Temperature dependence of the rheological properties of poly(vinylidene fluoride)/dimethyl acetamide solutions and their electrospinning

The effects of measuring temperature on the rheological properties of poly(vinylidene fluoride) (PVDF) solutions in dimethyl acetamide (DMAc) were investigated at three different concentrations. Ubbelohde vis-cometer and rotational rheometer were employed for dilute and concentrated solution regimes, respectively. In the dilute concentration regime, intrinsic viscosity was decreased with increasing temperature over the range of 30 to 70°C but Huggins constant was increased from 0.301 to 0.345. In the high concentration regime, however, dynamic viscosity (η') was increased with increasing measuring temperature. PVDF solutions exhibited almost Newtonian flow behavior at 30°C but a notable dependence of η' on frequency was observed at 50 and 70°C. In temperature sweep measurement, the critical temperature where η' was abruptly increased was reduced with increasing PVDF concentration. Increasing measuring temperature from 30 to 70°C decreased the value of loss tangent (tan δ). The Cole-Cole plot at 30°C revealed that PVDF solutions gave a single master curve of constant slope irrespective of PVDF concentration. On the other hand, the slope varied with polymer concentration at 50 and 70°C. As the electrospinning temperature was increased the average diameter of nanofibers in the electrospun web was decreased and the fiber surface got rougher and coarser.