Hierarchical Supramolecular Cross-Linking of Polymers
for Biomimetic Fracture Energy Dissipating Sacrificial Bonds and Defect
Tolerance under Mechanical Loading
posted on 2017-02-15, 17:37authored byTeemu
T. T. Myllymäki, Laura Lemetti, Nonappa, Olli Ikkala
Biological
structural materials offer fascinating models how to
synergistically increase the solid-state defect tolerance, toughness,
and strength using nanocomposite structures by incorporating different
levels of supramolecular sacrificial bonds to dissipate fracture energy.
Inspired thereof, we show how to turn a commodity acrylate polymer,
characteristically showing a brittle solid state fracture, to become
defect tolerant manifesting noncatastrophic crack propagation by incorporation
of different levels of fracture energy dissipating supramolecular
interactions. Therein, poly(2-hydroxyethyl methacrylate) (pHEMA) is
a feasible model polymer showing brittle solid state fracture in spite
of a high maximum strain and clear yielding, where the weak hydroxyl
group mediated hydrogen bonds do not suffice to dissipate fracture
energy. We provide the next level stronger supramolecular interactions
toward solid-state networks by postfunctionalizing a minor part of
the HEMA repeat units using 2-ureido-4[1H]-pyrimidinone
(UPy), capable of forming four strong parallel hydrogen bonds. Interestingly,
such a polymer, denoted here as p(HEMA-co-UPyMA),
shows toughening by suppressed catastrophic crack propagation, even
if the strength and stiffness are synergistically increased. At the
still higher hierarchical level, colloidal level cross-linking using
oxidized carbon nanotubes with hydrogen bonding surface decorations,
including UPy, COOH, and OH groups, leads to further increased stiffness
and ultimate strength, still leading to suppressed catastrophic crack
propagation. The findings suggest to incorporate a hierarchy of supramolecular
groups of different interactions strengths upon pursuing toward biomimetic
toughening.