Free Volume Investigation of Polymers of Intrinsic Microporosity (PIMs): PIM-1 and PIM1 Copolymers Incorporating Ethanoanthracene Units

Abstract

High free volume, film-forming copolymers were prepared in which a proportion of the spirounits of PIM-1 were replaced with units derived from 9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene-2,3,6,7-tetrol (CO1). A full investigation of free volume, utilizing N 2 sorption, positron annihilation lifetime spectroscopy (PALS), Xe sorption and 129 Xe NMR spectroscopy, was undertaken for copolymer PIM1-CO1-40 (spiro-units:CO1 = 60:40) and a comparison is made with PIM-1. All techniques indicate that the copolymer, like PIM-1, possesses free volume holes or pores on the nanometre length scale (i.e., microporosity as defined by IUPAC). For the batch of PIM-1 studied here, the sample as received showed anomalous N 2 sorption, Xe sorption and 129 Xe NMR behavior that could be interpreted in terms of reduced porosity in the size range 0.6-0.7 nm, as compared to the copolymer. The anomalous behavior was eliminated on conditioning or relaxation of the polymer, e.g., by Xe sorption at 100 °C and 3 bar. PALS for both PIM1-CO1-40 and PIM-1 indicates a maximum in the average free volume hole size, and in the width of the distribution of hole sizes, on increasing temperature. This maximum appears to be a feature of high free volume polymers and may be related to the onset of localized oscillations of backbone moieties.

Polymers of Intrinsic Microporosity

“…This method generally shows a smooth distribution with a peak at maximum 0.6 nm extending to 2.0 nm, although the apparent maximum at 0.6 nm is contributed to by micropores of smaller diameter for which analysis is limited by the difficulty in obtaining N 2 adsorption data at sufficiently low pressure. In order to probe smaller pores the HK method can be applied to data obtained from CO 2 absorption at 303 K [20] or Xenon at 298 K [75]. In addition, 129 Xe NMR can be used to demonstrate gas adsorption within PIMs [75].…”

“…In order to probe smaller pores the HK method can be applied to data obtained from CO 2 absorption at 303 K [20] or Xenon at 298 K [75]. In addition, 129 Xe NMR can be used to demonstrate gas adsorption within PIMs [75].…”

“…However, there seems little evidence from other techniques to support the formation of a bimodal distribution in microporous polymers. PALS has been used to investigate the pore size distribution of PIM-1 and closely related PIMs, under a range of environments, relative to other high free volume polymers [75,94,95]. An unusual reduction in the apparent size of free volume elements is noted from PALS above 100…”

“…• C for PIM-1 [75,94]. The use of X-ray scattering to characterise the amorphous structure of PIMs is also of value, especially in combination with computer simulations (see below) [96].…”

“…The relative flexibility of the spirobisindane unit may also assist in solubility [97]. It is notable that the PIM prepared from the ethanoanthracene monomer A4 and tetrafluoroterephthalonitrile B1 is insoluble [75], and this may be due to the greater rigidity of the bridged bicyclic ethanoanthracene unit. It may also be attributed to the fact that the polymer chain is contorted only within two dimensions allowing the polar nitrile groups to interact more fully thus increasing polymer cohesion, whereas soluble PIMs form random coils in all three dimensions.…”