Makromol. Chem. 182,665 - 668 (1981) 665

Rapid Communication

Viscometric and Potentiometric Study of Dilute Solution Behaviour of Poly(acry1amide-co-methacrylic acid)

Hernan Rios, Ligia Gargallo *, Deodato Radic

Departamento de Fisico Quimica, Instituto de Ciencias Quimicas, Pontificia Universidad Cat6lica de Chile, Casilla I 14-D, Santiago, Chile

(Date of receipt: September 30, 1980)a)

Introduction

For ionic polymers, the shapes of the macromolecules are influenced by the electro- static potential of the fixed charges on the macromolecular backbone'-3). The con- formation determination of these macromolecules is possible in dilute solutions only in the presence of simple electrolytes, as a result of screening by co-ions.

Previous investigations on the behaviour of ionic polymers and copolymers show conformational transitions4), e.g., the unperturbed dimensions of some copolymers (ethyl acrylate and acrylic acid (mole ratio 3 : 1)) in its unionized state in an organic theta-solvent were 1,3 to 1,4 times those obtained for the fully ionized polymer in an aqueous theta solvent5).

This physico-chemical behaviour of polyelectrolytes is generally attributed to interactions due to electric charges. In recent years, considerable interest has been shown in relation to the fact that both the ionic and hydrophobic groups attached to the main chain are concerned closely with conformational changes which polyelec- trolytes in solution undergo.

The purpose of the present communication is to study by viscometric and potentio- metric measurements the behaviour of poly(acry1amide-co-methacrylic acid) having ionic and non-ionic units in various ratios, in aqueous solutions.

Experimental Part

Polymerization conditions

A series of samples of acrylamide/methacrylic acid systems with various compositions were prepared by copolymerization according to Pichot et aL6) at 56°C in aqueous solutions under nitrogen ( 1 bar) using sodium persulfate as initiator: 0,16 g/100 cm3 with different mole ratios of monomers at pH 4.

After 30 min the polymerization was stopped by pouring the material in methanol at pH t. 10 or adjusting the pH of the solution at 2 with HC1.

a) Revised manuscript of December 5, 1980.

666 H. Rios, L. Gargallo, D. Radic

The polymers precipitated in basic media were dissolved in water and reprecipitated twice in CH,OH and those obtained in acid media were only washed with dilute HC1.

Polymer characterization

An approximate estimation of the molecular weight was obtained from viscometric measure-

The copolymer compositions were obtained from the nitrogen content by elemental analysis. Tab. 1 summarizes the synthesis conditions and chemical compositions of the copolymers. Intrinsic viscosity measurements of the copolymer samples had to be performed in the

ments using 0,s M aqueous NaCl as solvent.

presence of low molecular weight electrolytes.

Tab. 1. (AA) and methacrylic acid (AM). (Time of copolymerization: 30 min)

Synthesis conditions and chemical compositions of the copolymers from acrylamide

~ ~ ~~~~~~~

Samples Mole ratio of Yo Conversion [q]z'oc/(dl. g- ') Copolymer monomers 0,s M NaCl composition [-4Al/[AMl

A 1 : l 27,4 B 1 : 4 14,l C 4 : 1 15,8

0,70 0964 0,83

1 : 6 1 : 12 1 : 1,4

Potentiometric titrations were carried out with a Radiometer PHM 62 pH-meter equipped with a Radiometer G2040B glass and calomel K W O electrodes. Titrants were added with a microburette Radiometer ABUl1 in a vessel thermostatted to within +0,1 "C.

Results and Discussion

Fig. 1 shows the intrinsic viscosity [q] as a function of the pH of the aqueous solutions for the three copolymers studied. The increase of [q] with increasing pH is an indication of increasing electrostatic potential inside the compact coil upon increased ionization of carboxylic groups. The sudden unfolding of the macromolec- ular chains can be observed, as revealed by the observed change in viscosity in all the systems.

From these curves one should estimate the average pK, of the copolymers, considered as the pH value at which [q] has an average value. This approximation assumes that the macromolecular coil has a minimum hydrodynamic volume at a = 0 and a maximum volume at a = 1, a being the ionization degree.

We are aware of the fact that this is an approximate characteristics, because of the high ionic force of the solution and the fact that the polyelectrolyte is a copolymer and the pK, should be affected by the presence of the acrylamide unit in the molecule.

Fig. 2 shows the typical curves of the potentiometric titration. From the basic range one gets the solubility of the copolymer.

Viscometric and Potentiometric Study of Dilute Solution.

4%

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1 I I

0. ’ 94 96 90 IP q2Addition of 0.3 M HCI in mI

Fig. 2

Fig. 1. Intrinsic viscosity [q] as calculated according to Sol~mon-Gotesman~) of copolymer samples A, B and C (see Tab. 1) as a function of the pH of the aqueous solutions

Fig. 2. Curves for the potentiometric titration of the copolymer samples A, B, C (see Tab. 1) studied. Initial volume of solution: 25 ml

The pK, values were obtained from potentiometric titrations (PK,,~), the “average viscosimetric pK,” values (PI&), were compared with PK,,~. The pKa values for the three copolymer samples are summarized in Tab. 2.

Tab. 2. pK, values of the copolymers from acrylamide (AA) and methacrylic acid (AM) as calculated for aqueous solutions in 0,5 M NaCl from viscometric (pKa,.) and potentiometric (pK,, p) measurements

Sample [fil/[AMl PKa, v pKas p

A B C

~~~

l : t 4,61 5,35 1 : 4 5,15 5,50 4 : 1 4,63 5,19

The viscosimetric and potentiometric pK, values found in this work are in good agreement and coincide with the pK, values 4,67 and 4,60 obtained for methacrylic acid in 0,l M NaCl and in 0,Ol M NaCl, respectively@.

668 H. Rios, L. Gargallo, D. Radic

The authors wish to express their acknowledgements to “Direccidn de Znvestigacidn de la Ponti3cia Universidad Catdlica de Chile”, DIUC, Grants No 107/79 and 314/80, for financial support.

‘) H. Eisenberg, J . Polym. Sci. 23, 579 (1957) 2, D. 0. Jordan, T. Kurucsev, R. L. Darcus, Polym. J. 6, 303 (1965) 3, A. Takahashi, M. Nagasawa, J. Am. Chem. SOC. 86, 543 (1964) 4, E. Prokopova, M. Stol, E. Knizakova, M. Bohdanecky, Makromol. Chem. 180, 615 (1979) ’) J. S. Tan, S. P. Gasper, J. Polym. Sci., Polym. Phys. Ed. 12, 1785 (1974) 6, C. Pichot, A. Hamoudi, Q. T. Pham, A. Guyot, Eur. Polym. J. 14, 109 (1978) ’) 0. F. Solomon, B. S. Gotesman, Makromol. Chem. 104, 177 (1967) *) M. Nagasawa, T. Murase, K. Kondo, J. Phys. Chem. 69, 4005 (1965)