Staining Polymers for Microscopical Examination

Abstract Polymers find widespread use in blends, graft copolymers, polymer alloys, and composites. Staining is an important tool for microscopically visualizing polymer phases and morphologies in these multi-component systems. Three stains have been used in most investigations. These stains (and the functional groups they are aimed at) are: osmium tetroxide for chemical unsaturation; ruthenium tetroxide for styrenics; and phosphotungstic acid for polyamides. Beyond these three basic stains there are a significant number of additional ones that have been developed for special cases. Aside from reacting chemically with functional groups, stains have been used as: negative (outline) stains; stains to delineate amorphous-crystalline regions; stains for microvoids, and stains for phase boundaries. Stains are most commonly transported to the target functional groups in tow ways — dissolved in liquid media or by direct contact with stain vapors. Improvisations have been developed to meet special staining problems. These improvisations most often involve: 1) chemical alteration of a polymer backbone to introduce a stainable functionality; 2) tailoring of staining media to provide adequate diffusion into the matrix; 3) preferential swelling of a blend component in order to accept a stainable moiety; 4) tactical use of a staining temperature to take advantage of differences in glass transition temperatures of blend components; and 5) optimization of chemical equilibria to expedite staining time or extent. Examples of these modifications are presented along with some cautionary notes regarding some artifact that may arise. A plea is made for more adequate descriptions of staining protocols in published articles. In too many cases there is inadequate description of staining methods and experimental duplication is put in jeopardy.