Article pubs.acs.org/Macromolecules
Molecular Alignment in Polyethylene during Cold Drawing Using InSitu SANS and Raman Spectroscopy Carlos R. López-Barrón,*,† Yiming Zeng,‡ Jonathan J. Schaefer,† Aaron P. R. Eberle,∥ Timothy P. Lodge,‡,§ and Frank S. Bates‡ †
ExxonMobil Chemical Company, Baytown, Texas 77520, United States Department of Chemical Engineering and Materials Science and §Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States ∥ ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, United States ‡
S Supporting Information *
ABSTRACT: Changes in the crystalline and mesoscale lamellar structure during plastic deformation of semicrystalline polymers have been extensively studied by X-ray diffraction techniques. However, direct measurements of single chain conformations during stretching have not been realized, although they are key to fully understand the structural transitions during cold drawing and their relation with the state of uniaxial stress. We report direct measurements of molecular alignment of a semicrystalline polymer during cold drawing by combining in-situ smallangle neutron scattering (SANS) and polarized Raman spectroscopy. The sample investigated is a linear low-density polyethylene (LLDPE) with density of 918 kg/m3 and melt index of 1.0 g/10 min. A multifaceted protocol consisting of hydrogen−deuterium exchange, followed by fractionation (by molecular weight, MW) and blending of selected deuterated fractions with protonated LLDPE, was used to elucidate, via SANS measurements, the response of the different fractions to uniaxial deformation. Under tensile deformation significant chain stretching occurs in the initial elastic regime. Further plastic deformation causes additional chain stretching, but to a lesser degree, that eventually plateaus in the strain hardening regime. Concurrently, the fraction of trans conformers increases linearly, as measured by in-situ Raman spectroscopy. The total orientation, quantified using an alignment factor, is lower for the lower MW fractions. We hypothesize through simple geometric arguments that this is directly related to the probability of forming intercrystal tie chains.
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INTRODUCTION
Several hypotheses have been proposed to explain the relation between the stress response and structural evolution during cold drawing of PE. Sun et al.12 recently proposed a model that considers stretching of two interpenetrating (crystalline and amorphous) networks to compare the structural evolution during stretching of two ethylene−octene copolymers with different octene mole fractions. The volume fraction of the amorphous network increases with comonomer content, which explains their observation that the critical strain for crystal fragmentation is much larger for the polymer with higher comonomer content.12 Seguela et al. proposed a different deformation model to explain the double yielding phenomenon observed in ethylene−α-olefin copolymers,11,13−15 which considers that the onset of plastic deformation is due to two simultaneous processes: slipping of the crystal blocks past each other in the mosaic crystalline structure and a homogeneous shearing of the crystal blocks. A
Under uniaxial deformation semicrystalline polymers undergo a series of microstructural transitions that are directly linked to the bulk stress response. In this work, we focus on polyethylene (PE). At small extensional strain ( 1), the tie chains are fully stretched, and Af reaches a plateau value. Additionally, in this regime most of the trans sequences probed by Raman are well-aligned along the stretching direction, which is the reason ⟨P2⟩ reaches such high orientation values. The mechanism proposed by Seguela et al.13−15 involving slipping and shearing of crystalline blocks to account for the yielding in ethylene−α-olefin copolymers correlates well with the initial rapid increase in Af. Indeed, Seguela and Rietsch speculated “the chain folds that bridge the slip planes operating in the block boundaries gradually hampers the relative displacement of the crystal blocks and activates the plastic deformation within the core of the crystal blocks”.13 This hindering of crystal block displacement (not only by chain folds but also by tie chains) could explain the fact that the Af plateau starts to develop after the second yield, at which point the tie chains begin to reach their maximum elongation, as discussed above. Our results also support the deformation mechanism
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CONCLUSIONS We present the first in-situ measurements of molecular alignment on the length scale of Rg during cold drawing of a semicrystalline polymer and its relation with the uniaxial stress state. The combination of in-situ SANS and Raman measurements allows us to identify three regimes of alignment in commercial LLDPE: (i) initial strains in the elastic regime show significant chain stretching, followed by (ii) a slower but continuous increase in the total orientation during the plastic deformation, and (iii) a final stage where further chain stretching stops at the onset of strain hardening. Similar evolution is observed in the total fraction of trans conformers by Raman spectroscopy. Also, we observed a noticeable difference in alignment between the low- and high-MW fractions in the polymer, which is attributed to a lack of tie chain formation below a critical MW. Finally, SANS and Raman measurements both show evidence of amorphous chain recoil and loss of orientation upon long time relaxation of dog bone samples. Our experimental procedures provide new insights into the structural evolution of PE when deformed and these methods are readily applicable to other semicrystalline polymers. Indeed, we are currently exploring the effect of H
DOI: 10.1021/acs.macromol.7b00504 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00504. Complementary plots: molecular weight distributions (Figure S1), SANS profiles of unstretched samples (Figure S2), illustration of WAXS peak analysis (Figure S3), Raman spectra of sample stretched and after 1 day of relaxation (Figure S4), and complete set of 2D SANS profiles (Figure S5) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (C.R.L.-B.). ORCID
Carlos R. López-Barrón: 0000-0002-9620-0298 Timothy P. Lodge: 0000-0001-5916-8834 Frank S. Bates: 0000-0003-3977-1278 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Hasnain Rangwalla and Jamie Ambriz for performing the preparative fractionation of the DPE sample, Joseph Throckmorton for WAXS analysis, Shuhui Kang for the GPC analysis, and Yimin Mao, Yun Liu, and Paul Butler for their help with the SANS experiments. We also thank Patrick Brant and Alex Norman for useful discussions.
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DOI: 10.1021/acs.macromol.7b00504 Macromolecules XXXX, XXX, XXX−XXX
