Molecular-level structures of polymer surfaces and interfaces studied by sum frequency generation vibrational spectroscopy.

In this thesis, the molecular-level surface/interface structures of several important polymeric systems, including widely used macromolecular surfactants (Pluronics), adhesion promoters for polymer materials (silanes), anti-biofouling polymer coatings, and a few important model polymers such as poly(methacrylate)s and polystyrene (PS) have been investigated using sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM). We have demonstrated that SFG is a powerful technique to elucidate molecular-level structures of polymer surfaces and buried interfaces. Chapter I is a brief introduction of the studies on polymer surfaces and interfaces and the basic theory of SFG nonlinear optics. The proposed research is also presented in this chapter. The study of the molecular conformations of liquid polyethers including poly(ethylene glycol), poly(propylene glycol), and their block copolymers (known as Pluronics) at different liquid/air and solid/liquid interfaces are summarized in Chapters II and III. The relationship between such interfacial conformation and interfacial environment has been established. The different interfacial conformations of Pluronics at different concentrations have been related to the surface tensions of the solutions. In Chapters IV and V, we further extend SFG studies to several polymer/silane interfaces. Silanes are widely used as adhesion promoters for polymer materials. The conformations of silane molecules have been compared at different polymer/silane interfaces to understand molecular interactions at these interfaces. After the examination of polymer/silane interfaces using SFG, research on solid polymer/solid polymer interfaces and solid/solid polymer blend surfaces are summarized in Chapters VI and VII. Using the poly(n-butyl methacrylate)/PS or deuterated PS (d-PS) interface as an example, we have elucidated molecular conformations of polymers at the buried polymer interface. In addition, the surface morphology and chemical structures of PBMA/PS polymer blends with different compositions have been investigated by SFG and AFM. In Chapters VIII and IX, surface restructuring behaviors of several important polymer materials in water are discussed. The molecular-level structures at the polymer/air and polymer/water interfaces have been compared. This section presents many details on what happens when a polymer surface is brought into contact with water, which can improve molecular-level understanding of surface properties for important polymers used in wet environments.