Temperature modulated optical refractometry: A quasi-isothermal method to determine the dynamic volume expansion coefficient

The volume expansion coefficient is a thermodynamic key parameter yielding insight into molecular
cohesion and organization of condensed matter. We present here the novel experimental technique temperature
modulated optical refractometry (TMOR) to determine not only the static but also the dynamic
volume expansion coefficient of transparent condensed matter of cubic or isotropic symmetry. A specialty
of TMOR is its capability to measure the volume expansion coefficient under quasi-isothermal
conditions. In this experimental mode TMOR is able to differentiate between static, dynamic and kinetic
contributions to the volume expansion coefficient. Due to these features TMOR especially qualifies for
the investigation of structural changes provoked by structural phase transitions, glass transitions and
other structure-related transformations in transparent condensed matter. The scientific potential of this
novel experimental technique is demonstrated by evidencing anomalies of the complex volume expansion
coefficient accompanying the chemically induced glass transition occurring during the formation of
a covalently cross-linked epoxy network.