Poly friction anisotropy

For polymers, anisotropic friction 10) was observed on lamellar crystals II -13 as well as on extended-chain crystals obtained by friction transfer deposition 14). The anisotropic friction for lamellar crystals was explained by the occurrence of chain-folding oriented preferentially in planes parallel to the corresponding crystal face. For extended-chain crystals friction anisotropy observed parallel and perpendicular to the chain direction was interpreted by the interlocking asperity model. In this paper we give an overview regarding our nanotribological observations on oriented high density polyethylene (HDPE), poly(tetrafluoroethylene) (PTFE) and on crystals of polyethylene (PE) and transcrystallized poly(ethylene oxide) (PEO). 

Faint contrast in the error signal is a good indication for an optimized feedback loop. At the same time the dimension and the habit and possible defects of the crystal, including the fold domain boundaries, are very well visualized. For several polymers including PE [49] and poly(4-methyl-l-pentene) [50], the friction forces measured on the surface of the lamellar crystals obtained from solution are anisotropic (Fig. 3.20 bottom right). This anisotropy is related to the direction of the folds on the crystal surface. 

In nanostructured parylene columnar films, the columns tilted at an angle of 57-63° relative to the surface, during sliding contact in some cases a mechanical anisotropy emerges [115]. Microscale sliding friction experiments were performed on nanostructured poly(chloro-/7-xylylene) films. 

Polymer Lamellar Crystals Grown from Solution. Similarly to the regular array of polymer chains exposed at extended-chain fibrillar crystals, the folded sections of polymer molecules at the fold plane of solution-grown single crystals can cause anisotropy of friction (Figures 6). This has been observed for a variety of materials, such as POM, PE, and poly(4-methyl-l-pentene) (11 -13). The explanation proposed for this observation is based on the presence of oriented folds at the surface of the fold plane. 

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