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Solubility of electrolytes in polymers
0032-3950/79/1201-3087507.50[0
Polymer Science U.S.S.R. Vol. 21, pp. 3087-3102. © Pergamon Press Ltd. 1980. Printed in Poland
SOLUBILITY OF ELECTROLYTES IN POLYMERS* A. L.
IORDAI~'SKII,
A. L. SHTERENZOI~ and G. YE. ZAIKOV
Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 30 August 1978) A general analysis was made of the solubility of electrolytes in non-ion exchange polymers of various kinds (hydrophobie, hydrophilic and those slightly swelling in water). The distribution coefficient of the electrolyte between the polymer and a dilute solution is determined for hydrophobic polymers by the interaction of neutral molecules in the polymer and for slightly swelling polymers--by chemical and electrostatic interaction. Chemical and osmotic interactions in the polymer matrix am predominant for strongly swelling hydrophilic polymers. It was shown how existing theories describe the effect of specific adsorption, Donnan and electrostatic potentials on solubility and the role of the type and molecular weight of the polymer was noted. INTRODUCTION
The quantitative description of diffusion and solubility of electrolytes in polymers is a necessary condition for forecasting the chemical stability of polymers and operational parameters of anti-corrosive polymer coatings. For the successful use of medicinal polymers as implants and elements of artificial organs information is also required about absorption parameters of polymers to be implanted, which are in contact with the physiological medium. A s t u d y is made in this paper of the equilibrium distribution of electrolyte between the polymer and a dilute solution. Most attention is given to acid distribution since the acid-basic condition of the medium determines the intensity of protolytic processes in the polymer [1]. As far as possible, we omitted the description of electrolyte adsorption b y ion exchange resins, since this problem has been examined on a high level in a number of monographs comparatively recently published [2-4]. We also omit the kinetic aspect of adsorption: a description of diffusion of electrolytes in non-ion exchange polymers has been published recently [5, 6]; these studies also deal with methodical problems. There is no single theory available at the present time dealing with the ratio between the amount of electrolyte adsorbed and physical and chemical properties of the polymer. Adsorption has been described within the framework of the classification proposed [5], which is convenient, but somewhat conventional, and groups the polymers into hydrophilic, hydrophobic and intermediate ones. I f types o f polymer of opposite hydrophobic properties markedly differ in transfer mechanisms even with the same water content (for hydrophilic ones these will be the initial times of adsorption), the intermediate type shows very different adsorp* Vysokomol. soyed. A21: No. 12, 2797-2809, 1979. 3087
~088
A.L.
IORDANSKIt et al.
tion properties approaching, according to water content, to hydrophilic or hydrophobic type. Special features of the polymer-electrolyte systems studied, compared with systems such as, for example, polymer-solvent systems, are as follows: 1) existence of charged particles (ions) and consequently, formation of electric potentials; 2) adsorption by the polymer of a specific solvent such as water. Aa a consequence of the interaction with hydrophilie groups of the polymer at significant osmotic pressures the latter may markedly change the structure of the polymer forming clusters with low water concentrations, more or less elongated sections with a high dielectric constant with moderate concentrations and then, continuous polymer regions filled with water, for example in gels (quasi-homogeneous model). In strongly swelling polymers water functions as a screening factor in the interaction of the ion and the polymer. Dealing with the role of water during adsorption is beyond the scope of this paper; readers who are interested in the mechanism of water absorption in polymers may be recommended separate studies [7-9]. A Table showing results concerning water content of polymers at a temperature of 25 ° and different relative moisture contents [10] may be used for practical purposes. GENERALIZED FORMULATION OF SOLUBILITY
T h e r m o d y n a m i c aspect. T h e role of the electrostatic factor. The solubility o f electrolytes in polymers is described from a thermodynamic point of view as the distribution of ions between a dilute solution of the electrolyte and the polymer. I n a state of equilibrium the following equality holds good / O + R T In as -4-P V~Jr-z~F~,= -o - ~ p- -V~Jr z~F~] - , I~ ~ R T 1n as
(1)
where/~ is the standard chemical potential of an i ion with charge z,; V , - - t h e partial molecular volume of the ion; p, y--pressure and electric potential, respectively [4]. Parameters without a line above the latter correspond to the solution and those with a line, to the polymer phase. In order to simplify the study, it is assumed that V, is constant in both phases and ionic concentrations c, arc used instead of activities as. From eqn. (1)a formula follows for the distribution coefficient of the electrolyre, Kiwi
where u is the osmotic pressure; A/l~A--the difference of standard chemical potentials; d~--difference of electric potentials in the polymer and in solution; cxA--the concentration of the electrolyte. Let us examine first in more detail the first item in the right hand side o f eqn. (2) since this is related to specific sorption effects of electrolytes in polymers~
Solubility of electrolytes in polymers
308~
In the general case the potential variation in the system of electrolyte solutior~ 1-polymer-electrolyte solution 2 without external field may be presented as the total of interphase potential differences at the polymer=solution interface ~'1, ~'2 and of the membrane potential itself ~m
Z~=~l+~m+~'~
(3)
With symmetrical boundary conditions the membrane potential fully determines Aq]. According to the classical Nernst theory ~tm is due to the differences in t h e solubility of positive and negative ions in each of the contacting phases. From conditions of equality of electrochemical potentials and electric neutrality for the polymer phase
RT
~m= ~
K+
In K_
(4)
where K+, K - are the distribution coefficients of the cation and anion, respectively. For example, for a tetramethyl ammonium ion with considerable affinity with t h e nonpolar phase, compared with the C1- ion, K + > K_ and therefore, ~gm>0. The distribution coefficient of the ion in first approximation may be calculated bythe Born equation [11, 12]. The value of K~ ~ 10 -2° gives an estimate for an ion of radius 2 A, distribute4 between the aqueous phase ( e : 80) and a nonpolar polymer (e----3) at 25% However, as indicated [11], this approach ignores the chemical nature of the ion. In practice, it is more convenient to use KxA
KxA:~/K-+K-
(5)
Using eqns. (4) and (5) it was shown [13] that for electrolytes with the same ior~ (KI, NaI) the following formula holds good:
RT
K~aI
~u~= - ~ - l n KKI
(6)
Equation (6) was verified experimentally [13] and it was shown that, for example, for K + and N(C2H5) + ions the value of ~Um----124mV, while for I - and C1- the value of ~ m - ~ - 95 inV. For a number of polymers there may be a case when ions 40 not penetrate into the polymer in practice and ~m=-0. However, there may be a potential difference at the phase interface produced by the formation of a double electric layer. Nonpolar polymers such as polyethylene and fluoroplastic may acquire a surface charge in the solutions as a result of the formation of oxide groups, or eonversely~ by the primary adsorption of ions from solution [14], which affects ~t1 and ~ . . Cations differ in their ability to displace counter ions of the surface layer in a corresponding lyotropic series AlS*>Ba~÷> Ca2+> Mg~*>K+>NH+ > l ~ a + > Li ÷
(7~
:~090
A. L. IORDANSKII et al.
'This series may be explained using Coulomb's law. Counter ions are retained on Che external surface by a force which is proportional to the charge and inversely ::proportional to the radius of hydrated ions [14]. The H + ion does not occupy a certain position in the lyotropic series. The :point is that in polyamide type polymers acid groups undergo dissociation with low constants, while salts of these acids are dissociated considerably. However, in .ion exchange resins of strong acid type (R--SOaH) the H+ ion does not have "high bond energy and is situated at the end of the series among monovalent ca:tions. A similar pattern has also been described for OH- anions [14]. For hydrophilic polymers with dissociating groups of the zQ charge between t h e solution and the polymer the Donnan potential is established
RT
(
zQ
In _ -[- - + 1 ~D = F \ 2tEA [_4C~h
(8)
:For example, for sulphonated phenol-formaldehyde resin Q--0.3 mole/l, CNAcl----102 ~nole/1. and ~YD=--87 inV. There are two limiting cases: low fixed charge zQ <<.CKA ~D----ln[
cxA
~
z___Q_Q
(8a)
a n d zQ ~CKA
Three types of error are possible when determining the Donnan potential ~ince: a) calculation is carried out via activity and in the polymer it is complicated • o determine the activity of the electrolyte; b) it is difficult to estimate the diffu"~ion potential on the boundary of polymer phase-measuring electrode; c) the Condition of quasi-homogeneous charge distribution may not hold good in the :polymer, particularly for dilute solutions, when the error in determining VD r e a c h e s 40% [15]. Kinetic aspect. The equilibrium state is characterized by the eqnality of transfer velocities of the electrolyte through the polymer-solution boundary in two ,opposite directions
I,=klc,--k2~=O ,
(9)
..~ence
KI
-
c~
,
(lO)
vchere k 1 and k2 are the rate constants of electrolyte transfer from the solution ~o the polymer and in the opposite direction, respectively [16].
