Dynamically Cross-Linked Polyolefins via Hydrogen Bonds: Tough yet Soft Thermoplastic Elastomers with High Elastic Recovery
Giuseppe Leone, Benedetta Palucci, Giorgia Zanchin, Adriano Vignali, Giovanni Ricci, Fabio Bertini
Abstract
The fabrication of polyolefin thermoplastic elastomers (P-TPEs) with superior robustness (high strength and high toughness) is challenging. Integrating dynamic (reversible) noncovalent cross-links into P-TPEs may solve the trade-off between strength and toughness and permanent (irreversible) cross-linking and elasticity. Here, we report a two-step synthesis of P-TPEs that contain flexible polymer chains and different thiol branches (less than 2.0 mol %) that cross-link the polymer chains through dynamic hydrogen bonding. The cross-linked polymers exhibit negligible hysteresis after being circularly stretched 10 times at low strain, that is, few dynamic H-bonds break per cycle and delocalize the stress concentration to withstand load and delay premature fracture. At large deformation, the polymers dissipate vast stress energy by the sacrificial H-bond scission: the H-bonds break and reform to prevent failure and to dictate simultaneously high fracture strength (σ up to 10.2 MPa) and high toughness (UT up to 22.6 MJ/m3). Meanwhile, the resultant materials present low stiffness (E ≈ 2.5 MPa), good extensibility (ε > 600%), and elastic recovery of 90% even at 680% strain. The cross-linked polyolefins are readily (re)processable, and tensile and elastic properties are largely recovered after being remolded at least twice.