Instability of aqueous solutions of cationic acrylamide copolymers

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27. I. Ya, SLONIM, S. G. ALRKSEYEVA, L. G. URMAN, B. M. ARSHAVA, B. L. AKSEL'ROD, L M. G U R ~ and L. N. SMIRNOVA, Ibid. A20: 2286, 1978 (Translated in Polymer Sci. U.s,S.R. 20: 10, 2569, 1978) 28. i. Ya. SLONIM, S. G. ALEKSEYEVA, L. G. URMAN, B. M. ARSHAVA and B. L. AKSEL'ROD, Ibid. A20: 1477, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 7, 1661, 1978) 29. M. GORDON~ T, G. PARKER and W. B. TEMPLE, J, Combinatorial Theory Bll: 1971 30. S. V. KOROLEV, S. I. KUCHANOV and M. G. SLtN'KO, Polymer J. 15: 775, 785, 1983

Polymer Science U.S.S.R. Vol. 26, No. 2, pp. 296-301, 1984

Printed in Poland

0032-3950/84 $10.00+.00 © 1985 Pergamon Press Ltd

INSTABILITY OF AQUEOUS SOLUTIONS OF CATIONIC ACRYLAMIDE COPOLYMERS* O. V. KLENINA, V. I. FOMrNA, V. I. KL~n'~IN,P. K. AVETISYAN, G. P. MEDVEDEV, S. I. KLENIN,'YE. N. BYKOVA and YE. B. MILOVSKAYA Chernyshevskii State University, Saratov "Vodokanal" Board of Water-Pipe Canalization Management Institute of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences (Received 3 June 1982)

Instability with time in aqueous solutions of cationic acrylamide copolymers has been studied by viscometry. The ageing effect is manifest to a higher degree for solutions of lower concentration and for copolymers with a higher content of acrylamide units. The introduction of NaNOa into freshly prepared aqueous solutions of the copolymers stabilizes the viscosity of the system. The absence of change in the viscosity of dilute solutions of the copolymers in 1 M NaNOa obtained from aqueous solutions at different stages of ageing excludes the degradation mechanism of their instability. IN THE last few years water-soluble polymer floqulants have been widely used in various branches of the national economy. One o f the most important areas o f application is the purification o f drinking water and sewage. Analysis of the published and patent d a t a indicates that in this field the principal role is played by polyacrylamide (PAA) and ionogenic acrylamide copolymers. A number of factors contributing to the wide a d o p t i o n o f polymers o f this class for purifying drinking water and sewage m a y be singled out. They include the non-toxicity of PAA, the possibility of synthesizing P A A by the most accessible and currently best studied method o f radical polymerization, the possibility through uncomplicated chemical reactions o f transforming P A A into * Vysokomol. soyed. A26: No. 2, 271-275, 1984.

Instability of aqueous solutions of cationic acrylamide eopolymers

~7

anionic or cationic flocculants and the unique ability of acrylamide to form polymers with a very high MW. As shown by practice, the efficacy of the flocculating action of the polymer depends on its chemical nature, MW, the size and sign of the charge, etc., although the scientific bases of the use of polymer flocculants have dearly not been researched enough. The problem of so-called ageing of the aqueous solutions o f PAA and its derivatives attracting the attention of many investigators is unresolved [1-7]. By ageing is understood the instability of the properties of these solutions with time most frankly manifest in fall in their viscosity and flocculating capacity. Some authors have observed clouding of solutions on storage [5, 6]. Solutions both of industrial and carefully purified laboratory samples are subject to ageing [6]. The ageing process gradually wanes in the course of several days or weeks at room temperature in resting solution [2, 6, 7]. Some investigators are inclined to attribute the fall in viscosity of PAA solutions in time to rupture of the main chain due to particular factors [3-5] and others to aggregative phenomena [1, 2]. References [6, 7] suggest that the mechanism of ageing of solutions be linked with the conformational transition occurring in time of the PAA macromolecules and hydrolyzed PAA (HPAA) from more elongated and rigid to more flexible and compact conformations as a result of slow rearrangement of the system o f hydrogen bonds. All these explanations are by way of hypotheses none of which may yet be regarded as sufficiently substantiated. It may be that different causes are at work in different cases. This apparently explains the difference in the effectiveness of particular stabilizing additives [5, 6]. The solution of the problem of ageing of aqueous solutions of PAA and its derivatives calls for the accumulation of new experimental data. The aim o f the present work is to make a systematic study of the instability of aqueous solutions of cationic acrylamide copolymers by the viscometric method. CHARACTERIZATION OF SAMPLES

Sample No.

