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New approach to the molecular characterization of hydrophobically modified polyacrylamide
Polymer 45 (2004) 5897–5904 www.elsevier.com/locate/polymer
New approach to the molecular characterization of hydrophobically modified polyacrylamide I.V. Blagodatskikha,*, O.V. Vasil’evaa, E.M. Ivanovaa, S.V. Bykova, N.A. Churochkinaa, T.A. Pryakhinaa, V.A. Smirnovb, O.E. Philippovab, A.R. Khokhlovb a
Nesmeyanov Institute of Organoelement Compounds of RAS, Vavilova 28, 119991 Moscow, Russian Federation b Physics Department, Moscow State University, 119992, Leninskie Gory, Moscow, Russian Federation Received 22 January 2004; received in revised form 17 May 2004; accepted 16 June 2004
Abstract A new approach to the molecular characterization of hydrophobically associating copolymers of acrylamide is developed. It is based on the study of associative properties: the formation of intermolecular aggregates was followed by dynamic and static light scattering (DLS and SLS), while the formation of hydrophobic domains was detected by fluorescence spectroscopy with pyrene as a probe. In aqueous media, hydrophobic aggregation begins at concentrations much lower than the overlap concentration. The addition of co-solvent, acetonitrile, shifts the aggregation to the semi-dilute region. The dissolution of hydrophobic aggregates is controlled both by fluorescence spectroscopy with pyrene as a probe and by DLS and SLS. Absolute Mw values are measured by SLS in mixed solvent of optimal composition. Molecular weight distribution (MWD) is characterized by GPC using calibration with secondary standards characterized by SLS. This approach allowed us to follow MWD evolution during a micellar copolymerization of acrylamide, N-nonylacrylamide and acrylic acid. It is found that the molecular weight heterogeneity remarkably growths with the increase of conversion. q 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrophobically associating polymers; Molecular weight characterization
1. Introduction Hydrophobically modified water-soluble polymers and polyelectrolytes have attracted increased interest over past decades due to their ability in controlling viscosity at various shear rates [1–3]. Among these polymers, hydrophobically modified polyacrylamides (HMPAAm) are especially attractive, in particular, for enhanced oil recovery. There are two main methods of synthesis of HMPAAm: (1) micellar radical polymerization and (2) chemical modification of polyacrylamide. The vast majority of investigations in the field of HMPAAm are concerned the micellar polymerization technique, which offers a real versatility in tuning such characteristic parameter of copolymers as monomer sequence distribution. A peculiar * Corresponding author. Tel.: C7-95-1358119; fax: C7-95-1355085 E-mail address: [email protected] (I.V. Blagodatskikh). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.040
feature of this method is as follows: water-soluble monomers (acrylamide or acrylamide and ionazable comonomer, e.g. sodium acrylate) are dissolved in water, while insoluble hydrophobic comonomer is solubilized in micelles of surfactant. The most detailed revue of micellar polymerization one can find in the work [4]. HMPAAm prepared by chemical modification are discussed in the work [5]. The key parameters controlling rheological behaviour of binary and charged ternary polymers are the chemical nature, the content and the blockiness in the distribution of hydrophobic monomer units and the molecular weight of polymer. Inhomogeneity in these parameters is also an important factor. To provide the synthesis of copolymers with the controllable properties, it is necessary to estimate the correlation between synthesis conditions and molecular characteristics of the prepared polymer. One can note that MW and MWD characterization of these polymers is rather difficult and, to our knowledge, no
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information about MWD is available in the papers dealing with HMPAAm. The main reason of this situation is the high ability of HMPAAm to intra- and intermolecular aggregation in aqueous media, very limited number of solvents for polyacrylamide, high ability to macroscopic segregation of dilute solutions in the presence of salts. All these reasons hamper the molecular weight characterization of binary and ternary polymers of acrylamide. The most reliable data on molar masses of HMPAAm provides SLS measurement in formamide. Firstly, this method was used in the work [6]. An alternative approach to molecular weight characterization of HMPAAm is proposed in the present work. The approach is based on the application of organic co-solvent acetonitrile (AN) that depressed hydrophobic aggregation. AN itself does not dissolve PAAm. Its role as a co-solvent consists in a partial disruption of water structure. As a result, the tendency to hydrophobic aggregation of side chains diminishes and molecular dissolution of copolymers occurs. Combination of fluorescence spectroscopy and DLS, being able to follow the aggregation into hydrophobic domains as well as into intermolecular species, provide a control of dissolution of hydrophobic aggregates. Further, SLS is used for Mw determination and GPC—for determination of molecular weight heterogeneity. The additional advantages of our co-solvent are low refractive index providing high value of dn/dc, isorefractivity with water (nH2OZ1.333, nANZ1.344) (which is important for light scattering), rather low viscosity of its aqueous mixtures and an ability to depress the reverse phase interaction in chromatographic columns (which is important for chromatographic application).
