Colloid & Polymer Science Colloid Polym Sci 269:131-137 (1991)

fluorescence and viscometry study of complexation of poly(acrylic add)with poly(acrylamide) and hydrolysed poly(acrylamide)

K. Sivadasan4)', P. Somasundaran', and N. TurroZ)

1) Langmuir Center for Colloids and Interfaces. Henry Krumb School of Mines and~ Department of Chemistry, Columbia University, New York, USA

Abstract: Interpolymer complexation ofpoly<acryiic acid) with poly(acrylarnide) andhydrolysed poly<acrylarnide) was studied by fluorescence spectroscopy and visco-metry in dilute aqueous solutions. Changes in chain conformation and flexibility dueto the interpolymer association are reflected in the intramolecular excimer fluores-cence of pyrene groups covalently attached to the polymer chain. Both poly<acryla-rnide) and hydrolysed poly(acrylamide) form stable complexes with poly(acrylicacid) at low pH. The molecular weight of poly(acrylic acid) and solution propertiessuch as pH and ionic strength were found to influence the stability and the structureof the complexes. In addition. the polymer solutions mixing time showed an effect onthe mean stoichiometry of the complex. The intrinsic viscosity of the solutions ofmixed polymers at low pH suggested a compact polymer structure for the complex.

Key words: !nterpolymer fomplexation; ~xcimer fluorescence; fooperative !!inding;~quilibration !ime; mean !toichiometry.

Introduction

Macromolecules in solutions often interact witheach other, resulting in excluded volumes andordered structures through the formation of inter-macromolecular complexes. It is well known thatintermacromolecular complexes play an importantrole in many biological functions [1, 2]. An investiga-tion of the mechanism and the factors affectingcomplexation using well-characterized syntheticpqlymers is of importance as these systems serve asmodels for complex biopolymers. In solutions con-taining mixtures of two chemically different macro-molecules, the interactive forces responsible forcomplexation are usually secondary binding forcesclassified as electrostatic, hydrogen bonding, andhydrophobic. The structure, as well as the stabilityof the complexes, depends on the type of interac-

tions, the solution conditions, and macromolecularcharacteristics such as pH, ionic strength, tempera-ture, solvent, molecular weight, and conformation.For example, in systems containing polyelectrolytesthe stability of the complexes depends on the pH ofthe medium since the interactions of the bindingsites with the solvent and the conformation of thepolyelectrolytes are affected greatly by pH.

Several studies have been reported [1-5] on inter-polymer complexations between hydrogen bonddonor and hydrogen bond acceptor polyr:ners. Fluo-rescent methods [6-10] based on the photophysicaland photochemical properties of covalently bondedfluorescent probes to the polyr:ners have been foundto be both sensitive and informative for investiga-tion of interpolyr:ner complexations. In mixed poly-mer systems, information on the behavior of poly-mer chains on a molecular level can be obtainedusing fluorescence methods, while the conventionalmethods (e.g., viscometry) provideimormation onlyon average properties representing the whole sys-

.) Post doctoral research scientist from Chemistry Section.Oil &; Natural Gas Commission. Bombay. India.

K 191

132 Colloid 11M Polymer Sdena, Vol. 269 . No.2 (1991)

tern. In our own earlier investigations [11,12) we haddemonstrated the use of pyrene-labeled polymers tostudy polymer-surfactant interactions and interpo-Iymer complexations. Oyama et al. also studiedinterpolymer complex formation by the reduction ofexcimer emission in pyrene-labeled polymers [13).Complexation of poly(acrylic acid) with poly(acry-lamide) and poly(acrylic acid-coacrylamide) usingpyrene-labeled polymers is reported in this paper.Effects of complexation conditions such as pH, ionicstrength, and the polymer molecular weight on thecomplexes are discussed. The extent ofexcimer for-mation is used as a parameter which measures thechange in conformation and flexibility of the poly-mer chain.

Polymer solutions were prepared in triple distilled water 24 hbefore the experiments to ensure complete dissolution of thepolymers. The mixing of solutions and the adjustments of pHand ionic strength were carried out just before measurements ofphotoemission of the label.

