Probing Flexural Properties of Cellulose Nanocrystal–Graphene Nanomembranes with Force Spectroscopy and Bulging Test

The flexural properties of ultrathin freely standing composite nanomembranes from reduced graphene oxide (rGO) and cellulose nanocrystals (CNC) have been probed by combining force spectroscopy for local nanomechanical properties and bulging test for global mechanical properties. We observed that the flexural properties of these rGO–CNC nanomembranes are controlled by rGO content and deformational regimes. The nanomembranes showed the enhanced mechanical properties due to the strong interfacial interactions between interwoven rGO and CNC components. The presence of weak interfacial interactions resulted in time-dependent behavior with the relaxation time gradually decreased with increasing the deformational rate owing to the reducing viscous damping at faster probing regimes close to 10 Hz. We observed that the microscopic elastic bending modulus of 141 GPa from local force spectroscopy is close to the elastic tensile modulus evaluated from macroscopic bulging test, indicating the consistency of both approaches for analyzing the ultrathin nanomembranes at different spatial scales of deformation. We showed that the flexible rGO–CNC nanomembranes are very resilient in terms of their capacity to recover back into original shape

Template-Assisted Assembly of the Functionalized Cubic and Spherical Microparticles

The patterned template-assisted assembly of the cubic microparticles driven by the competing capillary, Columbic, and van der Waals forces had been studied in comparison with the traditional spherical colloidal microparticles. We observed that the spherical and cubic microparticles assembled with different probability in the channels of the hydrophobic–hydrophilic patterned substrates due to differences in a balance of adhesive and capillary forces. In contrast to highly selective assembly of spherical microparticles, selective deposition of cubic microcrystals with channels is impeded by strong adhesive forces facilitated by large specific interfacial areas between cube facets and substrate. The modification of the patterned substrate by functionalized coatings with oppositely charged topmost layers significantly increases the probability (to 86%) of the cubic microparticles to assemble into chemically modified channels. The introduction of ultrathin LbL shells on cubic microparticles and functionalization of patterned substrates are critical for the directed colloidal assembly of anisotropic microparticles into ordered aggregates

Inkjet-Assisted Layer-by-Layer Printing of Encapsulated Arrays

We present the facile fabrication of hydrogen-bonded layer-by-layer (LbL) microscopic dot arrays with encapsulated dye compounds. We demonstrate patterned encapsulation of Rhodamine dye as a model compound within poly­(vinylpyrrolidone)/poly­(methacrylic acid) (PVPON/PMAA) LbL dots constructed without an intermediate washing step. The inkjet printing technique improves encapsulation efficiency, reduces processing time, facilitates complex patterning, and controls lateral and vertical dimensions with diameters ranging from 130 to 35 μm (mostly controlled by the droplet size and the substrate hydrophobicity) and thickness of several hundred nanometers. The microscopic dots composed of hydrogen-bonded PVPON/PMAA components are also found to be stable in acidic solution after fabrication. This facile, fast, and sophisticated inkjet encapsulation method can be applied to other systems for fast fabrication of large-scale, high-resolution complex arrays of dye-encapsulated LbL dots

Morphology and Properties of Microcapsules with Different Core Releases

The morphology, mechanical properties, and permeability of hydrogen-bonded layer-by-layer (LbL) microcapsule shells assembled on cubic CdCO₃ cores have been studied in comparison with traditional shells assembled on spherical SiO₂ cores. We observed that the morphology of LbL shells is dramatically affected by the different release processes with highly porous and softened LbL shells as a result of the intense CO₂ gas formation and ion release during the removal of cubic CdCO₃ cores. A substantial increase in porosity is reflected in a dramatic change in the mesh size of LbL shells, from 2 nm for spherical capsules to above 35 nm for cubic capsules. Shells also possess enhanced permeability with a many fold increase in diffusion coefficient for dextran molecules and enhanced softening with the elastic modulus dropping by almost an order of magnitude for cubic capsules. These dramatic changes in shell morphology, porosity, permeability, and stiffness, observed in this study for the first time, are all important for the intelligent projection of controlled loading and unloading behavior of microcontainers with different shapes and composition, a component usually overlooked in current studies

Cellulose Nanocrystal Microcapsules as Tunable Cages for Nano- and Microparticles

We demonstrate the fabrication of highly open spherical cages with large through pores using high aspect ratio cellulose nanocrystals with “haystack” shell morphology. In contrast to traditional ultrathin shell polymer microcapsules with random porous morphology and pore sizes below 10 nm with limited molecular permeability of individual macromolecules, the resilient cage-like microcapsules show a remarkable open network morphology that facilitates across-shell transport of large solid particles with a diameter from 30 to 100 nm. Moreover, the transport properties of solid nanoparticles through these shells can be pH-triggered without disassembly of these shells. Such behavior allows for the controlled loading and unloading of solid nanoparticles with much larger dimensions than molecular objects reported for conventional polymeric microcapsules

