Viscoelastic Properties And Solidification Of Macromolecular Layers At Solid-Solution Interfaces

The viscoelastic properties of polymer solutions play important roles in many technologies and applications. The behaviors of polymer solutions over a wide range of concentration have been extensively studied for quite a long time, but still draw considerable interests from both theorists and experimentalists because of a number of unresolved issues in the field. The viscoelastic behaviors of a polymer solution can be affected by the presence of a boundary or an interface. Depending on the interaction between polymer molecules and the boundary, and that between polymer and solvent molecules, a boundary or an interface can attract or repel the polymer molecules, which is different from that of in bulk solutions. The main component of this dissertation is the research work of the viscoelastic properties of the macromoleculars layers in the solid-solution interface. The technology we used is called quartz crystal resonator, which can detect the viscosity and shear modulus of polymer solutions with different molecular weights and configurations, and compare to the viscoelastic properties in bulk solutions. The novelty of this research work acan be summaried as follows:1. Viscoelastic properties of polyethylene glycol (PEG) solutions with different molecular weights and molecular configurations were studied using a quartz crystal resonator technique. The technique probes the solution boundary layer with a thickness approximately equal to the viscous penetration depth adjacent to the surface of the quartz crystal. Two different types of the electrode surfaces on the quartz crystal were used in the study:one is bare gold, and the other is thiol-group (SH)-attached polyethylene glycol (SH-PEG) plated gold surface. The results show that different surfaces affect the characteristic behaviors of the bottom layer and have little effect on the viscoelastic properties of the fluid layer over a wide concentration range.2. We have studied viscosity and dynamic shear modulus of solution boundary layers of polyethylene glycol (PEG) with different molecular weights near a solid substrate using a quartz crystal resonator technique. On the basis of the resonant frequency shift and the dissipation broadening of the quartz crystal resonator, the viscosity and shear modulus of the solution boundary layers as a function of the concentration and the molecular weight of PEG molecules are determined. The results show that, near the semidilute concentration of the solution, the viscosity of the boundary layer increases rapidly; the rise of the viscosity follows a power law with an exponent which depends on the molecular weight of PEG molecules. The exponents for solution boundary layers with smaller PEG molecules are coincident with that of bulk solutions, while the exponent for larger PEG molecular boundary layers appears to be consistently smaller than that of bulk solutions. The shear modulus of boundary solution layers consisting of smaller PEG molecules remains nearly zero in the concentration range covered in this study, while the shear modulus for solution boundary layers consisting of larger PEG molecules displays a rapid increase above the semidilute concentration. These results indicate that over the acoustic penetration depth (a few hundred nanometers) the viscoelastic properties of solution boundary layers for small PEG molecules are indistinguishable from that of bulk solution, but for large PEG molecules, their viscoelastic properties differ distinctly from that of bulk solutions.3. Viscosity and shear modulus of a linear PEG solution boundary layer is larger than that of a solution boundary layer of 3-arm dendrites-shaped PEG with the same molecular weight, because the gyration radius plays the most important role in these effections.4. These results can be explained as due the increase of PEG. density in the solution boundary layers. The density increase in the boundary layer also causes a solid-like behavior with apparent nonzero shear modulus and reduced viscosity comparing to the bulk solution. An estimate of the density increase base upon the scaled semidilute concentration finds that the density increases of PEG in the boundary layers are consistent with the findings in several other recent studies.