ANOMALOUS ADHESION IN ADSORBED POLYMER LAYERS

Anomalous adhesion behaviour observed in the polydimethylsiloxane/hept ane/silica system is reported. This behaviour is characterised by the infrequent appearance of one or more elastic minima which occur in add ition to the ever-present primary adhesion. The resulting elastic forc e is attributed to loops connecting the tip to the substrate and can b e described by two models; a worm-like chain, and a freely jointed cha in. Both models give similar values for the characteristic segment len gth of around 0.24 nm and indicate that the chains have been extended by between 90 to 95% of their chain contour length before desorption. In some cases multiple adhesions were observed between tip and substra te and have been used to suggest a method for determining the distribu tion of adsorbed polymer loops.