Multilevel and Multicomponent Layer-by-Layer Assembly for the Fabrication of Nanofibrillar Films

In this study, we demonstrate multilevel and multicomponent layer-by-layer (LbL) assembly as a convenient and generally applicable method for the fabrication of nanofibrillar films by exploiting the dynamic nature of polymeric complexes. The alternate deposition of poly(allylamine hydrochloride)–methyl red (PAH-MR) complexes with poly(acrylic acid) (PAA) produces nanofibrillar PAH-MR/PAA films, which involves the disassembly of PAH-MR complexes, the subsequent assembly of PAH with PAA, and the PAA-induced assembly of MR molecules into MR nanofibrils via a π–π stacking interaction. The aqueous solution of weak polyelectrolyte PAA with a low solution pH plays an important role in fabricating nanofibrillar PAH-MR/PAA films because proton transfer from acidic PAA to MR molecules induces the formation of MR nanofibrils. The generality of the multilevel and multicomponent LbL assembly is verified by alternate assembly of complexes of 1-pyrenylbutyric acid (PYA) and PAH with PAA to fabricate PAH-PYA/PAA films with organized nanofibrillar structures. Unlike the traditional static LbL assembly, the multilevel and multicomponent LbL assembly is dynamic and more flexible and powerful in controlling the interfacial assembly process and in fabricating composite films with sophisticated structures. These characteristics of multilevel and multicomponent LbL assembly will enrich the functionalities of the LbL-assembled films

One-Step Synthesis of Healable Weak-Polyelectrolyte-Based Hydrogels with High Mechanical Strength, Toughness, and Excellent Self-Recovery

Excellent self-recovery is critically important for soft materials such as hydrogels and shape memory polymers. In this work, weak-polyelectrolyte-based hydrogels with high mechanical strength, toughness, healability, and excellent self-recovery are fabricated by one-step polymerization of acrylic acid and poly­(ethylene glycol) methacrylate in the presence of oppositely charged branched polyethylenimine. The synergy of electrostatic and hydrogen-bonding interactions and the in situ formed polyelectrolyte complex nanoparticles endow the hydrogels with a tensile strength of ∼4.7 MPa, strain at break of ∼1200%, and toughness of ∼32.6 MJ m–3. The hydrogels can recover from an ∼300% strain to their initial state within 10 min at room temperature without any external assistance. Moreover, the hydrogels can heal from physical cut at room temperature and exhibit a prominent shape-memory performance with rapid shape recovery speed and high shape-fixing and shape-recovery ratios

One-Step Synthesis of Healable Weak-Polyelectrolyte-Based Hydrogels with High Mechanical Strength, Toughness, and Excellent Self-Recovery

Excellent self-recovery is critically important for soft materials such as hydrogels and shape memory polymers. In this work, weak-polyelectrolyte-based hydrogels with high mechanical strength, toughness, healability, and excellent self-recovery are fabricated by one-step polymerization of acrylic acid and poly­(ethylene glycol) methacrylate in the presence of oppositely charged branched polyethylenimine. The synergy of electrostatic and hydrogen-bonding interactions and the in situ formed polyelectrolyte complex nanoparticles endow the hydrogels with a tensile strength of ∼4.7 MPa, strain at break of ∼1200%, and toughness of ∼32.6 MJ m–3. The hydrogels can recover from an ∼300% strain to their initial state within 10 min at room temperature without any external assistance. Moreover, the hydrogels can heal from physical cut at room temperature and exhibit a prominent shape-memory performance with rapid shape recovery speed and high shape-fixing and shape-recovery ratios

Ion-Triggered Exfoliation of Layer-by-Layer Assembled Poly(acrylic acid)/Poly(allylamine hydrochloride) Films from Substrates: A Facile Way To Prepare Free-Standing Multilayer Films

A facile way to prepare sheet- and tubelike free-standing films of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) was developed by exfoliating PAA/PAH multilayer films from substrates in acid aqueous solution containing copper ions. The exfoliation of the PAA/PAH film from the substrate was achieved by breaking the electrostatic interaction of the PAA layer with the underlying substrate while keeping the integrity of the resultant films. Further study shows that thermally cross-linked free-standing PAA/PAH film can be prepared by treating the film in acid aqueous solution with a pH of 2.0. The ion-triggered exfoliation of PAA/PAH multilayer film provides a simple and flexible way to prepare layer-by-layer (LbL) assembled free-standing multilayer films

Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables <i>cis–trans</i> Isomerization of Azobenzenes

Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzene-containing polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly­{1–4­[4-(3-carboxy-4-hydroxyphenylazo)­benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)–poly­(acrylic acid) (PAA) complexes (noted as PAZO–PAA) with poly­(allylamine hydrochloride) (PAH) produces azobenzene-containing PAZO–PAA/PAH films. UV irradiation induces <i>trans–cis</i> isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the <i>cis–trans</i> back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the <i>cis–trans</i> back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for <i>cis–trans</i> azobenzene isomerization can be helpful for designing novel polymeric actuators

Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables <i>cis–trans</i> Isomerization of Azobenzenes

Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzene-containing polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly­{1–4­[4-(3-carboxy-4-hydroxyphenylazo)­benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)–poly­(acrylic acid) (PAA) complexes (noted as PAZO–PAA) with poly­(allylamine hydrochloride) (PAH) produces azobenzene-containing PAZO–PAA/PAH films. UV irradiation induces <i>trans–cis</i> isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the <i>cis–trans</i> back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the <i>cis–trans</i> back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for <i>cis–trans</i> azobenzene isomerization can be helpful for designing novel polymeric actuators

Patterning of Layer-by-Layer Assembled Organic−Inorganic Hybrid Films: Imprinting versus Lift-Off

