Influence of cooling rate on pVT-data of semicrystalline polymers

For numerical simulations of the injection-molding process, an accurate description of the specific volume is needed to predict differential shrinkage during and after molding, which causes thermally induced stresses and controls the dimensional accuracy and long-term dimensional stability. For amorphous polymers, for which it can often be assumed that cooling-rate dependence can be ignored, standard techniques enable measurements of the specific volume as a function of temperature and pressure. For semicrystalline polymers, the situation is more complicated since the specific volume depends strongly on the degree of crystallinity, which itself depends on the thermomechanical. history, that is, temperature and pressure (for quiescent crystallization). This requires the use of a combined experimental-numerical technique to interpret the data and to determine the specific volume. Standard equipment can only be used at relatively low cooling rates. Since high cooling rates are present during injection molding, improved experimental techniques must be designed. A setup based on the confining fluid technique is built, which can reach cooling rates to 60 Ks(-1) and pressures to 20 X 10(6) Pa. During an experiment, the specific volume is measured together with the temperature history and pressure. Using an accurate model to calculate the crystalline structure, together with a specific-volume model which depends on this structure, enables the determination of model parameters. Comparing both the measurements and the model predictions, leads to the conclusion that modeling of the crystallization kinetics results in accurate predictions of the specific volume