The effect of pre-thermal history on shear and uniaxial elongational viscosity of a tetrafluoroethylene/hexafluoropropylene copolymer near the crystal melting transition

The melt rheology of a commercially available tetrafluoroethylene/hexafluoropropylene copolymer, which is known as Teflon FEP copolymer, was studied to examine the effect of pre-thermal history during sample preparation conditions on dynamic shear and uniaxial elongational measurements. The first experimental series includes the sample preparation under hot press at 320 °C followed by a rapid cooling. The master curves were successfully obtained at 300 °C from the time-temperature superposition principle. The loss modulus G″ was found to be proportional to the angular frequency in a double-logarithmic plot toward 0.01 (rad/s), while the slope of the storage modulus G′ did not become 2. The elongational viscosity as a function of time under constant strain rates showed weak strain-hardening, which was enhanced with larger strain rates. The second experimental series contain three kinds of samples with different pre-thermal history to control rheological properties. All samples were hot-pressed at 320 °C followed by a rapid cooling to room temperature for the sample A and a slow cooling for the sample B and C. The dynamic shear and elongational measurements were performed at 270 °C for all samples, which were heated from room temperature for the sample A and B, but heated up to 280 °C and cooled down to 270 °C for the sample C. The distance between G″ and G′ become narrower in the order of the sample C, B, and A. In the same order, unexpectedly, the strain-hardening in the elongational viscosity measurements became the strain-softening. These unusual properties were discussed from a residual crystallinity.