2. Experimental 2.1. Materials Binary and ternary polymers of the following structures
where RZ–(CH2)nCH3, nZ8, 11; XZ–O, –NH; YZ–H, –
CH3, xZ0.002–0.015, yZ0–0.2 were prepared by freeradical copolymerization of monomers in aqueous micellar media using the known [6–8] technique. Ammonium persulfate (PSA) was used as an initiator, sodium dodecyl sulfate (SDS) was used as a surfactant. Acrylamide (AAm), PSA, SDS and (acrylamido)-2methylpropanesulfonic acid (AMPS) (Aldrich) and acrylic acid (AA) (Fluka) were used as received. n-Dodecylmethacrylate (DDMA) and n-nonylmethacrylate (NMA) were obtained from the Institute of Polymer Chemistry (Dzerzhinsk, Russia). N-Nonylacrylamide (NAAm) and N-dodecylacrylamide (DDAAm) were synthesized using the method described in Ref. [7]. Copolymers and charged terpolymers of various compositions were studied. The content of hydrophobic monomer was chosen to maintain solubility in water: up to 0.3 mol% of DDAAm or DDMA in uncharged copolymers and up to 1 mol% in charged terpolymers; up to 0.5 mol% of NMA and NAAm in uncharged copolymers and up to 1.5 mol% in charged terpolymers. The content of sodium acrylate (SA) or SAMPS was up to 20 mol%. The synthesis procedure was similar to that described in a previous paper [9]. The synthesis conditions were as follows: the total monomer concentration was 2–6 g/dl, PSA concentration was 4.4!10K4–4.4!10K3 mol/l. AA monomer was used either in its acidic form (at pHZ3.5) or in the salt form (at pHZ9.5, which was adjusted before polymerization by the addition of concentrated NaOH). AMPS was converted to the salt form sodium(acrylamido)2-methylpropanesulfonate (SAMPS) before polymerization by the same manner as AA. As an example, we describe below the procedure for the synthesis of terpolymers 1.5C9(Am)-11/13SA. The synthesis was carried out into a 100–150 ml flask equipped with a reverse condenser, an attachment for bubbling argon, a thermometer, a magnetic stirrer, and a thermostating jacket. A solution of SDS (1.14 g, 4 mmol) in 20 ml of water was placed into the flask and stirred under a flow of argon for 15 min. NAAm (0.0706 g, 0.358 mmol) was added to the as-prepared micellar solution. Solution of AAm (1.4485 g, 20 mmol) and acrylic acid (0.2333 g, 3 mmol) in 30 ml of water was prepared in a separate flask and poured into the reaction flask to a constantly stirred micellar solution of NAA under a flow of argon. The reaction mixture was heated to 50 8C and stirred at this temperature in a flow of argon until complete solubilization of the hydrophobic monomer in surfactant micelles was achieved. A solution of ammonium persulfate (0.0563 g, 0.25 mmol) in 1 ml of water was then added. The total concentration of the monomers in the synthesis was 3 wt% based on the total volume of the mixture. The series of synthesis with different reaction duration at 50 8C was performed (10, 20, 30 min, 1, 5 h). The reaction mixture was cooled to room temperature and diluted with water to the polymer concentration of about 1.5–2%. Acrylic acid units were converted to sodium acrylate by the addition of NaOH solution to pHZ9.5 under
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stirring, then the stirring were continued for 1 h. After that, reaction solution was added dropwise to a fivefold excess (by volume) of a methanol–acetone (1:1) mixture, filtered, washed with methanol, and dried under vacuum (2– 5 mmHg) over 5–7 h at 60 8C. Usually, the duration of synthesis was 5 h and the yield was about 90%. The following designations are assumed in this work: 1.5C9(Am)-11/13SA denotes copolymer with 1.5 mol% of NAAm and 13 mol% of SA units, 0.5C9(MA)-67/5SA denotes terpolymer with 0.5 mol% of NMA and 5 mol% of SA units according to the reaction feed composition. The surfactant to monomer ratio, SMRZ[SDS]/[hydrophobe] according to McCormick et al. [10], is shown through the hyphen. 2.2. Dynamic and static light scattering DLS and SLS experiments were performed on PhotoCor Complex (PhotoCor, Russia) setup equipped with the real time multiple tau correlator PhotoCorFC and an automatic goniometer. A Uniphase 1135P He–Ne laser (lZ633 nm, 20 mW power) was used as a light source. Milli-Q water was used for the preparation of solutions. Solvents were filtered through Durapore (Millipore) 0.22 mm membrane, polymer solutions were filtered through Durapore 0.45 or 0.65 mm membrane. The homodyne intensity correlation function G(2)(t) was measured within the range of delay times from 2!10K8 to 5!103 s. The normalized homodyne intensity correlation function g(2)(t) is related to the normalized electric field time correlation function g(1)(t) by the Siegert relation [11]: gð2Þ ðtÞ Z 1 C bjgð1Þ ðtÞj2 ; where g(2)(t)ZG(2)(t)/G(2)(N), G(2)(N) is an experimentally determined baseline, b is a coherence factor. In dilute solutions of monodisperse particles, field correlation function is connected with the translation diffusion as follows: gð1Þ ðtÞ Z expðKt=tÞ Z expðKGtÞ Z expðKDq2 tÞ;