Solubility of electrolytes in polymers
3091
In diffusion measurements parameter KKa is determined from kinetic results, according to the equation P0
KKA: D '
(11)
where P0 is permeability; D--the diffusion coefficient of the electrolyte. For the system consisting of poly-2-hydroxyethylmethacrylate-dilute KC1 solution K ~ values determined by a standard method and dynamically (using eqn. (11)) were the same [17]. The majority of diffusion experiments suggests that the equilibrium concentration in the surface layer of the polymer is established instantaneously and has no effect on the rate of diffusion. Special cases, when the rate of establishing equilibri~un surface concentration is comparable with the rate of diffusion, have been examined previously [18, 19]. When comparing kinetic and equilibrium measurements it should be considered that it is not the entire electrolyte absorbed by the polymer c~ which may take part in the transfer process, but only part of it co, the remaining electrolyte may not take part in transfer as a consequence of interaction between components of the diffusion system. The true diffusion coefficient D O is related in this case with the effective value, Deft determined experimentally from the equation
D°~-Def~ dc~'
(12)
where c~ is the overall concentration of the diffusate dissolved in the polymer; c0--the concentration of the "free" diffusate, which is normally determined by desorption. Experimental results which are in agreement with the equation of diffusion (12) are described in previous studies [20-22] for hydrophobie (PE), moderately ~welling (keratin) and hydrophilic (PVA) polymers, respectively. Let us examine in particular the absorption of electrolytes in various polymers. HYDROPHOBIC POLYMERS
Information about electrolyte concentration in hydrophobic polymers is scanty and not systematic. The weight variation of samples was determined in m a n y studies while being used in electrolyte solutions. These results are of known interest only for the evaluation of the corrosion resistance of materials. They are really useless, however, from the point of view of determining the concentration of the electrolyte in the polymer since the weight increase of the sample may be due both to the absorption of the electrolyte and the absorption of water and it is impossible in many, cases to predict their ratio in the polymer. Components absorbed have been determined separately in a number of studies. It was established [23, 24] that on keeping polypropylene in solutions of non-volatile electrolytes the weight variation of samples is completely due to water absorption and is
3092
A . L . IO~DANSKn e t a l .
clearly determined by steam pressure over the electrolyte solution, independent of the type of this solution. According to results previously described [25], the coefficient of solubility of HC1 in PVC determined as the ratio of HC1 concentrations in the polymer and in gas, is 5±0.5 at 20 ° . Nitric acid during absorption from solutions with a concentration close to 100% is absorbed by polymers in significant proportions (1-10~/o) [26-28]. If we assume that nitric acid absorption conforms to Henry's law, the order of absorption constants may be evaluated from these results (10-4-10 -S g/g tort). Swelling of fluorine containing rubbers in concentrated nitric acid reaches 17-19~/o [23]. There are no results available concerning the absorption of dilute nitric acid. solutions. Absorption by low density polyethylene of hydrogen chloride from concentrated solutions of hydrochloric acid was determined previously [20]. It is 0.01-0.05~/o, which corresponds to a constant of absorption of 10 -a g/g tort. Constants o f nitric acid absorption therefore exceed by two and higher orders of magnitude the constant of absorption of hydrochloric acid. An attempt was made [30] to evaluate electrolyte absorption by low density polyethylene by the Flory equation in a simphfied form --ln fp2= 1 +
RT
'
(13)
where ~2 is the volume fraction of the material dissolved; lY2--the molecular volume; 5~ and gl--solubilityparameters of the dissolved material and the polymer, respectively. According to the calculation made by eqn. (13) the solubility of nitric acid in polyethylene is 3.7%, whereas the value found experimentally is 3.1% [26]. T h e calculated solubility of sulphuric acid is 4 x 10-S~o . The low permeability of polyethylene in relation to sulphuric acid is, apparently, related precisely to the low solubility of sulphuric acid in polyethylene. Equation (13) was also used for the calculation of the solubility of sulphuric acid in Ftorlon-26* [31]. Experimental values of (z2 were close to calculated values (1.6 x 10 -a and 1.2 × 10 -a, respectively). The dependence of electrolyte concentration in the polymer on concentration in an external solutio~ was only studied in one paper [20]. HC1 concentration in the polymer increases very rapidly with an increase in the concentration o f hydrochloric acid. In contrast with this, the dependence of acid concentration in the polymer on vapour pressure over the acid is close to the linear (in the range of increased HC1 pressure). During absorption from the dry gaseous phase Henry'a law applies. Theoretical calculation of Henry's constant for a HCl-polyethylene system carried out by methods previously described [32] gave a value of 1.29 × * F t o r l o n - - a fluorine containing copolymer fibre.
Solubility of electrolytes in polymers
3093
× 10 -e g/g torr, which is close to the value found experimentally, i.e. 1.30-t-0.07 g/g torr. During absorption from hydrochloric acid HCI concentration in the polymer is higher than during absorption from the dry gaseous phase under the same partial tiC1 pressures in the external medium. This fact is explained assuming that part of HC1 molecules is combined in the polymer containing water molecules to form hydrates and in order to establish equilibrium, a further amount of HCI is transferred from the acid solution in the polymer. Equation (14) which is based on the assumption indicated, gives a satisfactory description of experimental results; the number of water molecules in the hydrate was near to five
•
//CHCI
log ~ -
-- 1
) =log(MKa~,o)-~nlogh~i,o, n
(14)
6HC1 0
where cftcl is the overall concentration of HC1 in the polymer; C~Cl, the concentration of "free" HCI in the polymer (uncombined in hydrates); M - - t h e molecular weight of HC1; K--constant of equilibrium of hydrate formation; all,o--constant of water absorption by the polymer; n--number of water molecules in the hydrate; h~,o--water vapour pressure over the acid solution. It slmuld be noted t h a t views concerning hydrate formation in the organic phase were confirmed experimentally in a number of studies dealing with extraction (see e.g. former paper [33]). There is no systematic information available concerning the effect of properties of polymers on electrolyte absorption. Results obtained by various authors are sometimes conflicting. It was shown [34] that a quenched Ftorlon-3 film absorbs up to 1.47% nitric acid, while a slowly cooled film (i.e. one with higher crystallinity) only absorbs 0.62%, however, it is indicated [28] that absorption of nitric acid increases with an increase in crystallinity. According to a former paper [28], quenched films of Ftorlon-42 absorb considerably more nitric acid than films subjected to slow cooling. The dependence on composition of nitric acid absorption by copolymers of fluorovinylidene with trifluorochloroethylene and hexafluoropropylene is described by a curve with a maximum, which corresponds to the minimum on the curve showing the dependence of tensile strength on composition. The authors explain the experimental nature of curves by amor10hization of copolymers containing 15-20 to 80-85 reel. % fluorovinylidene [27]. For the rough evaluation of the effect of the chemical composition of the amorphous polymer on electrolyte absorption eqn. (13) may, apparently, be used. The difference in permeability in relation to H~SO4 of polymers such as polyolefins (g~ 7.9) and "Pentaplasts" (J~ 9.9) can be readily explained by this equation. According to eqn. (13) it may be expected that the absorption of sulphurie acid by "Pentaplast" is considerably higher than by polyethylene (almost 40 times), it is therefore not surprising that the penetration of HzSO4 into polyolefins is very difficult to detect, whereas in respect of "Pentaplasts" this fac~ has been repeatedly established (see, e.g. previous study [35]).
A. L. IORDANSE~I ~ al.
3094
There are very few results available concerning the temperature dependenc~ of electrolyte absorption by hydrophobie polymers. The AH value given [36~ for the solution of 98% nitric acid in polyethylene, 22.6 kcal/mole, is very high, which may be due to a chemical reaction. The heat of HC1 absorption by polyethylene [30] has been calculated from an equation, which ~elates the heat of gas absorption by polyethylene with the Leonard-Jones potential for gas [32]. This value was 2.3 kcal/mole. The heat of solution of HC1 in PVC is 10± 1 kcal/mole
[2#]. INTERMEDIATE CLASS OF SLIGHTLY SWELLING ,POLYMERS
Slightly swelling polymers absorb electrolytes in accordance with eqn. (2). A special feature of polymers of this class is the fact that most of them contain ionogen functional groups (for example--NH2, - - C 0 0 H in polyamide). The presence of these groups complicates the description of absorption, compared with hydrophobic polymers as a consequence of the need for taking into account electrostatic and specific interactions of ions and macro-ions. Let us examine the best known theories of electrolyte absorption by polymers. T~BLE l. S P E C I F I C E N E R G Y OF ADSOI~PTION AND POLARIZATION ~ A N D VAN D E R W A A L S W COMPOI~7"ENTS :FOR VARIOUS SYSTEMS
System
C1--wool H +-wool Anion of orange I I dye-wool Anion of methyl yellow I I - w o o l
Cl--nylon H+-nylon Cu++-bull serum albumin K + - p h o s p h a t e colloid
Li+-
,,
,,
Ag+-
,,
,,
Overall specific energy 0 10.2 • 7.3
8.9 0 16.7 10.0 1.8 2.6 3.6 4.8 5.5
W /k T
0 0 7.0 8.6 0 0 0 0 0 0 0 0
~/kT
0 10.2 0.3 0.3 0 16.7 10.0 1.8 2-6 3"6 4-8 5.5
According to the Gilbert-Ridcal theory [37] it is assumed that ionic adsorption takes place on a few statistically distributed groups of polymer in conformity with the equation ( 0 ) (2+W)--e~, log ~ =logc+ 2.3kT '
(15),
where 2 and W are the energies of polarization and van der Waals interaction, respectively; O - - t h e proportion of free places, where adsorption may take place.
