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

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

Intumescent Flame-Retardant and Self-Healing Superhydrophobic Coatings on Cotton Fabric

Flame-retardant and self-healing superhydrophobic coatings are fabricated on cotton fabric by a convenient solution-dipping method, which involves the sequential deposition of a trilayer of branched poly(ethylenimine) (bPEI), ammonium polyphosphate (APP), and fluorinated-decyl polyhedral oligomeric silsesquioxane (F-POSS). When directly exposed to flame, such a trilayer coating generates a porous char layer because of its intumescent effect, successfully giving the coated fabric a self-extinguishing property. Furthermore, the F-POSS embedded in cotton fabric and APP/bPEI coating produces a superhydrophobic surface with a self-healing function. The coating can repetitively and autonomically restore the superhydrophobicity when the superhydrophobicity is damaged. The resulting cotton fabric, which is flame-resistant, waterproof, and self-cleaning, can be easily cleaned by simple water rinsing. Thus, the integration of self-healing superhydrophobicity with flame retardancy provides a practical way to resolve the problem of washing durability of the flame-retardant coatings. The flame-retardant and superhydrophobic fabric can endure more than 1000 cycles of abrasion under a pressure of 44.8 kPa without losing its flame retardancy and self-healing superhydrophobicity, showing potential applications as multifunctional advanced textiles

Layer-by-Layer Assembly of Fluorine-Free Polyelectrolyte–Surfactant Complexes for the Fabrication of Self-Healing Superhydrophobic Films

Fluorine-free self-healing superhydrophobic films are of significance for practical applications because of their extended service life and cost-effective and eco-friendly preparation process. In this study, we report the fabrication of fluorine-free self-healing superhydrophobic films by layer-by-layer (LbL) assembly of poly­(sodium 4-styrenesulfonate) (PSS)–1-octadecylamine (ODA) complexes (PSS–ODA) and poly­(allylamine hydrochloride) (PAH)–sodium dodecyl sulfonate (SDS) (PAH–SDS) complexes. The wettability of the LbL-assembled PSS–ODA/PAH–SDS films depends on the film structure and can be tailored by changing the NaCl concentration in aqueous dispersions of PSS–ODA complexes and the number of film deposition cycles. The freshly prepared PSS–ODA/PAH–SDS film with micro- and nanoscaled hierarchical structures is hydrophilic and gradually changes to superhydrophobic in air because the polyelectrolyte-complexed ODA and SDS surfactants tend to migrate to the film surface to cover the film with hydrophobic alkyl chains to lower its surface energy. The large amount of ODA and SDS surfactants loaded in the superhydrophobic PSS–ODA/PAH–SDS films and the autonomic migration of these surfactants to the film surface endow the resultant superhydrophobic films with an excellent self-healing ability to restore the damaged superhydrophobicity. The self-healing superhydrophobic PSS–ODA/PAH–SDS films are mechanically robust and can be deposited on various flat and nonflat substrates. The LbL assembly of oppositely charged polyelectrolyte–surfactant complexes provides a new way for the fabrication of fluorine-free self-healing superhydrophobic films with satisfactory mechanical stability, enhanced reliability, and extended service life

Rapid and Efficient Multiple Healing of Flexible Conductive Films by Near-Infrared Light Irradiation

Healable, electrically conductive films are essential for the fabrication of reliable electronic devices to reduce their replacement and maintenance costs. Here we report the fabrication of near-infrared (NIR) light-enabled healable, highly electrically conductive films by depositing silver nanowires (AgNWs) on polycaprolactone (PCL)/poly­(vinyl alcohol) (PVA) composite films. The bilayer film has sheet resistance as low as 0.25 Ω·sq^–1 and shows good flexibility to repeated bending/unbending treatments. Multiple healing of electrical conductivity lose caused by cuts of several tens of micrometers wide on the bilayer film can be conveniently achieved by irradiating the film with mild NIR light. The AgNW layer functions not only as an electrical conductor but also as a NIR light-induced heater to initiate the healing of PCL/PVA film, which then imparts its healability to the conductive AgNW layer

Antifogging and Frost-Resisting Polyelectrolyte Coatings Capable of Healing Scratches and Restoring Transparency

