Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Compatibilization of Isotactic Polypropylene (iPP) and High-Density Polyethylene (HDPE) with iPP−PE Multiblock Copolymers Jun Xu,† James M. Eagan,‡ Sung-Soo Kim,† Sanshui Pan,† Bongjoon Lee,† Kristine Klimovica,‡ Kailong Jin,† Ting-Wei Lin,‡ Micah J. Howard,† Christopher J. Ellison,† Anne M. LaPointe,*,‡ Geoffrey W. Coates,*,‡ and Frank S. Bates*,† †
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Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States S Supporting Information *
ABSTRACT: A series of isotactic polypropylene (iPP) and polyethylene (PE) diblock, tetrablock, and hexablock copolymers (BCPs) were synthesized with tunable molecular weights using a hafnium pyridylamine catalyst. The BCPs were melt blended with 70 wt % high-density PE (HDPE) and 30 wt % iPP commercial homopolymers at concentrations between 0.2 and 5 wt %. The resulting blend morphologies were investigated using TEM, revealing uniformly dispersed iPP droplets ranging progressively in size with increasing BCP content from three-quarters to one-quarter of the diameter of the uncompatibilized mixture. Tensile tests revealed a dramatic enhancement in toughness based on the strain at break which increased from 10% for the unmodified blend to more than 300% with just 0.2 wt % BCP and over 500% with the addition of 0.5 wt % BCP or greater. Incorporation of BCPs in blends also improved the impact toughness, doubling the Izod impact strength to a level comparable to the neat HDPE with just 1 wt % additive. These improved blend properties are attributed to enhanced interfacial strength, which was independently probed using T-peel adhesion measurements performed on laminates composed of HDPE/BCP/iPP trilayers. Thin (ca. ≤100 nm thick) BCP films, fabricated by high-temperature spin coating and molded between the homopolymer films, significantly altered the laminate peel strength, depending on the molecular weight and molecular architecture of the block copolymer. Multilayer laminates containing no BCP or low molecular weight diblock copolymer separated by adhesive failure during peel testing. Sufficiently high molecular weight iPP−PE diblock copolymers and iPP−PE−iPP−PE tetrablock copolymers with significantly lower block molecular weights exhibited cohesive failure of the HDPE film rather than adhesive failure. We propose adhesion mechanisms based on molecular entanglements and cocrystallization for tetrablocks and diblocks, respectively, to account for these findings. These results demonstrate exciting opportunities to recycle the world’s top two polymers through simple melt blending, obviating the need to separate these plastics in mixed waste streams.
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INTRODUCTION
incompatible and phase separate at all commercially relevant molecular weights, resulting in products with poor mechanical properties. Polyolefin compatibilization has drawn tremendous attention in the past three decades,4−22 with approaches typically divided into two main categories: reactive and nonreactive compatibilization. Reactive compatibilization is conducted by introducing chemically reactive functional groups4−11 that promote grafting or cross-linking reactions with iPP and PE, thus stitching together the interfaces between phase-separated domains and enhancing the mechanical properties of the blends. Many strategies have been developed for incorporating chemically
More than 70 million metric tons (MMT) of polyethylene (PE) and 50 MMT of isotactic polypropylene (iPP) are produced annually worldwide, which accounts for approximately twothirds of the global plastics market.1,2 Applications of these materials range from packaging and textile products to cuttingedge automotive and aircraft components. They have brought great convenience to modern lifestyles but have also caused significant environmental pressures due to the combined effects of chemical stability and difficulty in separating these plastics in recycling streams. At the present time, 10 wt % EPR or EPDM is required to elicit desirable compatibilization performance, compromising the tensile strength of the materials. Block copolymers composed of iPP-miscible and PE-miscible blocks are more attractive and effective compatibilizers than EPR and EPDM since BCPs are thermodynamically driven to immiscible interfaces as shown by theory31,32 and experiment24,33,34 in various blend systems. Earlier works in this field focused on using readily available block copolymers, such as B
DOI: 10.1021/acs.macromol.8b01907 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Synthetic scheme for the preparation of −(iPP−PE)− block copolymers
Table 1. Synthetic Parameters and Molecular Characteristics of −(iPP−PE)− Block Copolymersa entry
sample PPkg/molPEkg/mol
cat. (μmol)
propylene (g)
Pethylene (atm)
trxn (min)
yield (g)
Mn(theo) (kg/mol)
Mn(tot) (kg/mol)
Đ
Tm (°C)
1 2 3 4 5 6 7 8 9
PP24PE31 PP60PE80 PP73PE50 PP103PE113 PP36PE20PP34PE24 PP60PE80PP75PE90 PP73PE120PP167PE141 PP100PE81PP113PE108 PP52PE70PP37PE114PP34PE36
75 30 30 20 25 30 50 25 30
1.5 2.0 2.3 1.5 1.0, 1.0 2.0, 2.0 4.0, 4.0 2.5, 2.5 1.0, 1.0, 1.0
2.0 2.7 2.7 5.4 1.4 2.7 5.4 2.7 5.4
3 4 5 2 4, 4 4, 4 1.5 4 1, 1, 1
3.3 4 3.9 3.8 3 8.5 15.8 9.2 6.2
44 134 130 191 120 283 316 368 207
55 139 123 217 113 306 502 402 345
1.32 1.40 1.29 1.43 1.38 1.29 1.58 1.64 1.84
132 126 131 132 124 126 108, 129 103, 130 74, 127
a
Abbreviations: cat., catalyst; Pethylene, ethylene pressure; trxn, ethylene reaction time; theo, theoretical; tot, total.
and 0.2 wt %) would affect the blend morphology and tensile and impact mechanical properties. High-temperature spin coating enabled us to prepare very thin (ca.
