A high-performance epoxy vitrimer was facilely prepared from a renewable lignin derivative vanillin, and its carbon-fiber composites were nondestructively recycled.
Conventional thermosets
are built by nonrenewable fossil resources
and are arduous to be reprocessed, recycled, and reshaped due to their
permanent covalent cross-linking, and their flammability makes them
unsafe during use. Here, for the first time, we synthesized a novel
Schiff base precursor from abundant and renewable lignin derivative
vanillin and produced malleable thermosets (Schiff base covalent adaptable
networks (CANs)) combining high performance, super-rapid reprocessability,
excellent monomer recovery, and arbitrary permanent shape changeability
as well as outstanding fire resistance. The Schiff base CANs exhibited
high glass transition temperatures of ∼178 °C, tensile
strength of ∼69 MPa, tensile modulus of ∼1925 MPa, excellent
flame retardancy with UL-94 V0 rating and V1 rating, and high LOI
of ∼30%. Meanwhile, three Schiff base CANs showed high malleability
with the activation energy of the bond exchange of 49–81 kJ
mol–1 and could be reprocessed in 2–10 min
at 180 °C. These Schiff base CANs provide a prime example to
foster the development of advanced thermosetting materials from renewable
bioresources.
Epoxy thermosets containing a two-benzene-ring-conjugated Schiff base structure combined excellent controlled degradability, stability, antibacterial properties, and thermal and mechanical properties.
Vitrimers undergoing dynamic bond
exchange enable reprocessing
and recycle of thermosets. However, vitrimers are susceptible to creep,
leading to their poor dimensional stability, which limits their applications.
Here, a facile method via integration of metal complexes was utilized
to address this issue, and cross-linked polyimine was selected as
an example of vitrimer. Three different metal complexes were introduced
into a polyimine vitrimer via a one-pot preparation involving the
formation of metal complexes and cross-linking of polyimine. The addition
of 0.5 mol % Cu2+ relative to imine bond reduced creep
degree from 30% to 20% at 60 °C, and the creep resistance was
enhanced with increasing Cu2+ content. Loading 5 mol %
Cu2+ increased the initial creep temperature from 60 to
about 100 °C and raised the Arrhenius activation energy (E
a) for stress relaxation from 52.3 to 67.7 kJ
mol–1. The ability of different metal complexes
to suppress creep followed the order of Fe3+ > Cu2+ > Mg2+, and the initial creep temperature
reached around
120 °C for vitrimer with 5 mol % of Fe3+. Meanwhile,
the polyimine–metal complex vitrimers still exhibited excellent
reprocessing recyclability. Moreover, the introduction of coordination
structures enhanced the thermal and mechanical properties, solvent,
and acid resistance. Thus, metal coordination is an efficient approach
to achieve high-temperature creep resistance, excellent thermal and
mechanical properties, and chemical stability for vitrimers based
on the Schiff base.
Recycling thermosets have become extremely important due to their ecological and economic benefits. The development of thermosets that undergo reversible polymerization provides a solution to the end-life disposal issue of thermosetting materials. However, the synthesis and recycling of the current chemically recyclable thermosets are harsh, complex, and energyintensive, and their stability is often low. Here, we designed asymmetric acetal-containing thermosets (PRCs) from general phenolic resin and 1,4-cyclohexanedimethanol divinyl ether through one-step "click" cross-linking without using catalysts and solvents and without releasing small-molecule byproducts. PRCs exhibited conspicuous stress relaxation via a dissociative mechanism, corresponding to the superior malleability and reprocess recyclability. Importantly, PRCs presented excellent creep resistance even at 100 °C. In addition, PRCs could be readily and highly efficiently recovered to original phenolic resin via hydrolysis under specific mild acidic conditions but possessed high chemical stability under neutral conditions and even weak acidic conditions or acidic conditions in the absence of organic solvents with outstanding wettability and swellability toward the samples. Thermosets with different properties could be easily achieved via regulating raw materials. This work provides a promising dynamic covalent motif and a practical method to produce readily dual-recyclable (reprocess recyclable and chemically recyclable) thermosets with superior performance and stability.
Covalent adaptable networks (CANs)
represent a transition material
combining favorable features of thermosets and thermoplastics. However,
it is still a huge challenge to simultaneously achieve fast reprocessability
and high performance for CANs. Here, we designed catechol-based acetal
CANs to achieve continuous reprocessing without sacrificing thermal
and mechanical properties. A small-molecule model study demonstrated
the significantly accelerated acetal exchange by neighboring group
participation (NGP) of phenolic hydroxyl. Using this internally catalyzed
acetal chemistry, a series of CANs with a broad range of properties
were simply prepared from bio-based epigallocatechin gallate (EGCG)
and tri(ethylene glycol) divinyl ether (TEGVE) via one-step “click”
cross-linking without using catalysts or releasing small-molecule
byproducts. The dynamic nature of the CANs was confirmed via stress
relaxation and multiple recycling methods including extrusion. While
the dense cross-link density and high rigidity of the network provided
high solvent resistance and mechanical properties. This work provides
a promising and practical method to produce fast-reprocessing dynamic
covalent polymer networks with dense cross-link density and superior
performance.
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