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A systematic investigation to determine the importance of molecular weight on the isothermal crystallization kinetics of PEEK across a broad temperature range is presented for three commercial PEEKs (Victrex 150G, 450G, and 650G). The Avrami crystallization model is fit to the isothermal crystallization kinetics of PEEK as a function of crystallization time. To describe the secondary crystallization kinetics, a modified Avrami model is suggested by introducing a second Avrami exponent. The primary and secondary Avrami exponents of PEEK are 3.3 ± 0.4 and 2.3 ± 0.3. Using both standard differential scanning calorimetry (DSC) and fast scanning chip calorimetry (FSC), isothermal PEEK crystallization kinetics are investigated in a wide range of crystallization temperatures (158°C < T c < 336°C). As the molecular weight is increased, the crystallization kinetics decrease. The crystallization half‐times from DSC and FSC are well described by the Hoffman‐Lauritzen model over the entire range of possible crystallization temperatures.

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When the molten state of a semicrystalline polymer is subjected to sufficiently intense flow before crystallization, the crystallization kinetics are accelerated and the crystalline superstructure is transformed from spherulites to smaller anisotropic structures. In this study, flow-induced crystallization (FIC) of polyamide 66 (PA 66) was investigated using rheology and polarized optical microscopy. After an interval of shear flow at 270 °C, above the melting temperature (T m = 264 °C) and below the equilibrium melting temperature, small-amplitude oscillatory shear time sweeps at 245 °C were used to monitor FIC kinetics. As specific work was imposed on a PA 66 melt at 270 °C from 10 Pa to 40 kPa, the onset of crystallization at 245 °C did not change. Above the critical work of 40 kPa up to 100 MPa, the onset of crystallization at 245 °C was progressively shifted from 628 to 26 s, as the applied specific work was increased. For quantitative analysis of the acceleration, the Avrami equation was used with Pogodina’s storage modulus normalization method, revealing the transition of Avrami exponent from ∼3 to ∼2 at the critical specific work of ∼40 kPa. Strong FIC acceleration was observed after the transition. After applying very low shear rates, large spherulites were observed without cylindrites, while a mixture of small spherulites and large anisotropic cylindrites was seen after applying a shear rate of 10 s–1.