Experimental and computational study on direct fluorination of PEEKWC membranes

Abstract Membranes modification of poly-ether-ether ketone with cardo group (PEEK-WC) by direct fluorination and simulations was achieved in order to investigate the fluorinations effect on the morphology and gas perm-selectivity of PEEK-WC membranes. The thicknesses of the fluorinated layers as function of the fluorination extent were measured and then their average densities were obtained. The structures of two fluorinated PEEK-WC monomers were defined through quantum chemical calculations exploiting the in situ evolution of the membranes composition. The fluorinated monomers were then used to build chain models via molecular dynamics simulations realizing the scale increase. The analysis of free volume and permeability of light apolar gases (He, H2, O2, N2 and CH4) were finally performed. The fractional free volume drastically increases after the direct fluorination of the PEEK-WC and the free volume connections show that the high fluorinated PEEK-WC models present well-defined channels inside their structure similarly to fluorinated polymers obtained not by direct fluorination. The gases permeability resulted significantly larger with respect to pristine PEEK-WC; in particular, for low fluorination membranes, the permeability is significantly lower compared to samples with high fluorination amount. This result is in agreement with the free volumes analysis. For high degree of fluorination, the polymer shows an increase in free volume and, in parallel, an increase in permeability however a reduction in selectivity compared to low fluorinated polymer. This behaviour is in agreement with experimental data available for other polymer membranes.