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THE SELF-DIFFUSION OF WATER AND FEATURES OF STRUCTURE OF ION-EXCHANGING POLYAMIDE MEMBRANES ACCORDING TO EPR AND NMR WITH A PULSE GRADIENT OF A MAGNETIC FIELD

Авторы:
Город:
Воронеж
ВУЗ:
Дата:
11 марта 2016г.

Abstract

The peculiarities of structure of new ion-exchanging polyamide acid membranes, received by thermal processing of a solution of a copolymer 1,2,4,5-benzoltetracarbonic acid with 4,4'-diaminediphenyloxide in dimethylformamide were researched. By means of methods of a nuclear magnetic resonance (NMR) with a pulse gradient of a magnetic field and electronic spin resonance (ESR) structural characteristics of transport channels in gel phase of membranes are determined.


Keywords: Membrane; Synthesis; Polyamide acid; Imidization; Ion-exchanging; Structure; NMR-spectroscopy; Transport channels; Self-diffusion coefficients; ESR-spectroscopy.

1. Introduction

The receiving of new membrane materials, possessing high selectivity, thermo stability, mechanical durability, is a necessary condition for wide use of membrane processes of division in the solution of urgent technological and ecological problems. In this connection the big interest represents polyamide membranes, received by the thermal processing of aromatic polyamide acids (PAA) [1, 3]. The information on structural peculiarities as to a surface, and a volumetric phase of membranes is necessary for revealing mechanisms of functioning of the given membranes in various processes of division. With this purpose the whole complex of physical and chemical methods of research has been involved: sorption, NMR-, EPR-spectroscopy, reference porometry, scanning probe microscopy [2-8].

2. Experimental Part

2.1.   The thermal synthesis polyamide acid membranes

The polyamide acid membranes were synthesized by two-leveled thermal processing of 12-20% solution of polyamide acids – an aromatic copolymer 1,2,4,5- benzoltetracarbonic acid with 4,4'- diaminediphenyloxide in dimethylformamide (DMPА), produced on NPO "Plastic", Moscow. Molecularly-mass distribution of a copolymer was within the limits of 15-55 ths. c.u. The weight of an elementary link made up 418 c.u., and the contents of carboxyl groups

4.18 mmol/g of dry substance. The substrate with the solution of PAA was located in an autoclave, where as a result of thermal processing, was exposed to imidization. At the first step at the temperature of 353K within one hour the gradual removing of a great mass of solvent took place. Then the temperature in the autoclave was increased to the set one on the second step, at which within 30 minutes the process of elimination of waters with formation of imid links, and also removing of the rests of solvent and formed water from a product of thermal processing took place. The temperature of the second step of synthesis changed within the limits of 383-573K. After the consummation of the process the formed polymers together with the substrate were cooled, and then processed by the distilled water with the purpose of exfoliation from a substrate, washed and dried up on air. As a result came out thin pellicles with a smooth surface. 2.2. Method of NMR with a pulse gradient of a magnetic field

In the study of membranes by NMR pulse magnetic field gradient membrane samples were placed in weighing bottles with distilled water and incubated for 48 hours. To measure the NMR spectrometer swollen membranes after removing the external water were placed in standard vials, which were immediately sealed. To measure self-diffusion coefficients of water molecules was used the pulse sequence "stimulated echo" – 90(gδ)τ-90-δτ1-90-(gδ)τ-echo, where g – amplitude, δ – duration of pulses of magnetic field gradient. Frequency of proton NMR was 100MHz. The values of self- diffusion coefficients Dsi and the relative share of the diffusant pi in the membrane were determined from the analysis of the dependency of the spin-echo signal amplitude on g2 (diffusion decay), which was approximated by the following equation:

T1, T2 – the times of the nuclear spin-lattice and spin-spin relaxation, g – gyromagnetic ratio of the resonating nuclei,

τ – the time interval between the first and second RF pulses, τ1 – the time interval between the second and the third ones,

td = D-d/3 – the time of diffusion, where D – the interval between the pulses of the magnetic field gradient, N –

number of phases, if the membrane is not uniform [9].

2.3. Method of ESR

ESR-spectra were registered on spectrometer ER-420 Endor of firm Bruker  at temperature 293K [7]. PAA membranes have been studied in the copper-sour form, received by partial washing out of ions of copper from membranes by 0.1M solution of a hydrochloric acid.

1. Results and discussion

The small molecules are entered into a phase of a membrane as diffusant, are an original molecular probe by means of which it is possible to receive the information on structure of transport channels in a membrane [5-7].

For samples of membranes in Na+- form, with factor of a moisture capacity 5-6 molecules of water falling an average molecular part of polymer, the dependence of amplitude of a signal of a spin echo on size of a square of a gradient of a magnetic field (diffusion decay) is two-exponential (Figure 1). This fact proves the presence of transport channels of two various types in structure of membranes, time of an exchange of molecules between which is much more than time of diffusion td, sold in experiment. Thus, mobility of molecules of water can be characterized by two values of self-diffusion coefficients Ds1 and Ds2. At 20°С self-diffusion coefficients Ds1 and Ds2, and also relative shares of water in channels 1and 2 are accordingly equal 9.1·10-10m2/s and 3.5·10-11m2/s, 0.54 and 0.46. As value of coefficient Ds1 is close to similar size for free water at the same temperature (Ds=2.3·10-9m2/s), it is possible to assume, that the channel 1 represents large inter gel sites in a phase of polymer (defective areas). The size of coefficient Ds2 is comparable to similar values in ionogenic transport channels of membranes on the basis of sulpho-containing aromatic polyamides [5]. Therefore with the larger probability it can be confirmed, that the transport channel 2 represents areas with the increased local concentration of charged functional groups (carboxylate anions СОО¯) and connected with them hydrated contranions .



