University of North Texas 
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, 0302 Cartagena, Spain
(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
ABSTRACT
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.