Conventional cross-linked polymers or thermosets cannot be melt-processed because of their permanent cross-links. Covalent adaptable networks (CANs) with covalent cross-links that are dynamic under a stimulus allow for reprocessing. Disulfide...
Growing environmental concerns and the goal of a circular
economy
for polymers necessitate the development of biowaste-based materials
and efficient recycling of polymer materials. Here, we developed a
series of self-blowing network polyhydroxyurethane (PHU) foams by
leveraging the aminolysis and decarboxylation of cashew nutshell liquid
(CNSL)-based cyclic carbonate with thiols to release CO2 as a blowing agent; these foams contain up to 80 wt % bio-based
content. By systematically varying the blowing agent concentrations,
we demonstrated the tunability of the morphologies and mechanical
properties of CNSL-based PHU foams. Using dynamic mechanical analysis
(DMA), compression testing, and hysteresis testing, we showed that
these foams fall into the category of flexible foams with potential
as memory foams or resiliency foams. To address the recyclability
challenges of thermoset foams, we repurposed these CNSL-based PHU
foams into bulk materials and reprocessed them by exploiting the dynamic
chemistries of the hydroxyurethane linkages. Notably, the reprocessed
bulk networks exhibited full property retention. Moreover, the systematic
inclusion of permanent linkages to substitute dynamic cross-links
presents an avenue to study the interplay of permanent linkages and
cross-link density toward the dynamic characteristics. We showed that
average relaxation times and activation energies increase with increasing
levels of permanent linkages in the system, demonstrating highly tunable
dynamic behaviors in PHU network materials.
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