Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly­(styrenesulfonate) (PSS) and poly­(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation.

The properties of polyelectrolyte complexes, PECs, made from blended polycations, Pol+, and polyanions, Pol–, are routinely studied under conditions where they are at least partially swollen with water. Water plasticizes PECs, transforming them from an intractable, glassy, and brittle state when dry to tough and viscoelastic when wet. In the present work the supreme efficiency of water, compared to other solvents on a polarity scale, in swelling a PEC is illustrated. Using a PEC of poly­(diallyl­dimethyl­ammonium) and poly­(styrene­sulfonate) with precisely determined density, we show that swelling tracks a Dimroth–Reichardt polarity scale until the molecular volume exceeds ∼50 Å3, whereupon the degree of swelling drops sharply. Long-term (>1 year) swelling of this PEC in pure water reveals an instability, wherein the material substantially inflates, generating large pores even though T < T g. The mechanism for this instability is attributed to a small population of counterions, resulting from slight nonstoichiometry of polyelectrolytes, as well as the polymers themselves, a contribution estimated using Des Cloizeaux’s theory of osmotic pressure for overlapping chains. Low concentrations of salt in the bathing solution are enough to overcome the osmotic pressure within the PEC, and it remains dimensionally stable over the long time periods studied. The universal practice of rinsing PECs, whether they are in macroscopic or thin-film morphology, in pure water should be re-evaluated.

A layer-by-layer adsorption and reaction method was developed for the preparation of titanium phosphate ultrathin films. By repetitive adsorption of hydrated titanium from aqueous Ti(SO 4 ) 2 solution and subsequent reaction with phosphate groups, ultrathin films of titanium phosphate were fabricated. Regular film growth was verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. The film composition is mainly Ti(HPO 4 ) 2 , as confirmed by X-ray photoelectron spectroscopy and FT-IR spectroscopy. The as-prepared titanium phosphate films are uniform, highly stable, and their thickness can be well controlled by adjusting the number of adsorption-reaction cycles and pH value of Ti(SO 4 ) 2 and phosphate salt solutions. The layer-by-layer adsorption and reaction method provides a facile way to prepare ultrathin films of titanium phosphate and other kinds of metal phosphates.

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Hydrophilic polymer gels are attractive materials for a wide range of applications in the life sciences and bioengineering. The gels are environmentally responsive and have high potential in numerous applications such as drug delivery, sensors, actuators, absorbents, and scaffolds. The transport of ions and molecules through hydrated polymer gels has been investigated by many researchers, as has the bioaffi nity and mechanical properties of these gels. [ 1 ] Poly(4-vinylpyridine) (P4VP) forms typical hydrated polymer gels. This polymer is readily cross-linked with alkyl dihalide at room temperature via quaternization of the pyridine groups. Cross-linkers of various size and shape can be used, resulting in P4VP gels with diverse structures and properties. Thin fi lms made of cross-linked poly(vinylpyridine) gels have been widely investigated as a means of applying stimuli-evoked optical, impedance, and volume changes to sensors and biodevices. [ 2 ] P4VP has intrinsic selective permeability for gases, [ 3 ] and the network structures of these gels can be controlled by controlling the extent of cross-linking. Therefore, this polymer has also been studied for its utility in gas separation, desalination, and pervaporation membranes. [ 4 ] However, like most polymer gels, P4VP gels are mechanically weak, and the gel membranes, which are generally supported on a microfi ltration membrane, have thicknesses of a few to several tens of micrometers. In order to be practical, the separation layer needs to be as thin as possible to realize high liquid/gas fl uxes, which requires that the gels are mechanically stable. We here report ultrafi ltration membranes with thicknesses of several tens to hundreds of nanometers, prepared from P4VP gels by means of a two-step cross-linking of the polymer chains. The detailed structure of the polymer network was characterized using spectroscopic techniques and permeation and rejection experiments. Furthermore, the gel membranes were demonstrated to distinguish between water-soluble proteins of different molecular masses.The methodology for preparing ultrathin ultrafi ltration membranes of the cross-linked P4VP gel is shown in Figure 1 . First, P4VP was cross-linked with 1,3-dibromopropane (DBP) in dimethyl sulfoxide (DMSO), then the resulting weakly gelled solution was diluted with water (Figure 1 a). To form P4VP gel membranes, we used an extremely fl at sacrifi cial layer of metal hydroxide nanostrands. Without this layer, the gel readily penetrates into the submicrometer pores of the microfi ltration membrane, resulting in decreased permeability to water. The nanostrands spontaneously form in dilute aqueous solution of copper, zinc, or cadmium nitrate (or chloride) upon neutralization of the solution with base (see Supporting Information). [ 5 ] In the present study, cadmium hydroxide nanostrands with a diameter of 1.9 nm and lengths of a few micrometers were fi ltered on a polycarbonate (PC) membrane fi lter (Figure 1 b). [ 6 ] The resultant ultrathin nanofi brous layer guaranteed the...