Fluorescence measurements

The steady-state fluorescence emission spectra of the pyrenelabel were recorded on a SPEX Fluorolog Spectrofluorometerusing excitation at 333 nm. The ratio of excimer (480-490 nm) tomonomer (376 nm) intensities, 1./1.,. was detennined for varioussolution conditions. The concentration of the labeled polymer inthese measurements was kept at low levels (5 x 10-4 monomermol/L) to avoid intermolecular excimer formation. AU the meas-urements were carried out at room temperature (25 1: 2.C).

.I'

,Viscosity mt4sUrements

Viscosity measurements of polymer solutions were carriedout using an Ubbelohde suspension-type capillary viscometer at30 :f: 0.05 °C. The intrinsic viscosity [1]1 and the Huggins constantk' were determined from Huggins equation. Shear rate correc-tions were not applied to these data.

ExperimentalMaterials

Poly(acrylamide) (PAAm), pyrene-labeled poly(acrylamide)(PyP AAm), and hydrolysed poly(acrylamide) (PyHP AAm) wereprepared at the National Chemical Laboratory, Pune, India aspart of an Indo-US joint research program. 2-{4-(I-pyrene)buta-noyl)aminopropenoic acid used as the label was synthesized asdescribed elsewhere [11). Acrylamide and 2-{4-(I-pyrene)buta-noyl) aminopropenoic acid were copolymerized by precipita-tion polymerization in DMF at 6S °C using azobis(isooutyroni-trile) as initiator. Pyrene-labeled hydrolysed poly(acrylamide)was obtained by controlled hydrolysis [14] of labeled pOly(acry-lamide). Molecular weights and the pyrene content estimatedfrom UV absorption are listed in Table 1 along with the manufac-turer specified molecular weights of poly(acrylic acid) (PAA)samples purchased from Polysciences. Molecular weights of theunlabeled poly(acrylamide) and commercial samples are desig-nated using a numerical index and the actual molecular weight is1000 times this number.

Results and discussions

Poly(acrylamide) and partially hydrolysed poly(-acrylamide) behave as random coils in dilute solu-tions in the presence of simple electrolytes. The ran-domly distributed pendant pyrene groups at lowconcentrations on the polymer chain did not affectits dissolution characteristics significantly. Pyrenegroups are constantly in motion due to the highmobility of the polymer segments to which they arecovalently attached. An excited pyrene group inter-acts with a ground state pyrene group to form anexcimer when they approach each other withinabout 4-5 A The distance between the pyrenegroups on the polymer chain is determined by itsconformation and the mobility of the segments.Thus, the extent of intramolecular excimer forma-tion (given by the parameter I J I".) provides a meas-ure of the statistical conformation of the labeledpolymer chain. A large value of I ell m suggests poly-mer chain contraction and/or segmental mobility,whereas a small value of I JI m suggests polymerexpansion and rigidity. The interaction of the functi-onal groups is depicted in Fig. 1. Complexatio.nincreases the rigidity of the polymer coil and effecti-vely separates the pyrene labels attached to it.

Figure 2 shows the plots off JIm ratios of pyrene-labeled poly(acrylamide) and hydrolysed poly(-acrylamide) in the presence and the absence of

Table 1. Molecular weight. pyrene content. and source of poly.mer samples used in the study

Polymersample

Molecularweight, M.,

Pyrene content,we~ht%')

Source

PyPAAm 7.3 x 1~PyHPAAma) 6.8 x I~PAAm-l000 1.0 x loA)PAA-5 5.0 x 10'PAA-90 9.0 x lotP AA-800 8.0 x 10'PAA-3000 3.0 x l~

NCL"}NCLiNCLiPolysciencesPolysciencesPolysciencesPolysciences

}.1~.---:;~

O) extent of hydrolysis, 14.1 mole % by potentiometric titra-tion; , viscosity average molecular weight calculated from (13);') by IN absorption method, ~ 342 nm (IlL 4.{1-pyrene)buta-noic acid as standard; i National Chemical Laboratory, Pune,India

high pHlow pH

COOK...J ""'&JK ..:1!:!"~---r-a>oH

--~~

to

-':(!ON;;~~~~---~~NH 2

HPAAm+

-~~-r ~H2a>oH

-'~~~!~."'.""'-_.~~)O'

