Aqueous dye diffusion in thin films of water-soluble poly(vinyl pyrrolidone) copolymers: A dynamic secondary ion mass spectrometry study

We examine the effect of solute binding on aqueous dye diffusion in model p oly(vinylpyrrolidone) (PVP) copolymer films. The dye-binding property assoc iated with PVP derives from the unique combination of hydrophilicity, polar ity, and hydrophobicity contributed by the lactam ring, thereby enabling th e complexation of polarizable, anionic organic dyes. On the basis of dynami c secondary ion mass spectrometry (SIMS), we rationalize the depth profile of the aqueous dye solute in both charged and uncharged copolymer systems. Utilizing the essentially invariant carbon signal of the polymer to normali ze the sulfur signal that uniquely identifies the dye molecule, we compare the depth profiles for each of the individual copolymer films in terms of r elative dye concentrations. The opposing and intricately related processes of solute dye-binding and aqueous solvent imbibition preclude the retention of anionic dye molecules at the surface of any system characterized by a m easurable amount of firm free volume, by the dilution of VP moieties, or by the presence of positively charged pendant groups. Therefore, the number o f dye molecules diffusing into the bulk for each particular PVP copolymer i s not constant. The intrinsic thermodynamic capacity of the more hydrophili c copolymers to imbibe water molecules is manifested by the beneficial effe ct of water plasticization on diffusion, reflecting the relationship betwee n dye mobility and free volume. Because of the significant difference in mo bility distinguishing dye molecules from water molecules, the transport pro cess is also influenced by subtle variations in polymer microstructure and morphology.