Specifics of positron studies of microheterogeneous systems as applied to new polymeric sorbents

Abstract

Positron annihilation lifetime spectroscopy (PALS) has been used for a long time to investigate microspores in polymers [1][2][3][4]. The present study is in continuation of this line of research as applied to het erogeneous modified systems, such as polymeric sor bents. They have been modified in such a manner that to impart new, unique properties. For example, in the case of sorbents, the aim is to alter the size distribution of free volume voids (mesopores) and reach more sta ble functional properties of polymer blends. Earlier, it was shown that a promising way of improving perme ability for other systems that also have a developed free volume (membrane polymer materials) is to prepare nanocomposites, that is, polymers containing finely divided inorganic additives (mixed matrix membranes (MMMs)) [5][6][7]. Another way of modification (for mation of additional pores, increase in specific surface area) suggests polymer saturation with a gas in the superfluid state followed by temperature elevation resulting in "foaming" [8]. In our case, we deal with divinylbenzene copolymers modified with linear rub ber. The common feature of these cases is the creation of new pores that are untypical of the base material and usually more distant from one another.An advantage of this study is the use of a combina tion of three experimental techniques: positron anni hilation (PALS), thermally stimulated luminescence (TSL), and low temperature nitrogen vapor adsorp tion (BET). The fundamentals of the BET technique are described in [9]. The TSL effect is light emission induced by heating a material pre irradiated at a low temperature. A feature that is common to PALS and TSL is their radiation chemical nature, namely, track processes. These processes result in the formation of positronium Ps (hydrogen like atom composed of a positron and an electron) [10-13] or luminescence, respectively. At a low temperature, the positronium formation rate and the integrated (over wavelength) TSL intensity [14,15] depend on the buildup of weakly bound electrons (track electrons) and are prone to optical quenching (bleaching). These results are another piece of evidence for the spur mechanism of positronium formation [16,17].In [18][19][20], we showed the effectiveness of the combined use of the room temperature PALS and low temperature TSL methods for investigation of some new membrane materials including materials with a heterogeneous structure. The following logic was adopted. Permeability of polymer membranes depends (varies by more than an order of magnitude) upon the size of free volume holes (FVHs). This size usually ranges from a few angstroms to a few nanome ters. This parameter determines the lifetime of ortho positronium (o Ps, the long lived triplet state) trapped NANOSTRUCTURED SYSTEMS AND MATERIALSAbstract-The size distribution of free volume holes in mesoporous, microheterogeneous polymer sorbents based on linear rubber modified divinylbenzene copolymers has been studied. Analysis of the results of the study and published data has ...

Exceptionally High CO2 Capture in an Amorphous Polymer with Ultramicropores Studied by Positron Annihilation

“…Therefore, the information of the pore structure can be accurately provided by lifetime and relative intensity of the pore-sensitive o-Ps. In contrast to the N 2 adsorption measurements, the positronium probe can evaluate precisely not only the ultramicropores but also closed pores which gas molecules cannot enter. , …”

“…In contrast to the N 2 adsorption measurements, the positronium probe can evaluate precisely not only the ultramicropores but also closed pores which gas molecules cannot enter. 32,33 In this study, a series of amorphous SNW materials were synthesized with different strut lengths. The parameters of the pore structure were analyzed in depth by various probing methods, such as gas physisorption and PALS, as illustrated in Scheme 1.…”

On the Limits of Sensitivity of PALS to Incorporated Free Volume Holes in Polymeric Membranes and Sorbents: Effect of <i>sc</i>-CO₂ in Polyhexafluoropropylene

“…In difference with the case described in literature for poly [1-(trimethylsilil) propine] [3], we used not only PALS technique, but a combination of LTGS data, treated according to Barett-Joyner-Halenda (BJH) [4] and Horwath-Kawazoe (HK) [5], and result on permeation of different gases for the same samples [6]. In this study, along with the attempt to find the limits of the PALS application for the studies of incorporated FVH [7], we expected to learn more about the role of FVH of different size in formation of different permeations.…”

“…These values obviously correlate with permeation (Table 2). For the PALS method, variations of meso-porosity (5-10 nm) are not seen, since meso-pore density (calculated according to [7]) is about 25 times lower than that of micro-pores (3×10 19 g -1 , w ≈1nm), and pr-Ps is not able to find these FWH during the lifetime.…”

Nanoporosity of Polymer Sorbents and Membrane Materials as Seen by PALS and Low Temperature Gas Sorption

“…4 . The last column shows specific surface of the studied compounds [7,8]. Interestingly, areas of PALS and LTGS application partly overlap (see area τ 4 → D 4 , V 4 in Fig.…”