Although many polymers are not miscible
or chemically
incompatible,
some form miscible blends, which feature tunable properties relative
to those of their constituent polymers. Here, we introduce an approach
to enhance the mechanical properties of miscible blends of a thermoplastic
polyurethane and a sustainable polyester, polycaprolactone (PCL),
by introducing a copolymer of the two corresponding monomers. Rather
than polymerizing the copolymers from new monomers, they were generated
by a solvent-free, ester/urethane bond-exchange process conducted
during the coextrusion of polyurethane and PCL. Urethane/ester exchange
was confirmed by quantitative 13C NMR spectroscopy, along
with well-defined size-exclusion chromatography of the copolymers.
Extrusion time and temperature affected the extent of exchange, with
shorter residence times and lower temperatures giving limited bond
exchange and blockier copolymers. In contrast, longer extrusion times
and higher temperatures provided more extensive exchange and approximately
random copolymers. Mixtures of the polyurethane and polyester homopolymers
with 10 wt % copolymer demonstrated improved tensile stress and strain
relative to coextruded mixtures of thermoplastic polyurethane and
polyethylene because of the enhanced crystallization of the miscible
homopolymer blend facilitated by the added copolymer compatibilizer.
This approach represents a simple and general strategy for obtaining
copolymers from homopolymers rather than using specialized polymerization
techniques, allowing for products from mixed polymer waste to be used
as compatibilizers or otherwise enhance the properties of the original
homopolymer mixtures.