Conclusions

117

The C1s spectra of brush surfaces before and after the friction tests showed similar peak patterns, indicating that the chemical structure of PMIS still remained. We also confirmed that the atomic ratio of carbon and fluorine decreased drastically after 1000 friction cycles, whereas the sulfur component still remained on the worn surface. These results reveal that the chemical decomposition of the PMIS and counter anions took place after the brush layer peeled off.³⁵

As mentioned in the Introduction, the tribological properties of an ionic liquid largely depend on counter anions. Minami³⁶ and Itoh et al.³⁷ reported excellent tribological properties when using hydrophobic anions, such as trifluorotris(pentafluoroethyl) phosphate and perfluoroalkyl sulfate, probably because the low moisture content of ionic liquids retards unfavorable chemical reactions. In this work, only TFSI was investigated as a counter anion for an imidazolium-type poly(ionic liquid). Further improvement in the tribological properties of poly(ionic liquids) can be expected by optimizing the combination of organic anions and cations.

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The influences of solvent quality on the tribological properties have been well studied, and in general, a polymer brush immersed in a good solvent would afford extremely low friction coefficient. The affinity of the polymer brush for the solvent (or lubricant) plays an important role in controlling the interaction between the brush and the friction probe, as well as the frictional properties. XPS analysis of the wear tracks revealed that the PMIS brush layer was gradually abraded away by sliding the glass ball probe in a dry N2 atmosphere under a load of 1.96 N, and the layer was peeled off after 1000 friction cycles, which resulted in a much higher friction coefficient.

119

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Chapter 6

Conclusion and Perspective

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In this thesis, the methods for controlled radical polymerization of nonionic and ionic monomers in polar protic solvents have been first established, and then the chain dimensions and chain conformations of well-defined cationic polyelectrolytes and cationic polyelectrolyte brushes in aqueous NaCl solutions have been investigated. The main aim of this research was to obtain more detailed understanding of how the chain conformations of polyelectrolyte brushes are different from those of unbound polyelectrolyte chains in aqueous solutions and how the chain conformations of the polyelectrolyte brushes is related to their excellent surface properties.

In Chapter 2, the methods for controlling atom transfer radical polymerization (ATRP) in polar protic solvents have been demonstrated in order to synthesize well-defined polyelectrolytes and polyelectrolyte brushes directly from electrolyte monomers. Unfavorable side reactions often take place in protic solvents because of their polar character, however, electrolyte monomers are soluble only in polar protic solvents. The addition of organic chloride salts into ATRP system with protic solvents effectively suppresses the side reactions; moreover the halogen exchange occurs between added chloride salt and growing alkyl bromide chain ends, which largely improves control of ATRP.

In Chapter 3, chain dimensions and chain conformations of single cationic polyelectrolytes in aqueous NaCl have been investigated by dynamic light scattering and small-angle X-ray scattering (SAXS). Theoretical analysis using helical wormlike chain model revealed that not only excluded volume strength and chain stiffness but also the local chain conformation of cationic polyelectrolytes largely affected by the ionic strength of the solution. This fundamental research is essential for in-depth understanding in a characteristic chain conformation of polyelectrolytes brushes as discussed in Chapter 4.

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In Chapter 4, well-defined high-density polyelectrolyte brushes on silica nanoparticles have been characterized by synchrotron radiation ultra-small-angle X-ray scattering and SAXS. The swollen brush thickness in aqueous NaCl solutions (0.05–0.5 M) was more than twice chain dimensions of corresponding single cationic polyelectrolyte chain, and as large as about 50% of the full contour length of the grafted chain. These results indicated a highly extended conformation of the grafted polyelectrolyte chains. The relationship between the brush thickness and molecular weight of the grafted chains was qualitatively explained by the modified Daoud–Cotton scaling theory. It was confirmed that the crossover from a high-density regime to an intermediate-density regime occurs with increasing concentration of the added salt.

In Chapter 5, novel high-density polyelectrolyte brushes bearing ionic liquid moieties were prepared in order to improve tribological properties. The polyelectrolyte brushes with ionic liquid moieties showed extremely low friction coefficient in an ionic liquid, and the brushes also showed better wear resistance than nonionic high-density polymer brushes. As ionic liquids have low volatility, low flammability, and high thermal stability, this lubrication system is expected to be used under extreme conditions such as high temperatures and aerospace.