Crystallization and Dissolution of Flow-Induced Precursors

Abstract

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

“…The application of flow (shear or elongation) can change drastically the crystallization kinetics and the morphology of semicrystalline polymers (Kumaraswamy et al, 2004;Somani et al, 2005;Kimata et al, 2007;Balzano et al, 2008). Fibrillar precursors, composed of bundles of stretched molecules, are formed when some critical flow conditions are exceeded.…”

Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

“…35 The formation of a metastable suspension of shishes by imposing the shear close to the thermodynamic melting temperature was also reported by Balzano et al for a bimodal HDPE. 36 The presence of threadlike precursors was proved with rheo-optical techniques by Kornfield et al 17,37 in i-PP melt submitted to duct flow. They observed a peculiar upturn in the birefringence during flow up to a temperature of 215°C; nevertheless, for temperatures higher than 205°C, the flow induced structures relax so fast that no oriented structures were found in the recovered samples.…”

“…This notwithstanding, several direct evidences of the existence of long-living metastable structures in sheared polymer melts have been reported. [30][31][32][33][34][35][36][37][38] On segmental scale, rheo-Raman spectra of sheared polyethylene melts indicate the presence of chain sequences in trans conformation, featuring the ordered crystalline state, which persist for several hours after cessation of the shear. 30 Recently, shear-induced conformational ordering in the molten state was also observed in samples of isotactic polypropylene by Li et al [31][32][33] In this case, flow enhances the concentration of long sequences of repeating units in the 3/1 helical conformation required to form the monoclinic crystalline structure.…”

Crystallization and Precursors during Fast Short-Term Shear

“…Recently, we studied FIPs in a bimodal polyethylene (PE) melt with this protocol. 35 It was found that FIPs are bundles of stretched chains that, although not crystalline, have a density higher than the melt. In agreement with the classical view on crystallization, 47 after cessation of flow, a selection takes place among FIPs: those exceeding some critical dimensions transform into crystals, whereas the remainders dissolve into the melt.…”

“…It is known that stretch of the longest (highest molecular weight) molecules in a undercooled melt promotes shish-kebab formation, but the events in the early stages of the process are not yet fully clarified and certainly not quantified, an important issue for modeling. Several authors invoked the formation of precursors of crystallization, 31,[35][36][37][38][39][40] i.e., an intermediate phase directing the subsequent developments of the crystallization process. For instance, flow-induced precursors (FIPs) were observed by Lee and Schultz 41 studying fiber spinning of poly(ethylene terephthalate) (PET).…”