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spectra of decay times, efficient diffusion coefficient D and hydrodynamic radius Rh were calculated using relations DZ1/q2t, DZkT/6phRh, where h is a solvent viscosity. The true values of D and Rh were obtained by extrapolation to qZ0 and cZ0. SLS study was performed at the same instrument in the range of scattering angles from 30 to 1408. Weight average molar mass Mw, z-average root mean square radius of gyration Rg and second virial coefficient A2 were determined by double extrapolation using Zimm method, based on the following relations: 1 1 Kc=RQ y 1 C hR2g iq2 C 2A2 c; Mw 3 where K Z 4p2 ðdn=dcÞ2 n2o =No l4 : Refractive index increment dn/dc was measured using Chromatix KMX-16 (Milton Roy) differential refractometer. dn/dcZ0.169G0.002 were determined for PAAm and HMPAAm in water and dn/dcZ0.165G0.004—in 0.1 M NaNO3–AN 8:2 (v/v). 2.3. GPC study GPC study was performed with Waters liquid chromatograph equipped with M501 pump, UK6 injector and M2410 DRI detector. The purge with He was applied to degas the eluent. Millenium software was used for data processing. For GPC/MALLS experiments, Waters M501 pump, UK6 injector with M410 DRI coupled with Dawn-F light scattering photometer (Wyatt Technology) was used. The data obtained were processed with ASTRA 2.01 program. Ultrahydrogel 2000 and 1000 (Waters Ass.) were used as hydrophilic columns. 0.05–0.1 M NaNO3–AN (8:2 or 7:3) mixture was used as an eluent at flow rate 1 ml/min. Eluent was filtered through 0.22 mm Durapore membrane (Millipore). Samples were dissolved in eluent, concentration of probes was 1 mg/ml and the volume was 100 ml. Sample solutions were filtered through 0.45 or 0.65 mm Durapore membrane (Millipore).
D Z limq/0 ðG=q2 Þ:
2.4. Fluorescence spectroscopy
Here, t is the decay time, GZ1/t is the decay rate, D is the diffusion coefficient, qZ ð4p=lÞsinðq=2Þ is the wave vector, q is the scattering angle. If there is a large number of independent decay processes in the system, g(1)(t) is a weighed sum of individual contributions. In continuous form, one can write ðN gð1Þ ðtÞ Z AðtÞexpðKt=tÞdt;
The fluorescence spectroscopy measurements were performed with the Hitachi MPF-3 spectrofluorimeter using 5 and 1.5 nm bandpass settings for excitation and emission, respectively. The excitation wavelength was 338 nm. Pyrene was used as fluorescent probe. Pyrene obtained from Fluka was recrystallized three times from absolute ethanol. Solutions for fluorescence measurements were prepared by first pipetting 0.012 ml of pyrene stock solutions (2!10K4 mol/l in ethanol). Then, 3 ml of polymer solution of a given concentration was added to the flask and stirred for 1 day before the fluorescence measurements were made. Polarity parameter of pyrene I1/I3 equal to the ratio of the first to the third emission peak
0
where A(t) is a distribution of scattered light over decay times. CONTIN program [12] based on the inverse Laplace transform was used for determination of this distribution function. For every relaxation maximum found in the
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intensities (I1Z371 nm, I3Z383 nm) was measured. In the mixtures water–AN, I1/I3 was determined in a similar way. It was previously found that I1/I3Z2 in these mixtures. The limiting solubility of pyrene in the mixture water–AN (8:2) was shown to be equal to 2!10K4 mol/l. 2.5. Rheological study Rheological experiments were performed with the Haake RS150L rheometer equipped with a cone and plate measuring unit. The dynamic measurements were conducted over the frequency range 5!10K3–10 Hz. All the measurements were made within the linear viscoelastic region. The temperature TZ20G0.1 8C was adjusted by an external thermostat.
