Tunable Interpenetrating Polymer Network HydrogelsBased on Dynamic Covalent Bonds and Metal–Ligand Bonds
2020
A series
of tunable interpenetrating polymer network (IPN) hydrogels
are designed by the orthogonal incorporation of two distinct types
of reversible bonds, i.e., Schiff base bonds and metal–ligand
coordination bonds. Two copolymers based on poly(N,N-dimethyl acrylamide) (PDMA) are synthesized and
used as building blocks for the IPN hydrogels. The first one bears
ketone pendant groups and is cross-linked by dihydrazide or bishydroxylamine
compounds to form, respectively, acylhydrazone- or oxime-based dynamic
covalent bond (DCB) networks. The second one bears terpyridine side
groups and is cross-linked by the addition of two different transition-metal
cations to obtain supramolecular networks based on metal–terpyridine
bis-complexes. Several IPN hydrogels are prepared by combining these
different types of reversible bonds to investigate how the two subnetworks
influence each other. To this end, the influence of the cross-linker
nature and of the hydrogel preparation protocol on the rheological
properties of these IPNs are also studied in detail. In particular,
we show that the obtained IPN hydrogels exhibit a higher modulus compared
to the simple addition of the moduli of the single networks, which
we attribute to the entanglements between the two networks. We then
study how these IPN hydrogels can disentangle and partially relax
if one of the reversible subnetworks is composed of cross-links with
a shorter lifetime. Finally, pH is used as a stimulus to affect the
dynamics of only one of the subnetworks, and the impact of this change
on the properties of the IPNs is investigated.
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