Measurements of the rheological behavior of a crystallizing polymer by an "inverse quenching" technique

In this paper we show that a suitable thermal history can be used to produce a polymer melt in which a fixed amount of crystalline phase has been frozen. We call this novel method "inverse quenching," since a stable arnorphous/crystalline system is obtained by heating up the sample rather than cooling it down. If the inverse quenching temperature is suitably chosen, the polymer can remain stable for a long time, thus allowing different types of experimental measurements. Here we first prove the validity of the inverse quenching method in quiescent crystallization conditions, and then we use the inverse quenching method to perform rheological measurements on an isotactic polypropylene at a constant degree, of crystallization. In particular, steady-state viscosity measurements in the early crystallization stages are reported for the first time, showing that the viscosity at low shear rates is much larger than that of the purely amorphous melt even for small values of crystallinity. The technique is also used to study the liquid-to-solid transitional behavior of the crystallizing polymer, which can be seen as a gelation process, at temperatures that are forbidden to traditional techniques. Such measurements are shown to provide further, robust validation of this novel method. (C) 2003 The Society of Rheology.