Vitrimers
form a promising class of dynamic polymer networks, but they have
an Achilles’ heel: elastomeric vitrimers exhibit significant
creep under conditions where permanently cross-linked, elastomeric
networks exhibit little or no creep. We demonstrate that vitrimers
can be designed with strongly suppressed creep and excellent reprocessability
by incorporating a substantial yet subcritical fraction of permanent
cross-links. This critical fraction of permanent cross-links, which
has little or no detrimental effect on reprocessability, is defined
by the gelation point of only permanent cross-links leading to a percolated
permanent network. Via a modification of classic Flory–Stockmayer
theory, we have developed a simple theory that quantitatively predicts
an approximate limiting fraction. To test our theory, we designed
vitrimers with controlled fractions of permanent cross-links based
on thiol–epoxy click chemistry. We characterized the rubbery
plateau modulus before and after reprocessing as well as stress relaxation
of our original vitrimers. Our experimental results strongly support
our theoretical prediction: as long as the fraction of permanent cross-links
is insufficient to form a percolated permanent network, the vitrimer
can be reprocessed with full recovery of cross-link density. In particular,
with a predicted limiting fraction of 50 mol %, a vitrimer system
designed with 40 mol % permanent cross-links achieved full property
recovery associated with cross-link density after reprocessing as
well as 65–71% creep reduction (for both original and reprocessed
samples) relative to a similar vitrimer without permanent cross-links.
In contrast, a system with 60 mol % permanent cross-links could not
be reprocessed into a well-consolidated sample, nor did it recover
full cross-link density; it failed by breaking at early stages of
creep tests. The ability to predict an approximate limiting fraction
of permanent cross-links leading to enhanced creep resistance and
full reprocessability represents an important advance in the science
and design of vitrimers.
We developed reprocessable polyhydroxyurethane (PHU) networks with full property recovery and incorporating both associative and dissociative dynamic chemistry.
A series
of isotactic polypropylene (iPP) and
polyethylene (PE) diblock, tetrablock, and hexablock copolymers (BCPs)
were synthesized with tunable molecular weights using a hafnium pyridylamine
catalyst. The BCPs were melt blended with 70 wt % high-density PE
(HDPE) and 30 wt % iPP commercial homopolymers at
concentrations between 0.2 and 5 wt %. The resulting blend morphologies
were investigated using TEM, revealing uniformly dispersed iPP droplets ranging progressively in size with increasing
BCP content from three-quarters to one-quarter of the diameter of
the uncompatibilized mixture. Tensile tests revealed a dramatic enhancement
in toughness based on the strain at break which increased from 10%
for the unmodified blend to more than 300% with just 0.2 wt % BCP
and over 500% with the addition of 0.5 wt % BCP or greater. Incorporation
of BCPs in blends also improved the impact toughness, doubling the
Izod impact strength to a level comparable to the neat HDPE with just
1 wt % additive. These improved blend properties are attributed to
enhanced interfacial strength, which was independently probed using
T-peel adhesion measurements performed on laminates composed of HDPE/BCP/iPP trilayers. Thin (ca. ≤100 nm thick) BCP films,
fabricated by high-temperature spin coating and molded between the
homopolymer films, significantly altered the laminate peel strength,
depending on the molecular weight and molecular architecture of the
block copolymer. Multilayer laminates containing no BCP or low molecular
weight diblock copolymer separated by adhesive failure during peel
testing. Sufficiently high molecular weight iPP–PE
diblock copolymers and iPP–PE–iPP–PE tetrablock copolymers with significantly lower
block molecular weights exhibited cohesive failure of the HDPE film
rather than adhesive failure. We propose adhesion mechanisms based
on molecular entanglements and cocrystallization for tetrablocks and
diblocks, respectively, to account for these findings. These results
demonstrate exciting opportunities to recycle the world’s top
two polymers through simple melt blending, obviating the need to separate
these plastics in mixed waste streams.
A nitroxide-mediated polymerization strategy allows one-step synthesis of recyclable crosslinked polymeric materials from any monomers or polymers that contain carbon-carbon double bonds amenable to radical polymerization. The resulting materials with dynamic covalent bonds can show full property recovery after multiple melt-reprocessing recycles. This one-step strategy provides for both robust, relatively sustainable recyclability of crosslinked polymers and design of networks for advanced technologies.
Cross-linked polyurethane (PU) is extensively used as thermoset foam; however, methods to directly reprocess PU foam waste derived from commercial sources into similar value materials have not been developed. We demonstrate that introducing dibutyltin dilaurate (DBTDL) into cross-linked PU foams and films enables their reprocessing at elevated temperatures via dynamic carbamate exchange reactions. Both model and commercial cross-linked PU foams were continuously reprocessed using twin-screw extrusion to remove gaseous filler and produce PU filaments or films with elastomeric or rigid thermoset mechanical properties. The properties of microcompounded model PU foam were in excellent agreement with PU film synthesized using the same monomers, indicating that this process occurs efficiently. These findings will enable the bulk reprocessing of commercial thermoset PU waste and inspire the further development of reprocessing methods for other thermosets and the compatibilization of chemically distinct cross-linked materials.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.