Makromol. Chem., Macromol. Symp. 39, 155-169 (1990) 155

STRUCTURE FORMATION AND GELATION PHENOMENA IN SOLUTIONS OF TERNARY INTERPOLYELECTROLYTE COMPLEXES

V.A.Kabanov*, A.B.Zezin, V.A.Izumrudov, T.K.Bronitch, N.M.Kabanov, 0.V.Listova

Department of Polymer Science, Moscow State University, Moscow, USSR

Abstract: The representative of the new family of mechanically reversible gels is described. The gel is formed by mixing of an aqueous solution of non-stoichiometric interpolyelectrolyte complex of po- ly(sodium methacrylate) and poly(N-ethyl-4-vinylpyridinium bromide) containing a certain amount of covalent links between oppositely charged polyions, with aqueous solution of poly(potassium vinylsulfate). The gelation mechanism arises to the partial replacement of the electrostatic contacts between the polycation and poly(methacry1ate) anion in the original polycomplex with those between the polycation and poly(vinylsu1fate) polyanion. The network of the gel is most probably formed by the nodes consisting of covalent links and interpolyelectrolyte double-strand electrostatic clusters, united by poly(sodium methacrylate) tie-chains.

It is a well-known fact that the cooperative interaction between

oppositely charged polyelectrolytes results in formation of in-

terpolyelectrolyte complexes (IPC). The properties of IPC are to

great extent determined by their composition. Depending on the

mole ratio of the oppositely charged repeating units one can

distinguish the stoichiometric and nonstoichiometric interpoly-

electrolyte complexes ( SIPC and NIPC, respectively ) . In this

paper we will consider mainly NIPC consisting of relatively long

polyanions (host polyelectrolyte ( HPE)), which units are incor-

porated in NIPC species in some excess, and relatively short po-

lycations (guest polyelectrolyte (GPE)). Such NIPC have been

studied in detail and described in literature [l-51. They are

soluble in aqueous solutions of simple salts when their composi-

tion, defined the ratio of amount concentrations of repeating

units v=[GPE]/[HPE] , does not exceed some critical value which

0 1990 Hiithig & Wepf Verlag, Basel CCC 02.58-0322/90/$02.00

156

is less than 1; p=l corresponds to S I P C .

In the simplest case an N I P C macromolecule in dilute solutions-

considered to be a peculiar segmented copolymer, containing

single-strand and double-strand blocks[1,2,4]:

The relatively hydrophobic double-strand blocks are the coupled

regions of HPE and GPE polyions. They alternate with the charged

hydrophilic single-strand blocks consisting of the 88100ps9t and

t8tails88 of HPE polyion. The double-strand blocks in their turn

may involve the defects presented in Scheme (1) as small loops.

The reversibility of the electrostatic binding provides the abi-

lity of relatively short GPE chains to migrate along the HPE

chain. It causes the conformational transformations characteri-

stic for N I P C and the effects of intra- and intermolecular seg-

regation of the hydrophobic blocks [5-71.

It is worth to mention that besides the migration within one

N I P C macromolecule, GPE polyions are able to migrate from one

N I P C species to another. This effect manifests itself in dispro-

portionation reactions which can be observed for example under

the phase separations in N I P C solutions [5,6]. The processes

which take place under N I P C desalination are presented in Scheme

(2):

in solution in solution in precipitate

A s it follows from the scheme, on addition of the low molecular

weight salt, some polycomplex species of NIPC transform into

stoichiometric ones (SIPC) due to transfer of GPE polyions from

the others, and then precipitate. Those NIPC species which now

carry less amount of GPE than the original ones, remain in the

solution. Such re-distribution of GPE polyions between the simi-

lar HPE polyions represents an example of interpolyelectrolyte

exchange reaction.

GPE polyion interchange may also occur between HPE chains of a

different chemical structure. Such a process presented in Scheme

( 3 ) is considered to be an interpolyelectrolyte substitution re-

action:

NIPC (HPE2 - GPE ) + HPEl + HPE-’ ( 3 ) _.

NIPC ( H P E ~ - GPE

The equilibrium, kinetics and mechanism of substitution were co-

nsidered in [ 8-10 1. In particular, it was shown that the di-

rection and rate of the reaction depend upon the nature and con-

centration of low molecular weight salts in the reaction mixture

and also upon the degree of polymerization of HPE1, HPE2 and

GPE polyions.

