The photooxidation of polymers. IV. A note on the coloration of polystyrene

Yellow coloration develops in polystyrene under 2537 A. radiation a t comparable rates whether or not oxygen is present. Thus the commonly accepted view that coloration is a manifestation of oxidation is wrong. The coloration increases in intensity with time but does not move toward the red end of the spectrum, behavior which is typical of poly(vinyl chloride), polyacrylonitrile, etc., and which is due to the development of long conjugation sequences. It is suggested that the color in polystyrene is due to conjugated carbon–carbon unsaturation in the polystyrene backbone but that the lack of mobility of the molecules within the rigid polymer films prevents long sequences of double bonds becoming coplanar and color consequently moving through the spectrum. The rapid darkening of irradiated polymer which occurs on melting is probably due to relaxation of this rigidity allowing coplanarity of longer sequences.