Potentiometric titration of a suspension of crosslinked polymethacrylic acid

Crosslinked potymethacrylic acid

125

5. M. G. KRAKOVYAK, Ye. V. ANUFRIYEVA a n d S. S. SKOROKHODOV, Vysokomol. soyed. A l 1 : 2499, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 11, 2842, 1969) 6. Ye. V. ANUFRIYEVA, M. V. VOL'KENSHTEIN and T. V. SHEVELEVA, Biofizika 7: 372, 1965 7. Ye. V. ANUFRIYEVA, T. M. BIRSHTEIN, T. N. NEKRASOVA, O. B. PTITSYN a n d T. V. SHEVELEVA, J. Polymer Sci. C16: 3519, 1968 8. M. G. KRAKOVYAK, Ye. V. ANUFRIYEVA, M. V. VOL'KENSHTEIN, T. D. ANANYEVA, Yu. Ya. GOTLIB, R. A. GROIKOVA, S. P. KOZEL, G. I. LASHKOV, V. B. LUSHCHIK, V. D. PAUTOV, S. S. SKOROKHODOV and T. V. SHEVELEVA, Dokl. AN SSSR 224: 873, 1975

POTENTIOMETRIC TITRATION OF A SUSPENSION OF CROSSLINKED POLYMETHACRYLIC ACID* N. P . KUZNETSOVA, R . N. MISI-IAYEVA, L. R. GUDKIN,

YE. V. ANUFRIYEVA,

V. D. I:)AUTOV and G. V. SAMSONOV

High Polymer Institute, U.S.S.R. Academy of Sciences

(Received 31 .May 1976) An aqueous suspension of a macro-mesh carboxylic cationite with particles of size N 0.5/1 has been prepared from methacrylic acid and ethylene dimethaerylamide (2.5 mole ~o). The degree of dispersity was such that methods of polarized luminescence and potentiometric titration could be used for the investigation. Ionization of the polymethaerylic acid (PMAA) chain of the ionite network is accompanied b y transition from the form with crosslinking due to hydrophobic interaction to a disordered form. This transition takes place as a cooperative process, and occurs within a narrow interval of ionization degrees (~), as in the case of soluble PMAA. Three regions corresponding to the crosslinked and disordered forms, and to the transition, are readily distinguishable on the potentiometric titration curves plotted with the coordinates p K - a a n d pH-log (l---a)/g. The extent to which the ionic strength of solutions influences ionization of the suspension was investigated. The experimental results apply equally to coarse particle suspensions of the studied cationite.

SOME problems of the physico-chemical and electrochemical properties of network polyelectrolytes call for ful~her investigation in view of the limited amount of work so far accomplished. Among these problems are some relating to the mobility and conformational state of polymer chain segments located between chemical crosslinks produced by erosslinking agents such as polyvinyl compounds. This paper relates to a potentiometric titration analysis of the ionization of a macro-mesh carboxylic cationite, namely the copolymer of methacrylic acid and * Vysokomol. soyed. A19: No. 1, 107-11 1, 1977.

126

N. P. KUZI~ETSOVA et aL

ethylene dimethacrylamide (2-5 mole %) [l]. Results obtained b y the potentiometric titration method are compared with data on the intramolecular mobility (IMM) of the network polymer chains investigated by the polarized luminescence method (PL).

I/F o.-

lq

KS

302O 10 I0 8, I

I

0.2

0.6 tx ~G.

1

I

I

I

t

0.1

0.3

0"5

0"7

I

0"9

FIO. 2

FZG. 1. Swelling of the macro-mesh carboxyl cationite during ionization. FIG. 2. Reciprocal of the luminescence polarization velsus degree of ioriization for the suspension of PMAA-C with covalently bonded luminescing groups. It is known t h a t the ionization of carboxylic network polyelectrolytes is accompanied b y swelling of the polymers. In the present instance it was desired t h a t the ionization process should be investigated with ionization degrees (=) ranging from 0 to 0.4 as it is in this interval t h a t changes in the volume swelling coefficient (Ks) are most marked (see Fig. 1). I n the specified range of= values there is practically an 8 fold rise in K,, and a m a x i m u m value limited by the network carcass is attained. The direct titration of carboxylic cationites has proved difficult in view of the slow attainment of a hydrogen-sodium exchange equilibrium (5-7 days) [2], and the process is normally carried out by titration of separate weighed portions of the cationites. To carry out the potentiometric titration directly we prepared a finely dispersed fractionated non-precipitating suspension of a carboxylic cationite with particles of size ~ 0.5/~. This suspension has been given the conventional code name PMAA-C in this paper (crosslinked polymethacrYlic acid). The partitles were small enough to prevent kinetic difficulties in neutralization of the PMAA-C suspension with an alcohol. A further advantage obtained with the suspension lay in the f a c t t h a t it could be investigated by spectroscopic methods, e.g. the polarized luminescence method could be used for the analysis of IMM in PMAA-C [2]. The results of an IMM analysis of PMAA-C with covalently bonded luminescent groups * in the process of ionization showed t h a t changes in the reci• The authors thank M. G. Krakovyak for incorporating the luminescent group in the PMAA-C.