Low molecular weight anion

Content of acrylamide units, %

[r/l, dl/g (1 M NaNOa, 30°C)

MW x 10-6 (0"5 M NaNOa)

CHaSO~" C1C1-

60 30 10

6"2 4-5 4"8

2"45 2-10 3"37

The test samples 1-3 (Table) are the copolymers of acrylamide and N,N-dimethylaminoethylmethacry~ate alkylated by dimethylsulphate (sample 1) or methylchloride (samples 2, 3). The content of the ionogenic units was found by the method of colloidal titration [8]. The MW of the copolymers was determined by sedimentation and diffusion. The viscosity of the aqueous solutions was measured at 30°C in the Ubbelohde viscometer by the standard technique [9]. The solutions were prepared at room temperature by agitating the system with a magnetic stirrer. Then the solutions were filtered through a Sehott No. 1 filter without pressure and the concentration checked against evaporation.

298

O.V. ~ A

et al.

Taking into account the instabilityof the solutions,in all cases we strictlyobserved the regions of preparing the solutions and the time before the start of the measurements.The water-salt solution of the polymer was obtained from the aqueous by adding to it the appropriate amount of NaNOa solution. From the freshly prepared aqueous solution of concentration 0.2 g/dl by dilution we obtained solutions of lower concentrationsthe viscositiesof which were measured at specifiedtime intervals. The results of the measurements are presented in Fig. 1. For clarity of inspection the ordinate gives the relative values of the viscosity numbers A-(q,p/c)t/(~7,p/C)o , where (qop/c)t is the viscosity number of the solution at the moment of time t; 07,pfc)o is the viscosity number at the initial moment of time. The viscosity of the solutions of the samples studied at all concentrations fairly rapidly fell at first followed by a period when the viscosity changed slowly or reached a practially constant value. A similar character or change in viscosity in time was observed for aqueous solutions of PAA and HPAA [2, 6, 7]. This apparently gives some grounds for assuming that their ageing is of a common nature. The findings presented show that with fall in the concentration of the solutions the rate of drop in the relative viscosity number in the first stage of ageing and the overall change in this value increase, i.e. more dilute solutions are unstable to a higher degree. Evidently the higher viscosity slows the rearrangement of the structure of the concentrated solutions. From this follows the conclasion of practical importance that where necessary it is desirable to store solutions of sufficiently high concentration.

A

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Fie. 1. Kinetic functions of change in the relative viscosity number of solutions of samples 1 (a), 2 (b) and 3 (c) of concentration 0.094 (1), 0.045 (2), 0-023 (3), 0.014 (4), 0.009 (5), 0-0047 (6) and

0"0023 (7) g/dl.

Another notable aspect is that higher instal~ility with time is shown by the*aqueons solutions of the samples with a higher content of acrylamide units. This is made clear by comparing the curves in Fig. 1 corresponding to the same concentration of polymer in solution of 0.023 and 0.009 g/di. Thus, the viscosity number of the solutions 0.009 g/dl of samples 1-3 containing 60, 30 and 10 ~o acrylamide units fell in seven days by

Instability of aqueous solutions of cationic acrylamide copolymers

299

90, 50 and 12% respectively. This and the general character of the kinetics of ageing of PAA solutions and anionic and cationic acrylamide copolymers suggest that the acrylamide units are mainly responsible for the instability of the solutions. The literature provides information indicating that additions o f certain substances, in particular, the salts of monovalent metals to aqueous solutions of PAA and its derivatives reduce the ageing effect [5]. The influence of the salt NaNO3 on the viscosity of solutions of cationic acrylamide copolymers was studied. Figure 2 presents the kinetic functions ~7,p/c o f aqueous and water-salt solutions o f samples 1 and 3 of concentration 0.045 and 0.047 g/dl respectively for different concentrations o f NaNOa. These solutions were obtained from freshly prepared aqueous solution of concentration 0-2 g/dl and then stored in the appropriate water-salt medium. It will be seen that a sufficiently high concentration of salt has a stabilizing effect on the s2cstem. The value rhp]C o f sample 3 no longer depends on time in 0.01 M solution of NaNO3 ; for sample l at this concentration o f salt stability of the solution was not reached. Measurements o f rhp]C with time of all samples in 1 M solution of salt at 0.02 g/dl ~
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Fio. 2. Dependence of rhp/e on time of storage of samples 1 (a) and 3 (b) of the aqueous solutions (1) and water-salt solutions at concentrations of NaNO3 0-01 (2), 0.1 (3) and 1 (4) mole/l. Fro. 3. Dependence of q~p/c of aqueous solution (1) and water-salt (1 M NaNO3) solutiorts (2) of concentration 0.041 (a) and 0-021 (b) g/dl on the duration of storage of solution. The experiments on the effect of the salt additives on the stability of tl-,e aqueous solutions o f the ionogenic PAA derivatives may give some information on the nature o f ageing but salts apparently do not find application as stabilizers in practice since

300

O.V. KLE~NA et al.