Polymeric antifogging/frost-resisting coatings are suitable for use on flexible substrates but are vulnerable to accidental scratches and cuts. To solve this problem, we present the fabrication of healable, highly transparent antifogging and frost-resisting polymeric coatings via the layer-by-layer assembly of poly­(ethylenimine) (PEI) and a blend of hyaluronic acid and poly­(acrylic acid) (HA-PAA). Due to their remarkable water-absorbing capability, the highly transparent and flexible (PEI/HA-PAA)*50 coatings show excellent antifogging and frost-resisting capabilities even under aggressive fogging and frosting conditions. Meanwhile, these coatings can conveniently and repeatedly heal scratches and cuts several tens of micrometers deep and wide in the same region upon exposure to water because of the dynamic nature of the PEI/HA-PAA coatings. The healability of the (PEI/HA-PAA)*50 coatings provides a new way to design transparent antifogging/frost-resisting polymeric coatings with high flexibility, enhanced reliability, and extended service life

Counteranion-Mediated Intrinsic Healing of Poly(ionic liquid) Copolymers

Fabrication of self-healing/healable materials using reversible interactions that are governed by their inherent chemical features is highly desirable because it avoids the introduction of extra groups that may present negative effects on their functions. The present study exploits the inherently featured electrostatic interactions of the ion pairs in polymeric ionic liquids (PILs) as the driving force to fabricate healable PIL copolymers. The healable PIL copolymers are fabricated through the copolymerization of the IL monomers with ethyl acrylate followed by the replacement of Br^– counteranions with bulkier ones such as bis­(trifluoromethanesulfonyl)­imide (TFSI^–). Without modifying the chemical structures of the PIL moieties, the healing performance of the as-prepared PIL copolymers can be effectively mediated by their counteranions. The PIL copolymers that do not possess healability when paired with Br^– counteranions become healable after exchanging the Br^– counteranions with larger-sized ones (e.g., TFSI^–). The PIL copolymers paired with bulky counteranions exhibit enhanced chain mobility and highly reversible ion-pair interactions, which facilitate the healing process. The PIL copolymers paired with TFSI^– anions can completely heal the damage/cut upon heating at 55 °C for 7.5 h. Meanwhile, the counteranions with larger sizes not only benefit the healing performance of the PIL copolymers but also enhance their ion conductivity. The ion conductivity of the PIL copolymers paired with TFSI^– is an order of magnitude higher than that of the PIL copolymers paired with Br^–. Therefore, the as-prepared healable PIL copolymers are potentially useful as solid electrolytes in PIL-based energy devices to improve their safety and reliability

Oil-Repellent Antifogging Films with Water-Enabled Functional and Structural Healing Ability

Healable oil-repellent antifogging films are fabricated by layer-by-layer assembly of hyaluronic acid (HA) and branched poly­(ethylenimine) (bPEI), followed by immersion in the aqueous solutions of perfluorooctanesulfonic acid potassium salt (PFOS). The loading of PFOS endows the HA/bPEI films with oil repellency while maintaining its original hydrophilicity. The resulting films have an excellent antifogging ability, and various organic liquids can easily slide down the slightly tilted films (<10°) without any residue. Through water-assisted migration of PFOS and polyelectrolytes, oil-repellent antifogging films are able to repetitively and autonomously recover their damaged oil repellency and transparency caused by plasma etching, cutting, or scratching, prolonging their life span. The as-developed healable oil-repellent antifogging films have potential application as antifingerprint coatings for touch screens, antigraffiti coatings for signs and shop windows, and antifogging coatings for lenses, mirrors, and windshields

Highly Transparent, Nanofiller-Reinforced Scratch-Resistant Polymeric Composite Films Capable of Healing Scratches

Integration of healability and mechanical robustness is challenging in the fabrication of highly transparent films for applications as protectors in optical and displaying devices. Here we report the fabrication of healable, highly transparent and scratch-resistant polymeric composite films that can conveniently and repeatedly heal severe damage such as cuts of several tens of micrometers wide and deep. The film fabrication process involves layer-by-layer (LbL) assembly of a poly(acrylic acid) (PAA) blend and branched poly(ethylenimine) (bPEI) blend, where each blend contains the same polyelectrolytes of low and high molecular weights, followed by annealing the resulting PAA/bPEI films with aqueous salt solution and incorporation of CaCO₃ nanoparticles as nanofillers. The rearrangement of low-molecular-weight PAA and bPEI under aqueous salt annealing plays a critical role in eliminating film defects to produce optically highly transparent polyelectrolyte films. The in situ formation of tiny and well-dispersed CaCO₃ nanoparticles gives the resulting composite films enhanced scratch-resistance and also retains the healing ability of the PAA/bPEI matrix films. The reversibility of noncovalent interactions among the PAA, bPEI, and CaCO₃ nanoparticles and the facilitated migration of PAA and bPEI triggered by water enable healing of the structural damage and restoration of optical transparency of the PAA/bPEI films reinforced with CaCO₃ nanoparticles