We investigated polypropylene (PP)'s strain-induced crystallization behavior under the influence of supercritical carbon dioxide (CO2) in extrusion by using visualization techniques. It was observed that crystallites first formed in the middle section of the flow channel, where extensional strain was dominant. In the absence of CO2, this strain-induced crystallization occurred even at 178 degrees C, which was much higher than the T-m. As the CO2 content was increased, we had to decrease the temperature significantly to induce strain-induced crystallization. The effect of the strain rate on the crystallization behavior in a tandem extruder was studied by decoupling the strain rate from the degree of cooling via a sudden increase in the flow rate. We observed that the effect of the stain rate on the strain-induced crystallization was significant. We also observed that the strain-induced crystallization kinetics about 10 degrees C higher than the melting temperature was very sensitive to the temperature because of the fierce competition between melting and crystallization. (C) 2017 Elsevier Ltd. All rights reserved.