Later we will discuss mainly substitution reactions, in which

HPEl and HPE2 are high molecular weight poly(methacry1ate) (PMA)

and poly(vinylsu1fate) ( P V S ) , respectively, and GPE is poly(N-

ethyl-4-vinylpyridinium) (PEVP) of a lower degree of polymeriza-

tion:

NIPC ( PMA - PEVP ) + PVS -> IPC ( PVS - PEVP ) + PMA ( 4 )

Let us note that poly(su1fonate)- and poly(su1fate)- containing

158

polyanions, as a rule, replace completely poly(carboxy1ate)-

containing polyanions in IPC. In other words, the reaction ( 4 )

equilibrium is completely shifted to the right. The driving

force for such a shift probably arose from the difference not i.n

purely electrostatic but rather specific interactions of -CO@

and - S e groups with a repeating unit of a polycation.

The reaction ( 4 ) was studied by means of fluorescence quen-

ching measurements. The fluorescent anthryl tags were introduced

into poly(sodium methacrylate) (PMANa). The tagged PMANa(PMA*Na)

with weight-average degree of polymerization,Fw= 3 0 0 0 , contained

one anthryl group per 500 repeating units on the average. PEVP

polycations themselves are found to be the effective fluorescen-

ce quenchers [11,12]. In this work we used PEVP bromide, Fw=lOO.

It was shown that the direction and kinetics of reaction ( 4 )

with participation of PMA*Na can be followed by measuring of a

fluorescence intensity of the reaction mixture. The transfer of

PEVP-quencher from PMA*Na-PEVP species to PVS polyanions intro-

duced into the reaction mixture, for example, in the form of

poly(potassium vinylsulfate) (PVSK) , (Fw=1300) is accompanied

by an increase of the fluorescence intensity. This process is

represented in the following Scheme:

+ (5)

NIPC *

159

The kinetic curves of the reaction (5) are represented in Fig.1

as time dependences of ( It - 1, ) / ( Im - I. ) , where I. is the

fluorescence intensity of the original solution of NIPC*, It and

I, are the fluorescence intensities of the reaction mixtures at

the instant t and at t-oo. - 1

150 160 t/min

Fig.1. The kinetic curves of the substitution reaction between NIPC* ( PMA* - PEVP ) , (p=O,33 and PVSK in NaCl solutions of various concentrations: without NaCl (a) : 0,002M (b) ; 0,004 M (c); 0,006 M (d); 0,Ol M (e); pH 10, 2OoC. [PMA*Na]=0,004 base mol/l, [PEVP]=[PVSK].

It can be seen that in the salt-free systems no substitution re-

action occurs between the above mentioned reagents. The reaction

rate noticeably increases in the presence of the low molecular

weight salt. When NaCl concentration equals O.OlM, this reaction

completes in a few seconds after the mixing of NIPC* and PVSK

solutions [8,9].

When PEVP(GPE) and PVSK(HPEJ units are taken in equivalent ra-

tio, the reaction (5) completion according to the fluorescence

measurements corresponds to practically complete replacement of

PMA*-PEVP salt bonds with PVS-PEVP ones. It follows from the da-

ta shown in Fig.1 and Fig.2.

160

0,5 180 [ PVSK] / [ PEVP]

Fig.2.The dependence of the infinite relative fluorescence in- tensity I m p o (NIPC*(PMA*-PEVP), q=0,33,+PVSK) upon [PVSK]/[PEVP] ratio: pH=lO; 25OC. [PMA*Na]=0.004 base mol/l in 0.025M NaC1. The measurements were accomplished 5 min after the reagent mixing and then were repeated after 10 hours. Io-fluorescence intensity of PMA*Na solu- tion the concentration of which equals its concentration in the reaction mixture.

% F

t 0,5 1,O [ PVSK]/ [ PEVP]

Fig.3. The specific viscosity of the mixture of NIPC (PMA-PEVP) , (p=0.33, and PVSK YS. [PVSK]/[PEVP] ratio in the reaction mixture. pH=lO at 25OC in 0.025M NaC1. The total concentration of polyelectrolytes is 0.4 g/100 om3.

However, it is worth to mention that the viscosity of the reac-

tion mixture which was measured 5 min after mixing of NIPC (PMA-

PEVP) and PVSK, i.e. when according to the fluorescence quen-

ching data the replacement of one type of interpolyelectrolyte

161

contacts with another one was practically completed, was noti-

ceably higher than the viscosity of the NIPC original solution

(Fig.3).

I I :: I I

I r n ln :::I 1,45 I I I

I I T) at t - m o I - - - - - - - - - - - - - - - - - ' E - - - -

Fig.4. vSp of the solution of NIPC, (p=0.33 (PMA+PEVP)+PVSK vs. time. [PEVP]=[PVSK]=0.003 mol/l, pH=lO at 25OC in 0.025M NaC1, t=O corresponds to the moment of mixing of the re- agents. The total polyelectrolyte concentration is 0.14 g/ io0cm3.