Solubility of electrolytes in polymers
3095
Equating 0 values for two oppositely charged ions and excluding the electrostatic term, the energies of adsorption were calculated for ions indicated i~ Table 1. In studies by Wall et al. [38, 39] titration curves of nylon-66 were published[ for the first time using NaOH and HC1. The interpretation of results was based on the Gilbert-Rideal theory, but a correction was introduced in eqn. (15) in respect of the inequality of the carboxyl and amino end groups in nylon-66. An ion exchange model [40] was proposed by Myagkov and Pakshver t o explain the adsorption of HC1, KOH, naphthalene-2-sulphonic acid, dyes containing nylon-6 and Kapron. This model suggests that log [x (Xo--X)-1]: l o g Kin-- 2pH,
(16)~
where x o is the overall number of places of adsorption; x--number of combined places; Kin--adsorption constant of equilibrium. The models mentioned show limited agreement with experimental results, which gave reason for developing the Matheson-Wkewell [41] polyelectrolyte model. Authors of this model assume that the energy of proton removal from t h e polymer into solution depends on electrostatic and osmotic interactions and chemical affinity (difference between standard chemical potentials of H+ in thepolymer and in solution). For a partially dissociated group in the polymer a n equation is proposed which is similar to eqn. (2). In contrast to strongly swelling gels, this theory enables the osmotic term to be ignored in eqn. (2) [41, 42] pH----pKo--log[(1 --a)a -1] --0.434/RT (F~+A]~°),
(17}~
where K0 is the constant of dissociation of the group in the polymer; a--degree. of dissociation. Calculation of the electrostatic term for cases of homogeneous potential in the polymer results in the f~rmula
~F/RT = sin h -1 (c//2zcs),
( 18)
where c! is the charge concentration in the polymer, cs--the molecular concentration of the electrolyte in solution; z--ionic valency. Irregular charge distribution. Interaction (association) may take place betweer~ fixed groups in the polymer, for example in wool, which distorts potential ~ and its distribution. There are two methods for taking into account this effect [41]_ 1. For a pair of combined groups of opposite charge local potential ~s will exceed ~ (the average potential examined) by a value of ~s--~. This measure i s also used in another study [43] for the description of ionic combination in polyelectrolyte solutions and gels. It has been indicated [44] that local charge density has a marked effect on the behaviour of dissociating carboxyl groups in copolymers: of maleic acid with vinylpyrrolidone and vinyl acetate. Coefficient fl--the proportion of charged groups included in ionic pairs (--0-0434 F~sfl/RT) should i~. this case be formally introduced in eqn. (17). It is difficult to calculate this tern~ since fl varies with the degree of dissociation a, however, if the value of fl is known,
~096
A.L. TORDANSKII6t a~.
the entire term may be calculated knowing the distances between ions, for example, in pairs of - - C O O - . . . H a I ~ + - and --COO-...H+ before recombination. 2. It is easier to examine carboxyl groups (free and connected) as two differe n t l y dissociating groups with dissociation constants K~ and Kq, respeetively. The concept of homogeneous ~ is retained in this case. I f p and q are the de~ e e s of dissociation of these groups, eqn. (17) takes the form'
(0.434/RT) (F~,÷A/~) pH----pgq--log[(1--q)q-l]-- (0.434/RT) (F~,+A/~)
pH----pK~--log[(1--p)p-1]--
(19)
Here p K 0 varies from pK~ to pKq as a function of ~. Effect of molecular weight. The molecular weight of polymers dissociating only a t the end groups influences solubility. The concentration of groups undergoing titration is inversely proportional to the degree of polymerization, the same way as c! (concentration of fixed charges) provided that the accessibility of groups to acids and bases does not vary with the degree of polymerization. Pao rameters pKo and Zl~Hare practically independent of the degree of polymerization therefore, combinig eqns. (17) and (18) gives pH~-const--0.434 sin
h-~(cf/2ca)
(20)
for various degrees of polymerization with the same degree of dissociation. When cf/2ca~200 and higher, i.e. when the concentration of the salt added is low sin h-~(cf/2ca)-~ln (cf/c~) (21) ~nd
pH----const--log (cf/cs)
(22)
e, being constant, eqn. (22) may be presented graphically in co-ordinates p H log cl. A former study [41] demonstrates satisfactory agreement between theory ~nd experiment. After the electrostatic potential has been calculated, theory enables chemical ~ffmity between various acids and the polymer to be established. Table 2 shows ~ values for an acid-wool system [41]. Titration curves without any specific interaction enable the p K value of acid groups of the polymer to be established. For example, for keratin the double pH value of a middle point on the titration curve is equal to the p K value of glutamic acid (this acid imitates the elementary unit of the polymer) [44]. However, the ~qu~lity mentioned is not valid for adipic acid--a model of the elementary unit o f nylon-66, even if a correction is made in respect of the shift of the isoelectric point of nylon-66 as a consequence of the inequality of the number of acid and basic groups in the polymer [45]. The most likely cause of deviation, in the ~nthors' opinion, is a reduction of the dielectric constant e in nylon-66, compared with' the keratin matrix; in the latter e dies not differ markedly from the 8 value f o r water (~ 80).
Solubility of electrolytes in polymers
3097
HYDROPHOBIC POLYMERS
Hydrophilie polymers containing uncombined water (e.g. hydrogels of poly~ oxyethylmethacrylamide type, PVA) absorb electrolytes according to the equation KKA--~O(OH,O,
(23)
derived using the theory of free volume [46] (qH,o--volume fraction of water in the polymer and a0, coefficient characterizing the interaction of electrolyte with the polymer in the absence of which a0----1). Graphical expression of eqn. (23) is given in a previous study [44]. I f the polymer matrix is a micro-heterogeneous system (porous model), t h e distribution coefficient m a y be expressed as KxA = Kwq~,o + K~(1-- q~,o ) ,
(24)
where qm,o is the volume fraction of water; Kw and K~--the individual coefficients of distribution [17] in water and in the polymer, respectively. The physical significance of the distribution coefficient in a neutral gel m a y be explained b y the following equation system [47]
K~--
~K~ _ y± e-~V,,/R~
(25)
RT ----- _ in p
(26)
6ILk
y+
V~.o Here ~ is the osmotic pressure; y ± - - a c t i v i t y coefficient; Vxx and I7~,o, molecular volumes of the electrolyte and water, respectively; p - - w a t e r vapour pressure; All parameters with a line correspond to the polymer phase. TABLE
2. CHEI~IICAZ A F F I N I T Y VALUES Z]//0 B E T W E E N VARIOUS
ACIDS
AND
I,ATED 1~1~O~ I~ESULTS P R E V I O U S L Y G I V E ~ ( [ 3 5 ] A ~ D
[39])
Acid HC1 Ethylsulphuric HBr HiSO~ Benzenesulphonic Nitric p-Toluenesulphonic o-Xylenesulphonic Trichloroacetic o-Nitrobenzenesulphonic 2,5-Dichlorobenzenesulphonie 4-Nitrobenzenesulphonic 2,4-Dinitrobenzenesulphonic Naphtalene -p-sulphonic
Ap°/RT O 0.13 0.42 0.57 0-65 0.66 0"75 0"86 0'97 1"12 1"65 1"65
1"72 1"96
WOOL,
CALCU-.
A~° [39] I A~0 [35] cal/mole O 160 510 710 810
200
830
940 1070 1210 1400 2060 2040 2150 2450
1200 120Q 1450 2100
2250 2250
:$098
A.L. IO~DAt+SKIIet d.
Water pressure in the polymer phase cannot, usually, be measured; as supposed by Gregor, this value may be determined by measuring vapour pressure over the polymer solution having the same chemical structure as the gel, but without erosslinks [48]. The coefficient of electrolyte distribution between the gel and the external .solution therefore depends on the chemical structure of the polymer and the electrolyte and on osmotic pressure, which may reach several hundreds of atmospheres in view of the high value of RT/Vw~ 1400 arm at 25 °. These arguments are also valid for polyelectrolytes and ion exchange resins; however, in the presence of charged ionogen groups in the polymer the Donnan distribution of electrolyte has to be considered [3, 4]._ Let us examine the effect of the electrolyte on Kr~ between the polymer and a ~dilute solution. Relative hydrophobic 19roperties of the ior~ or a neutral molecule. The solubility +of organic electrolytes in highly swelling polymers decreases in the following ~sequenee of groups incorporated in the anion (or cation)structure of this electrolyte: CHs--> CHsCHa--> CH3CH+CH,--> CH3(CHI).--
Aromatic phenyl groups are approximately equivalent as regards hydrophobie ~properties in relation to the n-propyl group, while the naphthyl group, in relation to the n-hexyl group. Substitution of sulphur for oxygen in the molecule of the ,diffusing substance also reduces hydrophilie properties, while the a~ldition of .+structural groups, which may form hydrogen bonds with water, or the other hand, +increases the solubility of electrolyte in the polymer [47] --OH >,CO~H >--NITs ~---SOzH~---SO2NH2~--CO2CHs-+--CONH2~--OCHB Inverse sequences are observed for hydropkobic polymers and with an increase hu hydrophobic properties of the polymer matrix, an increasing part of the energy +of interaction, the value of which may be very significant for organic ions, will be short range interaction. For example, two zig-zag carbon chains in 20 carbon .atoms interact with each other with an energy of ~ 10 kcal [~0]. S~z+ of ion. A description was given [51] of the effect, from a theoretical point of view, of the size of ion on absorption, compared with the absorption of a s t a n d a r d ion 1/~ in size. The steric effect may, in principle, be offset by the hydrophobic interaction described, however, no detailed investigation has been carried out of this effect. Structural aspevt. A study was made [52] of the interaction of PVA with dilute +solutions of various salts. The polymer matrix is, in first approximation, identical with water. This interpretation suggests that large single charged ions (CNS-, Br-, I--) influence the structure of amorphous regions of PVA the way they affect -water structure. Penetrating the polymer matrix ions change the structure of the ~polymer, rupture and redistribute hydrogen bonds of macromolecules. On in-
Solubility of electrolytes in polymers
3099
creasing water content in the polymer, the behaviour of the system consisting of PVA and a dilute salt solution is mainly determined by structural changes of the water itself, occurring by the action of electrolyte ions dissolved in it and temperature, i.e. a considerable screening effect is achieved in the ion-polymer interaction. It should be noted that large ions disrupt water structure (K +, Rb +, Cs+, I~H +, I - and Br-), whereas small ones and a few multivalent ions regulate it (Li+, Na +, Ca2+ and Mg~+); ~ 1.6 A is a critical size of transition of the ion from on~ group into the other. A study of solubility in a system, consisting of a polymer-solvent-electrolyte~ is of interest. Tager et al. [53] have shown that the ability of the electrolyte t o dissolve the polymer is determined by the type of constitutional diagram of th~ system and with an increase in the concentration of the system may first increase and then reduce mutual solubility until the system undergoes micro-separatiom The presence in cellulose acetate of bi- and polyvalent cations of Ca 2+, Ba ~+ a n d La s+ type in the region of high salt concentrations coordinates groups corresponding to adjacent macromolecules and increases the viscosity of the system. Interaction between nncleophilic atoms of organic polar groups of the polymer and electrophilic cations of salt with high charge density (high valency, small ionic radius) is confirmed [54] by IR spectra of cellulose diacetate. A shift to the long wave region is observed in the system of absorption bands of --OH and --C----O groups on adding Mg(Cl04)2 to the polymer which, in the authors' opinion is the consequence of the redistribution of charge density on these groups in the presence of a Mg ~+ ion. Anions have an indirect role in this case: the weaker the polarizing action of the anion (large radius, small charge), the slighter its effect on the cation and this way it contributes to a more active interaction o f this cation with functional groups of the polymer. Electrolyte absorption in the polymer in a number of cases changes crystalline packing in the polymer. For example, absorption of trifluoroacetic acid by poly-L-valine and poly-L-lencine increases the proportion of a-spirals of macromolecules, as shown by X-ray [55, 56], while absorption by polycaprolactam (PC-4) of sulphuric acid initiates a--~ crystalline transition [57]. Diffusion of dilute acid solutions (HC1, H3P04) in PVA results in practically complete amorphization of partially crystalline initial samples [58]. However, there are no detailed descriptions in the literature of the correlation between the: solubility of electrolytes and the crystalline structure of polymers. The effect of ionogenic charged groups in a highly swelling polymer on KKA is: characterized by the Donnan distribution mentioned. However, as a consequence of the considerable distance between these groups (e.g. in 90% gel the averag~ distance between adjacent chains is 12-13 • [59]) Coulomb interactions are less significant than in slightly swelling polymers [60]. Gels with high charge density and low degrees of cross]inking may be described as highly concentrated electrolyte solutions [2]. On increasing the degree of cross]inking, ion exchange resins may be regarded as ionic crystals [61].
