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

Abstract

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.

Roles of solution concentration and shear rate in the shear-induced crystallization of P3HT

“…This result implies that only ∼1% of the mechanical energy input reduces the entropy-relevant free energy due to the dominance of friction and relaxation effects. 27 In P3HT toluene solution, the high concentration solution has high viscosity, which may dissipate more mechanical energy under shear, namely, E is low in high concentration solution. To reduce the entropy to accelerate nucleation and crystallization, a higher W was needed.…”

“…Such enhanced crystallization kinetics are mainly attributed to the signicantly increased nucleus density and growth rate, namely, the shear ow would eventually lead to an increased nucleation rate once the shear rate exceeds a critical value. 21,27 Under shear, polymer chains aligned along the shear direction, reducing the entropy due to fewer possible chain congurations. From a thermodynamic point of view, Flory proposed that the reduction in entropy lowers the nucleation energy barrier ðDG * n Þ, resulting in nucleation acceleration.…”

Dual Nakamura model for primary and secondary crystallization applied to nonisothermal crystallization of poly(ether ether ketone)

“…Figure 1 depicts the time–temperature protocol to monitor non‐isothermal crystallization kinetics of PEEK at the cooling rates of 10, 30, 50, 70, and 90 K/min. Our previous publication [ 24, 25 ] reported that shear history can accelerate the crystallization kinetics of PEEK. Thus, non‐isothermal crystallization kinetics was monitored after erasing the shear history at 400°C above the equilibrium melting temperature of , [ 10 ] for 5 min.…”

Shear-Induced Isotropic–Nematic Transition in Poly(ether ether ketone) Melts

“…(II) As the shear rate reaches 1 s –1 , an abrupt increase in birefringence was clearly observed (see the birefringence image at γ̇ = 1 s –1 and Figure S2), implying a significant structural change attributed to the I–N transition. ,, The border between Regimes I and II has a relaxation time corresponding to 1/γ̇ = 1 s. This might represent the Rouse time τ R of the longest chains, where such long chains get significantly stretched along the flow direction. , As γ̇ increases, the failure of the Cox–Merz empirical rule manifests. On a similar PEEK melt, sheared at 350 °C, Nazari et al assigned the failure of the Cox–Merz rule to the onset of flow-induced crystallization (FIC) precursors, with accelerated crystallization kinetics.…”

“…12,28,40 The border between Regimes I and II has a relaxation time corresponding to 1/γ̇= 1 s. This might represent the Rouse time τ R of the longest chains, where such long chains get significantly stretched along the flow direction. 36,41 As γ̇increases, the failure of the Cox−Merz empirical rule 42 manifests. On a similar PEEK melt, sheared at 350 °C, Nazari et al 36 assigned the failure of the Cox−Merz rule to the onset of flow-induced crystallization (FIC) precursors, with accelerated crystallization kinetics.…”