Layer-by-layer (LbL) assembled organic−inorganic poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH)/Au nanoparticle hybrid films are patterned by using Norland Optical Adhesive 63 (NOA 63) polymer molds. Depending on the rigidity of the hybrid films, their patterning can be realized by a room-temperature imprinting or lift-off process. For [(PAA/PAH)1-(Au nanoparticle/PAH)3]∗10 and [(PAA/PAH)3-(Au nanoparticle/PAH)3]∗5 films which have a low content of Au nanoparticles, the films can be imprinted at room temperature to form patterned films with large areas because of the compressibility and fluidity of the films under high pressure. The Au nanoparticle/PAH films, which have an extremely high content of Au nanoparticles and are fragile, can be patterned by a lift-off process during which the film contacted with the NOA 63 mold was peeled off because of the strong adhesion between the film and the mold and the fragility of the film. The complementary room-temperature imprinting and lift-off methods with polymer NOA 63 molds provide facile and general ways to pattern LbL assembled organic−inorganic films with various film compositions

Nitrogen-Coordinated Boroxines Enable the Fabrication of Mechanically Robust Supramolecular Thermosets Capable of Healing and Recycling under Mild Conditions

The fabrication of mechanically robust polymeric materials capable of self-healing and recycling remains challenging because the mobility of polymer chains in such polymers is very limited. In this work, mechanically robust supramolecular thermosets capable of healing physical damages and recycling under mild conditions are fabricated by trimerization of bi-(ortho-aminomethyl-phenylboronic acid)- and tri-(ortho-aminomethyl-phenylboronic acid)-terminated poly­(propylene glycol) oligomers (denoted as Bi-PBA-PPG and Tri-PBA-PPG, respectively). The resultant supramolecular thermosets are cross-linked by dynamic covalent bonds of nitrogen-coordinated boroxines. The mechanical properties of the supramolecular thermosets can be systematically tailored by varying the ratios between Tri-PBA-PPG and Bi-PBA-PPG, which changes the cross-linking density of nitrogen-coordinated boroxines and the topology of the supramolecular thermosets. The mechanically strongest supramolecular thermosets with a molar ratio of Tri-PBA-PPG to Bi-PBA-PPG being 1:2 have a glass transition temperature of ∼36 °C, a tensile strength of ∼31.96 MPa, and a Young’s modulus of ∼298.5 MPa. The high reversibility of nitrogen-coordinated boroxines and the flexibility of poly­(propylene glycol) chains enable the supramolecular thermosets with the strongest mechanical strength to be highly efficiently healed at 55 °C and recycled under a pressure of 4 MPa at 60 °C to regain their original mechanical strength and integrity

Nitrogen-Coordinated Boroxines Enable the Fabrication of Mechanically Robust Supramolecular Thermosets Capable of Healing and Recycling under Mild Conditions

The fabrication of mechanically robust polymeric materials capable of self-healing and recycling remains challenging because the mobility of polymer chains in such polymers is very limited. In this work, mechanically robust supramolecular thermosets capable of healing physical damages and recycling under mild conditions are fabricated by trimerization of bi-(ortho-aminomethyl-phenylboronic acid)- and tri-(ortho-aminomethyl-phenylboronic acid)-terminated poly­(propylene glycol) oligomers (denoted as Bi-PBA-PPG and Tri-PBA-PPG, respectively). The resultant supramolecular thermosets are cross-linked by dynamic covalent bonds of nitrogen-coordinated boroxines. The mechanical properties of the supramolecular thermosets can be systematically tailored by varying the ratios between Tri-PBA-PPG and Bi-PBA-PPG, which changes the cross-linking density of nitrogen-coordinated boroxines and the topology of the supramolecular thermosets. The mechanically strongest supramolecular thermosets with a molar ratio of Tri-PBA-PPG to Bi-PBA-PPG being 1:2 have a glass transition temperature of ∼36 °C, a tensile strength of ∼31.96 MPa, and a Young’s modulus of ∼298.5 MPa. The high reversibility of nitrogen-coordinated boroxines and the flexibility of poly­(propylene glycol) chains enable the supramolecular thermosets with the strongest mechanical strength to be highly efficiently healed at 55 °C and recycled under a pressure of 4 MPa at 60 °C to regain their original mechanical strength and integrity

Mechanically Stable Antireflection and Antifogging Coatings Fabricated by the Layer-by-Layer Deposition Process and Postcalcination

Complexes of poly(diallyldimethylammonium chloride) (PDDA) and sodium silicate (PDDA−silicate) are alternately deposited with poly(acrylic acid) (PAA) to fabricate PAA/PDDA−silicate multilayer films. The removal of the organic components in the PAA/PDDA−silicate mulilayer films through calcination produces highly porous silica coatings with excellent mechanical stability and good adhesion to substrates. Quartz substrates covered with such porous silica coatings exhibit both antireflection and antifogging properties because of the reduced refractive index and superhydrophilicity of the resultant films. A maximum transmittance of 99.86% in the visible spectral range is achieved for the calcinated PAA/PDDA−silicate films deposited on quartz substrates. The wavelengths of maximum transmittance could be well tailored by simply changing the deposition cycles of multilayer films. The usage of PDDA−silicate complexes allows for the introduction of high porosity to the resultant silica coatings, which favors the fabrication of antireflection and antifogging coatings with enhanced performance. Meanwhile, PDDA−silicate complexes enable rapid fabrication of thick porous silica coatings after calcination because of the large dimensions of the complexes in solution. The easy availability of the materials and simplicity of this method for film fabrication might make the mechanically stable multifunctional antireflection and antifogging coatings potentially useful in a variety of applications