(739b) Enhanced Yield Strength in Polyethylene-Glassy Block Copolymers

Polyethylene is the worldâ??s most widely produced synthetic polymer in terms of annual production volume, and has tremendous utility for many applications as a low-cost, easily processable tough plastic. However, polyethylene is poorly suited to certain uses due to the relative ease of permanent deformation under an applied load. For example, linear, high-density polyethylene typically exhibits a yield stress of approximately 30 MPa.¹ In a semicrystalline material, this yield point corresponds to the critical stress at which the crystal stems making up the crystal lamellae dislocate and â??slipâ? in the direction of the applied force. Crystallite dislocation and eventual fragmentation are irreversible, and for most molded plastics this mode of permanent deformation effectively ruins the article.

The liquid-like amorphous fraction (T_g ~ -120 °C) filling the space between polyethylene crystal lamellae is incapable of providing mechanical reinforcement to the crystal fold surface, so the homopolymerâ??s yield stress is dictated largely by the crystal thickness. Slow cooling from the melt or annealing slightly below the melting temperature can be used to increase crystal thickness, allowing for some control over the yield stress.¹ However, these improvements are modest: the yield stress of a 70 kg/mol narrow-distribution linear polyethylene differs by only ~30% between a sample slowly cooled from the melt over the span of three hours versus a sample rapidly quenched from the melt over the span of a few seconds.

Our alternative strategy for controlling the mechanical properties of a semicrystalline polymer is the attachment of a short amorphous block with a high glass transition temperature. Upon crystallization from the melt, the glassy block will be excluded to the amorphous fraction, stiffening the domains between adjacent crystal surfaces. Block copolymers containing a high-T_g block and a perfectly linear polyethylene block can be readily prepared via sequential ring-opening metathesis polymerization of substituted norbornene monomer and cyclopentene, followed by catalytic hydrogenation.² The present work makes use of hydrogenated poly(norbornyl norbornene) as the glassy block (T_g = 115 °C), although ongoing work will consider other candidate glassy blocks such as hydrogenated poly(cyclohexyl norbornene) (T_g = 86 °C, though substantially more miscible with polyethylene). We investigate a range of minority glassy block contents, spanning ~10% â?? 20% by weight, with the molecular weights of these polymers selected to ensure single-phase melts for ease of processing.³

By simultaneously varying the glassy block content and the crystal thickness/crystallinity via thermal history, the mechanical properties of the block copolymers can be altered dramatically from those of a polyethylene homopolymer. Polymers with intermediate glassy block content (15%), with crystallinity maximized by slow cooling from the melt in order to further concentrate the glassy block within the amorphous layer, exhibit remarkably large increases in strength and stiffness relative to polyethylene of comparable molecular weight and crystal thickness. These enhancements can be as large as a doubling of the yield stress and a tripling of the Youngâ??s modulus, vastly more pronounced than can be achieved by thermal treatment of a polyethylene article alone. These changes can be understood in terms of enhancing the rigidity of the amorphous layer with an increasing concentration of glassy block, and our current work aims to understand the extent of block mixing within the amorphous layer (i.e., phase-separated glassy domains versus a homogeneous glassy block/amorphous polyethylene mixture) in order to explain the mechanical role of the high-T_g material.

The observed increases in strength and stiffness of the glassy-polyethylene block copolymers are typically associated with a decrease in the ultimate strain. Prepared by slow cooling from the melt, linear polyethylene homopolymer exhibits an extension at break of ~1000%. However, under the same thermal history, the block copolymers with 15% glassy block fracture at 45% extension, reduced to 2% extension for the materials with 20% glassy block. The loss of mechanical toughness for these polymers can be explained by a lack of tie molecules spanning the inter-lamellar distance. The most brittle materials were those with the lowest molecular weights (40 kg/mol for the 20% glassy block case), owing to the requirement to avoid microphase separation in the melt. At these low chain lengths, the predicted mean end-to-end distance for a polyethylene chain⁴ is significantly lower than the periodic domain spacing measured by small-angle X-ray scattering. Our ongoing work seeks to improve the material toughness by preparing the glassy block from a polymer that is more miscible with polyethylene than hydrogenated poly(norbornyl norbornene). This way, the block copolymer molecular weight can be increased to improve the tie molecule density while retaining a homogeneous melt.

In summary, the yield stress of polyethylene can be substantially increased by incorporation of a short, high-T_g block. Upon crystallization, the glassy block is excluded to the amorphous layer between crystal lamellae and helps to partially immobilize the crystal surface. By slowly cooling from the melt, the crystallinity of these block copolymers can be raised in order to further enrich the fraction of glassy block in the amorphous layer. The molecular weights of these polymers are tuned to avoid microphase separation in the melt, however the lowest molecular weight polymers become brittle under a slow cooling thermal history due to the lack of tie molecules between neighboring crystal lamellae.

This work was generously supported by the National Science Foundation, Polymers Program (DMR-1402180).

References

1. Kennedy, M. A.; Peacock, A. J.; Mandelkern L. Macromolecules 1994, 27, 5297.
2. Trzaska, S. T.; Lee, L.-B. W.; Register, R. A. Macromolecules 2000, 33, 9215.
3. Bishop, J. P.; Register, R. A. Macromolecules 2010, 43, 4954â??4960.
4. Fetters, L. J.; Lohse, D. J.; Richter, D.; Witten, T. A.; Zirkel, A. Macromolecules 1994, 27, 4639.