Molecular level understanding of polymer surfaces and their interactions with other molecules studied by sum frequency generation vibration spectroscopy and atomic force microscopy.

Abstract

It is necessary to study molecular surface structures of polymers because polymer surface structures determine surface properties. Polymer blends and copolymers are important polymer materials because they may exhibit surface properties that are different from those observed in the bulk. Sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM) have been applied to study the molecular structures and morphologies of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend surfaces and the copolymer between PS and PMMA (PS-co-PMMA) in air and in water. Results show that both PS and PMMA segregate within the blends and form distinct domains on the polymer surfaces depending on their bulk concentrations while the PS-co-PMMA surface remains flat. These polymer surface structures can be varied step-wisely by systematically altering the bulk concentrations of the two components and by annealing the surfaces. Design of such step-wisely changed surface structures offers the potential for fine tuning of the surface properties of polymer materials. With many polymers being used as biomaterials, it is important to probe the interaction of proteins adsorbed onto these surfaces once the biomaterial is implanted inside of the body. SFG and AFM have been applied to examine the structural and conformational information of various proteins after adsorption onto different polymer surfaces. Results suggest that adsorbed proteins adopt certain conformations and/or orientations. These conformations and/or orientations are characteristic of certain secondary structures of the protein and may change as a function of time. In addition to SFG studies on surface structures of polymer materials and polymer-protein interactions, SFG is also applied to study molecular structures of interfaces between poly (ethylene terephthalate) (PET) and individual as well as mixtures of model ink components to understand adhesion properties between ink and polymer substrates. Such ink components include urethanes, nitrocellulose, plasticizers, and adhesion promoters. Our results indicate that the PET-ink interactions are very complicated. Various factors, such as segregation and ordered alignment of ink components at the interface, interfacial hydrogen bonding, as well as interfacial Van der Waals interactions play roles in adhesion. Adhesion of inks on polymer substrates is achieved by a cooperative effect of the individual ink components.