Phase behavior and ion conductivity of electrolytes based on aggregating combshaped polyethers

The influence of temperature and salt concentration on the properties of solid polymer electrolytes based on lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) complexed by combshaped polyethers has been studied. The combshaped polyethers consisted of styrenic backbone polymers carrying pendant poly(ethylene oxide) (PEO) or poly(ethylene oxide-co-propylene oxide) (PEOPO) side chains terminated by hydrocarbon (C-16) chain ends. Analysis by DSC and polarized light microscopy showed that the structure and composition of the side chains had a large effect on the crystallization behavior of the electrolytes. For example, a slow crystallization process was noted in electrolytes containing PEO side chains after storage at ambient temperature. This transition was not observed in corresponding electrolytes containing PEOPO side chains. Polymer aggregation by phase separation of the hydrocarbon chain ends was indicated in all electrolytes by narrow melt endotherms. A maximum conductivity of 10(-4.6) S/cm at 20 degreesC, and 10(-3.2) S/cm at 80 degreesC, was found at [Li]/[O] = 0.050 for an electrolyte based on a polyether having side chains containing 20 wt% propylene oxide. The temperature dependence of the ionic conductivities was well described by Vogel-Tamman-Fulcher equations in the absence of crystallinity

Development of highly phosphonated polymers for fuel cell membranes.

Phosphonated polymers may show high intrinsic proton conductivities at low water contents provided that the local concentration of phosphonic acid groups is very high [1,2]. Moreover, the lower acidity of aryl- and alkylphosphonic acids in relation to sulfonic acids requires higher acid contents to reach high conductivities also at higher water contents. In this context, poly(vinylphosphonic acid) (PVPA) has emerged as an interesting component for fuel cell membranes because of its extremely high concentration of phosphonic acid, corresponding to 9.25 mmol –PO3H2 per g dry material. However, the high ionic content leads to complete water solubility as well as poor mechanical properties in the solid state. Consequently, it is necessary to develop synthetic strategies to efficiently immobilize the PVPA in the membranes before practical use. We have previously immobilized PVPA by preparing various block and graft copolymers with PVPA segments [3]. These copolymers where found to self-assemble and form robust membranes with nanostructured morphologies and high proton conductivities. Very recently we have pursued a number of novel synthetic strategies towards different highly phosphonated membranes. These approaches include phosphonated norbornene copolymers prepared via ring opening metathesis polymerization (ROMP) [4], multiblock copolymers selectively grafted with PVPA via anionic polymerization [5], as well as block and graft copolymers containing the more acidic poly(tetrafluorostyrenephosphonic acid) via atom transfer radical polymerization (ATRP) [6]. Challenges and selected results on the synthesis and properties of these copolymers and membranes will be presented and discussed along with future prospects

Porous polymaleimide networks

Porous network structures were prepared by in situ polymerisation of a mixture of N-phenylmaleimide and 1, 1'-(methylenedi-4,1-plienylene)bismaleimide (95:5 w/w) in poly(vinylidene difluoride-co-hexafluoropropylene) (PVDH). The mixture of the maleimide monomers was found to form thermoreversible gels with PVDH. Thermodynamic analysis of the gelation process using calorimetry data indicated the formation of complexes of N-phenylmaleimide and vinylidene difluoride repeat units with a molar ratio of 1:2, respectively. PVDH formed optically clear solutions in the mixture above the gelation temperature, observed at 80-120 degreesC depending on composition. Polymerisation of the maleimides was induced by increasing the temperature to 250 degreesC. During polymerisation, the resulting polymaleimide phase separated from the PVDH melt. A morphological study by microscopy revealed that blends with 80 wt% PVDH contained continuous networks of crosslinked polymaleimide. The porous networks were collected after removing the PVDH by solvent extraction. Thermogravimetrical analysis of the polymaleimide networks under N-2 atmosphere showed that the onset of weight loss occurred at 380 degreesC. These networks may thus be used to prepare thermally stable membranes

Ion conducting electrolytes based on aggregating comblike poly(propylene oxide)

Solid and gel electrolytes based on comblike poly(propylene oxide) (PPO) have been prepared and studied. The polymer consisted of a polyethylene backbone densely grafted with PPO side chains terminated by hexadecanoyl chain ends. Analysis by impedance spectroscopy of the solid polymer electrolytes containing lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt showed ion conductivities of 10(-5.5) S cm(-1) at 20 degreesC. The ion conductivity of polymer gel electrolytes containing 1 M LiTFSI in gamma -butyrolactone (gamma -BL) reached 10(-3.0) S cm(-1) at 20 degreesC with 50 wt% of electrolyte solution added. Thermal analysis showed that the comblike PPO aggregated through microphase separation of the hexadecanoyl units in both the solid and the gel electrolytes. This microphase had multiple melting points in the temperature interval between -45 and 40 degreesC. Furthermore, the crystallisation of gamma -BL was greatly suppressed in gel electrolytes. A study of the gel electrolytes by FT-IR spectroscopy implied that the lithium ions were preferentially coordinated by the PPO grafts, and not by gamma -BL