New polymer and gel electrolytes for potential applications in smart windows

Polymer electrolytes have many attractive features which enable them to be used in 'smart' window applications. Transparency, flexibility, durability and thin-film formation make polymer electrolytes the favoured option for use as the conducting layer in electrochromic devices. A number of polymer systems were investigated including comb-branch polymer acrylate glycols (2) with bi-ionic salts such as LiCF3SO3. Single-ion conducting co-polymers were also prepared from lithiated ion-pairs of triflate (8), sulfonate (12) and carboxylate (16) end-groups within acrylate monomers, in combination with the acrylate glycol monomer (1). Macroporous poly(ethylene) films and β-cyclodextrin systems with high concentrations of 'liquid' electrolyte were analyzed using poly(ethylene glycol) 300 or dimethylacetamide and triflate salts. The conductivity of these systems ranged from 10-4 to 10-6 S cm-1. Polyglucosan gels and films led to the most promising systems, with rigid gels A and B being formed with specific formulations of Cellulose, LiCI and Dimethylacetamide or N-methylpyrrolidinone. Cellulose gels showed conductivities > 10-4 S cm-1 at ambient temperatures and had the additional advantage of 'instantaneous' colouring responses when the gels were used as the conducting layer in electrochromic devices. This allowed excellent colouring and bleaching responses on the first cycle when a voltage > 2V was applied. A major development, as cells currently on the market often require a number of completed cycles before their optimum optical transmission is reached. An example from our own studies being the acrylate glycol homopolymer (2) containing LiCF3SO3 which did not show respectable colouring or bleaching responses until > 100 cycles had been completed. A superior system was prepared utilizing the photofunctional moieties of cellulose cinnamate (16) allowing a crosslinked film to be obtained in situ. This led to fewer processing difficulties with greater mechanical stability along with all the other properties the gels had shown. The 'instantaneous' colouring response and cinnamate film analysis has been filed as Patent Application Number 9709885-9. (author)