Matrix-free, polymer-grafted nanoparticle (PGN) assemblies show promise for a wide array of structural, photonic, and electrical applications. We examine the modulus, yield strength, and crazing of assemblies of polystyrene grafted Fe 3 O 4 (Fe 3 O 4 -PS) at low graft density (Σ < 0.15 chains/nm 2 ) where chain entanglements are maximized. From the wrinkling−cracking method (WCM) we show that modulus (E) and yield stress (σ y ) are independent of nanoscale film thickness (70 < h f < 250 nm) and graft molecular weight (30 kDa < MW < 370 kDa) and in good agreement with predictions from effective medium theory. Furthermore, thin film craze observations from TEM imply two critical length scales for maximum deformability of matrix-free PGN assemblies: (1) PGN core size should be less than the critical length scale of the craze microstructure, and (2) near-neighbor entanglements are optimized for the lowest graft length (N) where the PGN architecture exhibit intermediate graft densities (r 0 Σ 0.5 ∼ 3−6 and N/N e ∼ 4−6). These findings provide a foundation for optimizing designs that simultaneously maximize inorganic volume fraction, processability, and mechanical robustness for PGN thin film assemblies.

A single step monomeric photo-polymerization and crosslinking via thiol-ene reaction is developed for the preparation of hydroxide exchange membranes (HEMs) in a ternary system with a triallyl triazine, a quaternary ammonium diallyl, and a dithiol. This facile method enables reproducible tuning of the ion exchange capacity and crosslink density. These HEMs demonstrate reasonable hydroxide conductivity, limited alkaline stability, and good thermal stability and have lower water uptakes than other photocrosslinked HEMs produced with much longer reaction times. Furthermore, this new fabrication method allows the incorporation of catalyst nanoparticles in the hydroxide exchange materials to form thin catalyst layers that are resistant to dissolution in methanol which suggests these polymers can be used in direct alcohol fuel cells (DAFCs). Hydroxide exchange membrane fuel cells (HEMFCs) have the potential to decrease the overall fuel cell system cost through the utilization of non-precious metal catalysts (e.g., nickel) and inexpensive bipolar plates (e.g., aluminum).1 As one of the key components for a HEMFC, the hydroxide exchange membrane (HEM) has to be both reasonably ion conductive and have low water uptake (WU). Attempts to increase ion conductivity in HEMs typically lead to increased water uptake and excessive swelling that, in the extreme cases, results in the dissolution of the membrane. Covalent crosslinking of the polymeric HEM is an approach that effectively combats excessive swelling and dissolution that would otherwise occur during fuel cell operation. 2,3Typically, polymer crosslinking methods for HEM fabrication require separate polymerization and crosslinking steps necessitating careful manipulation of reaction conditions including the reaction time, reactant concentrations, and temperature. As a result of this complexity, the reproducible fabrication of cross-linked HEMs is still an ongoing challenge in the development of fuel cell membranes to achieve consistent ion exchange capacities (IECs) and degrees of crosslinking (DC).Several groups have already demonstrated that a single-step polymerization and crosslinking fabrication of HEMs is possible using photo-polymerization, 4-8 thermal polymerization, 9 and ring opening metathesis polymerization (ROMP).10,11 A single-step polymerization and crosslinking reaction enables precise control over the degrees of functionalization (and thus IEC) and crosslinking imparted by the use of small molecules rather than large polymer precursors. Moreover, photo-crosslinking, which allows for rapid initiation and propagation, has been demonstrated by numerous groups in the formation of solvent-resistant networks [4][5][6][7][8][12][13][14] and is a particularly attractive means to fabricate membranes because of the low cost, safety, and spatial-temporal control afforded by the use of light initiation.Thus far, all of the studies involving the photo-copolymerization and crosslinking reaction between vinyl-functionalized comonomers have proceeded under traditional ...

in numerous areas including electrochemical devices, switchable surfaces, sensors, catalysis, composites, and gas separation membranes. [2] Less studied is the formation of covalently crosslinked PILs which exhibit superior mechanical properties and resistance to dissolution. Crosslinked PILs are distinguished from polymers postsynthetically doped with ionic liquids [3] or polymerized in ionic liquid solvent [4] as in this case the ionic liquid is covalently bound to the polymer backbone which prevents leaching. As most PILs are highly hygroscopic, controlling the swelling of poly mers is important to obtain mechanical stability in these polymer electrolytes.The majority of crosslinked PILs are synthesized via the free radical copolymerization of vinyl-functionalized ionic species with a primary focus on the polymerization of imidazolium. For example, PILs have been synthesized using free radical polymerization of multifunctional (i.e., crosslinkable) cationic monomers [5] and vinyl-functionalized imidazolium ionic liquids with comonomers such as styrene and acrylonitrile. [6] Ultraviolet (UV) initiated Ene-functionalized ionic liquids with a range of different cationic groups and counteranions react stoichiometrically within a tetrathiol-divinyl ether formulation within 20 minutes to form thiol-ene polymers with measurable ionic conductivities via a photoinitiated polymerization and crosslinking reaction. Dynamic mechanical analysis indicates that these networks are more spatially heterogeneous and possess higher glass transition temperatures (T g ) compared with thiol-ene formulations without charge. While tuning the molar content of ionic liquid monomer is one method for adjusting the crosslink and charge densities of the thiol-ene polymeric ionic liquid networks, the presence of cation-anion interactions also plays a critical role in dictating the thermomechanical and conductive properties. Particularly, while cationic structure effects are not significant on the polymer properties, the use of a weakly coordinating hydrophobic anion (bistriflimide) instead of bromide-based networks results in an apparent decrease in hydrated ion conductivity (7.4 to 1.5 mS cm −1 ) and T g (−9.6 to −17.8 °C).

The photoinitiated copper(I)-catalyzed azide−alkyne cycloaddition (photo-CuAAC) is a "click" reaction that enables spatially and temporally controlled polymerizations. The solventless photopolymerization of multifunctional azide and cationic alkyne monomers results in the rapid formation of a charged polymer network. Full conversion of these monomers is achieved within 30 min under mild, blue-light irradiation conditions (470 nm light at 30 mW/cm 2 ). The modulus of the material is readily tuned by controlling the ratio of di-and trifunctional alkyne monomers. Facile exchange of the hydrophobic bistriflimide counterion for a hydroxide anion yields an ion conductive polymer network with photopatternable charged regions. The spatiotemporal nature of the ionic photo-CuAAC reaction coupled with the chemical stability and mechanical flexibility suggests this chemistry as a facile and novel approach for ion-containing material synthesis (e.g., alkaline fuel cell components).