Direct Force Measurements for the Characterization of Modified Poly(propylene)

Abstract

The interfacial properties of the technical polymer poly(propylene) and its blends were investigated by direct force measurements and complementary techniques, including streaming potential measurements and X-ray photoelectron spectroscopy (XPS). Special emphasis was put on the effect of physical treatment of poly(propylene), viz. oxygen plasma and flame treatment, which are often used in industry to modify poly(propylene). A scanning force microscope (SFM) was used to characterize the topography of the samples and for qualitative, comparative direct force measurements. In particular, the reproducibility of direct force measurements on these technical polymer samples and the effect of added salt (KCl) was investigated: The untreated and the plasma treated poly-(propylene)s show a long range electrostatic repulsion and a short range attraction, which vanishes at a certain KCl concentration. This concentration is smaller for the plasma treated poly(propylene), because of the stronger electrostatic repulsion of the incorporated, charged COOH groups. The flame treated poly(propylene) shows an attraction at all KCl concentrations. Quantitative direct force measurements were performed with the bimorph surface forces apparatus. The comparison with the DLVO (Deryaguin, Landau, Verwey, Overbeek) theory of colloid stability[1, 2] shows on the flame treated poly(propylene) an increase of the interfacial potential by 100 mV and of the charged groups by a factor of about 10, as compared to the untreated poly(propylene). A method for the preparation of thin, smooth poly(propylene) films which are suitable for direct force measurements was developed.