Rheological properties of poly(vinyl chloride)/plasticizer systems-relation between sol-gel transition and elongational viscosity

Poly(vinyl chloride) (PVC)/di-isononyl phthalate systems with PVC content of 45.5 (PVC8) and 70.4 wt% (PVC6) were prepared by a hot roller at 150 degrees C and press molded at 180 degrees C. The dynamic viscoelasticity and elongational viscosity of PVC8 and PVC6 were measured in the temperature range from 150 to 220 degrees C. We have found that the storage and loss shear moduli, G' and G '', of PVC8 and PVC6 exhibited the power-law dependence on the angular frequency omega at 190 and 210 degrees C, respectively. Correspondingly, the tan delta values did not depend on omega. These temperatures indicate the critical gel temperature T (gel) of each system. The critical relaxation exponent n obtained from these data was 0.75 irrespective of PVC content, which was in agreement with the n values reported previously for the low PVC concentration samples. These results suggest that the PVC gels of different plasticizer content have a similar fractal structure. Below T (gel), the gradual melting of the PVC crystallites takes place with elevating temperature, and above T (gel), a densely connected network throughout the whole system disappears. Correspondingly, the elongational viscosity behavior of PVC8 and PVC6 exhibited strong strain hardening below T (gel), although it did not show any strain hardening above T (gel). These changes in rheological behavior are attributed to the gradual melting of the PVC crystallites worked as the cross-linking domains in this physical gel, thereby inapplicability of the of time-temperature superposition for PVC/plasticizer systems.