成膜溶剂与氟化嵌段共聚物膜的表面富集行为

Abstract

The surface structure and properties of poly(methyl methacrylate)-b-poly (2-perfluorooctylethyl methacrylate) (PMMA-b-PFMA) films, prepared by their cyclohexanone, toluene and benzotrifluoride solutions, were investigated using contact angle measurement, X-ray photoelectron spectroscopy (XPS), sum frequency generation (SFG) vibrational spectroscopy, atomic force microscopy(AFM), and so on. The highest surface enrichment of fluorinated moieties was found when the film was prepared by benzotrifluoride solution. However, the film prepared by cyclohexanone solution exhibited the lowest surface segregation regardless of film-formation methods. The reason was mainly attributed to molecular aggregation structure of the copolymer in the solution and interfacial structure at the air/liquid, which affects formation of surface structure during film formation. When the copolymers in solution formed micelles composed of a PMMA corona and a PFMA core, the segregation rate of the fluorinated moieties was not high enough, the macromolecules were then 'frozen' into a specific structure as the solvent evaporated, in which parts of the fluorinated moieties were buried in the bulk. Therefore, the extent of fluorine enrichment at the polymer surface was affected greatly by film formation methods. When more free chains are in solution due to the presence of loose and unstable aggregates, faster chain exchange should accordingly occur, with more facile segregation of the perfluoroalkyl side chains on the surface. Thus the extent of fluorine enrichment at the copolymer surface was less affected by film formation methods.