Thus the data presented in Fig.3 apparently testify to the for-

mation of polyion associates the lifetime of which exceed the

time required for replacement of the overwhelming majority of

the electrostatic contacts in the original NIPC. Indeed, if mi-

xing of the reagents was not accompanied by additional polyion

association, the addition of PVSK solution, which had relatively

low viscosity, to NIPC solution should be followed not by an in-

crease but by a decrease of the viscosity of the resultant mix-

ture.

The formation of associates can be easily indicated from sedi-

mentograms of the reaction mixtures, where one can observe the

162

peak of the rapidly precipitating fraction (sedimentation coeff-

icient is in the range 10-20 Sv at total polyelectrolyte concan-

tration 1.26-1.44 g/100 cm3 and [PVSK]/[PEVP]=0.2-0.8). The se-

dimentation coefficient and the area under the sedimentation

peak are increased with an increase of the PVSK amount added to

the reaction mixture. However, as time goes on a gradual decrea-

se of the viscosity of the reaction mixture occurs (Fig.4), ap-

parently due to self-dispergation of these associates.

The data presented above are in a good agreement with the fol-

lowing schematic representation of an interpolyelectrolyte sub-

stitution reaction:

-- - I PC (HPE,-CPE) + HPE,, ($pee)

One may conclude that the above mentioned associates containing

all three types of the reacting polyions, are the intermediate

products ( A ) of a substitution reaction. Formation of such in-

termediates is regarded to be a peculiar feature of polyion rep-

lacement in IPC. They are formed as a result of collisions and

interpenetration of NIPC and HPEz coils, accompanied by the for-

163

mation of the new salt bonds between GPE and HPE2. If the life-

time of HPE2-NIPC mixed complex is long enough its species can

aggregate to form new larger associates (B) involving a consi-

derable number of HPEl chains. It is worth to mention that HPEl

chains lose to a great extent their connection with GPE because

of the rapid substitution of the majority of HPEl-GPE contacts

for HPE2-GPE ones. Then HPEl polyanions are slowly released from

the associated particles. The latter ensues from the compari-

son of the rates of fluorescence intensity and the viscosity

changes of the reaction mixture. Now, one may distinguish two

stages of the substitution reaction. The rapid stage corresponds

to formation of ternary associates in which the most of HPEl-GPE

interpolyelectrolyte contacts are replaced with HPE2-GPE ones.

The slow stage corresponds to spontaneous dispergation of the

associates formed at the first stage and a release of HPEl poly-

anions.

The slow stage can he considerably accelerated by addition of a

low molecular weight salt, i.e. by increase of the ionic

strength of the reaction system. In this case as it follows from

the kinetic study of exchange and substitution reactions of NIPC

[8,9], rearrangements of the units of any oppositely charged po-

lyanions are accelerated . In particular, the final equilibrium state in the reaction system shown in Scheme (6), which at

[GPE]=[HPE2] corresponds to formation of the new insoluble SIPC

(GPE-HPE2) and a release of HPEl polyions, can be achieved in

less than 1 min at [NaC1]=0.3M.

However, the reaction (6) can be fully terminated at the stage

of formation of a ternary polycomplex by introduction of a small

amount of covalent cross-links between GPE and HPEl polyions in

the original NIPC species.

164

For this purpose poly(4-vinylpyridine) , (Fw=170) (PVP) was syn-

thesized containting about 2% of the repeating units quaternized

by ethylene bromohydrine. The rest of PVP units were exhaustive-

ly quaternized by ethyl bromide. The solution of the modified

PEVP was mixed with PMA*Na (Pw=4100) solution to obtain the so-

lution of the modified NIPC (mNIPC).Then the water-soluble car-

bodiimide as a condensing agent was added to the solution and

the following cross-linking reaction between the carboxylate

groups of PMA anion and hydroxyl groups of 2-hydroxyethyl substi-

tuents of mPEVP occurred [13]:

The number of obtained ester bonds which essentially did not ex-

ceed 2 mol-% of the amount of GPE units, was quite sufficient to

immobilize covalently practically all PEVP polycations. The ab-

sence of NIPC dissociation to the constituent polyions in the

solution at high simple salt concentration was regarded to be a

criterion of the complete cross-linking. At the same time the

amount of the cross-links was so small that physico-chemical

properties (viscosity and sedimentation coefficient) of mNIPC

and corresponding NIPC practically coincided.

The presence of the few covalent cross-links did not prevent

nearly complete replacement of the PMA-PEVP electrostatic

contacts with PVS-PEVP ones and did not influence the rate of

such a reaction. This conclusion is a result of our investigati-

ons of the kinetics of the reaction between PVSK and fluores-

cent-tagged mNIPC. The substitution reaction between mNIPC and

PVSK results in formation of ternary polycomplexes of the type B

(see Scheme 6 ) . However, the introduction of PVSK into mNIPC so-

165

4.0

3.0

2.0

1.0

lution is accompanied by a much sharper increase of

sity of the

the corresponding non-modified NIPC (Fig.5).

