Single-molecule Force Spectroscopy On The Interfacial Adsorption Of Macromolecules

The science of macromolecules at interfaces is an interdisciplinary field that deals with the phenomena, composition, structure, morphology, properties and the energy of macromolecular materials at interfaces. It is regarded as one of the growth points in the modern science and technology. With the emersion of atomic force microscope (AFM), the study of single macromolecular chain at interfaces becomes possible. Utilizing the AFM based technique, single molecule force spectroscopy (SMFS), this dissertation attempted to detect directly the information of single macromolecular chain at interfaces.In Chapter 1, the science of macromolecules at interfaces is reviewed. Some common characterization methods are summarized. The AFM as well SMFS are specially introduced. The feasibility of SMFS on the study of single chain properties at interfaces is discussed.In Chapter 2, the SMFS of poly(4-vinylpyridine) (P4VP) is studied. The typical force curve of P4VP presents a saw-tooth pattern, which may correspondto the loop conformation of P4VP at interface. A statistical analysis finds that the desorption force of a single anchor point is ~180 pN, which is much larger than the s trength o f a s ingle H-bond. T his finding i ndicates t hat b esides H -bonding, other interactions also contribute to the interaction at interface.In Chapter 3, we used a novel amphiphilic segmented polymer -PNIPAM-seg-PS to study t he contribution of a m onomer unit on the interfacial interaction. This polymer behaves differently on various substrates, depending on the hydrophobicity of the substrate. On hydrophilic substrate, the PNIPAM segments anchor, while on hydrophobic substrate, the PS segments adsorb. The SMFS of PNIPAM-seg-PS on a hydrophobic substrate presents regular saw-tooth pattern, each peak corresponds to the desorption of an individual PS segment from the substrate. The desorption force increase with the rise of stretching velocity implies that the adsorption/desorption progress of PS segment at interface is a slow, dynamic one. We estimate that each styrene monomer contributes 1.3~2.1 pN to the desorption force.In Chapter 4, the SMFS of a strong polyelectrolyte (PAMPS) is studied, in terms of desorption and elongation. We find that the ionic strength and the linear charge density do not affect the interfacial force between the single PAMPS chain. This result implies that hydrophobic interaction dominates the adhesion force between polymer and substrate, even if strong polyelectrolyte is used. We attribute the phenomena to the introduced spacer between the polymer backbone and the charged groups.The preparation of a "single-molecule" sample is a key issue in SMFS. Due to the entanglement of polymer chains and the effect of surface concentration enrichment, the sample preparation of SMFS feazes the research community. In Chapter 5, we propose a novel method for the sample preparation by utilizing theIVdefects of the self-assembly monolayers. With this method, the polymer chains are separated individually at the surface, and the desorption force of single chain from substrate is successfully measured.In Chapter 6, the whole dissertation is summarized and discussed.