Synthesis, gas barrier and thermal properties of polyimide containing rigid planar fluorene moieties

<p>In order to obtain pristine polyimides with high barrier properties, pyromellitic dianhydride (PMDA) and 9H-fluorene-2,7-diamine (FDA) containing rigid planar fluorene moieties were used to prepare polyimide (FPI) via a conventional two-step polymerization process in this paper. The synthesized polyimide shows good barrier properties, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 1.01 cm<sup>3</sup> m<sup>−2</sup> day<sup>−1</sup> and 2.35 g m<sup>−2</sup> day<sup>−1</sup>, respectively. The effect of rigid planar structure in main chain on the barrier properties of polyimide was studied by means of wide angle X-ray diffractograms (WAXD), molecular dynamics simulations and positron annihilation lifetime spectroscopy (PALS), which was rarely reported before. The results reveal that the good barrier properties of FPI are mainly due to the high crystallinity, high chain rigidity and low free volume, which are resulted from the rigid planar structure. Additionally, the polyimide exhibits excellent thermal and dimensional stability with 5 wt% loss temperature of 519°C, glass transition temperature of 370°C and coefficient of thermal expansion (<i>CTE</i>) of 5.72 ppm/K. The good gas barrier and thermostability endow the polyimide with promising potential in flexible electronics encapsulation applications.</p>