Multiresponsive Star-Graft Quarterpolymer Monolayers

Multifunctional star-graft quarterpolymers PS_<i>n</i>[P2VP-<i>b</i>-(PAA-<i>g</i>-PNIPAM)]_<i>n</i> with two different arm types, shorter PS arms and longer P2VP-<i>b</i>-PAA block copolymer arms with grafted PNIPAM chains, were studied in terms of their ability to form micellar structures at the air/water and air/solid interfaces. Because of the pH-dependent ionization of P2VP and PAA blocks, as well as thermoresponsiveness of PNIPAM chains, these multifunctional stars have multiple responsive properties to pH, temperature, and ionic strength. We observed that the molecular surface area of the stars is the largest at basic pH, when the PAA blocks are strongly charged and extended, and PNIPAM chains are spread at the interface. At acidic conditions, the molecular surface area is the smallest because the P2VP blocks submerge into the water subphase and the PAA blocks are contracted and form hydrogen bonding with grafted PNIPAM chains. The molecular surface area of the stars at the air/water interface gradually increases at elevated temperature. We suggest that the transition across lower critical solution temperature (LCST) results in the emerging of PNIPAM chains from the water subphase to the interface due to the hydrophilic to hydrophobic transition. Moreover, at higher surface pressure, the stars tend to form intermolecular micellar aggregates above LCST. The graft density of PNIPAM chains as well as the arm number was also found to have strong effects on the thermo- and pH-response. Overall, this study demonstrates that the star block copolymer conformation and aggregation are strongly dependent on the intramolecular interactions between different blocks and spatial distribution of the arms, which can be controlled by the external conditions, including pH, temperature, ionic strength, and surface pressure

Biopolymeric Nanocomposites with Enhanced Interphases

Ultrathin and robust nanocomposite membranes were fabricated by incorporating graphene oxide (GO) sheets into a silk fibroin (SF) matrix by a dynamic spin-assisted layer-by-layer assembly (dSA-LbL). We observed that in contrast to traditional SA-LbL reported earlier fast solution removal during dropping of solution on constantly spinning substrates resulted in largely unfolded biomacromolecules with enhanced surface interactions and suppressed nanofibril formation. The resulting laminated nanocomposites possess outstanding mechanical properties, significantly exceeding those previously reported for conventional LbL films with similar composition. The tensile modulus reached extremely high values of 170 GPa, which have never been reported for graphene oxide-based nanocomposites, the ultimate strength was close to 300 MPa, and the toughness was above 3.4 MJ m^–3. The failure modes observed for these membranes suggested the self-reinforcing mechanism of adjacent graphene oxide sheets with strong 2 nm thick silk interphase composed mostly from individual backbones. This interphase reinforcement leads to the effective load transfer between the graphene oxide components in reinforced laminated nanocomposite materials with excellent mechanical strength that surpasses those known today for conventional flexible laminated carbon nanocomposites from graphene oxide and biopolymer components

Linear and Star Poly(ionic liquid) Assemblies: Surface Monolayers and Multilayers

The surface morphology and organization of poly­(ionic liquid)­s (PILs), poly­[1-(4-vinylbenzyl)-3-butylimidazolium bis­(trifluoromethylsulfonyl)­imide] are explored in conjunction with their molecular architecture, adsorption conditions, and postassembly treatments. The formation of stable PIL Langmuir and Langmuir–Blodgett (LB) monolayers at the air–water and air–solid interfaces is demonstrated. The hydrophobic bis­(trifluoromethylsulfonyl)­imide (Tf₂N^–) is shown to be a critical agent governing the assembly morphology, as observed in the reversible condensation of LB monolayers into dense nanodroplets. The PIL is then incorporated as an unconventional polyelectrolyte component in the layer-by-layer (LbL) films of hydrophobic character. We demonstrate that the interplay of capillary forces, macromolecular mobility, and structural relaxation of the polymer chains influence the dewetting mechanisms in the PIL multilayers, thereby enabling access to a diverse set of highly textured, porous, and interconnected network morphologies for PIL LbL films that would otherwise be absent in conventional LbL films. Their compartmentalized internal structure is relevant to molecular separation membranes, ultrathin hydrophobic coatings, targeted cargo delivery, and highly conductive films

Star Polymer Unimicelles on Graphene Oxide Flakes

We report the interfacial assembly of amphiphilic heteroarm star copolymers (PS_<i>n</i>P2VP_<i>n</i> and PS_<i>n</i>(P2VP-<i>b</i>-P<i>t</i>BA)_<i>n</i> (<i>n</i> = 28 arms)) on graphene oxide flakes at the air–water interface. Adsorption, spreading, and ordering of star polymer micelles on the surface of the basal plane and edge of monolayer graphene oxide sheets were investigated on a Langmuir trough. This interface-mediated assembly resulted in micelle-decorated graphene oxide sheets with uniform spacing and organized morphology. We found that the surface activity of solvated graphene oxide sheets enables star polymer surfactants to subsequently adsorb on the presuspended graphene oxide sheets, thereby producing a bilayer complex. The positively charged heterocyclic pyridine-containing star polymers exhibited strong affinity onto the basal plane and edge of graphene oxide, leading to a well-organized and long-range ordered discrete micelle assembly. The preferred binding can be related to the increased conformational entropy due to the reduction of interarm repulsion. The extent of coverage was tuned by controlling assembly parameters such as concentration and solvent polarity. The polymer micelles on the basal plane remained incompressible under lateral compression in contrast to ones on the water surface due to strongly repulsive confined arms on the polar surface of graphene oxide and a preventive barrier in the form of the sheet edges. The densely packed biphasic tile-like morphology was evident, suggesting the high interfacial stability and mechanically stiff nature of graphene oxide sheets decorated with star polymer micelles. This noncovalent assembly represents a facile route for the control and fabrication of graphene oxide-inclusive ultrathin hybrid films applicable for layered nanocomposites