Molecular Modeling of Free Volume Distributions of Amorphous Membrane Polymers
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AA7.4.1
Molecular Modeling of Free Volume Distributions of Amorphous Membrane Polymers Dieter Hofmann, Matthias Heuchel, GKSS Research Center, Institute of Chemistry, Kantstr. 55, D-14513 Teltow, Germany, and Yuri Yampolskii, A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Pr., 117912 Moscow, Russia. ABSTRACT The paper deals with important differences in the distribution of free volume in high and low free volume polymers as indicated by a joint investigation utilizing molecular modeling and positron annihilation lifetime (PALS) studies. The main focus of this paper is on the molecular modeling approach. The first set of polymers in question are the ultra-high free volume polymer poly(1–(trimethylsilyl)-1-propyne) (PTMSP) and two polystyrene derivatives containing Si and F. Extended equilibration procedures were necessary to obtain reasonable packing models for the polymers. The transition state Gusev-Suter Monte Carlo method was utilized to prove a reasonable agreement between simulated and measured diffusivity and solubility values for the model structures. The free volume distribution was analyzed for the validated packing models and compared with respective PALS data. In both cases a bimodal distribution of free volume was observed for PTMSP while the polystyrene derivatives as other conventional glassy polymers showed a more or less monomodal behavior. Good qualitative agreement is demonstrated between size distributions of free volume elements in these polymers obtained via molecular computer modeling and PALS experiments. In addition a series of conventional and high performance polyimides was considered. There a major result is that polyimides with conventional permeabilities for small molecules always show just a monomodal distribution of free volume element sizes in the radius range up to about 5 Å, while all polyimides with high permeabilities have a wider, often bimodal, distribution extending up to a radius between 6 and about 8 Å.
INTRODUCTION The free volume distribution in amorphous polymers is of paramount importance for their transport behavior towards small and medium sized penetrant molecules. There are several experimental methods to characterize the free volume of a polymer, with the positron annihilation lifetime spectroscopy (PALS) as the method, which gives the most direct and detailed information on free volume [1,2]. The data obtained by PALS methods have been only very rarely compared with the results of computer aided molecular modeling (CAMM) the nanostructure of polymers [3,4]. In this paper results of such a comparison for the ultra-high free volume polymer poly(1–(trimethylsilyl)-1-propyne) (PTMSP) and two polystyrene derivatives containing Si and F are discussed. In addition related molecular modeling data for seven polyimides are considered. All these polymers are glassy and amorphous at room temperature. Table 1 shows the chemical structure and some experimental data of the polymers involved.
AA7.4.2
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