Positron Annihilation Study of Hyper-Cross-Linked Polystyrene Networks

Abstract

Hyper-cross-linked polystyrene sorbents with various degree of cross-linking ranging from 25% to over 100% were studied using positron annihilation lifetime (PAL) spectroscopy. Long-lived components of the lifetime distribution of positron annihilation, orthopositronium (o-Ps) lifetimes, give information on the effective size of elementary free volumes (unoccupied spaces), responsible for the properties of sorbents. Experiments were carried out in a vacuum, in air and also in oxygen and nitrogen. Narrow size distribution of elementary free volumes having radius of about 1.5 nm and concentration estimated as ∼10 19 cm -3 , were found in the samples with cross-linking higher than 40%. Some of the hyper-cross-linked samples gave positronium lifetime τ5 > 50 ns, which is among the highest values ever measured for organic polymer compounds. Positronium quenching (shortening of o-Ps lifetime) due to Ps exchange interaction with oxygen, dissolved in sorbents, gave information about the character of distribution of elementary free volumes in polymers.

Positron Annilation Lifetime and Glass Transition Temperatures in CO2Sorption Polystyrene

“…It is also interesting to observe that the ratio of α above and below T g for free volume (α r /α g = 4.6) is also larger than that from the bulk (= 2.6). These observations of o-Ps data vs. T are also in general consistent with the existing PAL data in PS [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32].…”

“…While PS have been extensively studied using PAL in recent years [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32], for CO 2 exposure, only up to 50 psi has been reported [13]. In this paper, we report the result of free-volume size and distribution and determine the variation of glass transition temperature (T g ) in polystyrene with and without CO 2 sorption under 400 psi pressure at different thermal cycles and compare with the DSC data.…”

Some Aspects of Free Volume Studies in Molecular Substances Using Positron Annihilation Experiments

“…The number N 5 = 0.15 × 10 20 cm −3 was obtained for LPS200X. The names of the sorbents are explained elsewhere [10]. The largest free volume holes in the polymer sorbents were big enough (in LPS200X at RT, τ 5 = 73−75 ns, and, according to Tao-Eldrup equation, R 5 = 20Å) for comparison with the data of nitrogen sorption experiments.…”

“…Alternative approaches to the problem are illustrated on example of polyimides (temperature dependence experiments [4][5][6][7][8][9]) and on example of low-density cross-linked polymeric sorbents [10][11][12][13][14][15][16][17] (Table), TABLE PATFIT and MELT results for the lifetime spectra (first and second lines for RT and LT, correspondingly) of polymeric sorbents described in [11][12][13]. Letters ρ and S with subscripts are used to denote density (in g/cm 3 ) and specific surface of the samples (in m 2 /g) measured in two different ways [11].…”

“…3), if the model curve is normalized on high-temperature data, an increase in experimentallifetime at low temperature is not as strong as it is expected from the theory. For silica-based glasses and polystyrenes, an explanation of this discrepancy was found [10,11,15] in combination of the "large hall" effect with chemical interaction of positronium with radicals induced by irradiation and stored in a sample during the experiment with positron source at low temperature or with atmospheric oxygen strongly trapped between aromatic rings. However, another discrepancy comes out when the estimations of the pore size found on the bases of the ETE model and sorption experiments are compared.…”