From Molecular Entanglement Network to Crystal-Cross-Linked Network and Crystal Scaffold during Film Blowing of Polyethylene: An in Situ Synchrotron Radiation Small- and Wide-Angle X-ray Scattering Study

Abstract

Combining a homemade film blowing machine and an in situ synchrotron radiation source with small- and wide-angle X-ray scattering (SAXS and WAXS) capability, an investigation of film blowing of polyethylene (PE) has been studied. From the die exit to the positions above the frost line, four zones defined with different structural features are observed with SAXS and WAXS measurements. In zone I, precursor and crystal structures emerge from the polymer entanglement network during cooling and extension, which lead to the formation of a deformable crystal-cross-linked network at the boundary between zones I and II. The occurrence of the crystal-cross-linked network enhances the effective chain stretching during further deformation in zone II. Crystallization is largely accelerated, which generates crystals with high orientation. Further increasing the crystallinity results in the deformable crystal-cross-linked network transforming into a nondeformable crystal scaffold at the frost line (the boundary between zones II and III), which stabilizes the bubble and prevents further deformation. In zones III and IV, the scaffold and the entire sample are gradually filled up by crystals, respectively. Interestingly, increasing the take-up ratio (TUR) does not influence the critical crystallinity (χI–II) for the formation of the deformable crystal-cross-linked network, while the crystallinity (χf) at the frost line or for the formation of nondeformable scaffold does vary with TUR. This suggests that the former (χI–II) is mainly controlled by molecular parameters, while the latter (χf) is determined by both processing and molecular parameters of PE material.

Structure–Property–Process Relationship for Blown Films of Bimodal HDPE and Its LLDPE Blend

“…Thus, in our case, due to the broad MWD, the lamellar orientation is preferred. Moreover, the high‐MW chains can promote the shear‐induced crystallization in the necking zone, and this can be referred to the mechanism proposed by Li's group . They have proposed the mechanism of structure evolution in the LLDPE blend film as follows: i) The oriented precursor and crystal are firstly formed at the short distant of the melt exited from the die (zone I) due to the flow field and sharp cooling of the melt temperature; ii) these crystals (in zone I) act as deformable crosslinkers of a physical network, which can be stretched under flow (in zone II) and promote the crystallization as well as crystal orientation; iii) the deformable crosslinked network is solidified to strengthen the bubble stability at the frostline (in zone III); and iv) the crystals are filling up the entire film (in zone IV).…”

“…Later, Li et al performed the similar experiment of film‐blowing processing of LLDPE using in situ X‐ray scattering techniques. They proposed that the mechanism of structure development during the film‐blowing process could be divided into five zones, which will be discussed in detail in this study.…”

Multiscale characterization of semicrystalline polymeric materials by synchrotron radiation X‐ray and neutron scattering

“…However, limited by the available space in the SR facility, large industry equipment is unable to be coupled with SR SAXS and WAXD to capture the transient structure evolution during processing. In order to fill the gap between academics and industry, Li's group recently designed and manufactured a series of industry mimicking equipment . Here, we present two examples to demonstrate such kind of instruments and the corresponding applications.…”

“…Also, the in situ SAXS and WAXD can be captured simultaneously. Zhang et al investigated the film blowing of a PE blend, which is mixed by two linear low‐density polyethylenes: LLDPE 1 (the ethylene−octene copolymer, XUS61530, DOW, M w = 133 kg mol −1 , polydispersity index PDI = 4.2), LLDPE 2 (the ethylene−butene copolymer, 1001AV, ExxonMobil, M w = 151 kg mol −1 , PDI = 4.9), and one low density polyethylene: LDPE (0725 N, TASNEE, M w = 109 kg mol −1 , PDI = 5.1) with a weight ratio of 2:1:1.. As shown in Figure C, the detailed structure evolution of PE blend during film blowing can be well characterized. After analyzing the scattering data, Zhang et al proposed that the evolution of PE blend melts to the final crystalline structure with four distinct zones.…”

“…(A) Film blowing apparatus installed in BL16B1, SSRF, and (B) schematic diagram. (C) Selected 2D SR‐wide angle X‐ray diffraction and small angle X‐ray scattering patterns at a different position of blowing filmReproduced from Reference , with permission from Copyright© 2017, Chinese Chemical Society, Institute of Chemistry, Chinese Academy of Sciences and Springer‐Verlag GmbH Germany and Copyright© 2018, American Chemical Society …”