Polymer Behavior In Solution And Biological Surface

By using the combined techniques of laser light scattering (LLS), quartz crystal microbalance with dissipation (QCM-D) and ultra sensitive microcalmetry, we have inverstigated the properties of polymers in solution and biological surface science, which includes the polymer modification on drug delivery system, the potential cyclotoxicity of nanoparticles on cell membrane, and the structral evolution of intelligent polymers.First, we have examined the effect of hydrophobic interaction on the adsorption of hydrophobically end-capped poly(ethylene glycol) (HE-PEG) on lipid bilayer. Our studies reveals that both hydrophobic interaction between the lipid membrane and HE-PEG chains as well as the osmotic pressure drive the adsorption and insertion of such polymer chains. At a low HE-PEG concentration, the adsorption can not induce a vesicle-to-bilayer transition until the carbon number is up to 16. However, at a high HE-PEG concentration, the adsorption results in a vesicle-to-bilayer transition at m ? 12. ITC measurements also demonstrate that enthalpy change (ΔH) changes from positive to negative as the carbon number increases from 12 to 16, indicating that the HE-PEG insertion increases with its hydrocarbon end-chain length.Secondly, we have focused on the ion-mediated changes in nanomechanical properties of supported lipid bilayer. The effect of Ca²⁺ ions on structure changes of negatively charged POPG/POPC bilayer was discussed. We find that the viscoelastic and optical properties of SLBs containing POPG are sensitive to Ca²⁺ ions in bulk solution. The possible mechanism is the structural changes of bilayer caused by Ca²⁺ ions, which act as a bridge to link the POPG lipids and SiO₂ substrate. When they're removed, the linkage is broken and the repusion between the negativly charged bilayer and the negatively charged SiO₂ sensor surface lead to both SLB undulation and interfacial water layer change.Thirdly, we have studied the interaction between TiO₂ nanoparticles and differently charged surface-supported lipid membranes. In this project, we use supported lipid bilayer as simpified biological model system to examine the potential cytotoxicity of TiO₂ NPs. We demonstrate that TiO₂ NPs interacted transiently with POPG/POPC membranes via Ca²⁺ ions mediatated mechanism, leaving behind holes in the supported lipid membrane as verified by AFM imaging, due to the stronger affinity between Ca²⁺ ions and TiO₂ surface. Calculation based on DLVO theory is used to further support the mechanism.Moreover,we have examined the structure of thermosensitive mixed micelles by laser light scattering. Poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) diblock copolymers form micelles in water. The introduction of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymer leads to the formation of mixed micelles through hydrophobic interaction. The dimension of the mixed micelles varies with the weight ratio (r) of PEO-b-PPO-b-PEO to PI-b-PEO. The temperature dependence of the structural evolution of the micelles at different r was studied as well. At r < 10, the size of the mixed micelles decreases with temperature. At r > 10, due to the excessive PEO-b-PPO-b-PEO chains in solution, as temperature increases, the mixed micelles aggregate into larger micelle clusters.Finally, we have investigated the energy change in the periodic swelling-to-deswelling of thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels containing ruthenium (II) tris(2,2'-bipyridine) (Ru(bpy)₃) which is a catalyst for Belousov-Zhabotinsky (BZ) reaction. As temperature increases, the induction period and oscillation period of BZ reaction decrease because the reaction rate increases. However, the oscillation disappears at a temperature above the lower critical solution temperature (LCST) of the microgels since Ru(bpy)₃ is trapped in the microgels and can not react with BZ substrates. As microgel size increases or the cross-linking density decreases, the restriction of polymer networks on Ru(bpy)₃ decreases, so that Ru(bpy)₃ can readily contact with BZ substrates, leading the oscillation amplitude to increase. In addition, the so-called transient chaos occurs at a low stirring speed, and it wanes with the increasing stirring speed. All the facts indicate that the contact between Ru(bpy)₃ and BZ substrates determines the oscillation of the microgels.