IS THE MOLECULAR-SURFACE OF POLYSTYRENE REALLY GLASSY

The atomic force microscope (AFM) has been used to probe the surface of a glassy polymer, polystyrene (PS). We have found for the first time that the AFM tip-PS surface interaction produces a morphology on the nanometer scale dependent on the PS molecular weight (MW). In the range from 13k MW to 1000k MW, two distinct types of oriented patterns on the PS surface occur in the top 7 nm of the surface. The marked transitions in the patterning and the evolution of the patterning as a function of time are indications of the elastic nature of the PS surface. Two distinct patterns are observed: an abrasion pattern for MW's less than 24k and a fully developed oriented pattern for MW's greater than 100k. The onset of the oriented pattern occurs very near the entanglement MW (M(e)) for PS, approximately 19k. Between molecular weights of 24k and 100k the oriented pattern does not fully develop. The "wear" of the PS surface decreases with increasing MW, although the exact functional dependence is unknown. The proposed scaling of the oriented wave spacings compared to those observed for macroscopic spacings for rubbers suggest that the near surface of PS behaves more like a material exhibiting rubber elasticity than one in the glassy state. The oriented patterns using the AFM are postulated to be due to the stretching of polymer chains which possess an extra degree of freedom at the PS surface. With further refinements in fabricating tips with smaller radii, it may be possible to disassemble a polymer surface in a macromolecule by macromolecule fashion. Also, by investigating other glassy polymers above and below M(e), it should be possible to contribute to the development of polymer surface structural models.