At present there is no metallic ink that enables formation of conductive patterns at room temperature by a single printing step. Printing conductive features by metallic nanoparticle-based inks must be followed by sintering while heating to elevated temperatures, thus preventing their utilization on most plastic substrates used in plastic electronics. In this report we present a new silver nanoparticle-based conductive ink, having a built-in sintering mechanism, which is triggered during drying of the printed pattern. The nanoparticles that are stabilized by a polymer undergo self-sintering spontaneously, due to the presence of a destabilizing agent, which comes into action only during drying of the printed pattern. The destabilizing agent, which contains Cl(-) ions, causes detachment of the anchoring groups of the stabilizer from the nanoparticles' surface and thus enables their coalescence and sintering. It was found that the new metallic ink leads to very high conductivities, by a single printing step: up to 41% of the conductivity of bulk silver was achieved, the highest reported conductivity of a printed pattern that is obtained from nanoparticles at room temperature.

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In this work, the lateral mobility of polyelectrolyte multilayers was investigated by means of the fluorescence recovery after photobleaching (FRAP) technique, with special attention to the effect of relevant parameters during and after preparation. Different polyelectrolytes with respect to charge density, stiffness, and hydrophilicity were compared. From the experimental results emerged that the density of charged sites along the polymer is the most important parameter controlling the formation of polymer complexes. At higher charge density, more complexes are formed, and the diffusion coefficient decreases. It was observed that the intrinsic backbone stiffness reduces the interpenetration of polyelectrolyte layers and the formation of complexes promoting the lateral mobility. In addition, the lateral mobility increases with increasing ionic strength and with decreasing hydration shell of the added anion in the polyelectrolyte solution. The effect of heating or annealing in electrolyte solution after preparation was also investigated along with the embedding of the probing layer at controlled distances to the multilayer surface.

Water-soluble poly(acrylic acid) has been covalently labeled with a fluorescent hydrophobic chromophore, naphthalene (Np), randomly attached onto the polymer backbone with an amount of 3 mol %. The polymer, which is a new type of hydrophobically modified polymer denoted PAAMeNp-34, was investigated using steady-state fluorescence spectroscopy in aqueous solutions of different pH and in methanol. The fluorescence emission spectra of PAAMeNp-34 in water exhibit both Np monomer emission (with intensity I M) and Np excimer emission (with intensity I E). The excimer emission is mainly due to the association of Np groups, preformed in their ground electronic state as a result of the hydrophobic interaction. For a PAAMeNp-34 aqueous solution, the intensity ratio, I E/I M, decreases in the pH range where the electrostatic repulsive forces overcome the hydrophobic interactions between the Np groups and the polymer chain expands because of the intrapolymer repulsion between the negatively charged carboxylate groups. In methanol, the excimer emission is low because hydrophobic interactions are insignificant in this solvent. The interaction between PAAMeNp-34 and cationic surfactants of different alkyl chain length (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium chloride) was also studied in dilute aqueous solutions at pH 3.0 and pH 6.8. The addition of surfactants perturbs the Np−Np interactions because of polymer−surfactant associations. This causes a detectable change in the fluorescence emission, which is followed with increasing surfactant concentration. From the onset of the change, the force that dominates the interaction between the polymer and the surfactants at different pH can be examined. At low pH, PAAMeNp-34 is uncharged and hydrophobic forces dominate the polymer−surfactant interaction. The photophysical properties of the system therefore show a clear dependence on the hydrophobicity (or chain length) of the surfactants. On the other hand, at pH 6.8, where the polymer is negatively charged, almost no or very little difference between the three surfactants is observed at the onset of fluorescence change, which indicates that electrostatic forces dominate the interaction at the lowest surfactant concentrations.

Commercial poly(acrylic acid) (PAA) samples with M v = 150 000 and 450 000 were labeled randomly with small amounts (2−3 mol %) of 1-pyrenylmethyl (MePy) groups. A sample of PAA (M v = 450 000) labeled with 2 mol % MePy and modified with 2 mol % n-dodecyl (C12) groups was also prepared. The photophysical properties of the polymers have been investigated by steady-state fluorescence spectroscopy. The ratio (I E /I M) of excimer-to-monomer emission intensities was used to determine the effect of changes in pH and addition of sodium dodecyl sulfate (SDS) on the solution properties of the labeled polymers in the absence and presence of salt (NaCl). Changes in polymer conformation in aqueous solution upon addition of base were revealed by the curve I E /I M vs pH that had an inflection point at pH 4.7, the pK a value of labeled PAA. The ratio I 1/I 3 of the emission of MePy linked to PAA was monitored to obtain information on the micropolarity sensed by the pyrene label and how it is affected by external stimuli such as changes in pH and addition of surfactant. SDS interacts with labeled PAA in solution of pH 3 to form a polymer/SDS complex with a critical aggregation concentration (CAC) lower than the critical micelle concentration of SDS. The CAC values decrease further in the presence of NaCl but are not affected significantly by the molecular weight of the parent PAA or the grafted dodecyl along the PAA chain.

The effect of homogeneous nonionic surfactants (C n E8 with n = 10, 12, and 14) on the solution behavior of poly(acrylic acid) (PAA) was investigated by surface tension and viscometry. The first method allowed determination of the critical micelle concentration (cmc), the critical aggregation concentration (cac or T 1), the saturation of the polymer and the onset of free micelles into solution (T 2) and an intermediate concentration (T 2‘) that was defined as the stoichiometric concentration for binding. The T 1 points were lower than cmc's, and the change of T 1 vs the number of carbon atoms in the alkyl chain of the surfactants obeys the same rule as the cmc does. The energy change in transferring one methylene unit from micellar to water environment was nearly the same for micelles and complex and proves that both phenomena have similar driving forces. Viscometric data evidenced in turn the T 1 and T v points. T v is the minimum point that appears in the viscosity curve of PAA−nonionic surfactant systems and T 2 by surface tension was a little higher than T v by viscometry. The longer the alkyl chain of the surfactant, the lower was T v. The composition of complexes at T v was nearly constant and suggested that a considerable number of ethylene oxide groups do not participate in complex formation. The effect of surfactants on PAA at T v was compared to that of inorganic electrolytes (i.e., HCl and NaCl), and the following order was established:  surfactant < NaCl ≤ HCl. The results revealed the important role played by hydrogen bonding and hydrophobic forces in PAA−nonionic surfactant interaction, and the data were discussed in light of the latest experimental and theoretical achievements about the mechanism of complex formation.

Hexaethylene (C12E6) and octaethylene (C12E8) glycol dodecyl ethers in dilute aqueous solutions, with or without poly(acrylic acid) (PAA), were studied by steady-state fluorescence and time-resolved fluorescence quenching methods, using pyrene as probe and dimethylbenzophenone as quencher. The polarity parameter (I 1/I 3) pointed out a critical aggregation concentration lower than the critical micelle concentration of the surfactant. On polymer addition, the fluorescence lifetime increased, indicating that the polymer wraps around the micelle-like clusters of surfactants. The aggregation numbers of clusters were smaller than those of free micelles. For 10.2 mM PAA and surfactant concentrations higher than around 1 mM, free micelles appeared in solution, but cluster formation continued until polymer saturation. At the surfactant concentration selected (1 mM for C12E6 and 0.8 mM for C12E8) and over the temperature range chosen (8−45 °C for C12E6 and 8−60 °C for C12E8), the presence of polymer does not very much affect the temperature dependence of aggregation number.

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