Delay of re-entanglement kinetics by shear-induced nucleation precursors in isotactic polypropylene melt. (1st December 2020)

• Record Type: Journal Article
• Authors: Wang, Bao; Cavallo, Dario; Chen, Jingbo
• Is Part Of: Polymer. Volume 210(2020)
• Publication Date: 2020-12-01
• Subjects: Nucleation precursors -- Entanglements -- Large amplitude oscillatory shear flow; Polymers -- Periodicals; Polymerization -- Periodicals; Polymères -- Périodiques; Polymérisation -- Périodiques; 547.7
• DOI: 10.1016/j.polymer.2020.123000
• Languages: English
• ISSNs: 0032-3861

Abstract

Abstract: Rheological measurements were carried out to in-situ monitor the re-entanglement process of large amplitude oscillatory shear flow (LAOS) modified partially disentangled isotactic polypropylene (i-PP) melt. The characteristic entanglement recovery time ( τ ent ) was essentially longer than the average reptation time ( τ D ) of fully entangled melt. At temperatures above 200 °C, the Arrhenius temperature dependence of τ ent and τ D are the same, displaying a normal chain diffusion-controlled mechanism with flow activation energy of 41.4 kJ/mol. However, when the temperature is lower than 200 °C, τ ent is unexpectedly longer than expectation extrapolated from high temperatures, and a much higher activation energy of 98.6 kJ/mol is observed for the re-entanglement process. Based on our understanding, the re-entanglement process of shear-induced disentanglements in i-PP melt was postponed by the existence of quasi-ordered clusters created by shear at relatively low temperatures and their dissipation turns to be the rate-determining step for the whole recovery. Moreover, morphological study under polarized optical microscope showing much higher nucleation density which originates from the flow-induced precursors corroborates our hypothesis. Meanwhile, the surviving time of LAOS-generated nucleation precursors is comparable to the recovery time of disentangled chains, suggesting that the delay effect of nucleation precursors on chain diffusion may last over the entire re-entanglement process. Notwithstanding the obstruction of "extra" physical entanglements for chain diffusion, the molecular weight ( M w ) dependence of entanglement recovery still follows the reptation model-based power law: τ ent = (a+b) Mw 3.4, where a and b is the contribution of chain diffusion and dissipation of precursors, respectively. Graphical abstract: Image 1 Highlights: Large amplitude oscillatory shear flow was applied to obtain disentanglements. Characteristic re-entanglement time is longer than the average reptation time. Re-entanglement kinetics is postponed by the existence of flow-induced precursors.