they, heavily reducing the electrostatic interaction, tighten the coils and reduce the viscosity of the system thereby robbing the polyelectrolytes of their main advantages. The following series of experiments was so run that the solutions of the acrylamide copolymers were prepared and stored in an aqueous medium but at different stages of ageing before measuring the viscosity NaNOs was added to them. The limiting viscosity number [r/] of samples 1 and 3 in 1 M NaNOa was measured during storage of aqueous solutions of concentration 0-2 g/dl. The first concentration of 0.1 g/dl for measuring rlsp/c was obtained by mixing equal volumes of aqueous solution of polymer and 2 M NaNOa, then dilution was carried out with 1 M salt solution. Itwas found that [r/] of these solutions, at least in the course of tlarce weeks, remained unchanged. This means that in this case fall in the viscosity of the aqueous solutions was not connected with degradation of the macromolecules. Since the ageing effect is more clearly manifest in more dilute aqueous solutions, it was of interest to run such experiments on storage of solutions of lower concentration. For this purpose we measured rlsp/c of water--salt solutions (1 M NaNOa) of sample of concentration 0.02 g/dl obtained from stored aqueous solutions (c =0.041 and 0.021 g/dl). The minimal concentration of polymer (0.02 g/dl) was determined by the possibility of making a sufficiently precise measuremer~t of viscosity in the water-salt medium. To the aqueous solution of concentration 0.021 g/dl before measurement was added the corresponding amount of salt in dry form. Figure 3 shows that while ~hp/C for the aqueous solutions of the polymer considerably fell, rl~p/c for the solutions in the watersalt medium did not change with time. The magnitude [r/] (or rlsp/c) is very sensitive to the MW and if on ageing of the aqueous solutions the chain were ruptured, this would undoubtedly affect its value. It should be noted that in a period of time limited to 30 days no clouding of the aqueous solutions of the copolymers was observed. The authors of [6, 7] showed by light scatter that the MW of well purified laboratory PAA and HPAA samples on ageing of their aqueous solutions remains constant. Thus, the instability of the aqueous solutions of these samples like the cationic acrylamide copolymers studied by us is not connected with degradation of the macromolecules. The possibility of explaining the phenomenon of ageing of aqueous solutions o f PAA and its derivatives by conformational change in the macromolecules [6, 7] calls for further experimental and theoretical evidence. Such an explanation assumes that the relaxation time of the conformation of the polymer chain is of the order of several days. Relaxation times of such an order are characteristic of protein macromolecules with their multiplicity of types of interactions and complex secondary and tertiary structure [10], but for flexible synthetic polymers the intramolecular relaxation time is measured in fractions of a second [5]. It appears more likely that in this case the longterm processes (if they do not reflect change in the chemical structure of the macromolecules) are linked with the supramolecular fdrmations including a huge number of molecules. Many investigators consider that the instability of aqueous solutions of PAA and its derivatives is essentially due to the duration of the process of dissolution of the

Instability of aqueous solutions of cationic acrylamide copolymers ~

301'

polymer, the existence in the freshly prepared solution of incompletely dissolved swollen particles, unravelling of the long intertwined chains, etc. [1, 2]. In the case of cationic acrylamide copolymers the time taken to establish the equilibrium state of aqueous solutions may relate to the diversity of the character o f the interactions between the macromolecules and their role in influencing both the conformation of the macromolecules and the state of the system as a whole. Besides the hydrogen bonds, hydrophobic interactions between the alkyl groups are possible in cationic copolymers. The local hydrocarbon surrounding of the fixed charges on the chain may enhance the role of the ionic interactions which, on the one hand, will lead to intensification of the binding of counter-ions and, on the other, to ionic inter- and intramolecular crosslinks. Translated by A. CROZV REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

N. NARKIS and M. REBHUM, Polymer 7: 507, 1966 W. P. SHYLUK and F. S. STOW, J. Appl. Polymer Sci. 13: 1023, 1969 C. BRUCE and W. H. SCHWARZ, J. Polymer Sci. 7: 909, 1969 H. C. HAAS and R. L. MAC DONALD, J. Polymer Sci. 10" 461, 1972 M. CHMELIR, A. KUNSCHNER and E. BARTHELL, Angew. Makromolek. Chem. 89: 145, 1980 W.-M. KULICKE and J. KLEIN, Angew. Makromolek. Chem. 69: 189, 1978 W.-M. KULICKE and R. KNIEWSKE, Makromolek. Chem. 182: 2277, 1981 S. KAWAMURA, G. HANNA and K. $HUMATA, J. Amer. WaterWorks Ass. 59: 1003, 1967 S. R. RAFIKOV, S. A. PAVLOVA and I. I. TVERDOKHLEBOVA, Metody opredeleniya molekulyarnykh vesov i polidispersnosti vysokomolekulyamykh soyedinenii (Methods of Determining the Molecular Weights and Polydispersity of High Molecular Weight Compounds), p. 281, Akad. Nauk SSSR, Moscow, 1963 R. LAMRY and R. BILTONEN, Struktura i stabilnost' biologicheskikh makromoleku! (Structure and Stability of Biological Macromolecules). p. 7, Mir, Moscow, 1973