A study of the purity of some polystyrene samples by means of refractive index increment measurements and diffusion experiments, together with a calibration of a differential refractometer

In some recent diffusion experiments performed in this laboratory on dilute solutions of polystyrene fractions in toluene, ethylacetate and butanone very anomalous shapes of the interferometrically recorded concentration curves were obtained. Since the most reasonable explanation of the anomaly is the assumption of an impurity in the polystyrene fractions a closer investigation has been performed of the efficiency of one or several steps of purification by precipitation, freeze drying, extraction, and dialysis. Judging from the diffusion experiments the impurity – if it exists – seems to have a negative refractive index increment in toluene. Hence the efficiency of the purification has been followed by very careful measurements of the refractive index increment. However, even after the most extensive purification the anomaly in the diffusion experiments was only slightly changed and the refractive index increments of polystyrene in toluene were constant, 0.1129 ± 0.0002 ml./g. at 436 mμ and 0.1091 ± 0.0002 ml./g. at 546 mμ. No definite conclusion as to the nature of the anomaly can be drawn from the experiments, but as a practical consequence accurate diffusion investigations using these polystyrene samples are impossible. Included in this paper is also a description of the calibration of a differential refractometer which shows that the sensitivity of the instrument used is about ± 5·10−7 refractive index units. The usefulness of diffusion experiments in detecting impurities is discussed and a few results are shown. For the same purpose some pyrolysis experiments were performed using a mass spectrometer as detector.