Nanomechanics Of Dendritic Polymers And Organometallic Polymers

Nowadays, polymers are very important and indispensable part in material science. The mechanical properties of polymer play an important role in its application. The investigation of inter- and intermolecular interactions at molecular level will provide useful information for the molecular design of high performance polymer materials. In recent years, atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) has proven to be a versatile platform for studying the minute force in polymers as well as in supramolecular and bimolecular systems. The investigation of the relationship between the applied force and the corresponding length of single polymer chain provide more and more interesting information which cannot read from traditional methods. SMFS shows a new insight on polymer science. In chapter 1, the basic principle of SMFS are introduced in details, including the experimental control of single molecule detection, the conversion and analysis of data and the standard for the distinguishment of single molecule detection. Finally, some recent progresses in SMFS on polymers are briefly summarized, such as the interaction between small molecule and polymers, the interfacial conformation and energy of polymers, multiple H-bond strength and optomechanical molecular motor. In this thesis we have focused on dendritic polymers and organometallic polymers studying the single chain elasticity, interfical properties, and so on.In chapter 2, we have investigated the force spectroscopy of two second-generation dendronized polymers, hydrophobic and amphiphilic poly(p-phenylene). In THF, the individual amphiphilic polymer chain exhibits larger elasticity than the hydrophobic polymer, which is probably induced from the collapse of oligoethyleneoxy chains in THF, which is a poor solvent for them. While using CH2Cl2, all the side groups can be expanded; the elasticity of the amphiphilic polymer chain is therefore smaller than that in THF. The hydrophobic polymer exhibits the same elasticity in THF and CH2Cl2 because of the similar chain conformation in the two solvents. The adhesion-force measurements in a poor solvent at the interface reveal that the poor solubility of the polymer side groups as well as the hydrophobic interaction between the surface and the polymer side groups enhances the adhesion force.In chapter 3, we have investigated the single chain elasticity of a series of dendronized copolymers, which bear three type side groups, Fréchet type dendrons, amides and carboxyls. The single chain elasticity of the polymers in THF and CHCl3 has been obtained, respectively. The elasticity of the polymer chain can be different depending on the properties of side groups and solvents. The polymer with first-generation dendrons shows different elasticities in the two solvents because of the different H-bonds of the polymer in two solvents. When the dendron groups are changed to be second and third generations, the single chain elasticity of the polymers is the same in two solvents.In chapter 4, we have directly measured the interaction between two poly(benzyl ether) dendrons using SMFS. For this purpose one dendron was immobilized on AFM tip through a poly(ethylene glycol) spacer, while the other dendron was anchored on gold substrate. The interaction between two first-generation dendrons on tip and on gold substrate is broken by force of 224 pN at a loading rate of 40 nN/s. The interaction between second- and first-generation dendrons is broken by 315 pN at the same force loading rate. The most probable rupture force between dendrons is dependent on the loading rate, indicating that the rupture between dendrons is a dynamic process.In chapter 5, we have investigated the elastic properties of poly(ferrocenyldimethylsilane) and poly(ferrocenylmethylphenylsilane). The two polymers show similar elasticity in normal forms in THF. They exhibit different enthalpic elasticity after oxidation probably because of different steric effects of side groups.In chapter 6, we have performed the closed mechano-electrochemical cycles of single poly(ferocenesilane) chain, PFS100, by using electrochemical SMFS. A chain lengthening of oxidized with respect to neutral chains was observed, which is attributed to the electrostatic repulsion between the oxidized ferrocene centers along the chain. A force of ~ 200 pN on the AFM cantilever was detected upon redox stimulation, while a single pre-stretched PFS100 chain was held at a constant z-position. Electro-mechanical cycles were completed in both directions as closed loops and showed comparable efficiencies. The single chain efficiency was found to increase with increasing stretching ratio. Experimentally a maximum efficiency of 26% was observed.