:3100
A.L.
IORDANSKII et a~.
The solubility of electrolytes in p o l y m e r s of various classes m a y be described b y a generalized eqn. (2). I n h y d r o p h o b i c polymers, where t h e electrolyte is in t h e nondissociate4 state, t h e distribution coefficient is d e t e r m i n e d b y short r a n g e i n t e r a c t i o n of t h e electrolyte in the polymer, which is confirmed b y the equilibr i u m a b s o r p t i o n o f HCI in P E f r o m t h e gaseous phase. T h e f o r m a t i o n in P E o f h y d r a t e d s t r u c t u r e s during t h e a b s o r p t i o n o f h y d r o c h l o r i c acid is also described in t h e absence of osmotic a n d electrostatic interactions. I n slightly swelling pol y m e r s t h e description of solubility requires consideration in eqn. (2) o f the e l e c t r o s t a t i c potential, while on t r a n s i t i o n to highly swelling p o l y m e r s it becomes n e c e s s a r y to b e a r in m i n d osmotic pressure. W h e n t h e p o l y m e r contains ionogenic groups excess ( c o m p a r e d w i t h osmotic) pressure is f o r m e d as a result o f t h e I ) o n n a n effect. The influence of this effect m a y be reduced considerably if measu r e m e n t s are carried o u t w i t h a n electrolyte c o n c e n t r a t i o n of >>zc~, where z is t h e effective charge o f one macromolecule a n d ca, the molecular c o n c e n t r a t i o n of the polymer. Translated by E. S~MERE REFERENCES
1. N. M. EMANUEL' and D. C. KNORRE, Kurs khimicheskoi kinetiki (Course on Chemical Kinetics). Izd. "Vysshaya shkola", 1969 2. (L HLADIK (Ed.), Fizika elektrolitov (Physics of Electrolytes). Izd. "Mir", 1978 3. Yu. A. KOKOTOV and V. A. PASECHNIK, Ravnovesiye i kinetika ionnogo obmena (Equilibrium and Kinetics of Ion Exchange). Izd. "Khimiya", 1970 4. N. LAKSHMINARAYANAIAH, Transport Phenomena in Membranes, New York, 1969 5. C. A. REITLINCER, Pronitsayemost' polimernykh materialov, Izd. "Khimiya", 1974 6. A. L. IORDANSKII, A. L. SHTERENZON, Yu. V. MOISEYEV and G. Ye. ZAIKOV, Uspekhi khimii 48: 1460, 1979 7. J. CRANK and (L PARK, Diffusion in Polymers, New York-London, 1968 8. J. Macromol. Soc., Physics 133: No. 4, 1969 9. M. M. MIKHAILOV, Vlagopronitsayemost' organicheskikh dielektrikov (Moisture Permeability of Organic Dielectrics). Goscnergoizdat, 1970 10. D. V. VAN-KREVELEN, Svoistva i khimicheskoye stroyeniye polimerov (Properties and Chemical Structure of Polymers). Izd. "Khimiya", 1976 11. V. S. MARKIN, Yu. A. CHIZMADZHEV, Indutsirovannyi ionnyi transport (Induced Ion Transport). Izd. "Nauka", 1974 12. M. BORN, Z. Phys. 1: 45, 1920 13. K. RANDLES, Trans. Faraday Soc. 49: 823, 1953 14. D. A. FRIDRIKHSBERC, Kurs kolloidnoi khimli (Course on Colloid Chemistry). Izd. "Khimiya", 1974 15. J. T. DAVIES and E. K. RIDEAL, Interracial Phenomena, Interscience, New York-London, 1961 16. F. H. JOHNSON, H. EYRING and M. J. POLISSAR, The Kinetic Basis of Molecular Biology, Ed. J. Wiley, New York-London, 1954 17. P. SPACEK and M. KUBIN, Collect. Czechosl. chem. commun. 16: 705, 1976 18. A. L. I O R D A N S K I I , L. N. BULATNIKOVA, O. N. BELYATSKAYA, Yu. V. MOISEYEV and G. Ye. ZAIKOV, Dokl. AN SSSR 218: 671, 1974 t9. M. V. SEFTON and E. W. MERRILL, J. Polymer Sci., Polymer Chem. Ed. 14: 1581, 1976
Solubility of electrolytes in polymers
310i
20. A. L. SHTERENZON, S. A. REIIrIJNGER and L. P. TOPINA, Vysokomol. soyed. A l 1 : 887, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 4, 1002, 1969) 21. J. A. MEDLEY, Trans. F a r a d a y Soc. 53: 1380, 1957 22. A. L. IORDANSKH~ V. S. MARKIN, Yu. V. MOISEYEV and G. Ye. ZAIKOV, Vysokomol, soyed. A14: 801, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 4, 892, 1972) 23. A . A . SHEVCHENKO, E. A. SKURATOVA and I. Ya. J~LIMOV, Fiz. k h i m i y a i m e k h a n i k a mater. 9: 106, 1973 24. E. A. SKURATOVA, K a n d i d a t s k a y a dissertatsiya (Post-Graduate Thesis). Moscow, MIKhM, 1969 25. R. A. P A P K 0 and V. S. PUDOV, Vysokomol. soyed. B18: 864, 1976 (Not translated in Polymer Sci. U.S.S.R.) 26. N. S. TEg~OMIROVA, K. I. ZERNOVA and V. I. KOTRELEV, Plast. massy, No. 12~ 40, 1962 27. N. S. GrLINSKAYA, S. A. REITLINGER, F. A. G~,LIL-0GLY a n d A. S. NOVIKOV~ Vysokomol. soyed. B I I : 215, 1969 (Not translated in Polymer Sci. U.S.S.R.) 28. D. D. CHEGODAYEV and N. A. BUGORKOVA, Zh. fiz. khimii 33: 269, 1959 29. N. S. GILINSKAYA, G. A. GUBAI, F. A. GALYL-OGLY and A. S. NOVIKOV, K a u e h u k i rezina, No. 12, 7, 1964 30. A. L. SHTERENZON, K a n d i d a t s k a y a dissertatsiya (Post-Graduate Thesis), Sverdlovsk~ Urals State Un., 1969 31. A. L. SHTERENZON, Diffuzionnyye yavleniya v polimerakh (Diffusion Effects in Polymers). Theses of Papers R e a d at the I I I All-Union Conf., p a r t 2, Riga, 1977 32. A. S. MICHAELD and H. J. BIXLER, J. Polymer Sci. 50: 393, 1961 33. E. KHEGFEL'D, Syrup. I o n Exchange, Mir, 1969 34. D. L. CHEGODAYEV, Z. K. NAUMOVA and Ts. S. DUNAYEVSKAYA, F t o r o p l a s t y (Floroplastics). Goskhimizdat, 1960 35. V. K. PAVLOVA, E. A. SKURATOVA, I. Ya. •TJMOV and I. K. YARTSEV, T e k h n i k a zashchity ot korrozii, No. 2, l l , 1973 36. N. S. TIKHOMIROVA and V. I. KOTRELEV, Syrup. Plasticheskiye massy (Plastics). Izd. " K h i m i y a " , 1970 37. G. A. GILBERT and E. K. RIDEAL, Proc. Roy. Soc. A182: 335, 1944 38. F. T. W A L L a n d T. S. SWOBODA, J. Phys. Chem. 56: 50, 1952 39. F. T. W A L L and P. M. SAXTON, J. Phys. Chem. 57: 370, 1953 40. V. A. MYAGKOV and A. B. PAKSHVER, Zh. prikl, khimii 79: 845, 1329, 1956 41. A. R. MATHESON, C. S. W H E W E L L and P. E. WILLIAMS, J. Appl. Polymer Sci. 8:: 2009, 2029, 1964 42. A. KATCHALSKY, S. LIFSON and H. EISENBERG, Prog. Biephys. Bioehem. 4: 1, 1954 43. F. E. HARRIS and S. A. RICE, J. Phys. Chem. 61: 1360, 1957 44. M. NAGASAWA and S. A, RICE, J. Amer. Chem. Soc. 82: 5070, i960 45. J. MARSHALL, J. Polymer Sci. 6, A - l : 1583, 1968 46. H. YASUDA, C. E. LAMAZE and L. D. IKENBERRY, Makromolek. Chem. 118: 19,. 1968 47. E. LAITFUT, Yavleniya perenosa v zhidkikh sistemakh (Transfer Effects in Liquid: Systems). Izd. "Mir", 1977 48. H. P. GREGOR, J. Amer. Chem. Soc. 73: 642, 1951 49. V. H. COHN, In: F u n d a m e n t a l s of Drug. Metabolism and Drug Disposition, E d . B. N. L a Du, G. Mandel and E. L. W a y , Baltimore, 1972 50. L. SALEM, Canad. J. Biochem. and Physiol. 40: 1287, 1962 51. D. A. KITCHENER (book). N o v y y e problemy sovremennoi elektrokhimii (New Problems~ in Modern Electrochemistry). Ed. G. Bokris, Izd. inostr, lit., 1962
~102
A. yr,. SKOROBOGATOVia n d V. P. KOZLOV
52. Ye. A. KOPYTOVA, Kandidatskaya dissertatsiya (Post-Graduate Thesis). Moscow, MTILP, 1976 53. A. A. TAGER, B. LIROVA and N. S. VASYANINA, Vysokomol. soyed. A19: 2506, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 11, 2885, 1977) 54. R. KESTING, J. Appl, Polymer Sci. 9: 663, 1965 55. W. W. BRANDT and R. S. BUDRYS, J. Biol. Chem. 239: 1442, 1964 56. W. W. BRANDT and R. S. BUDRYS, J. Phys. Chem. 6 9 : 600, 1965 57. V. S. MARK[N, K a n d i d a t s k a y a dissertatsiya (Post-graduate Thesis). Moscow, I K h F AN SSSR, 1974 58. L. L. RAZUMOVA, Vysokomol. soyed. A18: 1739, 1976 59. V. ENKELMAN and G. WEGUER, J. Appl. Polymer Sci. 21: 997, 1977 60. J. KOPE(~EK, J. VACIK and D. LIM, J. Polymer Sci. 2, A - l : 2801, 1971 4}I. N. I. NIKOLAYEV, A. M. FILIMONOVA and N. N. TUNITSKII, Zh. fiz. khimii 43: 1249, 1972
7PolymerScienceU.S.S.R.Vol. 21, pp. 3102-3107.
