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.

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.

The role of an interval of shear flow in promoting the flow-induced crystallization (FIC) for poly(ether ether ketone) PEEK was investigated by melt rheology and calorimetry. At 350°C, just above the melting temperature of PEEK (T m ), a critical shear rate to initiate the formation of flow-induced precursors was found to coincide with the shear rate at which the Cox−Merz rule abruptly begins to fail. In cooling the sheared samples to 320°C, FIC can be up to 25× faster than quiescent crystallization. Using rheology and differential scanning calorimetry, the stability of FIC-induced nuclei was investigated by annealing for various times at different temperatures above T m . The persistence of shear-induced structures slightly above T m , along with complete and rapid erasure of FIC-induced nuclei above the equilibrium melting temperature, suggests that FIC leads to thicker lamellae compared with the quiescently crystallized samples. W ith attributes of excellent chemical resistance, high thermo-oxidative stability, and superb mechanical properties that are retained at elevated temperature, poly(ether ether ketone) (PEEK) merits further investigation for many high-end applications where toughness at high temperature is required. 1−4 The crystallization behavior of PEEK has been extensively studied and it has been suggested that the aromatic stiff chain of PEEK makes the dynamics of crystallization different than that of flexible chains such as PE and PP. 5 Changes in crystallization conditions are known to result in different crystal morphologies, which influence final product properties. 6,7 Flow-induced crystallization (FIC) is ubiquitous to semicrystalline polymers. Brief intervals of either shear or extensional flows can greatly accelerate isothermal crystallization kinetics 8 and increase the temperature at which the sample crystallizes when cooled at a constant rate. 9 Flow is thought to align and stretch the longest chains in the molecular weight distribution and this lowers the nucleation barrier, leading to faster (or higher temperature) crystallization. For a number of polymers, such as PEEK and poly(ethylene terephthalate) (PET), due to a larger Kuhn length (10.8 and 2.4 nm for PEEK 10 and PET, 11 respectively), the chain configuration between two entanglements (M e = 1490 and 1170 g/mol for PEEK and PET, respectively, 12 ) is stiffer than for flexible chains such as PE and PP. Do stiff chains show similar FIC effects as the flexible chains that dominate the FIC literature? In all semicrystalline polymers the structures produced by flow are not yet identified, 13 so herein, we refer to these structures as flow-induced precursors.In this Letter, we demonstrate the use of rheology to investigate the role of flow-induced precursors in FIC of PEEK. With the aid of a cone-and-plate rheometer, short-and longterm shearing periods are applied to determine the effect of the applied shear rate and specific work on accelerating crystallization of PEEK. Janeschitz-Kriegl and co-workers have shown the usefulness of t...

When a semicrystalline polymer melt is subjected to sufficient flow before crystallization, the nucleation rate is accelerated. In this study, the degree of acceleration is investigated with a commercial poly(ether ether ketone), using a rotational rheometer. With a constant shearing time (t s = 1 s), the nucleation rate increases with the shear rate (10 s −1 < γ̇< 200 s −1 ). At a constant shear rate (γ̇= 20 s −1 ), the nucleation rate increases with the shearing time (1 s < t s < 15 s). For a constant strain (γ = γṫ s = 300), high shear rates with short shearing times enhance the nucleation rate more than low shear rates with long shearing times. The specific work (W = σγ, where σ is the shear stress) reduces all nucleation times to a common curve. A flow-induced nucleation model is suggested based on the entropy reduction model of Flory and the isothermal nucleation model of Hoffman and Lauritzen. A key ingredient is the critical volume of the nucleus, found to be 8−10 nm 3 , which corresponds to 3−4 Kuhn segments for PEEK.

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