ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2012, том 54, № 8, с. 1297-1303
ТРАНСПОРТ В ПОЛИМЕРАХ
УДК 541.64:547.314
MACROCHAIN CONFIGURATION, STRUCTURE OF FREE VOLUME AND TRANSPORT PROPERTIES OF POLY(1-TRIMETHYLSILYL-1-PROPYNE) AND POLY(1-TRIMETHYLGERMYL-1-PROPYNE)
© 2012 г. S. M. Matson", K. Râtzke*, M. Q. Shaikh*, E. G. Litvinova", S. M. Shishatskiyc, K.-V. Peinemann^, and V. S. Khotimskiy"
a A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky prospect, 29, 119991 Moscow, Russia b Technical Faculty, University of Kiel, Kaiserstrafie 2, 24143 Kiel, Germany c Institute of Polymer Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany d Advanced Membranes and Porous Materials Center at KAUST, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia
e-mail: matson@ips.ac.ru Received November 15, 2011 Revised Manuscript Received January 30, 2012
Abstract—The relationship between poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1-trimethyl-germyl-1-propyne) (PTMGP) microstructure, gas permeability and structure of free volume is reported. ngButane/methane mixed-gas permeation properties of PTMSP and PTMGP membranes with different cis-/trans-composition have been investigated. The n-butane/methane selectivities for mixed gas are by an order higher than the selectivities calculated from pure gas measurements (the mixed-gas n-butane/methane selectivities are 20—40 for PTMSP and 22—35 for PTMGP). Gas permeability and n-butane/methane selectivity essentially differ in polymers with different cis-/trans-composition. Positron annihilation lifetime spectroscopy investigation of PTMSP and PTMGP with different microstructure has determined distinctions in total amount and structure of free volume, i.e. distribution of free volume elements. The correlation between total amount of free volume and gas transport parameters is established: PTMSP and PTMGP with bigger free volume exhibit higher n-butane permeability and mixed-gas n-butane/methane selectivity. Such behavior is discussed in relation to the submolecular structure of polymers with different microstructure and sorption of n-butane in polymers with different free volume.
INTRODUCTION
Creation of novel polymeric materials with specially designed properties is one of important driving forces for efficient development of various industrial processes. Nanostructured polymeric materials are of great interest for developers of membranes for selective gas and vapor separation. Glassy 1,2-disubstituted polyacetylenes show unique transport properties, namely exceptionally high permeability and a high selectivity (organic vapor/permanent gas), which is unusual for conventional glassy polymers [1—3]. Silicon-containing disubstituted polyacetylene poly(1-trime-thylsilyl-1-propyne) (PTMSP) exhibits the highest gas and organic vapor permeability and selectivity during recovery of C3+ from permanent gas [4—6]. Highly selective recovery of condensable hydrocarbon (e.g. «-butane) from their mixtures with permanent gas (e.g. methane) was investigated by a number of research groups and may be explained in terms of selective surface sorption mechanism, where the proposed transport is realized by competitive sorption and surface diffusion [7, 8]. According to this mechanism, de-
cline of methane permeability in mixtures with condensable hydrocarbons in comparison with pure methane results from blocking of pores induced by multilayer adsorption or capillary condensation of vapor (e.g. «-butane) on the inner surface of free volume elements.
Unique properties of disubstituted polyacetylenes are provided by a specific organization of nanospace in these polymers, namely, extremely high fractional free volume and interconnection of the free volume elements [1, 8]. The specific structure is formed by rigid polymer backbone containing C=C bonds and bulky substituents [9—12].
The main chain of1,2-disubstituted polyacetylenes contains alternating double bonds and therefore may have various geometric structures, i.e. units in cis- and trans-configuration. Detailed investigation of polymerization of 1-trimethylsilyl-1-propyne (TMSP) and its Ge-containing analogue 1-trimethylgermyl-1-propyne (TMGP) in the presence of catalytic systems based on Nb and Ta pentachlorides has shown that by varying synthesis conditions, e.g. co-catalyst, solvent polarity and temperature of polymerization process
7 ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ Серия А том 54 № 8 2012
one can regulate the geometric structure of macro-chains, i.e. the ratio of cis- and trans-units, that determines submolecular organization of the polymer [12]. Moreover it was shown that functional properties of disubstituted polyacetylenes such as gas and organic vapor transport characteristics, as well as stability towards organic solvents are controlled by the geometric structure [12, 13]. Subtle variation in packing of polymer chains of different microstructure can alter the polymer submolecular organization and influence the free volume structure. Therefore, the investigation of the free volume structure is very important for understanding the relation between polymer structure and its properties.
