Reprocessable, Self-Healing, and Creep-Resistant Covalent Adaptable Network Made from Chain-Growth Monomers with Dynamic Covalent Thionourethane and Disulfide Cross-Links.

We synthesized covalent adaptable networks (CANs) made from chain-growth comonomers using nonisocyanate thiourethane chemistry. We derivatized glycidyl methacrylate with cyclic dithiocarbonate (GMA-DTC), did a free-radical polymerization of n-hexyl methacrylate with GMA-DTC to obtain a statistical copolymer with 8 mol % GMA-DTC, and cross-linked it with difunctional amine. The dynamic covalent thionourethane and disulfide bonds lead to CAN reprocessability with full recovery of the cross-link density; the temperature dependence of the rubbery plateau modulus indicates that associative character dominates the dynamic response. The CAN exhibits complete self-healing at 110 °C with tensile property recovery and excellent creep resistance at 90-100 °C. Stress relaxation at 140-170 °C reveals an activation energy of 105 ± 6 kJ/mol, equal to the activation energy (Ea) of the CAN poly(n-hexyl methacrylate) backbone α-relaxation. We hypothesize that CANs with exclusively or predominantly associative dynamics have their stress-relaxation Ea defined by the α-relaxation Ea. This hypothesis is supported by stress relaxation studies on a similar poly(n-lauryl methacrylate)-based CAN.

Rapidly Self-Healable and Melt-Extrudable Polyethylene Reprocessable Network Enabled with Dialkylamino Disulfide Dynamic Chemistry.

Catalyst-free, radical-based reactive processing is used to transform low-density polyethylene (LDPE) into polyethylene covalent adaptable networks (PE CANs) using a dialkylamino disulfide crosslinker, BiTEMPS methacrylate (BTMA). Two versions of BTMA are used, BTMA-S2, with nearly exclusively disulfide bridges, and BTMA-Sn, with a mixture of oligosulfide bridges, to produce S2 PE CAN and Sn PE CAN, respectively. The two PE CANs exhibit identical crosslink densities, but the S2 PE CAN manifests faster stress relaxation, with average relaxation times ∼4.5 times shorter than those of Sn PE CAN over a 130 to 160 °C temperature range. The more rapid dynamics of the S2 PE CAN translate into a shorter compression-molding reprocessing time at 160 °C of only 5 min (vs 30 min for the Sn PE CAN) to achieve full recovery of crosslink density. Both PE CANs are melt-extrudable and exhibit full recovery within experimental uncertainty of crosslink density after extrusion. Both PE CANs are self-healable, with a crack fully repaired and the original tensile properties restored after 30 min for the S2 PE CAN or 60 min for the Sn PE CAN at a temperature slightly above the LDPE melting point and without the assistance of external forces.