| Polymer films had been widely used in many applications such as coating, sealing,adhesives, microelectronics, etc, in which the surface properties of polymer films usually playedan important role. It was obvious that the surface performance originated from the surfacestructure of polymer films. Studying the mechanism of polymer film surface formation could beof great significance in adjusting and optimizing the surface structure and performance.With the aid of sum frequency generation vibrational spectroscopy (SFG) which possesseda unique surface-sensitivity, we had investigated the surface structure of film solidified from thepolymer solution and the structure of polymer monolayer at air/water interface. And aessential linkage between the structure of film surface and that at air/water interface had beensummarized. Some conclusions were obtained as follows:(1) The structure of poly vinyl acetate (PVAc) and Poly(methyl methacrylate)(PMMA)monolayer at air/water interface had been investigated. For PVAc monolayer, methyl groups inside chains lay randomly on the interface when surface pressure was0mN/m. The tilt angle ofmethyl groups decreased from61°to36°during the compression, which indicated that themethyl groups of the PVAc could remarkably changed the conformation from pointing parallel tothe interface to pointing mainly parallel to the surface normal by compression. However, forPMMA monolayer, the ester methyl groups could be ordered even when the surface pressure wasonly0.8mN/m, and further compression could not alter the orientation angle. The disparity inside chain mobility could be the essence of the difference in monolayer structure of PVAc andPMMA at air/water interface. For PVAc monolayer, the methyl was adjacent to the carbonyl thatwas regarded as an “anchor point” at the air/water interface. Hence, the methyl groups in PVAcmonolayer existed in an irregular state and needed to be compressed to form an orientedstructure. Being far from the carbonyl, the ester methyl groups in PMMA processed a bettermobility, they could form oriented structure when surface pressure was low.(2) The surface structure and wettability of PVAc films (both cast and spin-coated). Whenthe solution concentration was0.5wt%, the PVAc cast films were found more hydrophobic withan obvious peak assigning to the vibrational mode of methyl groups in the SFG spectra (bothSSP and PPP polarization combinations). As the concentration increasing, the intensity of peak from methyl groups weakened in the spectra of PVAc cast films, indicating that the methylgroups of PVAc molecules lay randomly. And the surface of PVAc cast films became relativehydrophilic. For spin-coated films, we could find an analogous phenomenon. The moreconcentrated the solution was, the more disordered the methyl groups would be, and the filmsurface would be more hydrophilic. Besides, the lower the applied speed was, the more orientedstructure the methyl groups would be. And the film surface would be more hydrophobic.(3) The surface structure of PVAc and PMMA cast films showed two different tendencies ofsolution concentration. As the concentration increased, the methyl groups of PVAc molecules layrandomly and the resulting films became hydrophilic. For PMMA films, the higher the solutionconcentration was the more ordered the methylene would be, and the cast films would be morehydrophobic. Similar with the situation at A/W interface，the disparity in side chain mobilitycould be the essence of this difference. The increasing in solution concentration led to weakeningof side chain mobility during the solidification of polymer solution. Due to the faintish mobilityof PVAc side chain, the increase in solution concentration could markedly limited the ability ofside chain to adjust its conformation, making the methyl groups disordered on the surface ofPVAc films. However, the ester methyl groups in PMMA were of a stronger mobility, whichcould still result in an oriented arrangement of methyl groups on the surface of PMMA films.That would be the reason why PMMA showed a tendency that the intensity of methylene peak inmain chain increased with the increment of the solution concentration, while the amplitude of theester methyl groups almost kept constant. |
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