Article pubs.acs.org/Macromolecules
Chain Entanglements in Polyethylene Melts. Why Is It Studied Again? V. M. Litvinov,*,† M. E. Ries,*,‡ T. W. Baughman,§ A. Henke,† and P. P. Matloka§ †
DSM Resolve, P.O. Box 18, 6160 MD Geleen, The Netherlands School of Physics & Astronomy, University of Leeds, LS2 9JT Leeds, England § DSM Ahead Materials Sciences R&D, P.O. Box 18, 6160 MD Geleen, The Netherlands ‡
ABSTRACT: The aim of this study is to revisit the characterization of entanglement density in polyethylene melts by studying a series of well-defined, high molecular weight polyethylene materials via transverse NMR relaxometry in the melt state at 423 K. The comparison of the relaxometry data with high temperature SEC-MALLS characterization allows the measurement and correlation of the fraction of chain-end fragments by two independent methods. As compared with rheological methods that measure volume average characteristics, the 1H NMR method described here offers advantages for studying the entanglement molecular weight (Me) and chain dynamics in entangled polyethylene melts due to the selectivity of dynamics to entangled chain fragments and disentangled chain-end blocks. The calculated Me value for infinitely long chains equals 1760 ± 80 g/mol. This value is in the range of previously reported Me for polyethylene; however, it exceeds commonly accepted in rheology Me of 1250 g/mol. The difference can be explained (1) by the effect of chain branching and molecular weight distribution, if samples are not well characterized, and (2) by complex chain dynamics in polymer melts that require several assumptions in rubber-elasticity theory used for calculation of Me from the plateau modulus. current theory”.17 Monte Carlo simulations suggest Me values from 710 to 840 g/mol,12,13 and an NMR method suggests 1230 g/mol.18 These differences can be explained by variations in the length and time scale of chain dynamics as probed by these methods, known experimental artifacts, theoretical assumptions used for calculation of the Me values,17,19−21 and the variable molecular architecture of PEs studied, namely, Mw, MWD, and short- and long chain branching. Currently, the Me value of 1250 g/mol calculated from the average plateau modulus is the generally accepted value for linear polyethylene samples, most widely referred to as high density polyethylene (HDPE). “The systematic study of the relationships between molecular structure and the rheological behavior of polyolefins has been seriously limited in the past by the lack of samples with controlled and simply described distributions of molecular weight and long chain branching. Polymers prepared using traditional catalyst systems have a fairly broad molecular weight distribution.”22 Because of the many advances in polyethylene synthesis in the past few decades, polymer microstructure, modality, and functionalization can be controlled quite well within families of industrially relevant systems like traditional Ziegler−Natta, metallocene, and post-metallocene families of catalysts.23 Each of these have specific characteristics of chain growth and chain transfer that allow various architectures of polyethylene to be created through proper catalyst selection or modification of reactor conditions while applying the same catalyst.
1. INTRODUCTION The entanglement density of long chain molecules is an important characteristic of polymers that largely determines melt viscoelasticity, dynamic and fracture behavior, and the strength of interfacial adhesion in polymer composites among other properties. Flow-induced chain elongation of entangled macromolecules influences crystallization behavior of semicrystalline polymers and, as a result, causes differences in morphology and physical properties of the skin and core parts of injection-molded materials. Two structural characteristics affect the entanglement density: the chemical structure of polymer chains1 and the molecular architecture. These parameters are described by the number- and weight-average molecular weight of polymer chains, Mn and Mw, respectively, the molecular weight distribution (MWD), and the amount, length, and distribution of long chain branches.2−5 Historically, several methods have been applied for the quantification of the entanglement density. The traditional and most widely applied methods include melt rheology as well as characterization by neutron spin-echo,6 NMR relaxation methods,7−11 and Monte Carlo simulations.12,13 The experimental basis for characterization of chain entanglements from the rheological behavior of polymers has been described in Ferry’s textbook14 and reviewed by Graessley.15 The most widely studied material by these methods is polyethylene (PE), and a very large range of the molecular weight between apparent chain entanglements, Me, has been reported in the literature. Rheological studies provide Me values in the range of 830−2600 g/mol.16 The broad range of Me values determined by rheology can be explained by several methodological issues of this approach that were recently discussed in a review entitled “The great myths of polymer melt rheology, Part I: Comparison of experiment and © 2013 American Chemical Society
Received: November 21, 2012 Revised: December 17, 2012 Published: January 9, 2013 541
dx.doi.org/10.1021/ma302394j | Macromolecules 2013, 46, 541−547
Macromolecules
Article
Table 1. Typical Values for Branch Content in PEs in This Study NMR
FT-IR
catalyst
total methyl groupsa / 1000C
methyl branches (secondary)/ 1000C
ethyl branches (secondary)/ 1000C
α-olefin/ 1000C (end group)
1,2-olefin/ 1000C (backbone)
methyl branches (secondary)/ 1000C
α-olefin/ 1000C (end group)
1,2-olefin/ 1000C (backbone)
FI ZN
0.23 0.19
0.06 0.09
0 0.04
0 4
0 1
0.03
