Dynamic covalent polymer networks (DCPN) have historically attracted attention for their unique roles in chemical recycling and self-healing, which are both relevant for sustainable societal development. Efforts in these directions have intensified in the past decade with notable progress in newly discovered dynamic covalent chemistry, fundamental material concepts, and extension toward emerging applications including energy and electronic devices. Beyond that, the values of DCPN in discovering/designing functional properties not offered by classical thermoplastic and thermoset polymers have recently gained traction. In particular, the dynamic bond exchangeability of DCPN has shown unparalleled design versatility in various areas including shape-shifting materials/devices, artificial muscles, and microfabrication. Going beyond this basic bond exchangeability, various molecular mechanisms to manipulate network topologies (topological transformation) have led to opportunities to program polymers, with notable concepts such as living networks and topological isomerization. In this review, we provide an overview of the above progress with particular focuses on molecular design strategies for the exploitation of functional material properties. Based on this, we point out the remaining issues and offer perspectives on how this class of materials can shape the future in ways that are complementary with classical thermoplastic and thermoset polymers.

Dynamic Covalent Polymer Networks: A Molecular Platform for Designing Functions beyond Chemical Recycling and Self-Healing.

Spider silk is one of the most robust natural materials, which has extremely high strength in combination with great toughness and good elasticity. Inspired by spider silk but beyond it, a healable and recyclable supramolecular elastomer, possessing superhigh true stress at break (1.21 GPa) and ultrahigh toughness (390.2 MJ m-3 ), which are, respectively, comparable to and ≈2.4 times higher than those of typical spider silk, is developed. The elastomer has the highest tensile strength (ultimate engineering stress, 75.6 MPa) ever recorded for polymeric elastomers, rendering it the strongest and toughest healable elastomer thus far. The hyper-robust elastomer exhibits superb crack tolerance with unprecedentedly high fracture energy (215.2 kJ m-2 ) that even exceeds that of metals and alloys, and superhigh elastic restorability allowing dimensional recovery from elongation over 12 times. These extraordinary mechanical performances mainly originate from the meticulously engineered hydrogen-bonding segments, consisting of multiple acylsemicarbazide and urethane moieties linked with flexible alicyclic hexatomic spacers. Such hydrogen-bonding segments, incorporated between extensible polymer chains, aggregate to form geometrically confined hydrogen-bond arrays resembling those in spider silk. The hydrogen-bond arrays act as firm but reversible crosslinks and sacrificial bonds for enormous energy dissipation, conferring exceptional mechanical robustness, healability, and recyclability on the elastomer.

Healable and Recyclable Elastomers with Record-High Mechanical Robustness, Unprecedented Crack Tolerance, and Superhigh Elastic Restorability.

Thermally conductive, self-healing, and elastic Polyimide@Vertically aligned carbon nanotubes composite as smart thermal interface material

The effect of intercrystalline links containing tie molecules and entangled loops on polymorphic transition from form II to form I in polybutene-1 (PB-1) of different molecular weights has been investigated by differential scanning calorimetry and small-angle X-ray scattering techniques. The PB-1 samples were isothermally crystallized at a range of temperatures to develop metastable form II crystalline modification of different lamellar thickness, long spacing, and number of intercrystalline links in the amorphous phase. Tammann’s two-stage crystal nuclei development method was applied to promote a faster polymorphic transition from form II to I; that is, form I nuclei were developed at the early low temperature stage, and form I crystals were formed at the later high temperature stage. At a given annealing condition, the transition rate in the high molecular weight sample is found to increase with the increase of the prior form II crystallization temperature, while it shows a negative correlation between...

Intercrystalline Links Determined Kinetics of Form II to I Polymorphic Transition in Polybutene-1

The retardation of polybutene-1 form II to form I transition in the late stage has been investigated by means of time-resolved wide-angle X-ray diffraction. Form II samples with different lamellar thickness and constituent in amorphous phase were generated via varying crystallization temperature and molecular weight and aged at room temperature for the form II to form I transition. The II to I polymorphic transition in polybutene-1 undergoes two stages, where slow nucleation and rapid growth proceed in the first stage and extremely slow secondary nucleation and growth take place in the late stage. The degree of transition reaches a plateau value in the late stage of transition, which depends highly on crystallization temperature and molecular weight that low molecular weight and high crystallization temperature always contribute to a greater degree of transition, but is less affected by the kinetics in the first stage. According to the crystal unit cell parameters of form II and form I, there should be an...

