Fully-cycled depolymerization and repolymerization of a low ceiling temperature polymer, cyclic poly(phthalaldehyde) (cPPA), yielding high performance structural polymer is demonstrated. The facile conditions for cPPA depolymerization circumvent the extreme conditions required to break down and recycle traditional thermoplastics and thermosets. cPPA depolymerizes in as little as 14 min at 120 °C, with concurrent evaporation and quantitative recovery of the monomer. Polymerization of the recovered monomer produces cPPA with molecular and mechanical properties identical to the original material. Depolymerization of cPPA is also demonstrated in the presence of various carbon fiber reinforcements. Continuous carbon fibers retain 100% of their moduli and tensile strength through multiple generations of recycling, while fully recycled cPPA/carbon nanofiber composites exhibit mechanical properties equivalent to the original composite and show no degradation with cycling.
Cyclic poly(phthalaldehyde) (cPPA) is a metastable and stimuli
responsive polymer that undergoes rapid solid state depolymerization
and has been utilized as a packaging and encapsulating material for
transient applications. However, the early onset thermal depolymerization
of cPPA severely hinders the fabrication and processing of plastic
parts. Herein, the thermally triggered depolymerization of cPPA was
investigated and tailored to enable thermal processing and molding
of cPPA at moderate temperatures below the thermal depolymerization
temperature. Stabilization of cPPA at elevated temperature was accomplished
by removal of the latent Lewis acid catalyst BF3 and by
addition of radical inhibitors and a Lewis base. Addition of a plasticizer
to the stabilized cPPA enabled the fabrication of a monolithic solid
polymer via hot press molding. Importantly, it is shown that the thermally
processed cPPA retains its stimuli responsive depolymerization capability
and will enable future work in the fabrication of bulk plastic parts
that depolymerize and disintegrate on demand.
Thermoset polymers and fiber-reinforced polymer composites
possess
the chemical, physical, and mechanical properties necessary for energy-efficient
vehicles and structures, but their energy-inefficient manufacturing
and the lack of end-of-life management strategies render these materials
unsustainable. Here, we demonstrate end-of-life deconstruction and
upcycling of high-performance poly(dicyclopentadiene) (pDCPD) thermosets
with a concurrent reduction in the energy demand for curing via frontal
copolymerization. Triggered material deconstruction is achieved through
cleavage of cyclic silyl ethers and acetals incorporated into pDCPD
thermosets. Both solution-state and bulk experiments reveal that seven-
and eight-membered cyclic silyl ethers and eight-membered cyclic acetals
are incorporated efficiently with norbornene-derived monomers, permitting
deconstruction at low comonomer loadings. Frontal copolymerization
of DCPD with these tailored cleavable comonomers enables energy-efficient
manufacturing of sustainable, high-performance thermosets with glass
transition temperatures of >100 °C and elastic moduli of >1
GPa.
The polymers are fully deconstructed, yielding hydroxyl-terminated
oligomers that are upcycled to polyurethane-containing thermosets
having
a higher glass transition temperatures than that of the original polymer
upon reaction with diisocyanates. This approach is extended to frontally
polymerized fiber-reinforced composites, where large-fiber volume
fraction composites (V
f = 65%) containing
a cleavable comonomer are deconstructed and the reclaimed fibers are
used to regenerate composites via frontal polymerization that display
properties nearly identical to those of the original. This work demonstrates
that the use of cleavable monomers, in combination with frontal manufacturing,
provides a promising strategy to address sustainability challenges
for high-performance materials at multiple stages of their lifecycle.
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