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

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

Multicompartmental Microcapsules from Star Copolymer Micelles

We present the layer-by-layer (LbL) assembly of amphiphilic heteroarm pH-sensitive star-shaped polystyrene-poly­(2-pyridine) (PS_<i>n</i>P2VP_<i>n</i>) block copolymers to fabricate porous and multicompartmental microcapsules. Pyridine-containing star molecules forming a hydrophobic core/hydrophilic corona unimolecular micelle in acidic solution (pH 3) were alternately deposited with oppositely charged linear sulfonated polystyrene (PSS), yielding microcapsules with LbL shells containing hydrophobic micelles. The surface morphology and internal nanopore structure of the hollow microcapsules were comparatively investigated for shells formed from star polymers with a different numbers of arms (9 versus 22) and varied shell thickness (5, 8, and 11 bilayers). The successful integration of star unimers into the LbL shells was demonstrated by probing their buildup, surface segregation behavior, and porosity. The larger arm star copolymer (22 arms) with stretched conformation showed a higher increment in shell thickness due to the effective ionic complexation whereas a compact, uniform grainy morphology was observed regardless of the number of deposition cycles and arm numbers. Small-angle neutron scattering (SANS) revealed that microcapsules with hydrophobic domains showed different fractal properties depending upon the number of bilayers with a surface fractal morphology observed for the thinnest shells and a mass fractal morphology for the completed shells formed with the larger number of bilayers. Moreover, SANS provides support for the presence of relatively large pores (about 25 nm across) for the thinnest shells as suggested from permeability experiments. The formation of robust microcapsules with nanoporous shells composed of a hydrophilic polyelectrolyte with a densely packed hydrophobic core based on star amphiphiles represents an intriguing and novel case of compartmentalized microcapsules with an ability to simultaneously store different hydrophilic, charged, and hydrophobic components within shells

Nondestructive Light-Initiated Tuning of Layer-by-Layer Microcapsule Permeability

A nondestructive way to achieve remote, reversible, light-controlled tunable permeability of ultrathin shell microcapsules is demonstrated in this study. Microcapsules based on poly{[2-(methacryloyloxy)ethyl] trimethylammonium iodide} (PMETAI) star polyelectrolyte and poly(sodium 4-styrenesulfonate) (PSS) were prepared by a layer-by-layer (LbL) technique. We demonstrated stable microcapsules with controlled permeability with the arm number of a star polymer having significant effect on the assembly structure: the PMETAI star with 18 arms shows a more uniform and compact assembly structure. We observed that in contrast to regular microcapsules from linear polymers, the permeability of the star polymer microcapsules could be dramatically altered by photoinduced transformation of the trivalent hexacyanocobaltate ions into a mixture of mono- and divalent ions by using UV irradiation. The reversible contraction of PMETAI star polyelectrolyte arms and the compaction of star polyelectrolytes in the presence of multivalent counterions are considered to cause the dramatic photoinduced changes in microcapsule properties observed here. Remarkably, unlike the current mostly destructive approaches, the light-induced changes in microcapsule permeability are completely reversible and can be used for light-mediated loading/unloading control of microcapsules

Thermo-Induced Limited Aggregation of Responsive Star Polyelectrolytes

Poly­(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate) (PDMAEMA) star polyelectrolytes with dual thermo- and pH-responsive properties have been studied by <i>in situ</i> small-angle neutron scattering at different temperatures and pH conditions in order to reveal their conformational changes in semidilute solution. At pH values close to the p<i>K</i>_a, all PDMAEMA stars studied here are partially charged and show a core–shell quasi-micellar morphology caused by microphase separation with a collapsed core region with high monomer density and a hydrated loosely packed shell region. Upon increasing the temperature, the PDMAEMA star polyelectrolytes first experience a contraction in the shell region while the core size remains almost unchanged, and then start to form limited intermolecular aggregates. With decreasing pH values, the transition temperature increases and the size of the aggregates decreases (average aggregation number changes from 10 to 3). We suggest that these changes are triggered by the decrease in solvent quality with increasing temperature, which leads to the transition from an electrostatically dominated regime to a regime dominated by hydrophobic interactions. The observed phenomenon is in striking contrast to the phase behavior of linear PDMAEMA polyelectrolytes, which show macrophase separation with increasing temperature under the same conditions