The existence of ionogenic transport channels in a phase of a membrane proves by the data received by method of ESR of ions Cu2+, on samples of PAA membranes in the mixed copper-sour form. The analysis of the form of spectrum ESR demonstrates existence in a phase of polymer of areas with the increased local concentration of carboxylic groups and contra-ions of copper. The structure, formed by functional groups, contra-ions and molecules of water microclusters (complexes) corresponds to the formula [Cu(H2O)4](COO)2. Microclusters will organize ionogenic spending channels for molecules of water, which mobility is follows from the given above data of NMR, it is essential lower, than in free water. At a room temperature the rotation of complexes [Cu(H2O)4](COO)2 is complicated, hence, the sizes of ionogenic channels are comparable to the sizes of tetraaquacomplexes of copper (II) and make the size about 1 nm. The temperature dependences of average self-diffusion coefficients submit to the Arrhenius law. The average values of the factors of self- diffusion of water in PAA-membranes depend on temperature of their reception, and consequently, from their structure. For membranes received at temperatures 383, 403 and 423К, sizes of energy of activation of self-diffusion of water have made accordingly 25.3; 23.3; 19.0kJ/mol. These sizes well correlate with data for sulpho-containing aromatic polyamides, where carry of molecules of water at low moister containing (factor of a moisture capacity of 5-6 molecules of water on an elementary part of polymer) is carried out by overjump of molecules from one center of hydration to another [6,7]. Apparently, the similar situation is realized and in the PAA-membranes. The centers of hydration here are ionogenic groups and contra-ions. The diffusive moving of molecules of water is limited in the ionogenic transport channels.

 

List of references

1.     Дьяконова О.В., Соколова С.А., Зяблов А.Н., Жиброва Ю.А. Особенности структуры полиамидокислотных мембран в зависимости от температуры синтеза. Сорбционные и хроматографические процессы. 2007. Т. 7.№ 5. С. 873-877.

2.     Перегончая О.В., Котов В.В., Соколова С.А., Котова Д.Л., Кузнецова И.В. Состояние воды в ионообменных мембранах, сорбировавших полиэлектролиты. Журнал физической химии. 2004. Т. 78. № 7. С. 1289.

3.     Соколова С.А., Дьяконова О.В., Зяблов А.Н. Особенности структуры ионообменных полиамидокислотных мембран, синтезированных при различных температурах. Сорбционные и хроматографические процессы. 2009. Т. 9. № 6. С. 893-903.

4.     Y.M. Volfkovich, V.K. Luzhin, A.N. Vanyulin. Application of the Reference Porometry for the Study of the porous Structure of ion-exchange Membrane. Russ. J. Elektrohimiya, Vol. 29, No. 5, 1993, pp. 656-664.

5.     V.I. Volkov, S.A. Korotchkova, I.A. Nesterov, J.E. Kirsch, S.F. Timashev, The Particularities of water State and Mobility in sulfocontaining aromatic polyamide Membranes on the pulse NMR data. Russ. J. Phys. Chem., Vol. 68, No. 7, 1994, pp. 1310-1316.

6.     V.I. Volkov, S.A. Korotchkova, H. Ohya, Q. Guo, Self-diffusion of water-ethanol Mixtures in polyacrylic asid- polysulfone composite Membranes obtained by pulsed-field gradient nuclear magnetic Resonance Spectroscopy. Journal of Membrane Science, Vol. 100, 1995, pp. 273-286.

7.     V.I. Volkov, V. D. Skirda, E. N. Vasina, S.A. Korotchkova, H. Ohya, K. Soontarapa. Self-diffusion of water-ethanol Mixtures in chitosan Membranes obtained by pulsed-field gradient nuclear magnetic Resonance technique. Journal of Membrane Science, Vol. 138, 1998, pp. 221-225.

8.     Volkov, S.F. Timashev. Magnetic Resonance Methods in the Investigation of Perfluorinated Ion-exchange Membrane. Russ. J. Phys. Chem., Vol. 63, No. 1, 1989, pp. 108-116.

9.     O.V. Dyakonova, S.A. Sokolova, V.V. Kotov, V.I. Volkov. The Structure and electrochemical Properties of cation- exchange Membranes based on partially imidized polyamide acid. Russ. J. Elektrohimiya, Vol. 38, No. 8, 2002, pp. 994-997.

10. J.E. Kirsch, I.M. Malkina, Y.A. Fedotov, S.F. Timashev, Selective Transfer of monovalent and divalent Cations in the sulfonate-containing aromatic polyamides Membranes. Russ. J. Phys. Chem., Vol. 67, No. 11, 1993, pp. 2312- 2314.

11. A.I. Maklakov, V.D. Skirda, N.F. Fatkullin. Self-diffusion in Polymer Solutions and Melts. Kazan, Kazan State University Press, 1987, p. 224.