Title: Tough self-healing elastomers by molecular enforced integration of covalent and reversible networks,
Abstract: Self-healing polymers crosslinked by solely reversible bonds are
intrinsically weaker than common covalently crosslinked networks. Introducing
covalent crosslinks into a reversible network would improve mechanical
strength. It is challenging, however, to apply this design concept to dry
elastomers, largely because reversible crosslinks such as hydrogen bonds are
often polar motifs, whereas covalent crosslinks are non-polar motifs, and these
two types of bonds are intrinsically immiscible without co-solvents. Here we
design and fabricate a hybrid polymer network by crosslinking randomly branched
polymers carrying motifs that can form both reversible hydrogen bonds and
permanent covalent crosslinks. The randomly branched polymer links such two
types of bonds and forces them to mix on the molecular level without
co-solvents. This allows us to create a hybrid dry elastomer that is very tough
with a fracture energy $13,500J/m^2$ comparable to that of natural rubber;
moreover, the elastomer can self-heal at room temperature with a recovered
tensile strength 4 MPa similar to that of existing self-healing elastomers. The
concept of forcing covalent and reversible bonds to mix at molecular scale to
create a homogenous network is quite general and should enable development of
tough, self-healing polymers of practical usage.