M. D. Bermudez (a), W. Brostow (b), F. J. Carrion-Vilches (a, b) J. J. Cervantes (a) and D. Pietkiewicz (b)
(a) Grupo de Ciencia de Materials en Ingenieria Metalurgica, Departamento de Ingenieria de Materiales y Fabricacion,
Universidad Politecnica de Cartagena.
C/Doctor Fleming s/n, 30302
(b) Laboratory of Advanced Polymers
and Optimized Materials (LAPOM),
Department of Materials Science, University of North Texas, Denton, TX
76203-5310, USA; Denton, TX 76203, USA;
We have studied in turn: polystyrene (PS) styrene/acrylonitrile (SAN) and plyamide
6 (PA6), adding each time to the polymer 1,3,5,7 or 10wt% 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 modified with
various MLCs or with MoS2.
Multiple Scratching tests were performed 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 of
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.