Friction and Scratch Resistance of Polymer Liquid Crystals: Effects of Magnetic Field Orientation
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Magdalena Jaklewicz Department of Mechanical Engineering, McGill University, Montreal, Quebec, Canada H3A 2K6 (Received 16 July 2003; accepted 4 December 2003)
We have studied PET/0.6 PHB, an alternating copolymer in which PET is poly(ethylene terephthalate) and PHB is p-hydroxybenzoic acid with the mole fraction of 0.6 PHB. It is a longitudinal polymer liquid crystal (PLC) with the LC sequences in the main chain and oriented along the chain backbone. Material not subjected to the magnetic field, specimens oriented along and perpendicularly to the flux of the magnetic field, were investigated. Static friction, dynamic friction, scratch penetration depth, and healing of the material were determined. Static and dynamic friction parameters for oriented samples have significant higher values for oriented samples. The best scratch resistance is found for the sample aligned along the magnetic field. The results are explained in terms of morphology revealed by scanning electron microscopy and phase structures.
I. INTRODUCTION
A book reflecting extensively and well the contemporary status of polymer science and engineering in the world has been created by J.E. Mark.1 In the subject index of his book, items such as tribology, wear, friction, and scratch resistance are absent. The same statement applies to the book by Goldman,2 who discusses a variety of polymer deformation modes. On the other hand, Rabinowicz3 has thoroughly covered the current status of tribology, and his book deals with metals almost exclusively. Thus, tribology is far better developed for metals than for polymers—though it represents in the latter case a rapidly expanding field of research.4,5 Polymer tribology is difficult. For instance, contrary to expectations, in carbon-fiber–reinforced polymers the presence of fibers lowers the wear resistance of the neat polymers.6 A chapter on polymers in a collective Swiss book on tribology7 is largely limited to tabulated and diagramatically presented friction values. The need for understanding the tribological behavior of polymers is clear. In the first paper on polymer tribology from this group,8 we have demonstrated that addition of a fluoropolymer to an epoxy before curing causes a significant lowering of both
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0135 1038
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 4, Apr 2004 Downloaded: 14 Mar 2015
static and dynamic friction of the cured material. Then we have shown that the same additive increases the scratch resistance of the same commercial epoxy.9 This time, we work on polymer liquid crystals (PLCs) because of their service extending to high temperatures, better mechanical properties than those of engineering polymers, low thermal expansivity, and other advantages.10,11 In particular, we have studied orientation of PLCs in magnetic fields.12,13 Though some work on tribology of liquid crystals (LC) state has been reported,14,15 tribological properties strongly depend on the system in which a
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