(527e) Tie Chains and Ductility in Semicrystalline Polymers

Semicrystalline polymers of low glass transition temperature, such as polyethylene (PE), can be either brittle or ductile depending on their content of intercrystallite stress transmitters—such as tie molecules (TMs), chains that directly bridge the intercrystalline amorphous layer. On the molecular side, TM content will increase with increasing molecular weight (M), or with the fraction of high-M chains in a disperse polymer. On the morphological side, TM content will also increase with decreasing intercrystallite repeat spacing d, which can be manipulated through thermal history and by incorporating comonomer. Yet despite decades of work on the structure and properties of PE—a particularly well-studied semicrystalline polymer—the minimum molecular weight to achieve ductility is known only empirically, and only approximately (30-50 kg/mol in quenched specimens).

To understand the microstructural aspects required for ductility, we examine the failure mode of two model narrow-distribution semicrystalline polymers: PE and hydrogenated polynorbornene (hPN). The polymers are synthesized via living polymerization techniques to obtain narrow molecular weight distributions, and to allow the average molecular weight to be precisely targeted. Most of the polymers are synthesized by ring-opening metathesis polymerization (ROMP), followed by catalytic hydrogenation: the monomers are either cyclopentene (yielding linear PE, LPE) or norbornene (yielding hPN). The repeat spacing d is varied by thermal treatment (fast or slow cooling) and measured directly by small-angle x-ray scattering. Short-chain branching is also employed to reduce d; for hPN, this is accomplished by copolymerization with a small amount of 5-n-hexylnorbornene (HxN), while ethyl-branched PE is obtained by hydrogenation of low-vinyl polybutadiene (hPB) synthesized by anionic polymerization.

Consider first the PEs [1]. For each series (LPEs with different thermal histories and quenched hPBs), a rather sharp brittle-to-ductile transition (BDT) is observed with increasing M, at a value Mbdt. However, across the three series, the value of Mbdt does not depend solely on the value of d; remarkably, a higher M is required to achieve ductility in quenched samples of hPB than in LPE, despite the much lower values of d for hPB. Consequently, the calculated value of TM fraction at the BDT (Pbdt, calculated via the classic Huang-Brown approach) increases strongly as average crystal thickness decreases, by a factor of approximately 50 from slow-cooled LPE (thickness ~35 nm) to quenched hPB (thickness ~5 nm). Similar results are obtained for the hPNs, which span 0-5 mol% HxN incorporation, although the TM contents at the BDT are systematically lower for hPNs than for PE.

The strong dependence of Mbdt on crystal thickness is explained by considering the influence of TMs on the brittle fracture stress (Sb), with the BDT occurring when there are sufficient TMs for Sb to exceed the yield stress (Sy), which is strongly dependent on crystal thickness but independent of TM content. The finding that Pbdt increases systematically with decreasing crystal thickness indicates that TMs anchored in thinner crystals are less effective as stress transmitters, and therefore more such TMs are required to achieve ductility. We thus infer that thinner crystals are less effective as “anchors”—that TMs are more easily pulled out of thinner crystals—and that this is not a small effect (e.g., ~50x across the PE series), but rather is comparable to the long-recognized effect of d, hence explaining the nonmonotonic variation of Mbdt with d.

[1] S.H. Cho and R.A. Register, “Minimum Molecular Weight and Tie Molecule Content for Ductility in Polyethylenes of Varying Crystallinity”, Macromolecules, 55, 3249-3258 (2022).