Positronium Diffusion in Polystyrene at Low Temperatures

Abstract

ABSTRACT:Diffusion of ortho-positronium (o-Ps) in polystyrene (PS) at low temperatures (20-300 K) was studied by means of the positron annihilation lifetime technique. 2,2'-Dinitrobiphenyl was added to PS as a Ps quencher, and the diffusion coefficients were determined from the measured Ps quenching rate constants, assuming that the reaction between Ps and the quencher is entirely diffusion-controlled. The determined diffusion coefficient increases with increasing temperature, and is significantly influenced by such structural relaxation of PS as i5 and fl transitions. The estimated diffusion length of o-Ps, 1.5-2.5 nm, is only a few times larger than the Ps cavity size, indicating that the effect of the Ps diffusion on the Ps lifetime distribution is almost insignificant in PS at low temperatures. The size distributions of free volume in pure PS were obtained from the lifetime distributions of o-Ps without taking an effect of the diffusivity of o-Ps into consideration. Peculiar broadening of the size distribution of free volume for PS, which is induced by an inhomogeneous local structure, is observed at the transition temperatures, T0 and Tp. KEY WORDSPolystyrene/ Diffusion Coefficient/ Free Volume/ Positronium / Positron Annihilation / Structural Relaxation / The free volume concept has been extensively used to interpret various macroscopic physical properties 1 -7 of polymers such as viscosity, elasticity, glass transition, and structural relaxation, all of which are important from the view point of controlling mechanical properties of polymers, The data obtained by such microscopic techniques as fluorescence, 8 photochemical hole burning,9 and nuclear magnetic resonance 10 are often discussed in relation to the predictions of the free volume theory. Another empirical technique, which can sensitively probe the local structure of the polymers, is positron annihilation.The observed decay curve of positrons annihilating in a polymer contains a long-lived component with an average lifetime of 1-5 ns. The component is attributed to "pick-off" annihilation of ortho-positronium (o-Ps: the spin-parallel bound state between a positron and an electron) localized in a sub-nanometer size hole. A relation between the pick-off annihilation lifetime and the cavity size was derived based on a simple quantum mechanical model initially proposed by Tao 11 and later developed by Eldrup et al.,12 and Nakanishi et al. 13 • 14 Several empirical correlations between the average Ps cavity size estimated from the relation and free volume properties of polymers were reported. 15 · 16 In addition to this, attempts have been made to deduce the size distribution of the Ps cavity by inverse Laplace transformation of the positron lifetime data. This paper concerns diffusion of Ps in a polymer at low temperatures. Following Hirata et al. 1 7 and Kobayashi et al., 18 2,2' -dinitrobiphenyl (DNB) was added to polystyrene (PS) as a Ps quencher, and the diffusion coefficients were determined from the measured Ps quenching rate constants, assum...

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.…”

Antiplasticization of Polymer Materials: Structural Aspects and Effects on Mechanical and Diffusion-Controlled Properties

“…The trendlines for the specific volume bear a similarity with the experimental data reported in [59], showing a reduction in the gradient of the change in the specific volume at temperatures below 50 • C after physical ageing. In this latter case the trendlines for the aged and unaged samples allude to a likely intersection at lower temperatures, possibly about −50 • C in correspondence to a more realistic temperature for the β transition of polystyrene [60], in concordance with the values for other glassy polymers. It should also be noted that the graphs representing the variation of specific volume with temperature in [59] indicate that the Tg of the sample decreased with increasing ageing time.…”