Crosslinked polymethacrylic acid

127

lIP

procal o f t h e luminescence p o l a r i z a t i o n for t h e suspension are s i m i l a r t o changes in t h e l a t t e r for soluble PMAA, a n d come t o light m o s t m a r k e d l y in t h e same r a n g e o f a values (Fig. 2). I t should be n o t e d t h a t t h e n e t w o r k effect influences the absolute values of I t lowers t h e m o b i l i t y o f P M A A chains in t h e netw o r k p o l y e l e e t r o l y t e t o a considerable e x t e n t .

1/P.

pH

6.l~

/5

ii O'l

I

-0.8 0"3 0"5

0.7

Fro. 3

0

0"9•

0"5

1.6

log.l~a:

FIG. 4

FIG. 3. Change in pK values during titration of a PMAA-C suspension in aqueous solutions of the latter with NaC1 concentrations: 0-001 (1), 0.01 (2), 0.05 (3), 0.15 (4), and 0.5 ~ (5). FxG. 4. Titration curves for a PMAA-C suspension varying the ionic strength of the solution by means of NaCl: 0.001 (1), 0.1 (2), and 0.5 N (3) (I--~1= 0'13, II--~=0.36). I t is k n o w n f r o m t h e results o f IMM investigations r e p o r t e d in [5, 6] for P M A A t h a t w i t h low degrees o f ionization (a values u p t o 0.15) soluble PMAA is a crosslinked p o l y m e r on a c c o u n t o f H bonds a n d h y d r o p h o b i c i n t e r a c t i o n of m e t h y l groups. W i t h a values exceeding 0.4, P M A A is a disordered coil w i t h high IMM. T h e p o l y e l e e t r o l y t e undergoes transition f r o m one f o r m to t h e o t h e r when the e n e r g y o f electrostatic repulsion o f ionized g r o u p s becomes c o m m e n s u r a t e with t h e e n e r g y o f 'H b o n d s a n d o f h y d r o p h o b i e i n t e r a c t i o n b e t w e e n m e t h a c r y l i c acid residues. This t r a n s i t i o n t a k e s place as a c o o p e r a t i v e process within a n a r r o w in-

128

N.P.

KUZNETSOV~. e t a l .

terval of ionization degrees. A similar situation is observed (see Fig. 2) in the case of a change in intramolecular mobility in a PMAA-C suspension in the course of ionization. Curves based on the results of potentiometric titration of aqueous suspensions of PMAA-C are displayed in Figs. 3 and 4; a clear picture of the conformational transition is given by the curves. On comparing these findings with t h e results of IMM determinations (Fig. 2) it can be seen t h a t individual portions of the potentiometric titration curves (curve 1 in Figs. 3 and 4) correspond to definite areas of change in the reciprocal of the luminescence polarization. R~strLTs or rOTE~O~VE~IC =~ATIO~ Or susrr~sm~s or PMAA-C

P

pK

P/-fftr

0.001 0.01 0.05 0.10 0.15 0-5

6.95 6.69 6-25 6,10 5.93 ~5.52

6.8 6.4 6.0 5.9 5-8 5.5

Transition region

0.1.3-0.36 0.18-0.29 O.20-O.27 0.21-0-27 O.24-O-37 0.40-0.49

.

0'25 0.24 0"23 0"24 0"30 0"45

n I

nii

1"46 1"48 1"41 1 "40 1"45 1"20

1 "68 1 "67 1"65 1"75 1"66 1"64

.Note. ~ t , is the degree of ionization of PMAA for the mi~tdle of the I-II transition; /~ is the ionic strength of the

solution.

The first portion (region of ~----0-0.13) relates to the crosslinked form of PMAA-C (I), the range of a covering 0.13-0.36 correspor[ds to cooperative transition from crosslinked to uncoiled form (I-II) and, finally, the region of a > 0 . 3 6 corresponds ¢o the disordered form of the PMAA-C chains (II). Using the appropriate graphs one can determine the values of the apparent ionization constant at a = 0 - 5 (pK), and the value of pKt, for the I - I I transition. I t can be seen from Fig. 3 t h a t during the ionization of PM_AA-C p K values for the crosslinked and the disordered forms are increased, whereas the values relating to the transition region remain constant. We now plot the potentiometric titration results for PMAA-C on the coordinates p H versus log (1--a)/a (Fig. 4), and the parameter n, characterizing the mutual influence of polymer chains of the network for the two forms of PMAA-C (n~ and nix ) can be determined from the angle of slope of the straight line ( p K = p H + n log (1--a)/a). The Table gives the values of p--K, pKtr, nr, nil, Oh, ~¢2and a~' obtained from the titration curves for suspensions of PMAA-C. The difference in interaction of the polymer chains in the region of low (I) and high (II) degrees of ionization of PMAA-C is reflected in the values of n I 1.4-1.48 and nn----1.64-1.75 at all ionic strengths of the solutions, The ionic strength of the external solution produced b y NaOl influences the acidity of the ionite, and weakens the electrostatic field t h a t appears in the network as ionization of the carboxyl groups proceeds (screening effect). I t follows Chat the effect of repulsive forces will be inversely proportional to the ionic strength =

