Trans-Rich Structures in Early Stage Crystallization of Polyethylene

Abstract

The kinetic pathway by which molten polymers transform into multi-length-scale, semicrystalline structures upon cooling is critical to their processing, properties, and ultimate performance. However, for the case of polyethylene (PE), critical questions remain concerning early stage kinetics of isothermal crystallization. Here we utilize Raman spectroscopy, in conjunction with turbidity and depolarization measurements, to probe crystallization kinetics in PE because of its unique ability to simultaneously measure intrachain consecutive trans conformations (locally straight) and interchain orthorhombic crystallinity. We analyze the spectra within the context of a three-state model to extract the mass fraction of conformations that are locally straight but not part of the orthorhombic crystal, which we term noncrystalline consecutive trans (NCCT). We first validate this methodology on the n-alkane C 21 H 44 where the NCCT mass fraction is expected to be large and then apply it to the early stage crystallization kinetics of PE. We find that the NCCT conformations appear in conjunction with orthorhombic crystallinity, indicating that the growing early stage clusters are rich in NCCT conformations. ■ INTRODUCTIONThe structure and physical properties of semicrystalline polymers are determined during crystallization, in which the material transforms from an entangled melt into a semicrystalline, multi-length-scale structure. 1 While precise control of crystallization is key to the final properties, it is mainly empirical. Among the broad array of crystallizing polymers, PE holds a special place for economic, scientific, and life-cycle reasons. Despite its maturity, industrial producers continue to develop new strategies to improve properties and processing. Scientifically, PE is important because it is the simplest polymer that forms a crystalline structure, and it serves as a model system for testing our understanding of polymer crystallization in general. A large literature has thus developed for PE devoted to understanding the molten phase, the lamellar semicrystalline state, and the kinetics of the transformation of the former into the latter. Despite much research on PE, there remains vigorous debate on the crystallization pathway. In particular, the events that occur during the early stages of crystallization, known as the precrystalline or induction period, are little understood.Here we demonstrate that Raman spectroscopy is a powerful tool to probe the kinetics of isothermal crystallization of PE due to its ability to simultaneously measure the evolution of local conformational states and orthorhombic crystallinity. It can be used to measure the mass fraction of conformations that are locally straight but are not part of the orthorhombic crystalline structure, which we call the noncrystalline consecutive trans (NCCT) conformation. We are particularly interested in the early time behavior where early stage structures are predicted to be rich in trans conformational states that are noncrystalline.In this work, we ...

Raman analysis of bond conformations in the rotator state and premelting of normal alkanes

“…Our initial Raman spectroscopic measurements of a C 21 alkane in the rotator phase indicate (10 to 30) % amorphous conformers. 21 This disordered fraction significantly exceeds the mass fraction of gauche bonds estimated from IR measurements of the C 21 alkane, which are on the order of (1 to 7) %. 14, 22 These differences must be addressed to relate Raman measurements to trans and gauche conformers directly.…”

“…The Raman spectra can be used to calculate mass fractions of conformers using a method described in our previous work 21 based on a prior analysis of polyethylene spectra by Strobl and Hagedorn. 20 The analysis assumes that the chain segments will have one of three conformations: an orthorhombic crystalline conformation (cr), and amorphous gauche -rich conformation (am), or a trans -rich conformation that is not in an orthorhombic crystalline configuration, denoted as noncrystalline consecutive trans (NCCT).…”

“…18, 19 In polyethylene, Raman spectroscopy in the CH 2 twist and bend regions have been used to provide quantitative information on the mass fraction of three conformational states: amorphous gauche -rich conformers, orthorhombic crystalline conformers, and non-crystalline consecutive trans (NCCT) sequences. 20, 21 Through measurement of these three states in the alkanes as a function of n , we hope to shed light on the melting transitions.…”

Phase‐specific Raman analysis of n‐alkane melting by moving‐window two‐dimensional correlation spectroscopy

“…To compensate irregular scattering, the baseline-detrended spectra were normalized by the area of the CH 2 twisting band (1240 cm −1 to 1350 cm −1 ). [7,8] …”

“…[7] One of the split peaks, at 1417 cm −1 , is used as a metric to determine the mass fraction of the orthorhombic conformation. [7,8] For most other Raman bands, however, the crystal-field splitting in the orthorhombic phase is smaller than the Raman bandwidth at room temperature; therefore, the peak splitting has been observed only at extreme conditions, such as at liquid nitrogen temperature [9,10] or from single crystals. [11] Such subtle spectral changes are challenging for conventional fitting-based individual peak analysis to decipher.…”