Chemistry Reference
In-Depth Information
investigation of the Q
7
-based POM-ILs revealed notable differences between the
samples. It was observed that the RTILs
6
,
8
and
9
feature a well defined melting peak
with an onset temperature of around 24 °C in combination with a minor eutectic melt
located at around -16 °C. In contrast,
7
,
10
and
12
show a broad asymmetric melting
range between ca. -70 °C and 0 °C with a minimum located at ca. -50 °C to -60 °C. It is
proposed that the difference in melting behavior is due to the transition metal cations
incorporated in the cluster shells: compounds
6
,
8
and
9
feature metal cations which are
redox-inert under the given reaction conditions (Cu
II
, Ni
II
, Fe
III
, respectively). In contrast,
compounds
7
,
10
and
12
contain redox-active metals (Mn
II
, Co
II
, Cr
III
) and it is proposed
that under the given conditions, partial oxidation results in the in-situ generation of
“intrinsic” impurities which contribute to the unexpected melting behavior.
3.1.4.4
Rheological analysis
Rheology is an excellent tool to describe how matter is flowing, primarily in the liquid
state, but also as soft matter or solids under conditions in which they respond with
plastic flow to an applied external force. It generally accounts for the behavior of non-
Newtonian fluids that show a strain rate dependent viscosity. The relative movement of
different layers in a given material can cause a change of its viscosity, which may be
reduced upon strain (so called shear thinning materials) or which may even rise with
relative deformation (so called shear thickening or dilatant materials). The experimental
characterization of a material's rheological behavior (so called rheometry) is carried out
by dynamic mechanical analysis with an oscillatory force (stress) being applied to the
material and the resulting displacement (strain) being measured. In purely elastic
materials the stress and strain occur in phase, so that the response of one occurs
simultaneously with the other. By contrast in purely viscous materials, there is a phase
difference between stress and strain, where strain lags by a 90° phase lag. Most
materials exhibit a behavior somewhere in between with a phase lag in strain between
0° and 90° - so called viscoelastic materials.
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