3. Results and discussion 3.1. Fluorescence spectroscopy Aggregation of surfactants and associating polymers can be studied on the molecular level using fluorescence spectroscopy with pyrene as a probe due to its sensitivity to the polarity of microenvironment [13–15]. The formation of hydrophobic microregions (domains) in aqueous media and the penetration of pyrene molecules into these domains lead to the drop of I1/I3 value from 2 to appr. 1.1–1.2. Usually, the critical aggregation concentration (cac) is determined as an inflexion point of the curve I1/I3 as a function of concentration. Characteristic features of aqueous solutions of polymers under study are the low cac values and the broad transition range, where I1/I3 value decreases. In some cases, we did not follow the complete transition region of concentration (in particular, in mixed solvents that will be discussed below). Therefore, we determined critical aggregation concentration (cac) as a polymer concentration corresponding to the onset of the I1/ I3 decrease. Cac values determined in such a way were of appr. 1!10K3–1!10K2 g/dl. These values are much lower than the overlap concentrations of the parent PAAm (here and in what follows, overlap concentrations were estimated as c*Z1/[h], where [h]Z1!10K2M0.755 according to w Kulicke et al. [16]). Fig. 1 shows concentration curves of the polarity parameter of pyrene for terpolymers of various compositions. The higher is the content of hydrophobic units, the lower is the cac value. The low cac and the high sensitivity to salts leading to phase separation in dilute region make the molecular weight characterization of these polymers in aqueous solutions virtually impossible. In general, the hydrophobic aggregation is due to the specific structure of water. Therefore, the addition of a cosolvent with smaller ability to the formation of H-bonds leads to the partial disruption of water structure and weakens the hydrophobic aggregation. In the present
Fig. 1. Dependencies of the polarity parameter of pyrene I1/I3 on the concentration of aqueous solutions of terpolymers 0.5C12(Am)41/10SAMPS (1), 0.5C12(Am)-48/20SAMPS (2) and 1C12(Am)28/20SAMPS (3). Synthesis conditions: total monomer concentration cmonZ3.8%, initiator concentration cPSAZ4.4!10K3 mol/l.
work, AN was chosen as a co-solvent. At the addition of 20–30% of AN, the solubility of copolymers essentially improves. Solubility of a series of copolymers in water–AN mixtures was studied using the method of clouding points. It is found that the partial precipitation begins upon addition of 40% or more of AN depending on polymer composition and Mw. An optimal dissolving capacity demonstrates water– AN mixture 7:3. Fig. 2 shows the evolution of concentration dependencies of I1/I3 after the addition of AN to the solution of terpolymer 1C12-28/20SAMPS. One can see that the addition of 20% of AN to water shifts cac value from 1!10K3 up to appr. 1!10K2 g/dl, further increase of AN content up to 30%
Fig. 2. Dependencies of the polarity parameter of pyrene I1/I3 on the concentration of solutions of terpolymer 1C12(MA)-28/20SAMPS in water (1), in water–AN (8:2) (2) and in water–AN (7:3) (3).
