Nanotribology of Polysiloxane Ultrathin Films

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The organosilane monolayers are thermally and mechanically robust due to the presence of strong anchoring functional group at the hydrophilic part. When the alkyltrichlorosilane monolayer is polymerized on the water surface, the resulting polymer is polyalkylsiloxane. This organosilane monolayer can be immobilized onto the silicon wafer substrate by an upward drawing method. In this study, the alkyltrichlorosilane, fluoroalkyltrichlorosilane and their mixed monolayers were prepared by the Langmuir-Blodgett method at an air-water interface. The surface molecular motion of the monolayers were characterized by temperature dependence of IR spectra and lateral force. n-alkyltrichlorosilane (CHィイD23ィエD2(CHィイD22ィエD2)ィイD2n-1ィエD2SiClィイD23ィエD2 n=12 : DDTS, n=16 : HDTS, n=18 : OTS, n=22 ; DOTS, n=30 : TATS), 18-nonadecyltrichlorosilane (NTS, CHィイD22ィエD2=CH(CHィイD22ィエD2)ィイD217ィエD2SiClィイD23ィエD2), [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS, CFィイD23ィエD2(CFィイD22ィエD2)ィイD27ィエD2(CHィイD22ィエD2)SiClィイD23 … More ィエD2) and [γ-(1H,1H,2H,2H-perfluorododecyloxy)propyl]triethoxysilane (FDOPTES, CFィイD23ィエD2(CFィイD22ィエD2)ィイD29ィエD2(CHィイD22ィエD2)ィイD22ィエD2O(CHィイD22ィエD2)ィイD23ィエD2Si(OCィイD22ィエD2HィイD25ィエD2)ィイD23ィエD2) were used to prepare the monolayer. Toluene solutions of organosilane were spread on the pure water surface at a subphase temperature of 293 K. Surface pressure-area (π-A) isotherms were measured with a computer-controlled home made Langmuir-trough. The monolayers were transferred onto the silicon wafer substrate by an upward drawing method, and then chemically immobilized onto the substrate with Si-OH groups. The magnitude of lateral force of alkylsilane monolayer increased with an increase in alkyl chain length. This increase is due to an increase in van der Waals interaction of alkyl chains. The fluoroalkylsilane showed a large friction force compared with that of alkylsilane at the corresponding chain length. A large lateral force of fluoroalkyl silane can be ascribed to the high shear strength of fluoroalkyl silane due to the rigid nature of fluoroalkyl chain and high polarity of CFィイD23ィエD2 end group. The temperature dependence of IR spectra was measured in order to characterize the structural transition of organosilane monolayers. The wavenumber of the peak assigned to the CHィイD22ィエD2 antisymmetric stretching band was apparently shifted from 2915.5 cmィイD1-1ィエD1 to 2917.7 cmィイD1-1ィエD1 at around 240 K and from 2917.7 cmィイD1-1ィエD1 to 2919.2 cm ィイD1-1ィエD1 at around 330 K. This result indicates that the rectangular-hexagonal and hexagonal-amorphous phase transitions of the OTS monolayer occurred at these temperatures. The magnitude of lateral force reflects the state of thermal molecular motion. A decrease in the magnitude of lateral force of the OTS monolayer was observed at around 240 K and 330 K. It seems reasonable to consider that these decreases in the lateral force corresponded to a decrease in the molecular density in the OTS monolayer due to rectangular to hexagonal phase transition at 240 K and a decrease in shear strength by activation of molecular motion due to the hexagonal to amorphous phase transition at 330 K. Less