Facile Tailoring of Film Morphology and Release Properties Using Layer-by-Layer Assembly of Thermoresponsive Materials

Layer-by-layer self-assembly was used to prepare thermoresponsive thin films of poly(N-isopropylacryl-amide) (PNIPAAm) and poly(acrylic acid) (PAA) based on hydrogen bonding. The temperature of PNIPAAm adsorption was shown to significantly affect both the mass proportion of PNIPAAm in the film and the film surface morphology. When the adsorption was conducted at temperatures close to the lower critical solubility temperature of PNIPAAm, the amount of PNIPAAm in the film increased significantly (from 51 to 59%), and the total film mass increased by 30-40%. The films prepared at 30 °C also exhibited a lower surface roughness (1-2 nm) compared with 5-8 nm when prepared at 10 or 21 °C. The resulting multilayer films ([PAA/PNIPAAm]₁₀) were capable of being reversibly loaded and unloaded with dye (Rhodamine B) by exposure to solutions at elevated temperatures. The rate of loading and release was shown to depend on both the solution temperature and film preparation temperature, leading to tunable loading/release properties.