Simultaneous access to the low-frequency dynamics and ultraslow kinetics of the thermal expansion behavior of polyurethanes

Abstract

A long-lasting technological and scientific need exists for more detailed understanding of the dynamics of thermo-mechanical properties of polymers. This article demonstrates how low-frequency dynamics and ultraslow kinetics of thermal expansion of polymers are easily measured by the spectroscopic dilatometry technique called “Temperature Modulated Optical Refractometry”. The influence of molecular packing and dynamics is investigated on the kinetic and dynamic glass transition of three visco-elastic polyurethane networks of different crosslink density. A major observation is the potentially massive difference between kinetic and dynamic thermal expansion coefficients for the vitrifying polyurethane networks at a given temperature. The same holds true for the kinetic and dynamic glass transition temperatures Tg kin and Tg dyn, which are observed to differ by 15 K under given cooling and temperature modulation conditions. The here used cooling procedures show how in general the comparison of the low-frequency dynamics with the kinetics allows for a given material to estimate the dynamic response from the kinetic response, and vice versa. First insights are gained into how far the relaxation dynamics of the polyurethane networks spread over a considerably larger temperature interval than those of a linear chain polymer.