Mechanically strong and flexible membranes with very high local concentrations of immobilized proton-conducting phosphonic acid have been achieved by grafting poly(vinylphosphonic acid) side chains onto polysulfones. The graft copolymers were prepared by anionic polymerization of diethyl vinylphosphonate initiated from lithiated polysulfones, followed by quantitative cleavage of the ester functions. The resulting phosphonic acid units acted like monoprotic acids to indicate a high level of intramolecular interactions, and the phaseseparated nature of the copolymers was shown by dual glass transitions. Fully polymeric membranes were conveniently cast from solution and showed high proton conductivities, e.g., 5 mS/cm under nominally dry conditions at 120 °C and up to 93 mS/cm under 100% relative humidity at the same temperature. The corresponding conductivities measured for Nafion 115 under the same conditions were 0.04 and 105 mS/cm, respectively. The former membranes furthermore showed high thermal stability with decomposition temperatures exceeding 300 °C under air. Additions of 2-5 wt % of a perfluorosulfonic acid polymer to the phosphonated membranes were found to reduce the water uptake significantly, thus improving the mechanical properties. The conductivity of these fully polymeric doped membranes was generally observed to be enhanced, or at least to remain at the same level, under both humidified and nominally dry conditions. The findings of this study demonstrate that phosphonated membranes with a proper macromolecular design may potentially show some important advantages in relation to the more commonly studied sulfonated membranes in fuel cell applications.

A series of organic‐inorganic hybrid particles were synthesized by a self‐assembled layer of different initiators, immobilized on silica particles and used for controlled radical polymerization. We use three different initiator systems for atom‐transfer radical polymerization (ATRP), unimolecular nitroxide mediated polymerization (NMP), and bimolecular NMP, for the development of the hybrid inorganic/organic particles. After preliminary qualitative characterization by X‐ray spectroscopy (XPS) and Fourier‐transformed infrared (FT‐IR) measurements, the hybrid nanoparticles were studied by thermogravimetric analysis (TGA) to determine and discuss the initiator graft density in terms of steric hindrance.The coupling agents employed for the various approaches used here: a) NMP1‐bimolecular system, b) NMP2‐unimolecular system, and c) ATRP.magnified imageThe coupling agents employed for the various approaches used here: a) NMP1‐bimolecular system, b) NMP2‐unimolecular system, and c) ATRP.

AFM-based single molecule force spectroscopy has been applied to study the detachment of poly-(acrylic acid) (PAA) chains preadsorbed onto a silicon nitride tip surface at varied pH. In particular, monolayers of end-grafted chains prepared by nitroxide-mediated polymerization initiated from silica substrates were brought into close contact with the silicon nitride tip of an AFM. The detachment of PAA chains from the tip surface is reflected by a set of constant-force plateaus in the AFM force spectra. From those, comprehensive information about the molecular length distribution of surface-grafted polymers, about their conformation, and about their interaction with the tip as a function of pH is deduced. At neutral pH, the plateau-length distribution well reproduces the molecular weight distribution of the grafted polymers as obtained by GPC. At high pH, increasing electrostatic repulsion between PAA and the tip surface reduces the strength of adsorption, which is manifested in a decrease of average plateau lengths and measured detachment forces. At low pH, complex force spectra and a bimodal distribution of apparent molecular lengths are obtained, reflecting the poor solubility of protonated PAA chains in water as well as polymer-substrate interactions.

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We report the formation of polyacrylate (molten state polymers: Tg < room temperature) monolayers attached onto silica surfaces using covalently bonded initiators for radical‐chain polymerisation. In a first reaction step, the initiator is self‐assembled on the surface. In a subsequent reaction, the initiator is activated and the polymer formed in situ at the surface of the substrate with high grafting density (‘grafting from’ procedure). Moreover, the use of a living free radical process permits the molecular weight and polydispersity of the polymer chains to be controlled, and thus the polymer monolayer thickness also. We have also made a preliminary qualitative characterisation by X‐ray spectroscopy (XPS) and Fourier transformed infrared (FTIR) measurements. © 2002 Society of Chemical Industry

Poly(arylene ether sulfone)s have been functionalised with alkyl side chains carrying phosphonic acid and bis(phosphonic acid) in order to investigate their properties as membrane materials. The phosphonated and bisphosphonated polymers were synthesised via lithiation of poly(arylene ether sulfone)s, followed by reactions with triethyl 3-phosphonopropionate and tetraisopropyl vinylidenediphosphonate, respectively. Flexible and mechanically tough acidic membranes were cast from solution after hydrolysis of the ester groups. The significantly higher acidity and acid concentrations of the membranes containing the tetraprotic bis(phosphonic acid) led to higher conductivities in comparison to the membranes containing the diprotic phosphonic acid. Membranes containing 1.7 mmol of bis(phosphonic acid)/g dry polymer absorbed 28 wt% water when immersed at room temperature, and a conductivity of 25 mS/cm was measured at 120 C. Moreover, the bisphosphonated membranes did not decompose at temperatures of up to 240 C under air. The study also showed that high degrees of hydrolysis of the bisphosphonate units were crucial in order to reach the high proton conductivity and thermal stability necessary for fuel cell applications.

Summary: Hybrid silica‐particles comprising an inorganic core and an organic polymer shell have been synthesized by nitroxide mediated polymerization of butyl acrylate in the presence of a grafted alkoxyamine. The layer of initiator molecules attached to the surface of silica is based on an acyclic β‐phosphonylated nitroxide also referred to SG1. The synthesis and the characterization by NMR of this alkoxyamine structure are briefly described. Moreover, the grafted initiator particles have been studied by XPS, NMR 29Si CP/MAS, and TGA in order to determine the nature of the anchorage sites and the grafting density. With this stable radical as chain growth moderator tethered to the inorganic core, it is demonstrated that the grafting from polymerization exhibits a control character with a very low polydispersity index (I < 1.1). For the first time, a comparative study with a bimolecular system based on a grafted azoic initiator in presence of SG1 is also described. A different kinetic behavior has been demonstrated and interpreted in terms of variation of the initial [monomer]/[initiator] ratio.

AFM-based single molecule desorption measurements were performed on surface end-grafted poly(acrylic acid) monolayers as a function of the pH of the aqueous buffer to study the adhesion properties of polymers that bridge two surfaces. These properties were found to depend on the adhesion forces of both surfaces in a differential manner, which is explained with a simple model in analogy to the Bell-Evans formalism used in dynamic force spectroscopy. The measured interaction forces between the poly(acrylic acid) chains and silicon nitride AFM tips depend on the grafting density of the polymer monolayers as well as on the contour length of the polymer chains. This study demonstrates that the stability of polymer bridges is determined by the adhesion strengths on both surfaces, which can be tuned by using pH-dependent polyelectrolyte monolayers.