Friction and multiple scratch behavior of polymer plus monomer liquid crystal systems

We have studied in turn: polystyrene (PS), styrene/acrylonitrile (SAN) and Polyamide 6 (PA6), adding each time to the polymer 1,3,5,7 or 10 wt% of 4,4'-dibutylazobenzene (LC1) which is a monomer liquid crystal (MLC). LC1 reduces both static and dynamic friction of PS and SAN against stainless steels or polytetrafluoroethylene (PTFE). By contrast, friction values are lower for pure PA6 than for PA6 modified with various MLCs or with MoS2. Multiple scratching tests were carried out with a micro scratch tester on every system between 2.5 and 15 N. The presence of LC1 in PS reduces penetration depth and residual depth and increases the viscoelastic recovery. So far PS was the only polymer, which does not show strain hardening in multiple scratching. The present results confirms this, but it also shows that only 1 wt% of LC reduces the brittleness of PS so that strain hardening appears. This effect is maintained at all higher concentrations of LC1 investigated as well. For SAN or PA6, additions of LC1 reduce penetration depth values with respect to pure polymers, but do not have a significant effect on viscoelastic recovery. Scanning electron microscopy (SEM) was used to study the deformation and wear mechanisms, and to relate the data obtained in multiple scratch sliding wear tests. For PS we see in SEM that increasing the LC1 concentration causes a more ductile behavior, with less crack nucleation. For SAN the debris accumulation in sliding wear is mitigated by the presence of the liquid crystalline lubricant. No debris formation is observed in PA6, with or without a lubricant. (C) 2005 Elsevier Ltd. All rights reserved.