Crosslinked polymethacrylie acid

129

Of the solution. As was demonstrated in [7], the effect of neutral salts on the ionization of cationites can be described by the empirical equation

pK---p Ko--fl log g where pK o is the negative logarithm of the ionization rate constant for the ionite at log/~--0; fl is the proportionality fi~ctor. Figure 5a shows how the salt concentr:~tion influences the values of p K (curve 1) and pKtr (curve 2). As the ionic strength of the solution increases, the region of I - I I transition is displaced towards higher degrees of ionization, so tha~ at H = 1 the values of the apparent ionization constant at ~=0.5 become equal to the ionization constants for the I - I I transition. I t was found t h a t the va]ue of ~ , corresponding to the onset of conformational transition, is a lir~ear function of the salt concentration of the external solution {with the exception of highly dilute solutions of 0.001 NaC1) (Fig. 5b). CN~[

pK 7.0-o

O-5 I

a

b

6.2~ 0"3 5.# -

0.1 -

-z

I

o

Io~t,~ FIo. 5

X~

o.z

03

I

-o.z

o.z

0.6

o~T

f.o

1o9 ~x FIG. 6

FIG. 5. Plots of pK (~=0"5) (1) and pKtr (transition of form I to form II) {2) for a PMAA-C suspension vs. logarithm of the ionic strength of the solution (a) and the effect of the NaC1 concentration on the onset of I-II transition in a PMAA-C (~1) suspension. FIG. 6. Titration curves for the macro-mesh carboxyl cationite (particle size 80-125 Z) at NaC1 concentrations: 0.05 (1) and 0.15 ~ (2). I t is fair to assume t h a t a similar process of change in the IMM of the PMAA chains in a suspension of PMAA-C during ionization will likewise occur in the case of particles of size ~ 100/~. To verify this we carried out the titration of individual portions of the studied cationite with particles of size 80-1251~, using a standard method. I t can be seen from Fig. 6 t h a t in the region of ionization degrees 0-1-0.3 there is a spread of points t h a t could be attributable to the inflexion corresponding to ~he I - I I transition of polymer chains of the network. Thus, it appears from a comparison of results obtained by potentiometric

130

Yu. Y s . SV:E~-~OV

titration and b y the P L method that a fine particle aqueous suspension of PMAA-C m a y exist, like soluble PMAA, in two forms: a crosslinlced form in the case of low degrees of ionization, and a disordered form in the case of high ~ values. Transition from one form to the other takes place as a cooperative process within a narrow range of degrees of ionization. Investigations of the network structure of polyelectrolytes m a y lead to a better understanding of processes whereby the binding of proteins, including ferments, m a y take place without altering their native state. Similarly, the immobilization of ferments under reversible and irreversible sorption conditions may in this w a y be facilitated. Translated by R. J . A. HEI~DRY REFERENCES

1. A. A. VANSHEIDT, V. A. DINABURG, K. M. GENENDER and S. N. KOROBEINIKOVA, U.S.S.R. Pat. 168427, 1963 2. A. R. MATHIESON and R. T. SHET, J. Polymer Sei. 4, A - I : 2945, ].~]66 3. Ye. V. ANUFRIYEVA, M. V. VOL'KENSHTEIN, M. G. KRAKOVYAK and T. V. SHEVELEVA, Dokl. A N SSSR 182: 361, 1968 4. Ye. V. ANUFRIYEVA, Yu. Ya. GOTLIB, M. G. KAKROVYAK and S. S. SKOROKHODOV, Vysokomol. soyed. A14: 1430, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 6, 1604, 1972) 5. T. N. NEKRASOVA, Ye. V. ANUFRIYEVA, A. M. YEL'YASHEVICH and O. B. PTITSYN, Vysokomol. soyed. 7: 913, 1965 (Translated in Polymer Sei. U.S.S.R. 7: 5, 1008, 1965) 6. J. C. LEYTE and M. MANDEL, J. Polymer Sci. A2: 1879, 1964 7. Z. S. TABIDZE, L. F. YAKHONTOVA, B. P. BRUNS a n d K. M. SALDADZE, Plast. massy, No. 3, 333, 1963 8. "~N. N. K U Z N E T S O V A , K. M. R O Z H E T S K & Y A , B. V. M O S K V I C H E V , L. K. SHATAYEVA, A. A. SELEZNEVA, I. M. OGORODNOVA and G. V, SAMSONOV, Vysokomol. soyed. A18: 355, 1976 (Translated in Polymer Sei. U.S.S.R. 18: 2, 408, 1976)

DYNAMIC BIREFRINGENCE AND VOLUME EFFECTS* Y u . Y E . SVETLOV High Polymers Institute, U.S.S.R. A c a d e m y of Sciences

(Received 31 May 1976) I t has been demonstrated t h a t the ratio of the dynamooptical constant to intrinsic viscosity ([n]/[t/]) is unrelated to volume effects in the vicinity of the 0 point. I t appears from an a p p r o x i m a t e expression derived for [n]/[q] i n good solvents t h a t * Vysokomol. soyed. A19: No. 1, 112-117, 1977.