UPy motifs were able to be excited under the ultraviolet radiation and the absorbed
energy to convert into heat, leading to the reversible disengagement of the hydrogenbonding motifs and thus quickly and efficiently healed introduced defects. However,
achieving plastic toughening remained more challenging.
McKee et al. [163] designed a one-component system involving reinforcing
colloidal nanorod cores grafted with long polymer brushes, and these polymer
brushes formed the matrix polymer-containing supramolecular UPy binding units
(Fig. 9b). UPy groups acted as sacrificial bonds simultaneously providing adhesion
between the CNCs while allowing them to orient and then gradually slide past each
other, thus dissipating fracture energy. The strategy of supramolecular binding units
within side chains of polymer grafts attached to colloidal reinforcements opens
generic approaches for tough nanocomposites.
Cross-linking of cellulose-based physical gels via hydrogen bonding has attracted
great interest owing to the typical shear-thinning behavior and injectability, which
enables a substantial reduction in viscosity under applied shear stress with the
subsequent recovery of gel properties after stress removal. Khabibullin et al. [164]
reported a multifunctional physical hydrogel formed by CNCs and graphene quantum dots (GQDs). Although CNCs and GQDs are both negatively charged, hydrogen bonding and hydrophobic interactions overcome the electrostatic repulsion
between these nanoparticles and yield a physically cross-linked hydrogel with
tunable mechanical properties. Owing to their shear-thinning behavior, the
CNC-GQD hydrogel was used as an injectable material in 3D printing. The
hydrogels were fluorescent and exhibited the distinct anisotropic nanofibrillar structure. The combination of these advantageous properties makes this hybrid hydrogel a
promising material and fosters the development of new manufacturing methods such
as 3D printing.
3.2.2 Host-Guest Interactions
Host-guest interactions involving two structurally dissimilar but dimensionally
matched macromolecular units are often used to proceed molecular assembly via
non-covalent complementary inclusion binding [165, 166]. A unique structural
relationship exists, such that one moiety (the guest) is physically inserted and
included inside another moiety (the host). As the supramolecular assemblies are
held together by non-covalent interactions including hydrophobic interactions,
hydrogen bonding, π-π stacking, and reversible host-guest cross-links can be used
in designing dynamic cross-linked cellulose-based gels.
Cyclodextrin (CD) has been widely used to prepare host-guest supramolecular
gels since its hydrophobic internal cavity accommodates hydrophobic binding sites
for guest molecules. The hydrophobic cavity can envelope a variety of appropriate
guest molecules with various structural designs of CD and guest-functionalized
polymers. For instance, Himmelein et al. [167] prepared a supramolecular carbohydrate hydrogel containing cyclodextrin vesicles as 3D cross-linker. A cellulose
polymer is randomly modified with hydrophobic side groups that act as guests for
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C. Shao and J. Yang
energy to convert into heat, leading to the reversible disengagement of the hydrogenbonding motifs and thus quickly and efficiently healed introduced defects. However,
achieving plastic toughening remained more challenging.
McKee et al. [163] designed a one-component system involving reinforcing
colloidal nanorod cores grafted with long polymer brushes, and these polymer
brushes formed the matrix polymer-containing supramolecular UPy binding units
(Fig. 9b). UPy groups acted as sacrificial bonds simultaneously providing adhesion
between the CNCs while allowing them to orient and then gradually slide past each
other, thus dissipating fracture energy. The strategy of supramolecular binding units
within side chains of polymer grafts attached to colloidal reinforcements opens
generic approaches for tough nanocomposites.
Cross-linking of cellulose-based physical gels via hydrogen bonding has attracted
great interest owing to the typical shear-thinning behavior and injectability, which
enables a substantial reduction in viscosity under applied shear stress with the
subsequent recovery of gel properties after stress removal. Khabibullin et al. [164]
reported a multifunctional physical hydrogel formed by CNCs and graphene quantum dots (GQDs). Although CNCs and GQDs are both negatively charged, hydrogen bonding and hydrophobic interactions overcome the electrostatic repulsion
between these nanoparticles and yield a physically cross-linked hydrogel with
tunable mechanical properties. Owing to their shear-thinning behavior, the
CNC-GQD hydrogel was used as an injectable material in 3D printing. The
hydrogels were fluorescent and exhibited the distinct anisotropic nanofibrillar structure. The combination of these advantageous properties makes this hybrid hydrogel a
promising material and fosters the development of new manufacturing methods such
as 3D printing.
3.2.2 Host-Guest Interactions
Host-guest interactions involving two structurally dissimilar but dimensionally
matched macromolecular units are often used to proceed molecular assembly via
non-covalent complementary inclusion binding [165, 166]. A unique structural
relationship exists, such that one moiety (the guest) is physically inserted and
included inside another moiety (the host). As the supramolecular assemblies are
held together by non-covalent interactions including hydrophobic interactions,
hydrogen bonding, π-π stacking, and reversible host-guest cross-links can be used
in designing dynamic cross-linked cellulose-based gels.
Cyclodextrin (CD) has been widely used to prepare host-guest supramolecular
gels since its hydrophobic internal cavity accommodates hydrophobic binding sites
for guest molecules. The hydrophobic cavity can envelope a variety of appropriate
guest molecules with various structural designs of CD and guest-functionalized
polymers. For instance, Himmelein et al. [167] prepared a supramolecular carbohydrate hydrogel containing cyclodextrin vesicles as 3D cross-linker. A cellulose
polymer is randomly modified with hydrophobic side groups that act as guests for
340
C. Shao and J. Yang