Elongation-induced crystallization of a high molecular weight isotactic polybutene-1 melt compared to shear-induced crystallization

Abstract

The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point (93–101°C). An increase of the transient elongational viscosity compared to the Trouton viscosity 3η(t) is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol to prove that quasi-quiescent crystallization occurs at the same time for both types of flow. At Hencky strain rates above 10−3s−1, the onset time decreases rapidly with increasing strain rate. Compared to SIC, the decrease in EIC is much stronger (e.g., at 10s−1 the onset time in elongation is two powers of ten shorter than that in shear) and approaches a constant Hencky strain εC=4 regime. A temperature change by 4°C had no significant effect on εC. Following a procedure introduced for SIC, a temperature-invariant plot of the onset time versus normalized flow rate was constructed containing both flow types.