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shifts the cac to the value of 1.5!10K1 g/dl, which is appr. equal to the overlap concentration of the parent PAAm. The value of cac and the amount of AN, which is necessary to shift the cac to semidilute region, depend mainly on the hydrophobicity of copolymer, i.e. on the nature of hydrophobic units, its content and blockiness of their distribution along the chain. In particular, we can compare a pair of polymers of one and the same composition and Mw differing only in their blockiness: 1.5C9(Am)-11/13SA and 1.5C9(Am)-5.5/13SA with MwZ 1.4!106 and 1.6!106 correspondingly. We have found that the higher is the blockiness (i.e. the lower is SMR), the lower is cac determined from I1/I3 (0.03 and 0.007 g/dl, correspondingly). The content and the nature of charged units as well as the polymer molar mass are additional factors influencing cac and the width of the transition region. Therefore, the combined effect of all these factors is rather complicated. 3.2. Dynamic and static light scattering It is known that the combination of fluorescence spectroscopy with DLS is a very sensitive tool to the study of hydrophobic association of polymers. It is clear from the above consideration that we did not observe hydrophobic domains in mixed solvent within the rather wide concentration region. To be sure that we can reach a molecular dissolution at these conditions, we performed DLS and SLS study of a series of binary and ternary polymers. Fig. 3 shows distributions on decay times obtained by DLS for dilute solutions of terpolymer 0.5C9(MA)-67/5SA in aqueous 0.05 M NaNO3 and in 0.05 M NaNO3–AN (7:3). In mixed solvent, we obtained from these data the value of hydrodynamic radius Rh(cZ0, qZ0)Z35 nm, which is in agreement with our supposition about molecular dissolution.
Fig. 3. Distribution over decay times of 0.5C9(MA)-67/5SA terpolymer in aqueous 0.05 M NaNO3 (1) (cZ0.01 g/dl) and in the mixture 0.05 M NaNO3–AN (7:3) (2) (cZ0.04 g/dl). Synthesis conditions: cmonZ3%, cPSAZ4.4!10K3 mol/l.
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MwZ5.6!105 and RgZ70 nm were calculated from the regular Zimm plot obtained in this solvent (Fig. 4). Rg/Rh ratio is known to depend on polymer architecture, chain conformation and polydispersity. Obtained Rg/Rhz2 and A2Z3.8!10K4 cm3 gK2 mol do not contradict to the assumption about polydisperse coils in a good solvent. By contrast, in aqueous solvent we observed much larger species with broad distribution on the sizes centred at RhZ500 nm, which indicates the formation of supramolecular aggregates. Therefore, on the basis of DLS and SLS data, one can infer that the addition of co-solvent disrupts supramolecular species formed due to hydrophobic association. It should be noted that at conditions of LS measurements the solutions of copolymer 0.5C9(MA)-67/5SA have quite different values of polarity parameters of pyrene in water and in the mixed solvent. In the mixed solvent the I1/I3 value did not manifest hydrophobic domains (I1/I3Z2), while in water I1/I3 is much lower (I1/I3Z1.6) indicating the presence of hydrophobic domains. Prevailing of an intermolecular aggregation over intramolecular one, even at very high dilution, is a specific feature of HMPAAm under study. As a consequence, the formation of two-phase gel—supernatant system is often observed in aqueous systems in the presence of salt. In particular, this was observed for terpolymers 0.5C12(MA)/20SAMPS and 1C12(MA)/20SAMPS presented on Fig. 1. This is the reason why we could not compare light scattering of these charged copolymers in aqueous salt containing solutions before and after addition of co-solvent. In salt free solutions both with and without co-solvent, charged terpolymers demonstrated typical polyelectrolyte behaviour [17]. Fig. 5 illustrates this fact. One can see that distribution over decay times is significantly shifted toward higher t in salt free system as compared with the solution containing NaNO3. Apparent Rh values estimated from these data for terpolymer
Fig. 4. Zimm plot of 0.5C9(MA)-67/5SA terpolymer in 0.1 M NaNO3–AN (7:3) mixture.
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Fig. 5. Distributions over decay times of 1C12(MA)/20SAMPS terpolymer in the mixtures water–AN (7:3) (1) and 0.1 M NaNO3–AN (7:3) (2), cZ 0.1 g/dl.
