA(AZ) for the assembly of click polymer capsules.