Ionic Liquid-Induced Inversion of the Humidity-Dependent Conductivity of Thin PEDOT:PSS Films.

The humidity influence on the electronic and ionic resistance properties of thin post-treated poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films is investigated. In particular, the resistance of these PEDOT:PSS films post-treated with three different concentrations (0, 0.05, and 0.35 M) of ethyl-3-methylimidazolium dicyanamide (EMIM DCA) is measured while being exposed to a defined humidity protocol. A resistance increase upon elevated humidity is observed for the 0 M reference sample, while the EMIM DCA post-treated samples demonstrate a reverse behavior. Simultaneously performed in situ grazing-incidence small-angle X-ray scattering (GISAXS) measurements evidence changes in the film morphology upon varying the humidity, namely, an increase in the PEDOT domain distances. This leads to a detriment in the interdomain hole transport, which causes a rise in the resistance, as observed for the 0 M reference sample. Finally, electrochemical impedance spectroscopy (EIS) measurements at different humidities reveal additional contributions of ionic charge carriers in the EMIM DCA post-treated PEDOT:PSS films. Therefrom, a model is proposed, which describes the hole and cation transport in different post-treated PEDOT:PSS films dependent on the ambient humidity.

The Topology of Polymer Brushes Determines Their Nanoscale Hydration.

Time-of-flight neutron reflectometry (ToF-NR) performed under different relative humidity conditions demonstrates that polymer brushes constituted by hydrophilic, cyclic macromolecules exhibit a more compact conformation with lower roughness as compared to linear brush analogues, due to the absence of dangling chain ends extending at the polymer-vapor interface. In addition, cyclic brushes feature a larger swelling ratio and an increased solvent uptake with respect to their linear counterparts as a consequence of the increased interchain steric repulsions. It is proposed that differences in swelling ratios between linear and cyclic brushes come from differences in osmotic pressure experienced by each brush topology. These differences stem from entropic constraints. The findings suggest that to correlate the equilibrium swelling ratios at different relative humidity for different topologies a new form of the Flory-like expression for equilibrium thicknesses of grafted brushes is needed.

Salt-Dependent Phase Transition Behavior of Doubly Thermoresponsive Poly(sulfobetaine)-Based Diblock Copolymer Thin Films.

The water vapor-induced swelling, as well as subsequent phase-transition kinetics, of thin films of a diblock copolymer (DBC) loaded with different amounts of the salt NaBr, is investigated in situ. In dilute aqueous solution, the DBC features an orthogonally thermoresponsive behavior. It consists of a zwitterionic poly(sulfobetaine) block, namely, poly(4-(N-(3'-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), showing an upper critical solution temperature, and a nonionic block, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), exhibiting a lower critical solution temperature. The swelling kinetics in D2O vapor at 15 °C and the phase transition kinetics upon heating the swollen film to 60 °C and cooling back to 15 °C are followed with simultaneous time-of-flight neutron reflectometry and spectral reflectance measurements. These are complemented by Fourier transform infrared spectroscopy. The collapse temperature of PNIPMAM and the swelling temperature of PSBP are found at lower temperatures than in aqueous solution, which is attributed to the high polymer concentration in the thin-film geometry. Upon inclusion of sub-stoichiometric amounts (relative to the monomer units) of NaBr in the films, the water incorporation is significantly increased. This increase is mainly attributed to a salting-in effect on the zwitterionic PSBP block. Whereas the addition of NaBr notably shifts the swelling temperature of PSBP to lower temperatures, the collapse temperature of PNIPMAM remains unaffected by the presence of salt in the films.

Preparation of Monodisperse Giant Unilamellar Anchored Vesicles Using Micropatterned Hydrogel Substrates.

Giant unilamellar vesicles (GUVs) are model membrane systems consisting of a single lipid bilayer separating an inner lumen from the outer solution, with dimensions comparable to that of eukaryotic cells. The importance of these biomimetic systems has recently grown with the development of easy and safe methods to assemble GUVs from complex biorelevant compositions. However, size and position control is still a key challenge for GUV formation and manipulation. Here, a gel-assisted formation method is introduced, able to produce arrays of giant unilamellar anchored vesicles (GUAVs) with a predetermined narrow size distribution. The approach based on micropatterned gel substrates of cross-linked poly(N-isopropylacrylamide) allows performing parallel measurements on thousands of immobile unilamellar vesicles. Such power and flexibility will respond to the growing need for developing platforms of biomimetic constructs from cell-sized single bilayers.

Almost fooled again: new insights into cesium dodecyl sulfate micelle structures.

Replacing sodium with cesium as the counterion for dodecyl sulfate in aqueous solution results in stronger complexation and charge shielding, which should lead to larger micelles and ultimately to a cylindrical structure (cf. spheres for sodium dodecyl sulfate), but small angle X-ray scattering (SAXS) and small angle neutron scattering patterns previously have been interpreted with ellipsoidal micelle models. We directly image CsDS micelles via cryo-transmission electron microscopy and report large core-shell spherical micelles at low concentrations (≤2 wt %) and cylindrical micelles at higher concentrations (5.0 and 8.1 wt %). These structures are shown to be consistent with SAXS patterns modeled using established form factors. These findings highlight the importance of combining real and reciprocal space imaging techniques in the characterization of self-assembled soft materials.

Complex tracer diffusion dynamics in polymer solutions.

We employ fluorescence correlation spectroscopy (FCS) and coarse-grained molecular dynamics simulations to study the mobility of tracers in polymer solutions. Excluded volume interactions result in crowding-induced slowdown, depending only on the polymer concentration. With specific tracer-polymer attractions, the tracer is slowed down at much lower concentrations, and a second diffusion component appears that is sensitive to the polymer chain length. The two components can be resolved by FCS, only if the distance traveled by the tracer in the polymer-bound state is greater than the FCS focal spot size. The tracer dynamics can be used as a sensitive probe of the nature and strength of interactions, which-despite their local character-emphasize the role of chain connectivity.