1C12/20SAMPS were RhZ450 and 67 nm, correspondingly. At the same time, the intensity of the scattered light increased approximately 10-fold after the addition of salt. From SLS measurements performed in the mixture 0.1 M NaNO3–AN (7:3), regular Zimm plot was obtained and MwZ2.8!106 was determined. For the majority of polymers under study, one can obtain molecular solutions in an appropriate mixture containing AN. The value of thermodynamical parameter A2 essentially depended on the polymer composition. For example, small negative or almost zero A2 in water–AN (7:3) were characteristic for uncharged copolymers 0.2C12(MA) with SMRZ40–80 and MwZ0.3!106–1.0!106. In the case of C9 copolymers, positive A2 values were obtained. In the previous work [9], Mw values of a series of samples measured by conventional SLS method were compared with the results obtained by GPC/MALLS. Consistent data were obtained in the most cases. However, some copolymers of the highest hydrophobicity turned out to demonstrate great discrepancies: Mw measured by SLS were much higher than those measured by GPC/MALLS. These discrepancies were attributed to the disruption of aggregates existed in solutions in the course of chromatography.
different, but they diverge greatly in the high MW region. At the same time, peak positions of HMPAAm are situated between PSS and PEO. For this reason, we have used high MW HMPAAm’s (MwZ3!105–3!106) together with PSS and PEO in the range of Mpeak between 1.5!103 and 2! 105. Either linear or 3-d order fit was used. We have shown that the choice of the eluent composition (20 or 30% of AN) does not affect the results of analysis. Moreover, we were able to analyse MWD of polymers that could not be molecularly dissolved in eluent. For example, terpolymer 1C12(Am)-28/20SAMPS turned out to undergo phase separation into gel and supernatant in the mixture 0.1 M NaNO 3–AN (8:2) at concentrations cZ0.08– 0.1 mg/ml. We were able to study this sample by DLS and SLS only in 0.1 M NaNO3–AN (7:3). We prepared three solutions for GPC in various solvents: in 0.1 M NaNO3–AN (7:3), in water–AN (7:3) and in water–AN (8:2), and verified the results of GPC with mixtures 0.1 M NaNO3–AN (7:3 and 8:2) as eluents. The results were in good agreement with each other. One can infer that GPC being dynamic method promote dissociation of aggregates formed in solution. This circumstance can explain the mentioned above discrepancies is the results of conventional SLS and GPC/MALLS. Due to this fact, one can say that practically all of binary and ternary HMPAAm can be analysed by GPC in mixtures containing 20–30% of AN. The contribution of AN to the altering of associative properties of HMPAAm illustrates the following rheological experiment. Two solutions of terpolymer 1.5C9(Am)11/13SA (conversionZ45%,) were prepared in water and water–AN (7:3) at concentration cZ1.5 mg/ml which is close to overlap concentration for parent PAAm (c*z1/[h]Z2 mg/ml). Their rheological behaviour was examined by the method of low-amplitude oscillations. One can see from Fig. 6 that the crossing point G 0 ZG 00 shifts in mixed solvent to the higher frequency value (from appr. 0.7 up to about 15 rad/s). This shift indicates essential decrease of the effective lifetime of network junctions after
3.3. Study of molecular weight heterogeneity using GPC In this work, we have used GPC with water–AN mixtures (8:2 and 7:3) containing NaNO3 as eluents. A series of HMPAAm characterized with SLS and GPC/MALLS (MwZ3!105–3!106) were used for calibration as secondary standards together with sulfonated polystyrene standards (Polymers Laboratories) (PSS) and poly(ethylene oxide) standards (Polymer Standards Service-USA, Inc.) (PEO). In the range of M!2!105, calibration curves for these two types of standards (PSS and PEO) are not so
Fig. 6. Frequency dependencies of dynamic moduli of solutions of 1.5-C911/13SA (conversion 45%) in different solvents, cZ0.15 g/dl.
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the addition of AN. Simultaneously dynamic moduli get an order of magnitude lower. Both these facts evidence the weakening of network junctions. It is known that hydrophobical domains play a role of network junctions in HMPAAm solutions and gels. The observed changes of rheological properties in the presence of AN as compared with aqueous systems correlate with the disruption of hydrophobic aggregates, if still exist, in the course of chromatography. One can mention the work [18], where hydrophobically end-modified telechelic PAAm (Mwz3!104) was studied by GPC in water and its mixtures with AN. In water and water–AN (9:1), high molecular weight peaks on GPC traces reflected the presence of large aggregates, while in the mixture water–AN (7:3) only peaks attributed to molecular species were detected. These data are consistent with our conclusion about the high ability of water–AN mixture containing 30% of AN to disrupt hydrophobic aggregates of HMPAAm. As an example of the study of molecular weight heterogeneity of HMPAAm, MWD evolution in the course of micellar copolymerization of AAm, NAAm and AA can be presented (Fig. 7). The specific feature of MWD evolution was found to be a remarkable growth of molecular weight heterogeneity with conversion. The growth of conversion was also accompanied with the change of polymer composition as can be seen from Table 1. The observed evolutions are in agreement with those reported earlier by Candau et al. [4] and McCormick et al. [10]. At the same time, we have found that rheological properties of terpolymers deteriorate with conversion. Fig. 8 illustrates the evolutions of dynamic moduli of solutions of cZ1.5 mg/ ml. It is seen that the value of storage modulus essentially decreases with conversion and crossing point of G 0 and G 00
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Fig. 8. Effect of conversion on frequency dependence of dynamic moduli of aqueous solutions of 1.5-C9-11/13SA, cZ0.15 g/dl. Conversion: 45% (1), 64% (2), 86% (3).