0032-3950/79/1201-.3102507.50/0
.O Pergamon Press Ltd. 1980. Printed in Poland
REPORTS KARGIN LECTURES* A. Y~,. SKOROBOGATOVAand V. P. KOZLOV T~E eighth and n i n t h Kargin Lectures devoted to a n u m b e r of problems concerning polymer chemistry and physics were held in 1978 and 1979. The eighth Kargin Lectures were held on 24-25 J a n u a r y , 1978 in Dzherzhinsk in t h e Scientific Research I n s t i t u t e of Polymer Chemistry a n d Technology named after Acad. V. A. Kargin and the n i n t h Kargin Lectures on 28 January, 1979 at at the Chemistry F a c u l t y of Moscow University. Opening the eighth Kargin Lectures 1~. A. Plate, corr. member of the U.S.S.R. Academy of Sciences dealt with some aspects of the creative work of V. A. Kargin and the significance of his scientific heritage in the formation and development of Soviet polymer science a n d ~oviet polymer industry. On behalf of the Dzerzhinskii bureau of the Town Committee of the CPSU and the executive committe of the Town Council representing people's deputies A. A. Shlykov, Secretary of the Dzerzhinskii Town Committee of CPSU greeted all workers taking parb i n the Kargin Lectures. Two memorial lectures were concerned with some aspects of V. A. Kargin's scientific heritage. One of them entitled "V. A. Kargin's Studies of Polymer Deformation and their Present Day Significance" was read l~y G. V. Vinogradov; this was on his behalf and on behalf of V. E. Dreval'. An analysis was made in this paper of V. A. Kargin's studies so ira* Vysokomol. soyed. A21: No. 12, 2810-2814, 1979.
Polymer Science U.S.S.R. Vol. 21, pp. 3087-3102. © Pergamon Press Ltd. 1980. Printed in Poland
SOLUBILITY OF ELECTROLYTES IN POLYMERS* A. L.
IORDAI~'SKII,
A. L. SHTERENZOI~ and G. YE. ZAIKOV
Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 30 August 1978) A general analysis was made of the solubility of electrolytes in non-ion exchange polymers of various kinds (hydrophobie, hydrophilic and those slightly swelling in water). The distribution coefficient of the electrolyte between the polymer and a dilute solution is determined for hydrophobic polymers by the interaction of neutral molecules in the polymer and for slightly swelling polymers--by chemical and electrostatic interaction. Chemical and osmotic interactions in the polymer matrix am predominant for strongly swelling hydrophilic polymers. It was shown how existing theories describe the effect of specific adsorption, Donnan and electrostatic potentials on solubility and the role of the type and molecular weight of the polymer was noted. INTRODUCTION
The quantitative description of diffusion and solubility of electrolytes in polymers is a necessary condition for forecasting the chemical stability of polymers and operational parameters of anti-corrosive polymer coatings. For the successful use of medicinal polymers as implants and elements of artificial organs information is also required about absorption parameters of polymers to be implanted, which are in contact with the physiological medium. A s t u d y is made in this paper of the equilibrium distribution of electrolyte between the polymer and a dilute solution. Most attention is given to acid distribution since the acid-basic condition of the medium determines the intensity of protolytic processes in the polymer [1]. As far as possible, we omitted the description of electrolyte adsorption b y ion exchange resins, since this problem has been examined on a high level in a number of monographs comparatively recently published [2-4]. We also omit the kinetic aspect of adsorption: a description of diffusion of electrolytes in non-ion exchange polymers has been published recently [5, 6]; these studies also deal with methodical problems. There is no single theory available at the present time dealing with the ratio between the amount of electrolyte adsorbed and physical and chemical properties of the polymer. Adsorption has been described within the framework of the classification proposed [5], which is convenient, but somewhat conventional, and groups the polymers into hydrophilic, hydrophobic and intermediate ones. I f types o f polymer of opposite hydrophobic properties markedly differ in transfer mechanisms even with the same water content (for hydrophilic ones these will be the initial times of adsorption), the intermediate type shows very different adsorp* Vysokomol. soyed. A21: No. 12, 2797-2809, 1979. 3087
~088
A.L.
IORDANSKIt et al.
tion properties approaching, according to water content, to hydrophilic or hydrophobic type. Special features of the polymer-electrolyte systems studied, compared with systems such as, for example, polymer-solvent systems, are as follows: 1) existence of charged particles (ions) and consequently, formation of electric potentials; 2) adsorption by the polymer of a specific solvent such as water. Aa a consequence of the interaction with hydrophilie groups of the polymer at significant osmotic pressures the latter may markedly change the structure of the polymer forming clusters with low water concentrations, more or less elongated sections with a high dielectric constant with moderate concentrations and then, continuous polymer regions filled with water, for example in gels (quasi-homogeneous model). In strongly swelling polymers water functions as a screening factor in the interaction of the ion and the polymer. Dealing with the role of water during adsorption is beyond the scope of this paper; readers who are interested in the mechanism of water absorption in polymers may be recommended separate studies [7-9]. A Table showing results concerning water content of polymers at a temperature of 25 ° and different relative moisture contents [10] may be used for practical purposes. GENERALIZED FORMULATION OF SOLUBILITY
T h e r m o d y n a m i c aspect. T h e role of the electrostatic factor. The solubility o f electrolytes in polymers is described from a thermodynamic point of view as the distribution of ions between a dilute solution of the electrolyte and the polymer. I n a state of equilibrium the following equality holds good / O + R T In as -4-P V~Jr-z~F~,= -o - ~ p- -V~Jr z~F~] - , I~ ~ R T 1n as
(1)
where/~ is the standard chemical potential of an i ion with charge z,; V , - - t h e partial molecular volume of the ion; p, y--pressure and electric potential, respectively [4]. Parameters without a line above the latter correspond to the solution and those with a line, to the polymer phase. In order to simplify the study, it is assumed that V, is constant in both phases and ionic concentrations c, arc used instead of activities as. From eqn. (1)a formula follows for the distribution coefficient of the electrolyre, Kiwi
where u is the osmotic pressure; A/l~A--the difference of standard chemical potentials; d~--difference of electric potentials in the polymer and in solution; cxA--the concentration of the electrolyte. Let us examine first in more detail the first item in the right hand side o f eqn. (2) since this is related to specific sorption effects of electrolytes in polymers~
Solubility of electrolytes in polymers
308~
In the general case the potential variation in the system of electrolyte solutior~ 1-polymer-electrolyte solution 2 without external field may be presented as the total of interphase potential differences at the polymer=solution interface ~'1, ~'2 and of the membrane potential itself ~m
Z~=~l+~m+~'~
(3)
With symmetrical boundary conditions the membrane potential fully determines Aq]. According to the classical Nernst theory ~tm is due to the differences in t h e solubility of positive and negative ions in each of the contacting phases. From conditions of equality of electrochemical potentials and electric neutrality for the polymer phase
RT
~m= ~
K+
In K_
(4)
where K+, K - are the distribution coefficients of the cation and anion, respectively. For example, for a tetramethyl ammonium ion with considerable affinity with t h e nonpolar phase, compared with the C1- ion, K + > K_ and therefore, ~gm>0. The distribution coefficient of the ion in first approximation may be calculated bythe Born equation [11, 12]. The value of K~ ~ 10 -2° gives an estimate for an ion of radius 2 A, distribute4 between the aqueous phase ( e : 80) and a nonpolar polymer (e----3) at 25% However, as indicated [11], this approach ignores the chemical nature of the ion. In practice, it is more convenient to use KxA
KxA:~/K-+K-
(5)
Using eqns. (4) and (5) it was shown [13] that for electrolytes with the same ior~ (KI, NaI) the following formula holds good:
RT
K~aI
~u~= - ~ - l n KKI
(6)
Equation (6) was verified experimentally [13] and it was shown that, for example, for K + and N(C2H5) + ions the value of ~Um----124mV, while for I - and C1- the value of ~ m - ~ - 95 inV. For a number of polymers there may be a case when ions 40 not penetrate into the polymer in practice and ~m=-0. However, there may be a potential difference at the phase interface produced by the formation of a double electric layer. Nonpolar polymers such as polyethylene and fluoroplastic may acquire a surface charge in the solutions as a result of the formation of oxide groups, or eonversely~ by the primary adsorption of ions from solution [14], which affects ~t1 and ~ . . Cations differ in their ability to displace counter ions of the surface layer in a corresponding lyotropic series AlS*>Ba~÷> Ca2+> Mg~*>K+>NH+ > l ~ a + > Li ÷
