Highly fluorinated poly(arylene alkylene ether sulfone)s: Synthesis and thermal properties

A simple procedure has been developed for the preparation of highly fluorinated poly(arylene alkylene ether sulfone)s using a nucleophilic polycondesation reaction of decafluorodiphenyl sulfone (DPSO) with a series of fluorinated linear or branched alkylene diols containing 4-12 carbon atoms. The reactions are activated using cesium fluoride (CsF) or potassium fluoride (KF) as a base in N,N-dimethylacetamide (DMAc) at room temperature and are completed in a few hours or a few days, respectively. Polymers with number-average molecular weights (M-n) around 50 000 Da have been prepared. A broad molecular weight distribution and a low dependence of the reaction rate on temperature are observed and attributed to the heterogeneous nature of the reactions. Because of low solubility, CsF and KF exist in the reaction solution mainly in the form of solid particles, and the adsorption of the polymer on the surface of the particles hinders the diffusion of CsF and KF from solid into solution. This effect causes the polymers close to the solid surface to have greater growth rates and higher molecular weights than those in solution. Polymers with linear alkylene spacers display a marked tendency to crystallize, which increases with the length of the alkylene spacer. In fact, the polymer with the longest alkylene spacers (P12C(F)SO) crystallizes during the polymerization reaction, preventing the polymer chains from growing, resulting in a low molecular weight (M-n < 8000 Da). Raising the reaction temperature to 70 degrees C slightly decreases the crystallization tendency and allows the M-n to increase to 17 000 Da. The crystallization of P12C(F)SO is fast enough to be completed during a quenching process induced by immersing the molten sample in liquid nitrogen. However, the polymer with branched alkylene spacers does not show any crystalline structure even when annealed at a temperature slightly higher than its glass transition temperature (T-g). A secondary crystal peak is also found in the DSC curves of the crystalline polymers with linear alkylene spacers. The position of this peak can be shifted to higher temperatures by annealing and has been attributed to incomplete crystallites formed between the primary lamellar crystals.