화학공학소재연구정보센터
Journal of Rheology, Vol.46, No.4, 959-976, 2002
Role of droplet deformation on the transient rheological response of a polypropylene/liquid-crystal-polymer blend
The transient theological response and morphological development of 70/30 polypropylene-Rodrun 5000 liquid-crystalline emulsion-like blends have been analyzed. The droplet size distribution was broad. Our results indicate that the shear stress overshoot observed at gamma = 1.5 s.u. is not due to the liquid-crystalline nature of the droplets, but rather to their elongational deformation. However, quick retraction of the elongated droplets in the steady state is attributed to their nematic nature. As a consequence, the transients after resting for a short time resemble those of start-up experiments. Size polydispersity plays a significant role: shear stress undershoot and normal stress overshoot at gamma = 25 s.u. are only observed when droplets larger than 40 mum radius are present. Results of flow reversal experiments show that if the flow is reversed before the steady state is reached, then a delayed shear stress overshoot is produced. We relate this to the optically observed retraction of the elongated drops and subsequent stretching by the flow. On the other hand, the monotonic stress increase observed when the flow is reversed after the steady state is reached, is explained by the absence of retraction during flow reversal. The stability of the fibers (elongated droplets) during this particular flow reversal experiment is remarkable.