(7~
:~090
A. L. IORDANSKII et al.
'This series may be explained using Coulomb's law. Counter ions are retained on Che external surface by a force which is proportional to the charge and inversely ::proportional to the radius of hydrated ions [14]. The H + ion does not occupy a certain position in the lyotropic series. The :point is that in polyamide type polymers acid groups undergo dissociation with low constants, while salts of these acids are dissociated considerably. However, in .ion exchange resins of strong acid type (R--SOaH) the H+ ion does not have "high bond energy and is situated at the end of the series among monovalent ca:tions. A similar pattern has also been described for OH- anions [14]. For hydrophilic polymers with dissociating groups of the zQ charge between t h e solution and the polymer the Donnan potential is established
RT
(
zQ
In _ -[- - + 1 ~D = F \ 2tEA [_4C~h
(8)
:For example, for sulphonated phenol-formaldehyde resin Q--0.3 mole/l, CNAcl----102 ~nole/1. and ~YD=--87 inV. There are two limiting cases: low fixed charge zQ <<.CKA ~D----ln[
cxA
~
z___Q_Q
(8a)
a n d zQ ~CKA
Three types of error are possible when determining the Donnan potential ~ince: a) calculation is carried out via activity and in the polymer it is complicated • o determine the activity of the electrolyte; b) it is difficult to estimate the diffu"~ion potential on the boundary of polymer phase-measuring electrode; c) the Condition of quasi-homogeneous charge distribution may not hold good in the :polymer, particularly for dilute solutions, when the error in determining VD r e a c h e s 40% [15]. Kinetic aspect. The equilibrium state is characterized by the eqnality of transfer velocities of the electrolyte through the polymer-solution boundary in two ,opposite directions
I,=klc,--k2~=O ,
(9)
..~ence
KI
-
c~
,
(lO)
vchere k 1 and k2 are the rate constants of electrolyte transfer from the solution ~o the polymer and in the opposite direction, respectively [16].
Solubility of electrolytes in polymers
3091
In diffusion measurements parameter KKa is determined from kinetic results, according to the equation P0
KKA: D '
(11)
where P0 is permeability; D--the diffusion coefficient of the electrolyte. For the system consisting of poly-2-hydroxyethylmethacrylate-dilute KC1 solution K ~ values determined by a standard method and dynamically (using eqn. (11)) were the same [17]. The majority of diffusion experiments suggests that the equilibrium concentration in the surface layer of the polymer is established instantaneously and has no effect on the rate of diffusion. Special cases, when the rate of establishing equilibri~un surface concentration is comparable with the rate of diffusion, have been examined previously [18, 19]. When comparing kinetic and equilibrium measurements it should be considered that it is not the entire electrolyte absorbed by the polymer c~ which may take part in the transfer process, but only part of it co, the remaining electrolyte may not take part in transfer as a consequence of interaction between components of the diffusion system. The true diffusion coefficient D O is related in this case with the effective value, Deft determined experimentally from the equation
D°~-Def~ dc~'
(12)
where c~ is the overall concentration of the diffusate dissolved in the polymer; c0--the concentration of the "free" diffusate, which is normally determined by desorption. Experimental results which are in agreement with the equation of diffusion (12) are described in previous studies [20-22] for hydrophobie (PE), moderately ~welling (keratin) and hydrophilic (PVA) polymers, respectively. Let us examine in particular the absorption of electrolytes in various polymers. HYDROPHOBIC POLYMERS
Information about electrolyte concentration in hydrophobic polymers is scanty and not systematic. The weight variation of samples was determined in m a n y studies while being used in electrolyte solutions. These results are of known interest only for the evaluation of the corrosion resistance of materials. They are really useless, however, from the point of view of determining the concentration of the electrolyte in the polymer since the weight increase of the sample may be due both to the absorption of the electrolyte and the absorption of water and it is impossible in many, cases to predict their ratio in the polymer. Components absorbed have been determined separately in a number of studies. It was established [23, 24] that on keeping polypropylene in solutions of non-volatile electrolytes the weight variation of samples is completely due to water absorption and is
3092
A . L . IO~DANSKn e t a l .
clearly determined by steam pressure over the electrolyte solution, independent of the type of this solution. According to results previously described [25], the coefficient of solubility of HC1 in PVC determined as the ratio of HC1 concentrations in the polymer and in gas, is 5±0.5 at 20 ° . Nitric acid during absorption from solutions with a concentration close to 100% is absorbed by polymers in significant proportions (1-10~/o) [26-28]. If we assume that nitric acid absorption conforms to Henry's law, the order of absorption constants may be evaluated from these results (10-4-10 -S g/g tort). Swelling of fluorine containing rubbers in concentrated nitric acid reaches 17-19~/o [23]. There are no results available concerning the absorption of dilute nitric acid. solutions. Absorption by low density polyethylene of hydrogen chloride from concentrated solutions of hydrochloric acid was determined previously [20]. It is 0.01-0.05~/o, which corresponds to a constant of absorption of 10 -a g/g tort. Constants o f nitric acid absorption therefore exceed by two and higher orders of magnitude the constant of absorption of hydrochloric acid. An attempt was made [30] to evaluate electrolyte absorption by low density polyethylene by the Flory equation in a simphfied form --ln fp2= 1 +
RT
'
(13)
where ~2 is the volume fraction of the material dissolved; lY2--the molecular volume; 5~ and gl--solubilityparameters of the dissolved material and the polymer, respectively. According to the calculation made by eqn. (13) the solubility of nitric acid in polyethylene is 3.7%, whereas the value found experimentally is 3.1% [26]. T h e calculated solubility of sulphuric acid is 4 x 10-S~o . The low permeability of polyethylene in relation to sulphuric acid is, apparently, related precisely to the low solubility of sulphuric acid in polyethylene. Equation (13) was also used for the calculation of the solubility of sulphuric acid in Ftorlon-26* [31]. Experimental values of (z2 were close to calculated values (1.6 x 10 -a and 1.2 × 10 -a, respectively). The dependence of electrolyte concentration in the polymer on concentration in an external solutio~ was only studied in one paper [20]. HC1 concentration in the polymer increases very rapidly with an increase in the concentration o f hydrochloric acid. In contrast with this, the dependence of acid concentration in the polymer on vapour pressure over the acid is close to the linear (in the range of increased HC1 pressure). During absorption from the dry gaseous phase Henry'a law applies. Theoretical calculation of Henry's constant for a HCl-polyethylene system carried out by methods previously described [32] gave a value of 1.29 × * F t o r l o n - - a fluorine containing copolymer fibre.
Solubility of electrolytes in polymers
3093
× 10 -e g/g torr, which is close to the value found experimentally, i.e. 1.30-t-0.07 g/g torr. During absorption from hydrochloric acid HCI concentration in the polymer is higher than during absorption from the dry gaseous phase under the same partial tiC1 pressures in the external medium. This fact is explained assuming that part of HC1 molecules is combined in the polymer containing water molecules to form hydrates and in order to establish equilibrium, a further amount of HCI is transferred from the acid solution in the polymer. Equation (14) which is based on the assumption indicated, gives a satisfactory description of experimental results; the number of water molecules in the hydrate was near to five
•
//CHCI
log ~ -
-- 1
) =log(MKa~,o)-~nlogh~i,o, n
(14)
6HC1 0
where cftcl is the overall concentration of HC1 in the polymer; C~Cl, the concentration of "free" HCI in the polymer (uncombined in hydrates); M - - t h e molecular weight of HC1; K--constant of equilibrium of hydrate formation; all,o--constant of water absorption by the polymer; n--number of water molecules in the hydrate; h~,o--water vapour pressure over the acid solution. It slmuld be noted t h a t views concerning hydrate formation in the organic phase were confirmed experimentally in a number of studies dealing with extraction (see e.g. former paper [33]). There is no systematic information available concerning the effect of properties of polymers on electrolyte absorption. Results obtained by various authors are sometimes conflicting. It was shown [34] that a quenched Ftorlon-3 film absorbs up to 1.47% nitric acid, while a slowly cooled film (i.e. one with higher crystallinity) only absorbs 0.62%, however, it is indicated [28] that absorption of nitric acid increases with an increase in crystallinity. According to a former paper [28], quenched films of Ftorlon-42 absorb considerably more nitric acid than films subjected to slow cooling. The dependence on composition of nitric acid absorption by copolymers of fluorovinylidene with trifluorochloroethylene and hexafluoropropylene is described by a curve with a maximum, which corresponds to the minimum on the curve showing the dependence of tensile strength on composition. The authors explain the experimental nature of curves by amor10hization of copolymers containing 15-20 to 80-85 reel. % fluorovinylidene [27]. For the rough evaluation of the effect of the chemical composition of the amorphous polymer on electrolyte absorption eqn. (13) may, apparently, be used. The difference in permeability in relation to H~SO4 of polymers such as polyolefins (g~ 7.9) and "Pentaplasts" (J~ 9.9) can be readily explained by this equation. According to eqn. (13) it may be expected that the absorption of sulphurie acid by "Pentaplast" is considerably higher than by polyethylene (almost 40 times), it is therefore not surprising that the penetration of HzSO4 into polyolefins is very difficult to detect, whereas in respect of "Pentaplasts" this fac~ has been repeatedly established (see, e.g. previous study [35]).
