Rubbing process

Triboemission time is extremely short while after-emission time is much longer. The enhanced surface activity caused by rubbing processes produces exoelectrons emission, catalytic and structural factors, increased surface temperature, and pressure (Rowe and Murphy, 1974). 

Fig. 3.85 (a) Schematic of the polyimide rubbing process TM-AFM height images (z-scale 3 nm) of (b) the pristine polyimide film, and (c) and (d) a rubbed film. The rub direction in (c) was from upper left to lower right, in (d) vertical. Reprinted with permission of John Wiley Sons, Inc. from [176]. Copyright 2001. John Wiley Sons, Inc. 

The effect of temperature on friction, wear and lubrication can be looked at from two points of view. In one, temperature effects originate as a consequence of the rubbing process pe/t it in the other, temperature is part of the ambient environment. This difference governs the way the influence of temperature is analyzed. In some instances temperature enters the analysis as an external experimental variable, the role of which is introduced by postulation. But in other cases temperature changes are an intrinsic part of the rubbing process, and refined experimental technique is required to obtain the data which must be combined with correct analysis to obtain valid results. 

In the absence of any mirror symmetry breaking by the rubbing process, the molecular and the sample frame are identical. In this case, the experimental intensities for 100% linearly polarized x-rays with the electric field vector oscillating along the x, y, and 2 axes are directly proportional to the... 

The AFM nanorubbing can be easily overwritten by performing a subsequent scan in a different direction, as shown in Fig. 7.10. This behaviour is well known in normal cloth rubbing, where the entire surface is affected by the rubbing process. Surprisingly, such overwrite behaviour was also observed for AFM patterned ITO surfaces [34]. 

For a rubbed polymer surface we note that there are two principal, orthogonal planes perpendicular to the film surface. These are the planes oriented parallel and perpendicular to the rubbing direction (see Fig. 6.3A and B). The first one, which we will refer to as parallel plane, clearly possesses mirror symmetry, while the mirror symmetry of the plane perpendicular to the rubbing direction, referred to as perpendicular plane, may be broken by the directional nature of the rubbing process. Hence, the most general expression for the polarization dependence on rotation of the electric field vector within... 

In conclusion we note that for both polymers the rubbing process aligns the polymer chains preferentially along the rubbing direction. The observation of opposite polarization dependences for polyimide and polystyrene reflects the orthogonal orientation of their tt systems with respect to the chain directions. While the tt system is oriented parallel to the polystyrene chain direction, it is oriented perpendicular to the polyimide polymer chain. 

In this contribution, it is also shown how the EFM technique can be used to investigate in detail the secondary effects of the rubbing process on dielectric films used to align liquid crystals. 

The rubbing process was repeated on the same sample for ten times, and the result is shown in Fig. 7.22. As can be noticed, the electrostatic signal coming from the surface increases and so does the surface deformation. Following the time evolution of the charge domains, we found that the signal was almost constant for five days, except for a very small reduction. The same measurements were performed also on PI and PVA. 

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