Retardance of Form II to Form I Transition in Polybutene-1 at Late Stage: A Proposal of a New Mechanism

There is a huge requirement of elastomers for use in tires, seals, and shock absorbers every year worldwide. In view of a sustainable society, the next generation of elastomers is expected to combine outstanding healing, recycling, and damage-tolerant capacities with high strength, elasticity, and toughness. However, it remains challenging to fabricate such elastomers because the mechanisms for the properties mentioned above are mutually exclusive. Herein, the fabrication of healable, recyclable, and mechanically tough polyurethane (PU) elastomers with outstanding damage tolerance by coordination of multiblock polymers of poly(dimethylsiloxane) (PDMS)/polycaprolactone (PCL) containing hydrogen and coordination bonding motifs with Zn2+ ions is reported. The organization of bipyridine groups coordinated with Zn2+ ions, carbamate groups cross-linked with hydrogen bonds, and crystallized PCL segments generates phase-separated dynamic hierarchical domains. Serving as rigid nanofillers capable of deformation and disintegration under an external force, the dynamic hierarchical domains can strengthen the elastomers and significantly enhance their toughness and fracture energy. As a result, the elastomers exhibit a tensile strength of ≈52.4 MPa, a toughness of ≈363.8 MJ m-3 , and an exceptional fracture energy of ≈192.9 kJ m-2 . Furthermore, the elastomers can be conveniently healed and recycled to regain their original mechanical properties and integrity under heating.

Healable, Recyclable, and Mechanically Tough Polyurethane Elastomers with Exceptional Damage Tolerance.

Kinetics of II to I polymorphic transformation in isotactic polybutene-1 (PB-1) and its annealing temperature and time dependencies have been investigated by means of differential scanning calorimetry and in situ wide-angle X-ray diffraction techniques. The PB-1 samples were isothermally crystallized into metastable form II crystalline modification followed by annealing at a lower temperature (Tl) and at a higher temperature (Th) subsequently or at a single temperature (Ts) to promote polymorphic transition from form II to I. This solid-to-solid phase transition was shown to be a two-step process including nucleation and growth suggested by the result that more form I was obtained after being annealed at Tl and Th than annealed at Ts for the same period. Annealing at Tl benefits nucleation due to internal stress induced by unbalanced shrinkage of amorphous and crystalline phases because of their different thermal expansion coefficients, while annealing at Th is beneficial to growth owing to rapid segmenta...

Kinetics of Nucleation and Growth of Form II to I Polymorphic Transition in Polybutene-1 as Revealed by Stepwise Annealing

As for the solid–solid transformation from form II to form I in polybutene-1, the intercrystalline links in amorphous phase and the temperature difference between crystallization and phase transiti...

Spontaneous Form II to I Transition in Low Molar Mass Polybutene-1 at Crystallization Temperature Reveals Stabilization Role of Intercrystalline Links and Entanglements for Metastable Form II Crystals

Yongna Qiao

Papers

Healable, Recyclable, and Mechanically Tough Polyurethane Elastomers with Exceptional Damage Tolerance.

Kinetics of Nucleation and Growth of Form II to I Polymorphic Transition in Polybutene-1 as Revealed by Stepwise Annealing

Intercrystalline Links Determined Kinetics of Form II to I Polymorphic Transition in Polybutene-1

Retardance of Form II to Form I Transition in Polybutene-1 at Late Stage: A Proposal of a New Mechanism

Spontaneous Form II to I Transition in Low Molar Mass Polybutene-1 at Crystallization Temperature Reveals Stabilization Role of Intercrystalline Links and Entanglements for Metastable Form II Crystals