A. L. IORDANSE~I ~ al.
3094
There are very few results available concerning the temperature dependenc~ of electrolyte absorption by hydrophobie polymers. The AH value given [36~ for the solution of 98% nitric acid in polyethylene, 22.6 kcal/mole, is very high, which may be due to a chemical reaction. The heat of HC1 absorption by polyethylene [30] has been calculated from an equation, which ~elates the heat of gas absorption by polyethylene with the Leonard-Jones potential for gas [32]. This value was 2.3 kcal/mole. The heat of solution of HC1 in PVC is 10± 1 kcal/mole
[2#]. INTERMEDIATE CLASS OF SLIGHTLY SWELLING ,POLYMERS
Slightly swelling polymers absorb electrolytes in accordance with eqn. (2). A special feature of polymers of this class is the fact that most of them contain ionogen functional groups (for example--NH2, - - C 0 0 H in polyamide). The presence of these groups complicates the description of absorption, compared with hydrophobic polymers as a consequence of the need for taking into account electrostatic and specific interactions of ions and macro-ions. Let us examine the best known theories of electrolyte absorption by polymers. T~BLE l. S P E C I F I C E N E R G Y OF ADSOI~PTION AND POLARIZATION ~ A N D VAN D E R W A A L S W COMPOI~7"ENTS :FOR VARIOUS SYSTEMS
System
C1--wool H +-wool Anion of orange I I dye-wool Anion of methyl yellow I I - w o o l
Cl--nylon H+-nylon Cu++-bull serum albumin K + - p h o s p h a t e colloid
Li+-
,,
,,
Ag+-
,,
,,
Overall specific energy 0 10.2 • 7.3
8.9 0 16.7 10.0 1.8 2.6 3.6 4.8 5.5
W /k T
0 0 7.0 8.6 0 0 0 0 0 0 0 0
~/kT
0 10.2 0.3 0.3 0 16.7 10.0 1.8 2-6 3"6 4-8 5.5
According to the Gilbert-Ridcal theory [37] it is assumed that ionic adsorption takes place on a few statistically distributed groups of polymer in conformity with the equation ( 0 ) (2+W)--e~, log ~ =logc+ 2.3kT '
(15),
where 2 and W are the energies of polarization and van der Waals interaction, respectively; O - - t h e proportion of free places, where adsorption may take place.
Solubility of electrolytes in polymers
3095
Equating 0 values for two oppositely charged ions and excluding the electrostatic term, the energies of adsorption were calculated for ions indicated i~ Table 1. In studies by Wall et al. [38, 39] titration curves of nylon-66 were published[ for the first time using NaOH and HC1. The interpretation of results was based on the Gilbert-Rideal theory, but a correction was introduced in eqn. (15) in respect of the inequality of the carboxyl and amino end groups in nylon-66. An ion exchange model [40] was proposed by Myagkov and Pakshver t o explain the adsorption of HC1, KOH, naphthalene-2-sulphonic acid, dyes containing nylon-6 and Kapron. This model suggests that log [x (Xo--X)-1]: l o g Kin-- 2pH,
(16)~
where x o is the overall number of places of adsorption; x--number of combined places; Kin--adsorption constant of equilibrium. The models mentioned show limited agreement with experimental results, which gave reason for developing the Matheson-Wkewell [41] polyelectrolyte model. Authors of this model assume that the energy of proton removal from t h e polymer into solution depends on electrostatic and osmotic interactions and chemical affinity (difference between standard chemical potentials of H+ in thepolymer and in solution). For a partially dissociated group in the polymer a n equation is proposed which is similar to eqn. (2). In contrast to strongly swelling gels, this theory enables the osmotic term to be ignored in eqn. (2) [41, 42] pH----pKo--log[(1 --a)a -1] --0.434/RT (F~+A]~°),
(17}~
where K0 is the constant of dissociation of the group in the polymer; a--degree. of dissociation. Calculation of the electrostatic term for cases of homogeneous potential in the polymer results in the f~rmula
~F/RT = sin h -1 (c//2zcs),
( 18)
where c! is the charge concentration in the polymer, cs--the molecular concentration of the electrolyte in solution; z--ionic valency. Irregular charge distribution. Interaction (association) may take place betweer~ fixed groups in the polymer, for example in wool, which distorts potential ~ and its distribution. There are two methods for taking into account this effect [41]_ 1. For a pair of combined groups of opposite charge local potential ~s will exceed ~ (the average potential examined) by a value of ~s--~. This measure i s also used in another study [43] for the description of ionic combination in polyelectrolyte solutions and gels. It has been indicated [44] that local charge density has a marked effect on the behaviour of dissociating carboxyl groups in copolymers: of maleic acid with vinylpyrrolidone and vinyl acetate. Coefficient fl--the proportion of charged groups included in ionic pairs (--0-0434 F~sfl/RT) should i~. this case be formally introduced in eqn. (17). It is difficult to calculate this tern~ since fl varies with the degree of dissociation a, however, if the value of fl is known,
~096
A.L. TORDANSKII6t a~.
the entire term may be calculated knowing the distances between ions, for example, in pairs of - - C O O - . . . H a I ~ + - and --COO-...H+ before recombination. 2. It is easier to examine carboxyl groups (free and connected) as two differe n t l y dissociating groups with dissociation constants K~ and Kq, respeetively. The concept of homogeneous ~ is retained in this case. I f p and q are the de~ e e s of dissociation of these groups, eqn. (17) takes the form'
(0.434/RT) (F~,÷A/~) pH----pgq--log[(1--q)q-l]-- (0.434/RT) (F~,+A/~)
pH----pK~--log[(1--p)p-1]--
(19)
Here p K 0 varies from pK~ to pKq as a function of ~. Effect of molecular weight. The molecular weight of polymers dissociating only a t the end groups influences solubility. The concentration of groups undergoing titration is inversely proportional to the degree of polymerization, the same way as c! (concentration of fixed charges) provided that the accessibility of groups to acids and bases does not vary with the degree of polymerization. Pao rameters pKo and Zl~Hare practically independent of the degree of polymerization therefore, combinig eqns. (17) and (18) gives pH~-const--0.434 sin
h-~(cf/2ca)
(20)
for various degrees of polymerization with the same degree of dissociation. When cf/2ca~200 and higher, i.e. when the concentration of the salt added is low sin h-~(cf/2ca)-~ln (cf/c~) (21) ~nd
pH----const--log (cf/cs)
(22)
e, being constant, eqn. (22) may be presented graphically in co-ordinates p H log cl. A former study [41] demonstrates satisfactory agreement between theory ~nd experiment. After the electrostatic potential has been calculated, theory enables chemical ~ffmity between various acids and the polymer to be established. Table 2 shows ~ values for an acid-wool system [41]. Titration curves without any specific interaction enable the p K value of acid groups of the polymer to be established. For example, for keratin the double pH value of a middle point on the titration curve is equal to the p K value of glutamic acid (this acid imitates the elementary unit of the polymer) [44]. However, the ~qu~lity mentioned is not valid for adipic acid--a model of the elementary unit o f nylon-66, even if a correction is made in respect of the shift of the isoelectric point of nylon-66 as a consequence of the inequality of the number of acid and basic groups in the polymer [45]. The most likely cause of deviation, in the ~nthors' opinion, is a reduction of the dielectric constant e in nylon-66, compared with' the keratin matrix; in the latter e dies not differ markedly from the 8 value f o r water (~ 80).
Solubility of electrolytes in polymers
3097
HYDROPHOBIC POLYMERS
Hydrophilie polymers containing uncombined water (e.g. hydrogels of poly~ oxyethylmethacrylamide type, PVA) absorb electrolytes according to the equation KKA--~O(OH,O,
(23)
derived using the theory of free volume [46] (qH,o--volume fraction of water in the polymer and a0, coefficient characterizing the interaction of electrolyte with the polymer in the absence of which a0----1). Graphical expression of eqn. (23) is given in a previous study [44]. I f the polymer matrix is a micro-heterogeneous system (porous model), t h e distribution coefficient m a y be expressed as KxA = Kwq~,o + K~(1-- q~,o ) ,
(24)
where qm,o is the volume fraction of water; Kw and K~--the individual coefficients of distribution [17] in water and in the polymer, respectively. The physical significance of the distribution coefficient in a neutral gel m a y be explained b y the following equation system [47]
K~--
~K~ _ y± e-~V,,/R~
(25)
RT ----- _ in p
(26)
6ILk
y+
V~.o Here ~ is the osmotic pressure; y ± - - a c t i v i t y coefficient; Vxx and I7~,o, molecular volumes of the electrolyte and water, respectively; p - - w a t e r vapour pressure; All parameters with a line correspond to the polymer phase. TABLE
2. CHEI~IICAZ A F F I N I T Y VALUES Z]//0 B E T W E E N VARIOUS
ACIDS
AND
I,ATED 1~1~O~ I~ESULTS P R E V I O U S L Y G I V E ~ ( [ 3 5 ] A ~ D
[39])
Acid HC1 Ethylsulphuric HBr HiSO~ Benzenesulphonic Nitric p-Toluenesulphonic o-Xylenesulphonic Trichloroacetic o-Nitrobenzenesulphonic 2,5-Dichlorobenzenesulphonie 4-Nitrobenzenesulphonic 2,4-Dinitrobenzenesulphonic Naphtalene -p-sulphonic
Ap°/RT O 0.13 0.42 0.57 0-65 0.66 0"75 0"86 0'97 1"12 1"65 1"65
1"72 1"96
WOOL,
CALCU-.