Common experimental techniques of free volume investigation are pressure-volume-temperature measurements, where the equation of state is determined in a closed system, Xe-129 spectroscopy and positron annihilation lifetime spectroscopy (PALS) [14]. Whereas the first technique only gives information about the total free volume and its temperature dependence, the latter two techniques give information about the average microscopic size of free volume entities [14, 15]. In case of PALS once injected from a radioactive source, positrons form in most polymers hydrogen-like ortho-positronium (o-Ps) states. The pick-off lifetime of ortho-positronium, x0-Ps, is well correlated to the free-volume void size in polymers. The success of PALS in polymer research is largely due to the so-called standard model developed by Tao [16]. This simple quantum mechanical model assumes the o-Ps to be confined to spherical holes with infinitely high walls and gives a direct relationship between sand the size of the free volume elements.
To-Psc
— A -
1 -
Rh 1 . 2nRh
-h— + — sin-h—
Rh + 8R 2n Rh + 8 R
(1)
This equation includes the reciprocal o-Ps decay rate xo_Ps, the spin averaged decay rate in the electron layer A0, the hole radius Rh, and the thickness of the electron layer 5R, which has been calibrated using substances with known pore sizes [17]. Since void sizes in amorphous polymers are relatively broadly distributed, the discrete To_Ps obtained from fits to lifetime spectra and hence the void radius have to be considered as average values. We use the LT 9.0 evaluation program by J. Kansy and coworkers [18], which takes into account the distribution of free volume void sizes by using a log normal distribution including a dispersion sigma, which is a measure for the width of the distribution of free volume void sizes. This technique has been applied by some of the authors to several high free volume polymers including Teflon AF [19], polymers of intrinsic microporosity [15, 20] and polymer nano-composites [21]. For chemically identical polymers, one could also calculate the fractional free volume
(ffv) via a well known relationship between relative oPs Intensity (I3) and o-Ps lifetime
ffv = AI3V/T3), (2)
where A is a constant factor A = 1.8 nm-3 [22], vf is the mean cavity volume, t3 is third lifetime component. Although the absolute value of this factor might be questionable and can lead to unusual results (see Results and Discussion section), the approach can be used to compare the fractional free volume for the present polymers relative to each other.
To date, no data for structure of free volume and permeation properties of disubstituted polyacetylenes with different microstructure are available in the literature. The development of efficient and sustainable membrane materials for processes of separation of different gas mixtures containing hydrocarbons can provide means for successful solution of multiple industrial problems, including natural gas conditioning, with the help of membrane technology. Therefore, research of model gas mixtures transport through membranes is of importance for the development of new membrane materials with optimal properties. In this paper, we report on the pure and mixed-gas n-bu-tane/methane transport properties of PTMSP and PTMGP with various geometric structures and analyze the influence of the polyacetylene microstructure and the free volume structure derived from PALS data on gas and vapour transport parameters.
EXPERIMENTAL
Synthesis of Monomers, Polymers and Characterization of Polymer Samples
TMSP was synthesized by interreaction of methyl-acetylene from methylacetylene-allene fraction with alkylmagnesium halogenide succeeding treatment of reacting mixture with trimethylchlorosilane [23]. TMGP was synthesized by analogues procedure with the use of trimethylchlorogermanium [12].
Synthesis of polymers PTMSP and PTMGP, characterization of polymer samples and calculation of quantitative ratio of cis- and trans-units in polymer samples are described in our previous works [12, 13].
Preparation of PTMSP and PTMGP Membranes
Dense isotropic membranes were prepared for all polymer samples at the same conditions: controlled solvent evaporation for 72 h at ambient conditions, mechanical membrane removal from PTFE-surface, and overnight conditioning in vacuum at 30°C using an oil-free membrane pump. The polymer solution was prepared in cyclohexane for PTMGP and in toluene in case of PTMSP at room temperature (3 wt. % of polymer) by stirring for 24 h. Thickness of membranes was 30—50 ^m.
Table 1. Polymerization conditions and characteristics of PTMSP and PTMGP samples
Sample Catalytic system Solvent Yield, wt
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