.; r-

~,PAA

+PAAm

- . -

0"0I ~

:I: !J. ...+~/NH2---'--

Fig. L The interactingfunctionaIgroups ofthe polymers at low and high pH

pol)'<acrylic acid) asa function of pH in 0.05 M sodi-um chloride solutions. Poly(acrylamide) being anonionic polymer, pH variation is not expected toaffect its coil size. In the low pH region, the slightincrease in Ie/1m with decreasing pH for PyPAAmmay be due to a reduction in coil size that resultsfrom pH effects on polymer-solvent interactions orto an inC:rease in ground state pyrene interactions. Inthe presence of pol)'<acrylic acid), Ie/1m ratiosremain unchanged at higher pH. As the pH isdecreased below 5.0, the ratio increases sharply toreach a maximum at pH 4.3 and then decreases untilphase separation is observed at pH 3.4. The analysisof the precipitate confirmed that PyP AAm and P AAformed an equimolar complex at the phase separa-tion pH. The hydrolysed pol)'<acrylamide) (copoly-mer of acrylamide and acrylic acid) in solution COI)-tracts to a more coiled conformation as the pH isdecreased. This is due to the decrease in the chargedensity on the polymer chain as a result of thereduc-tion in number of ionized acid groups with pH. Inthe case of PyHP AAm-P AA mixture, the values fol-

3 ~ 6 8~

'1 . g

Fig.].. Change in intramolecular excimer formation as a funCtionof pH for pyrene-labeled poly(acrylamide) (PyP AAm) and pyre-ne-labeled hydrolysed poly(acrylamide) (PyHP AAm) in thepresence and the absence of poly(acrylic acid) (PAA-90). (Con-centrationsofpolymer:PyPAAmand~AAm.4 x 10-4 basemol/L; PAA-90, 4 x 10-3 base mol/L)

134 Colloid and Polymer Sciena, Vol. 269 . No.2 (1991)

low the same trend as that of PyHP AAm solutiondown to pH 4.2, below which a sharp increase andthen a decrease similar to PyP AAmlP AA isobserved.

The changes inl ell ",in the presence ofpoly(acryl-ic acid) can be attributed to the following changes inpolymer conformation. At high pH conditions, in-teractions between the polymer chains are neglig-ible. The number of undissociated acid groupsavailable for complex formation is low and thecharged carboxylate groups present on the polymerchain destabilize the complex through coulombicrepulsions. As the pH is decreased the poly(acrylicacid) coil contracts and the unionized acid groupsbegin to interact with the amide groups of poly(-acrylamide) forcing its chain to adopt, along withpoly(acrylic acid) chain, a more coiled conforma-tion. The increase in I ell", ratio below pH 5 is attri-buted to this initial coiling of PyP AAm . Furtherdecrease in pH increases the contacts between thepolymer chains producing a compactstructure. Dueto the decreased intramolecular mobility of thepolymer molecule in this compact structure of thecomplex, the motion of the polymer segments andpyrene probes attached to them is restricted [IOJ anda reduction in excimer formation results.

Poly(acrylamide) (P AAm) has been reported12,4,15) to form a complex with poly(methacrylicacid) (PMAA). The complex formation is throughhydrogen bonds as well as ion-dipole interactionsbetween some of the amide groups of poly(acryla-mide) which are partially protonated, and C = 0dipoles of the carboxyl groups of poly(methacrylicacid) [2,15). The hypercoiled structure of poly(me-thacrylic acid) also contributes to the stability of thecomplex. Recently, Wang and Morawetz [I6J report-ed complexation of poly(acrylic acid) with poly(-N,N-dimethylacrylamide-co-acrylamide) based onthe fluorescence properties of dansyl groupsattached to poly(acrylic acid). They observed thatcopolymers with more than 45 mole % acrylamidedid not form a complex with poly(acrylic acid) at pH4, even though at pH3 acrylamide contributed sig-nificantly to the stability. They assumed the acryla-mide group was a much weaker hydrogen bondacceptor than the dimethylacrylamide group andthe stability of the complex was entirely due to thelatter. Since the probe used in this study reports onlythe hydrophobicity of the complex, the complexformed with the copolymer of higher acrylamidecontent is less hydrophobic and the emission char-

acteristics of the dansyllabel may be only slightlyaffected by these complexes. Our results show thateven a copolymer of acrylamide and acrylic acid in-teracts with poly(acrylic acid) to form a stable com-plex at pH below 4. Oyama and coworkers [17) havealso reported pyrene label to be more sensitive tointerpolymer complexation than dansyllabel.

