4 research outputs found
Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with an Anti-Polyelectrolyte Effect
Some
polyzwitterionic brushes exhibit a strong “anti-polyelectrolyte
effect” and ionic specificity that make them versatile platforms
to build smart surfaces for many applications. However, the structure–property
relationship of zwitterionic polymer brushes still remains to be elucidated.
Herein, we aim to study the structure-dependent relationship between
different zwitterionic polymers and the anti-polyelectrolyte effect.
To this end, a series of polyzwitterionic brushes with different cationic
moieties (e.g., imidazolium, ammonium, and pyridinium) in their monomeric
units and with different carbon spacer lengths (e.g., CSL = 1, 3,
and 4) between the cation and anion were designed and synthesized
to form polymer brushes via the surface-initiated atom transfer radical
polymerization. All zwitterionic brushes were carefully characterized
for their surface morphologies, compositions, wettability, and film
thicknesses by atomic force microscopy, contact angle measurement,
and ellipsometry, respectively. The salt-responsiveness of all zwitterionic
brushes to surface hydration and friction was further examined and
compared both in water and in salt solutions with different salt concentrations
and counterion types. The collective data showed that zwitterionic
brushes with different cationic moieties and shorter CSLs in salt
solution induced higher surface friction and lower surface hydration
than those in water, exhibiting strong anti-polyelectrolyte effect
salt-responsive behaviors. By tuning the CSLs, cationic moieties,
and salt concentrations and types, the surface wettability can be
changed from a highly hydrophobic surface (∼60°) to a
highly hydrophilic surface (∼9°), while interfacial friction
can be changed from ultrahigh friction (μ ≈ 4.5) to superior
lubrication (μ ≈ 10<sup>–3</sup>). This work provides
important structural insights into how subtle structural changes in
zwitterionic polymers can yield great changes in the salt-responsive
properties at the interface, which could be used for the development
of smart surfaces for different applications
Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures
Development of smart
soft actuators is highly important for fundamental research and industrial
applications but has proved to be extremely challenging. In this work,
we present a facile, one-pot, one-step method to prepare dual-responsive
bilayer hydrogels, consisting of a thermoresponsive poly(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)propane-1-sulfonate) (polyVBIPS) layer.
Both polyNIPAM and polyVBIPS layers exhibit a completely opposite
swelling/shrinking behavior, where polyNIPAM shrinks (swells) but
polyVBIPS swells (shrinks) in salt solution (water) or at high (low)
temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS
bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional
bending in response to temperatures, salt concentrations, and salt
types. Such bidirectional bending, bending orientation, and degree
can be reversibly, repeatedly, and precisely controlled by salt- or
temperature-induced cooperative swelling–shrinking properties
from both layers. Based on their fast, reversible, and bidirectional
bending behavior, we further design two conceptual hybrid hydrogel
actuators, serving as a six-arm gripper to capture, transport, and
release an object and an electrical circuit switch to turn on-and-off
a lamp. Different from the conventional two- or multistep methods
for preparation of bilayer hydrogels, our simple, one-pot, one-step
method and a new bilayer hydrogel system provide an innovative concept
to explore new hydrogel-based actuators through combining different
responsive materials that allow us to program different stimuli for
soft and intelligent materials applications
General Strategy To Fabricate Strong and Tough Low-Molecular-Weight Gelator-Based Supramolecular Hydrogels with Double Network Structure
Low-molecular-weight
gelator (LMWG)-based supramolecular hydrogels,
self-assembled by small molecules via noncovalent interactions, have
recently attracted great attention due to their unique structure–property
relationship and potential applications spanning from functional materials
to biomedical devices. Unfortunately, many LMWG-based supramolecular
hydrogels are mechanically weak and can not even be handled by conventional
tensile and tearing tests. Here, we propose several design principles
to fabricate new LMWG-based hydrogels with a true double-network structure
(G4·K<sup>+</sup>/PDMAAm DN gels), consisting of the supramolecular
self-assembly of guanosine, B(OH)<sub>3</sub> and KOH as the first,
physical G4·K<sup>+</sup> network and the covalently cross-linked
poly(<i>N</i>,<i>N</i>′-dimethyacrylamide)
(PDMAAm) as the second, chemical network. Different from those LMWG-based
supramolecular hydrogels, G4·K<sup>+</sup>/PDMAAm DN gels exhibit
high tensile properties (elastic modulus = 0.307 MPa, tensile stress
= 0.273 MPa, tensile strain = 17.62 mm/mm, and work of extension =
3.23 MJ/m<sup>3</sup>) and high toughness (tearing energies = 1640
J/m<sup>2</sup>). Meanwhile, the dynamic, noncovalent bonds in the
G4·K<sup>+</sup> network can reorganize and reform after being
broken, resulting in rapid self-recovery property and excellent fatigue
resistance. The stiffness/toughness of G4·K<sup>+</sup>/PDMAAm
DN gels can be recovered by 65%/58% with 1 min resting at room temperature,
and the recovery rates are further improved with the increase of temperatures
and resting times. Interestingly, G4·K<sup>+</sup>/PDMAAm DN
gels also exhibit UV-triggered luminescence due to the unique G4-quartet
structure in the G4·K<sup>+</sup> supramolecular first network.
A new toughening mechanism is proposed to interpret the high strength
and toughness of G4·K<sup>+</sup>/PDMAAm DN gels. We believe
that our design principles, along with new G4·K<sup>+</sup>/PDMAAm
DN gel system, will provide a new viewpoint for realizing the tough
and strong LMWG-based gels
Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures
Development of smart
soft actuators is highly important for fundamental research and industrial
applications but has proved to be extremely challenging. In this work,
we present a facile, one-pot, one-step method to prepare dual-responsive
bilayer hydrogels, consisting of a thermoresponsive poly(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)propane-1-sulfonate) (polyVBIPS) layer.
Both polyNIPAM and polyVBIPS layers exhibit a completely opposite
swelling/shrinking behavior, where polyNIPAM shrinks (swells) but
polyVBIPS swells (shrinks) in salt solution (water) or at high (low)
temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS
bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional
bending in response to temperatures, salt concentrations, and salt
types. Such bidirectional bending, bending orientation, and degree
can be reversibly, repeatedly, and precisely controlled by salt- or
temperature-induced cooperative swelling–shrinking properties
from both layers. Based on their fast, reversible, and bidirectional
bending behavior, we further design two conceptual hybrid hydrogel
actuators, serving as a six-arm gripper to capture, transport, and
release an object and an electrical circuit switch to turn on-and-off
a lamp. Different from the conventional two- or multistep methods
for preparation of bilayer hydrogels, our simple, one-pot, one-step
method and a new bilayer hydrogel system provide an innovative concept
to explore new hydrogel-based actuators through combining different
responsive materials that allow us to program different stimuli for
soft and intelligent materials applications