Rotational isomeric state study of molecular dimensions and elasticity of perfluoropolyethers: Differences between Demnum-, Fomblin-, and Krytox-type main chains

Abstract

Molecular dimensions and elasticity of Demnum-, Fomblin-Z-, and Krytox-type perfluoropolyether (PFPE) molecules were studied by rotational isomeric state (RIS) and RIS Monte Carlo (MC) methods. The mean square of end–end distance, 〈R²〉, that of the radius of gyration, 〈R_g²〉, and the characteristic ratio C_n of the molecules were calculated by the RIS method. In the RIS calculations, the number of main chain atoms, N, was increased to about 100. The RIS results show that the Demnum- and Fomblin-Z-type molecules have similar 〈R²〉, 〈R_g²〉 and C_n values. However, the Krytox-type molecule has correspondingly smaller values at every N. The molecular elasticity of the molecules was evaluated by the RIS-MC method by estimating the force–elongation relation of the molecules. These RIS-MC calculations clarified that the Demnum- and Fomblin-Z-type molecules have similar molecular elasticity, but the Krytox-type molecule is more elastic. These distinct features of the Krytox-type molecule are attributed to the distorted rotational potential of its CF(CF₃)–O bonds. These calculations were compared with experimental results obtained with a surface force apparatus (SFA), and it is concluded that the Krytox-type molecule is the most desirable for boundary lubrication.