Figure 3 describes the effect of ionic strengthon complexation between poly(acrylamide) andpoly(acryl acid). As the ionic strength is increasedthe pH at which the interpolymer interaction beginsdecreases. Ionic strength affects both the dissocia-tion of polyacid and the ion-dipole interactionswhich favor the complex formation. Reported pKovalues [18] for poly(acrylic acid) in NaCI solutionsand in water show that poly(acrylic acid) is moredissociated in NaCI solutions. Ion-dipole interac-tions are due to protonated amide groups of poly(-acrylamide) and the dissolved electrolytes areexpected to screen these interactions. In the absenceof the ion-dipole interactions more hydrogen bondlinkages between the polymer chains are necessaryfor stabilizing the complexes. This can be achievedonly by a decrease in the pH to increase the unio-nized acid groups on the polymer chain. Thus, theincrease in ionic strength systematically shifted theonset of interpolymer association to lower pHvalues.

The molecular weight dependence of P AAm-P AA interactions is shown in Fig.4. Fluorescence be-havior of the PyP AAm in the presence of poly(acryl-

6 ~-M. O.OIM ~I

a ~-~. 0.(84 ~I

0 ~-~. 1.04 "'1:1

+ pI)~. 0.1&4 ~I

~R

~\~! ~:~~~~~::::~;;:::~~:::~..\ -- v . -0-

---+~._'-.,: .;!0;

Q...;;c~"c,'" .;,; .4

~ft'

~ 8 a.P4

fig. 3. Effect of ionic strength on pH dependence of intramolecu-lar excimer formation. (Concentration of polymers: PyPAAm,4 x 10-4 base mollL; PM 4 x 10-3 base mollL)

135SWIlJJlSIlII ef "!:: FlMDreSteIlU~lUll7iscomtfry sflldy of jllferpolymer complerlltioll

The effect of concentration of poly(acrylic acid)on P AAm/P AA complex structure is shown in Fig. 5at three pH values. The leI 1m ratio of PyP AAm/P AAmixture relative to pure PyP AAm solution underidentical pH and concentration, (I ell".) 0- is plotted inthe figure as a function of the concentration ratio ofthe mixture which is varied by increasing thepoly(acrylic acid) concentration. At pH 7, the ratio isconstant as the ionized polymer chains do not inter-ad with each other. At pH 5, the (IJI"JI(IJI"Joratioincreases initially, then decreases as the concentra-tion of poly(acrylic acid) in the mixture is increased.A sharper decrease in the excimer fluorescence ratiois observed at pH 3.5 at low concentration ratios.

! o.

V--J 4 5 6 7 8 9~

Fig. 4. Effect of molecular weight on intramolecular excimer for-mation. (Concentration of polymers: PyPAAm, 4 x 10-4 basemollL; PM 4 x 10-3 base mollL)

ic acid) of different molecular weights is plotted. Inthe case of lower molecular weight poly(acrylicacids), the interpolymer interactions begin almost atthe same pH, even though the structures of theircomplexes appear to be different. As evident fromthe 1./1 m ratios at low pH, the complex formed withP AA-90 is more rigid (less excimer formation) thanthat of PAA-5. Moreover, the phase separation ofP AAm/P AA-5 complex occurs at a lower pH (pH3.2) than that of PAAmlPAA-90 complex (pH 3.4).For the highest molecular weight poly(acrylic acid)(P AA-3000), the interactions begin at pH 6.0 and thecomplex precipitates at pH 4.5. The relatively high1./1 m ratio above pH 6.0, where complex formationis not expeded, is possibly due to the contraction ofpoly(acrylamide) coil in the presence of large inter-penetrating poly(acrylic acid) chains. The hydrogenbonding forces and ion-dipole interaroons are oflow energies and the stability of complexes involv-ing such interactions is believed to be due to the coo-perative binding [1] between the complementarypolymer molecules. Also, we observed that bothacetic acid and succinic acid did not affed the 1./1 mratios of PyP AAm or induce any phase separation,even at very low pH values. Hence, cooperative in-teractions between many amide and carboxylgroups are essential for complex formation and thepolymer chain length plays an important role indetermining the stability and structure of the result-ing complex.

