Existing positron annihilation lifetime spectroscopy (PALS) uses the orthopositronium components, lifetimes, and intensities observed in molecular substrates, such as in polymers, to determine free-volume properties based on an infinitive potential spherical model originally proposed by Tao in 1972. However, in many molecular systems, positronium is either quenched or inhibited by interacting with chemical functional groups and leads to no or nearly no orthopositronium component in PALS. In this Article, a newly modified equation is developed by following the Tao’s quantum model using the positron component (not orthopositronium) of PALS. This modified equation is examined by fitting free-volume results obtained from o-Ps lifetimes with the positron lifetimes in pressure- and temperature-dependent data in polymers and calibrated with known or calculated cavity sizes in zeolite materials. A newly modified positron lifetime-free volume correlation equation is established for the determination of free volumes up to the mean radius of 5 Å in polymeric systems, where no orthopositronium component is observed in PALS.

A thin SiO(x) selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiO(x) was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) microanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiO(x) increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiO(x) layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: the O(2) permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O(2)/N(2) selectivity was 1.9; after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiO(x) layer was investigated by novel slow positron beam. The results show that free volume size increased with the depth, yet the degree of siloxane conversion to SiO(x) does not affect the amount of free volume.

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We have studied the mixed matrix membranes (MMMs) consisting of zeolite imidazolate framework-71 (ZIF-71, Zn(cbIm) 2 ) and polymer of intrinsic microporosity (PIM-1) with and without ultraviolet (UV) irradiation for various gas separations. Well compatible MMMs are formed between ZIF-71 nanoparticles and PIM-1. The incorporation of ZIF-71 considerably enhances the gas permeability without compromising the gas pair selectivities of O 2 /N 2 , CO 2 /N 2 and CO 2 /CH 4 . The PIM-1 comprising 30 wt% ZIF-71 exhibits a 154% increase in CO 2 permeability. An excellent agreement is found between the measured permeability and the Maxwell prediction. FESEM and PAS confirm the formation of an ultrathin dense layer at the membrane surface after the UV treatment.The photo-oxidative PIM-1 has CO 2 /CH 4 , CO 2 /N 2 and O 2 /N 2 selectivities of 34.1, 29.8 and 6.7, respectively, while the UV-treated MMMs possess much higher permeabilities compared to those of photo-oxidative PIM-1 with comparable gas pair selectivities.Under mixed gas tests, the UV treated PIM-1 containing 20 wt% ZIF-71 has a CO 2 permeability of around 2000 barrers and a CO 2 /CH 4 selectivity of 32. This performance outperforms PIM-1 and UV treated PIM membranes. The UV-PIM-1/ZIF-71 membranes have superior separation performance with both high permeability and selectivity for CO 2 /CH 4 or other gas pair separation. 4 carboxylate, thioamide and aromatic nitrile groups. 11, 15-17 PIM-1 with those pendant substituents exhibited lower permeability but slightly increased or comparable selectivity.Cross-linking PIM-1 by either a nitrene reaction in the presence of diazide cross-linker or through the thermal treatment of carboxylated PIM-1 helped increase the CO 2 /N 2 selectivity and reduce the plasticization at elevated CO 2 partial pressures. 18 Polymer blending with polyetherimide or polyimide was another direct way to enhance PIM-1's selectivity, but the miscibility and resulted permeability were largely affected by blending materials. 19,20 In addition, syntheses of new monomers (with different unit lengths or spiro-centre angles) and incorporation of other polymer structures (polyimide, spirobischromane, tetrazoles) inside the monomers were also investigated to obtain PIMs with higher selectivity. 11, 21-23 Materials with a high fractional free volume such as PIM-1 are generally subjected to severe physical aging. 24 To suppress the aging process, a microporous microparticle (porous aromatic framework, PAF) was incorporated into PIM-1. The resultant selectivity increased along with aging while the permeability was much less affected. 25Compared to the aforementioned modifications, ultraviolet (UV) irradiation is a more straightforward method to improve the membrane selectivity without involving additional polymers. 24, 26 Liu et al and Li et al were pioneers in studying the UV treatment to increase PIM-1 selectivity but the permeability was largely sacrificed. 27, 28 After being treated by UV, the CO 2 /N 2 and CO 2 /CH 4 selectivity increased, but the CO 2 ...