Shear Flow-Induced Crystallization of Poly(ether ether ketone)

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 < γ&dot; < 200 s^-1). At a constant shear rate (γ&dot; = 20 s^-1), the nucleation rate increases with the shearing time (1 s < t_s < 15 s). For a constant strain (γ= γ&dot;t_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 nm³, which corresponds to 3-4 Kuhn segments for PEEK.