Fig. 6. The fluorescence intensity ratio after equilibration for 1week. (The solutions and experimental conditions are the sameas in Fig. 4)

136 Colloid and Polymer SdeJIa, Vol. 269 . No.2 (1991,-

It has been reported [1, 2] that when two polymersinteract to form a stable complex, often it occurswith a definite composition of the interacting mole-cules. In the case of poly(ethylene oxide)/P AA sys-tem, a stable complex with a stoichiometric ratio of1 : 1 is formed, irrespective of the polymer composi-tion of the mixture in which it is formed. Our resultsindicate that the composition of the complex canvery well depend on pH and equilibration time. The(lell...)/(lell...)o ratio, a measure of the segmentalmobility of the polymer chain varies withpoly(acrylic acid) concentration and becomes con-stant only at very high concentration of poly(acrylicacid). It appears that the time duration between thepreparation of the mixture and the fluorescence in-tensity measurements is not adequate for the systemto reach equilibrium and to rearrange the interpo-lymer contacts sufficiently to minimize the numberof chains involved in the complexation. Hence, fluo-rescence intensiti~ were measured for the polymersolutions equilibrated for 1 week and the result ispresented in Fig. 6. It can be seen that the fluores-cence intensity ratio remains unchanged at pH 7.The (I ell...)/(1 ell,,) 0 ratio becomes constant at pH 3.5and 5 above polymer concentration ratios 2 and 6,respectively. For mixtures of high concentrationratios the local concentration of poly(acrylic acid)molecules around the poly(acrylamide) chains ishigh and possibly most of these molecules particip-ate in hydrogen bonding with poly(acrylamide)molecules to form a rather crosslinked structure. Asthe system is equilibrated the poly(acrylamide)molecules apparently rearrange their contacts toaccommodate only a limited number of poly(acrylicacid) molecules, though the number of hydrogenbond linkages will be same in both cases. This effectis more pronounced at pH 5. We also obtained thecomposition of the complex separated under theprecipitation conditions (pH below 3.4) as follows.The supernatant solution after centrifugation wasanalyzed for total organic carbon to determine thetotal uncomplexed polymers in the solution. Thepyrene-labeled polymer was estimated from the UVabsorption. The composition of the precipitatedcomplex thus calculated was very close to equimo-lar. These results suggest that the complexes formedhave a fixed mean stoichiometry which dependsonly on pH when the system is equilibrated.

In Fig. 7, the intrinsic viscosity of P AA-800/PAAm-lOOO mixtures at pH 4 and 7 is plotted as afunction of its composition. The phase separation of

0 Pi ..0 A pi 7.0

& In

~).....

~>u

~0:!.

1.:" ~

~

"",-;,,"';~~..'~.'~~{~"!~' .0 20 40 ro ~ 100

~. BASE ~ %

Fig. 7. Intrinsic viscosity of P AA-3OOIP AAm-lOOO as a functionof polymer composition at pH 4 and i

this polymer couple was observed at pH 3.8. At pH4, the intrinsic viscosity decreases with increasingconcentration of poly(acrylic acid) in the mixtureand reaches a minimum when the mixture is equi-molar. The decreased intrinsic viscosity can be attri-buted to the compact structure of the polymer com-plex which has a smaller hydrodynamic volumecompared to free molecules. The larger values ofHuggins constant k' (given in Table 2) also indicate[19] strong interpolymer association. The inb:insicviscosity of the mixture at pH 7 shows a linearincrease with P AA concentration. Furthermore, for agiven mixture the intrinsic viscosity is equal to thesum of viscosities of the component polymer solu-tions as in the case of non interacting polymer mix-

