Rheological Investigation of Shear-Induced Crystallization of Poly(ϵ-Caprolactone)

Abstract

The isothermal crystallization behavior of poly(ϵ-caprolactone), PCL, has been investigated as functions of crystallization temperature, shear rate, and shearing time using a Rheometeric dynamic mechanical spectrometer. The characteristic rheological parameters—such as the dynamic shear moduli (G′, G″), dynamic shear viscosity (η *), and loss tangent (tan δ)—were found to be very sensitive to the structure change accompanying the crystallization process. At the early stage of the crystallization process, a rapid increase in the rheological parameters (G′, G″, and η *) was observed; however, tan δ decreases strongly. This substantial change in the rheological parameters during the crystallization process is attributed to the increase of volume fraction of the solid crystalline part that may act like cross-links. The effect of shear flow on the isothermal crystallization process of PCL was investigated by imposing different shear values on the molten sample before the isothermal crystallization process. It was found that the crystallization process was accelerated to a great extent under high shear rate and long shearing time. The induction time of the crystallization process, t 0, was found to be substantially decreased with increasing shear rate. The isothermal crystallization kinetics at various shear rates was analyzed using the Avrami equation. The Avrami exponent, which provides qualitative information about the nature of the nucleation and growth process, was found to be shear rate and crystallization temperature dependent. The Avrami exponent increased to as large as 13.7 at γ˙=3 sec−1 and T c =46°C. This value is very high, higher than the exponent value obtained from other techniques, such as differential scanning calorimetry and X-ray scattering (∼3). The high value of the Avrami exponent was attributed to orientation in the molecular chains that leads to an increase in the nucleation and growth rate of the crystallization process under shear flow.