A~° [39] I A~0 [35] cal/mole O 160 510 710 810
200
830
940 1070 1210 1400 2060 2040 2150 2450
1200 120Q 1450 2100
2250 2250
:$098
A.L. IO~DAt+SKIIet d.
Water pressure in the polymer phase cannot, usually, be measured; as supposed by Gregor, this value may be determined by measuring vapour pressure over the polymer solution having the same chemical structure as the gel, but without erosslinks [48]. The coefficient of electrolyte distribution between the gel and the external .solution therefore depends on the chemical structure of the polymer and the electrolyte and on osmotic pressure, which may reach several hundreds of atmospheres in view of the high value of RT/Vw~ 1400 arm at 25 °. These arguments are also valid for polyelectrolytes and ion exchange resins; however, in the presence of charged ionogen groups in the polymer the Donnan distribution of electrolyte has to be considered [3, 4]._ Let us examine the effect of the electrolyte on Kr~ between the polymer and a ~dilute solution. Relative hydrophobic 19roperties of the ior~ or a neutral molecule. The solubility +of organic electrolytes in highly swelling polymers decreases in the following ~sequenee of groups incorporated in the anion (or cation)structure of this electrolyte: CHs--> CHsCHa--> CH3CH+CH,--> CH3(CHI).--
Aromatic phenyl groups are approximately equivalent as regards hydrophobie ~properties in relation to the n-propyl group, while the naphthyl group, in relation to the n-hexyl group. Substitution of sulphur for oxygen in the molecule of the ,diffusing substance also reduces hydrophilie properties, while the a~ldition of .+structural groups, which may form hydrogen bonds with water, or the other hand, +increases the solubility of electrolyte in the polymer [47] --OH >,CO~H >--NITs ~---SOzH~---SO2NH2~--CO2CHs-+--CONH2~--OCHB Inverse sequences are observed for hydropkobic polymers and with an increase hu hydrophobic properties of the polymer matrix, an increasing part of the energy +of interaction, the value of which may be very significant for organic ions, will be short range interaction. For example, two zig-zag carbon chains in 20 carbon .atoms interact with each other with an energy of ~ 10 kcal [~0]. S~z+ of ion. A description was given [51] of the effect, from a theoretical point of view, of the size of ion on absorption, compared with the absorption of a s t a n d a r d ion 1/~ in size. The steric effect may, in principle, be offset by the hydrophobic interaction described, however, no detailed investigation has been carried out of this effect. Structural aspevt. A study was made [52] of the interaction of PVA with dilute +solutions of various salts. The polymer matrix is, in first approximation, identical with water. This interpretation suggests that large single charged ions (CNS-, Br-, I--) influence the structure of amorphous regions of PVA the way they affect -water structure. Penetrating the polymer matrix ions change the structure of the ~polymer, rupture and redistribute hydrogen bonds of macromolecules. On in-
Solubility of electrolytes in polymers
3099
creasing water content in the polymer, the behaviour of the system consisting of PVA and a dilute salt solution is mainly determined by structural changes of the water itself, occurring by the action of electrolyte ions dissolved in it and temperature, i.e. a considerable screening effect is achieved in the ion-polymer interaction. It should be noted that large ions disrupt water structure (K +, Rb +, Cs+, I~H +, I - and Br-), whereas small ones and a few multivalent ions regulate it (Li+, Na +, Ca2+ and Mg~+); ~ 1.6 A is a critical size of transition of the ion from on~ group into the other. A study of solubility in a system, consisting of a polymer-solvent-electrolyte~ is of interest. Tager et al. [53] have shown that the ability of the electrolyte t o dissolve the polymer is determined by the type of constitutional diagram of th~ system and with an increase in the concentration of the system may first increase and then reduce mutual solubility until the system undergoes micro-separatiom The presence in cellulose acetate of bi- and polyvalent cations of Ca 2+, Ba ~+ a n d La s+ type in the region of high salt concentrations coordinates groups corresponding to adjacent macromolecules and increases the viscosity of the system. Interaction between nncleophilic atoms of organic polar groups of the polymer and electrophilic cations of salt with high charge density (high valency, small ionic radius) is confirmed [54] by IR spectra of cellulose diacetate. A shift to the long wave region is observed in the system of absorption bands of --OH and --C----O groups on adding Mg(Cl04)2 to the polymer which, in the authors' opinion is the consequence of the redistribution of charge density on these groups in the presence of a Mg ~+ ion. Anions have an indirect role in this case: the weaker the polarizing action of the anion (large radius, small charge), the slighter its effect on the cation and this way it contributes to a more active interaction o f this cation with functional groups of the polymer. Electrolyte absorption in the polymer in a number of cases changes crystalline packing in the polymer. For example, absorption of trifluoroacetic acid by poly-L-valine and poly-L-lencine increases the proportion of a-spirals of macromolecules, as shown by X-ray [55, 56], while absorption by polycaprolactam (PC-4) of sulphuric acid initiates a--~ crystalline transition [57]. Diffusion of dilute acid solutions (HC1, H3P04) in PVA results in practically complete amorphization of partially crystalline initial samples [58]. However, there are no detailed descriptions in the literature of the correlation between the: solubility of electrolytes and the crystalline structure of polymers. The effect of ionogenic charged groups in a highly swelling polymer on KKA is: characterized by the Donnan distribution mentioned. However, as a consequence of the considerable distance between these groups (e.g. in 90% gel the averag~ distance between adjacent chains is 12-13 • [59]) Coulomb interactions are less significant than in slightly swelling polymers [60]. Gels with high charge density and low degrees of cross]inking may be described as highly concentrated electrolyte solutions [2]. On increasing the degree of cross]inking, ion exchange resins may be regarded as ionic crystals [61].
:3100
A.L.
IORDANSKII et a~.
The solubility of electrolytes in p o l y m e r s of various classes m a y be described b y a generalized eqn. (2). I n h y d r o p h o b i c polymers, where t h e electrolyte is in t h e nondissociate4 state, t h e distribution coefficient is d e t e r m i n e d b y short r a n g e i n t e r a c t i o n of t h e electrolyte in the polymer, which is confirmed b y the equilibr i u m a b s o r p t i o n o f HCI in P E f r o m t h e gaseous phase. T h e f o r m a t i o n in P E o f h y d r a t e d s t r u c t u r e s during t h e a b s o r p t i o n o f h y d r o c h l o r i c acid is also described in t h e absence of osmotic a n d electrostatic interactions. I n slightly swelling pol y m e r s t h e description of solubility requires consideration in eqn. (2) o f the e l e c t r o s t a t i c potential, while on t r a n s i t i o n to highly swelling p o l y m e r s it becomes n e c e s s a r y to b e a r in m i n d osmotic pressure. W h e n t h e p o l y m e r contains ionogenic groups excess ( c o m p a r e d w i t h osmotic) pressure is f o r m e d as a result o f t h e I ) o n n a n effect. The influence of this effect m a y be reduced considerably if measu r e m e n t s are carried o u t w i t h a n electrolyte c o n c e n t r a t i o n of >>zc~, where z is t h e effective charge o f one macromolecule a n d ca, the molecular c o n c e n t r a t i o n of the polymer. Translated by E. S~MERE REFERENCES
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Solubility of electrolytes in polymers
310i
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~102
A. yr,. SKOROBOGATOVia n d V. P. KOZLOV
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7PolymerScienceU.S.S.R.Vol. 21, pp. 3102-3107.
0032-3950/79/1201-.3102507.50/0
.O Pergamon Press Ltd. 1980. Printed in Poland
REPORTS KARGIN LECTURES* A. Y~,. SKOROBOGATOVAand V. P. KOZLOV T~E eighth and n i n t h Kargin Lectures devoted to a n u m b e r of problems concerning polymer chemistry and physics were held in 1978 and 1979. The eighth Kargin Lectures were held on 24-25 J a n u a r y , 1978 in Dzherzhinsk in t h e Scientific Research I n s t i t u t e of Polymer Chemistry a n d Technology named after Acad. V. A. Kargin and the n i n t h Kargin Lectures on 28 January, 1979 at at the Chemistry F a c u l t y of Moscow University. Opening the eighth Kargin Lectures 1~. A. Plate, corr. member of the U.S.S.R. Academy of Sciences dealt with some aspects of the creative work of V. A. Kargin and the significance of his scientific heritage in the formation and development of Soviet polymer science a n d ~oviet polymer industry. On behalf of the Dzerzhinskii bureau of the Town Committee of the CPSU and the executive committe of the Town Council representing people's deputies A. A. Shlykov, Secretary of the Dzerzhinskii Town Committee of CPSU greeted all workers taking parb i n the Kargin Lectures. Two memorial lectures were concerned with some aspects of V. A. Kargin's scientific heritage. One of them entitled "V. A. Kargin's Studies of Polymer Deformation and their Present Day Significance" was read l~y G. V. Vinogradov; this was on his behalf and on behalf of V. E. Dreval'. An analysis was made in this paper of V. A. Kargin's studies so ira* Vysokomol. soyed. A21: No. 12, 2810-2814, 1979.