Table 2. Intrinsic viscosity (71) and Huggins constant k' of P AA-800/P AAm-lOOOmixture in O.OS M NaCI solutions as a functionof polymer composition

pH 4.0PM(basemole%)

pH 7.0PM(basemole")

k' k'I'll(dJlgm)

('11

(dl/gm)

01.0

.3.33

SO

60

66.6

80

100

3.002.7S2.602.5S2.SS2.5S2.703.50

0.360.51.LIO1.191.211.191.010.20

2.954.305.205.806.257.157.709.90

0.400.450.490.460.450.360.320.29

02033.3U.8SO6066.6

100

1375illWs4Zn tt R1~fz.ort5Cma.nJ lIiscomttry stIIJy of inttrpolymtr compltxRtion

rures. Thus, the viscosity results are in agreementwith the results obtained from fluorescence meas-urements.

Conclusion

The excimer fluorescence and the viscosity stud-ies indicate that ~ly(acrylamide) forms a stablecomplex with ~ly(acrylic acid) at low pH values.The decrease in excimer fluorescence and intrinsicviscosity of the mixture of ~lymer solutions is attri-buted to the rigid compact structure of the complex.The fact that the hydrolysed ~ly(acrylamide) (co-~lymer of acrylamide and acrylic acid) also inter-acts with ~ly(acrylic acid) shows that the amidegroups are strong hydrogen bond acceptors. Theincrease in ionic strength shifts the onset of interpo-lymer interaction to lower pH range as the dissolvedsimple electrolytes affect both the dissociation of~lyacid and the interactive forces. The molecularweight dependence of the association behavior con-firms the cooperative nature of the complexation.The complexes formed have fixed mean stoichio-metry which depends only on pH if the system isequilibrated.

References

1. BekturovEv; Bimendina LA (1981) Allv Polym Sri 41:992. Tschuchida E, Abe K (1982) Allv Polym Sci 45:13. Bailey FE Jr, lundbergRD, Callard RW (1964) J Poiym Sci

Part A 2:3454. Klenina OV; Fain EG (1981) Vysokomol Soedin Ser A

23:12985. Jeon SH, Ree TJ (1988) Poiym Sci Poiym Chern Ed 26:14196. Bednar B, Li z. Huang Y; Chang lCP, Morawetz H (1985)

Macromolecules 18:18297. Chen Hl, Morawetz H (1982) Macromolecules 15:14458. Oyama HT, Tang WT, Frank CW (1987) Macromolecules

20:474 &: 18399. Iliopoulos L Halary Jl, Audebert RJ (1988) Polym Sci Part A

26:27510. Heyward fl, Ghiggino KP (1989) Macromolecules 22:115911. Turro NJ, Arora K5 (1986) Polymer 27:78312. Chandar p, Somasundaran p, Turro NJ (1988) Macromole-

cules 21:95013. Oyama HT, Tang WT, Frank CW (1937) Macromolecules

20:47414. Kulkarni RA. Gundiah S (1984) Makromol Chern 185:95715. Abe K. Koide M. Tschuchida E (1977) Macromolecules

10:125916. Wang Y; Morawetz H (1989) Macromolecules 22:16417. Oyama HT, Hemker OJ. Frank CW (1989) Macromolecules

22:125518. Mandel M (1970) Eur PoiylIt J 6:80719. Morawetz H (1975) Macromolecules in Solution, 2nd ed.

Wiley, New York, pp332

Received January 22, 1990;accepted April 16.1990

AckllOfDltdgemellt

We thank Dr. S. Gundiah and his colleagues (NCL. rune,India) for the preparation and characterlzationof the polymersused in this investigation. The financial support of this work byNalco, Engelhard. Aqualon.IBM and National Science Founda-tion (Program No NSF-CBT-86-l5524) is gratefully acknow-ledged. K. S. also thanks ONGC, India, for granting study leave.

Authors' address:

Proressor P. SomasundaranLangmuir Center for Colloids and InterfacesHenry Krumb School of MinesColumbia UniversityNew York. NY 10027, USA