Abstract :
The main goal of this work is to find a
reasonable explanation for the frequently reported
drastic deviations from the ‘‘rule of mixture’’ applied
for calculation of the overall microhardness, H, of
complex polymer systems comprising a soft, (with a
glass transition, Tg, or melting, Tm, temperatures below
room temperature) component and/or phase. According
to the common practice, the contribution to H of
the soft component and/or phase, Hs, is considered as
Hs = 0, which results in extremely large differences
between the measured and calculated H values for
systems comprising more than 20–25 wt% soft component
and/or phase. For such systems a different
deformation mechanism during indentation process is
postulated, namely ‘‘floating’’ of the solid particles in
the soft component and/or phase, in addition to their
plastic deformation. The contribution of the ‘‘floating
effect’’ to the overall H is accounted for by the
empirically derived relationship H = 1.97 Tg–571.
Using the reported data on H and Tg for homopolymers,
blockcopolymers and blends, the H values are
recalculated and a good agreement with the experimentally
measured values is found. A modified additivity
law is suggested, which contains a term
accounting for the contribution of the soft component
and/or phase to the overall microhardness via the
relationship between H and Tg; its application results
in much smaller differences between the measured and
calculated H values.
