Free Volume in C₆₀ Modified PPO Polymer Membranes by Positron Annihilation Lifetime Spectroscopy

Abstract

PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) is a well-known membrane material showing good gas separation properties. The incorporation of nanoparticles can enhance or deteriorate the performance of composite membranes, sometimes depending only on the way of the composite preparation. We have modified the PPO polymer with C60 fullerenes up to a content of 2 wt %. Previous investigations showed a strong dependence of permeability on whether the C60 is simply dispersed in the polymer or chemically bonded to the polymer chains. Free volume effects were suggested as an explanation but not experimentally confirmed. Here, we present free volume studies by positron annihilation lifetime spectroscopy. An additional long positron lifetime shows the increased free volume of composite samples, while the high electron affinity of C60 helps to indicate the homogeneity of the samples. Combining the presented results with permeability measurements refines the understanding of this promising membrane material.

Molecular modeling investigation of the fundamental structural parameters of polymers of intrinsic microporosity for the design of tailor-made ultra-permeable and highly selective gas separation membranes

“…Recent strategies [2,3] on membrane development are centered around the synthesis of new polymeric substances with specific structural parameters such as bulkiness, presence of electronegative atoms, alternating double bond, out-of-plane kinks, etc., that can create nano-voids or nano-scale channels within the polymeric matrix during the molecular mobility of the polymeric chains [4][5][6]. Polymers comprising such nanopores are referred to as polymers of intrinsic microporosity (PIMs).…”

Interphases in Polymer Solid-Contacts and Nanocomposites Probed by Positron Annihilation Lifetime Spectroscopy

“…The decay signal after positron-electron annihilation is used as the stop signal. Measurements are performed by standard equipment (25)(26)(27)32) within a high vacuum environment with a temperature controlled sample holder. Due to the high energy of positrons emitted during the decay of Na-22, penetration depth into the polymeric sample can be up to 1 mm (13,14).…”

“…Selected applications to membranes helped to correlate changes in permeability with free volume and solubility (23)(24)(25). In (26) a methodic investigation showed that that even in noninteracting systems without an interphase, the functionalization of the nanoparticles and out-diffusion of the positrons can cause artifacts.…”

Molecular Dynamics Investigation into the High Permeability and High Selectivity of Nano-Porous Polyimide Membranes for the “Green” Separation of Natural Gas

“…that can create nano-voids or nano-scale channels within the polymeric matrix during the molecular mobility of the polymeric chains [2][3][4][5][6]. One main property to be optimized is the permeability, which is commonly expressed as product of diffusivity and solubility.…”

“…Studies relating polymer properties and free volume have been carried out by many authors in order to understand the transport mechanism of gases through the membranes [3,[6][7][8][9][10][11]. The total free volume in a polymer is, as a first approach, defined as the unoccupied volume in the polymer.…”