Influence of semi-crystalline microstructure on gas permeability of Poly (Ether-Ketone-Ketone)

In a context of ecological transition, aeronautics is moving towards the development of hydrogen-powered aircraft for which lightweight, long-lasting and tough storage tanks must be developed. Among other properties, the tightness of tanks made of polymer and composite materials is of prime importance. This study thus aims at understanding the permeation phenomena through Poly-Ether-Ketone-Ketone (PEKK), which is one of the thermoplastic polymers considered for tank manufacture. In order to better understand the phenomena governing gas permeability, tests were carried out on different PEKK grades of different T/I ratios and degree of crystallinity. The results show that the crystallinity ratio is not the only factor that governs gas diffusion and solubility, but that permeability also depends on PEKK semi-crystalline microstructure. Even if considering crystallites organization through a tortuosity factor improves permeability predictions, permeability can be better understood using a 3 phases description of PEKK microstructure considering distinct contributions of the rigid and mobile amorphous fractions (RAF and MAF). It appears that the MAF permeation does not depend on PEKK T/I ratio, but that gas diffusion and solubility increase for RAF when increasing T/I ratio, thus counterbalancing the blocking effect of crystallites.