Structural heterogeneity in glassy polymeric materials revealed by positron annihilation and other supplementary techniques

Abstract

A disagreement between effective sizes of elementary free volumes found from the four component PAT-FIT analyses of positronium annihilation lifetime spectra and experiments on low temperature sorption for two membrane materials of internal microporosity poly(tri-methyl-silyl-propyne) PTMSP and spirolinked benzodioxane polymer PIM-1 is described and discussed. This disagreement can be minimized essentially using the fifth lifetime component but not the Gaussian distribution of the fourth component. 1 Introduction Elastic and glassy polymers are normally considered as homogeneous disordered materials with a unimodal (statistical) distribution of elementary free volumes. More complex (bicentral) distributions, obtained from mathematical analyses of experimental positron annihilation lifetime spectra (PALS), were explained recently as "artefacts", i.e. results of incorrect mathematical treatment (de-convolution) of the spectra. Accepting this conclusion as a reasonable formal explanation in some cases, we discussed [1] results of our PAL measurements for a number of elastic and glassy polymers, such as polystyrene (normal and cross-linked) poly-isobutylene poly-butadiene, synthetic rubbers, etc., where observations of the two long-lived ortho-positronium components in PALS, comparison with the results of sorption experiments and irregular (nonlinear) variations of annihilation characteristics in the vicinity of the glass-transition temperature actually revealed structural heterogeneity, confirmed by supplementary techniques: measurements of thermo-stimulated luminescence (TSL), thermo-mechanics, and mobility of penetrants in the systems. The heterogeneity was explained by slow relaxation of the polymer structures. Continuing this line of investigations, we present in this paper annihilation characteristics and sorption data for two membrane materials of internal microporosity: well known poly(tri-methylsilyl-propine) PTMSP [2-4] and a novel material spiro-linked benzodioxane polymer PIM-1 [5,6].A combination of PAL experiments and sorption data for membrane materials is very important for understanding of the mechanisms of Ps annihilation and potential applications of the positron annihilation research. This combination enables testing of the PAL data on the large (1-2nm) pores. Along with

Positron annihilation lifetime data and actual porosity of polymers

“…3 as an example, show that hole radius distribution is actually non-Gaussian, and position of maximum is different from that derived from the analysis of experimental curves, though very good fit was obtained in both cases. Sorption data [2], not shown here, give the same result, as modelling. In our opinion, two o-Ps lifetime components better corre-Invited Article spond to the complex, non-Gaussian distribution of EFV.…”

“…Thus, TSL data are used in this study as a measure of rigidity while PAL measurements are those to learn about the free volume parameters. Along with this, comparison of PALS and gas sorption data and also mathematical modeling of size distribution of elementary free volumes reveal some discrepancy of the data [2]. The possible reasons of this disagreement can be found, for example, in wrong estimation of the pore shapes, in the character of approximation used in the simulation procedure ("V-connect" or "R-max") and also in positronium mobility before final localization EFV.…”

“…Results of PAL and low temperature gas sorption measurements Large pores in the materials studied made it possible to compare positron annihilation and low temperature gas (N 2 ) sorption data and results of dynamic simulation of size distribution of the fractional free volume for a number of porous polymer glasses [1,2,15]. The result for AF1600 and AF2400 displayed in Fig.…”

“…Application of three-component LT 9.0 analysis and discrete four-component PATFIT analysis (two o-Ps lifetimes) to PAL distributions in some porous polymers (PIM-1, AF2400, AF1600, PTMSP) give practically the same good fitting [1]. In the conditions of this uncertainty, some additional experiments (thermomechanical tests, NMR on the protons of the adsorbed water, thermo-stimulated luminescence (TSL), lowtemperature gas sorption BET [2], mathematic moleculardynamic simulation) give some arguments in favor of nonGaussian distribution of elementary free volumes in some porous polymers and their possible heterogeneity. Furthermore, a comparison of the PAL and TSL data confirm the interconnection between the rigidity of the polymer chains, effective size of EFV and the dependence of intensity of TSL on temperature.…”

Gas transport behavior of mixed-matrix membranes composed of silica nanoparticles in a polymer of intrinsic microporosity (PIM-1)

“…Compared to lowfree-volume polymers, such as polysulfone, PIM-1 and substituted polyacetylenes have a large fraction of interconnected free volume elements, which allow for high diffusion coefficients as well as high gas solubility [37,38].…”