Hydrophobic Polymer Modification with Ionic Reagents:  Polystyrene Staining with Water-Soluble Dyes

Homopolymer polystyrene films tinted with methylene blue and other hydrophilic cationic dyes are made following a new two-step procedure: dye adsorption in latex particles, followed by polymer plasticization with a suitable solvent. The dyed polymer is soluble in nonpolar solvents, and the dyes do not phase separate, even in nonsolvents for the pure dye salts. Microanalytical data on the particles and films were acquired using energy-loss spectroscopy imaging (ESI-TEM) and scanning electric potential microscopy (SEPM). The results are interpreted considering that (i) the polystyrene latex particles have a mild ionomer character, due to the charged sulfate groups arising from the polymerization initiator and bound to the chain ends; (ii) there is formation of ion pairs made out of dye cation and chain-end sulfate; (iii) the cationic dye self-associates, as evidenced by a pronounced metachromatic behavior. A nonpolar polymer can thus be made compatible with a basic dye without any special chain modification, just by using this new procedure based on dye sorption followed by diffusion.