It is seen that experimental values of qsp noticeably exceed

the corresponding additive values which were calculated on the

the visco-

reaction system than it is observed in the case of

-

-

-

-

assumption

% F

that there is no interaction between the reagents.

2 . 0 .

/ /

0

0.1 0.2 'NIPC in wt. -s(

Fig.5. qsp of the mixture of the cross-linked mNIPC and PVSK (1) and calculated additive q of the same mixture (2)

vs. the total polyelectrolyte concentration. pH=lO at 25OC in 0.025M NaC1, [PVSK]=[PEVP].

SP

The value of q of the reaction mixture does not change in ti-

me. When polyelectrolyte concentration exceeds some certain

value Cc,, the overall reaction mixture becomes a gel [14]. The

gelation takes place even at very low polyelectrolyte concentra-

tions (of the order of 0.1 g / l O O cm3). The probable structure of

such a gel fragment is represented by Scheme (8).

SP

The covalent cross-links between GPE and HPEl involved in clus-

ters formed by the fragments of IPC(GPE-HPE2), are most probably

166

i

the nodes that form the gel. At the scheme presented above these

nodes are depicted by the dashed circles. The free fragments of

HPE,(PMA) chains are the tie-chains of such three-dimensional

structure.

The results of the investigations concerning the properties of

the interpolyelectrolyte gels are found to be in a full agree-

ment with the given structural representations. Indeed, the gels

do collapse under acidification (pH<5) and recover under the ad-

dition of an alkali (pH=lO). This collapse is due to the rever-

sible change of PMA tie-chain conformations as a result of proto-

nation of carboxylate groups and loss of their negative charge.

It is known that formation of a specific compact structure is

quite characteristic for poly(methacry1ic acid) in acidic media.

The state of the network nodes , i.e. PEVP-PVSK interpolyelect-

rolyte clusters formed by the two strong polyelectrolytes, of

course, does not change under pH variation within the defined

range. Addition of simple salts (for example NaCl), which

shields the electrostatic repulsion between CO@ groups of PMANa

tie-chains, also results in contraction of the gel. When NaCl

concentration exceeds 0.15 M the gel spontaneously dispergates

167

into the discrete particles of 0.1 mm in size.

The introduction of the additional amount of PEVP polycations

(GPE) to the gel is followed by its sharp contraction and for-

mation of a compact precipitate containing all three polyele-

ctrolytes. This phenomenon is due to interaction of added PEVP

with PMANa tie-chains and in the end to the formation of rather

poorly hydrated IPC(PMA-PEVP).

The mechanical properties of the interpolyelectrolyte gels are

also found to be in a good agreement with the model presented

above (see Scheme 8). The dependencies of the shear stress upon

the time of the gel deformation at various shear rates are rep-

resented in Fig.6.

r / 8

Fig.6. Shear stress (P) vs. deformation time at various shear- rates (1: 10s-' ; 2 : 0.4s-' : 3 : lOOOs-') . The moments of unloading for 1 min are depicted by arrows. Gel concent- ration 1 g/100 cm3, pH 10 at 25OC in 0.025 M NaC1.

It is seen that when the shear rate is 10s-l there is a distinct

yield point (overstrain peak) on the stress-time curve 1, which

disappears at smaller shear rates (curve 2). One can also notice

that these curves exhibit regions of the steady viscous flow.

It is worth to mention that after 1 min "rest" (depicted by

168

arrows in Fig.6) these curves are reproduced. The recovery of

the gel structure is regarded to be due to a reversible charac-

ter of the destruction and recovery of the salt bonds involved

in the network nodes. The yield point being extrapolated to the

zero shear rate is accounted for 10 Pa at 25'C. The viscous flow

of the system under shear stresses exceeding the yield point is

governed by the mechanically activated interchange of GPE ( P E V P )

polycations between the polycomplex nodes of the network. At the

high shear rates the value of the shear stress is gradually dec-

reased (Fig.6, curve 3 ) . The 88restff of such systems does not

lead to a complete recovery of the gel structure. It gives

evidence to an irreversible character of the destructive proces-

ses most likely consisting of the mechano-chemical rupture of

PMANa tie-chains or mechanically activated hydrolysis of ester

cross-links between PMA and PEVP. Under the experimental condi-

tions in the latter case the rate of the polyion exchange reac-

tion is not sufficient to promote the reversible rearrangement

of the gel network. However, the region where the gel is charac-

terized by complete mechanical reversibility (thixotropy) is

found to be rather wide.

Thus, studying the above ternary polyelectrolyte system we found

out the new possibility of formation of reversible gels sensiti-

ve to changes of pH and ionic strength. The studied gel is able

to undergo a reversible transition to the viscous state under

applied stresses.

169

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