frequency dependencies shifts to higher frequencies. The similar changes in the viscoelastic properties with conversion were reported earlier in the work of Candau et al. [19]. These data were obtained for copolymers containing N,Ndihexylacrylamide as hydrophobic comonomer, i.e. for copolymers which did not demonstrate a drift in composition with conversion. Therefore, evolutions in rheological behaviour could not be connected with the compositional heterogeneity and were ascribed to the variations in the length of hydrophobic blocks. In our case, the observed evolutions in composition and the growth of polymolecularity of terpolymers allow us to suppose that the low MW fraction forming at the late stage of reaction consists mainly of macromolecules with small amount of hydrophobe or even of pure PAAm. Therefore, one should say that all types of molecular heterogeneity: compositional, structural, and molecular weight ones contribute to the deterioration of rheological properties.
4. Conclusions
Fig. 7. Evolutions of MWD in the course of synthesis of terpolymer 1.5-C911/13SA. 1—conversionZ20%, 2—conversionZ54%, 3—conversionZ 64%, 4—conversionZ86%. Curves are normalized for the values of conversion. Synthesis conditions are described in the experimental part and in Table 1.
It is shown by fluorescence spectroscopy, DLS, SLS and GPC that the addition of AN as a co-solvent can shift hydrophobic association of hydrophobically modified polyacrylamides to semi-dilute region. These copolymers can be successfully characterized by SLS, DLS and GPC in aqueous solutions containing optimal fraction of AN (about 30%). Hydrophobic aggregates, if still exist, can be disrupted during the chromatography in this eluent due to the weakening of hydrophobic junctions. The elaborated technique allowed us to reveal the broadening of MWD with the growth of conversion in the course of micellar terpolymerization. This is an additional factor along with the compositional and structural heterogeneity, which leads to the deterioration of thickening properties of the polymers prepared at high conversion.
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Table 1 Time evolutions of 1.5C9-11/13SA terpolymer composition and molecular weights Time, min
Conversion, wt%
NAAm,a mol%
SA,b mol%
Mw !10K6 (GPC)
Mw/Mn (GPC)
10 20 30 60 300
19.8 45.3 54.1 64.3 86.2
2.0 3.4 – – 1.3
14.94 13.88 13.71 13.59 12.53
1.8 1.8 1.6 1.6 1.2
3.1 4.3 4.4 4.6 5.8
a b
Determined by titration. Determined by 1H NMR.
Acknowledgements This work was supported by CRDF (RP0-13011) and by Russian Foundation for Basic Research (project no.03-0332683).
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[6] Biggs S, Hill A, Candau F, Selb J. J Phys Chem 1992;96:1505–11. [7] McCormick ChL, Nonaka T, Johnson CB. Polymer 1988;29(4):731– 9. [8] Hill A, Candau F, Selb J. Macromolecules 1993;26(17):4521–32. [9] Blagodatskikh IV, Sutkevich MV, Sitnikova NL, Churochkina NA, Pryakhina TA, Philippova OE, Khokhlov AR. J Chromatogr A 2002; 976:155–64. [10] Branham KD, Shafer GS, Hoyle CE, McCormick CL. Macromolecules 1995;28:6175–82. [11] Stepanek P. In: Brown W, editor. Dynamic light scattering. The method and some applications. Oxford: Clarendron Press; 1993. Chapter 4. [12] Provencher SW. Comput Phys Commun 1982;27(3):229–42. [13] Kalyanasundaram K, Thomas GK. J Am Chem Soc 1977;99:2039–44. [14] Winnik FM, Winnik MA, Tazuke S. J Phys Chem 1987;91:594–7. [15] Winnik F, Ringsdorf H, Ventzner J. Langmuir 1991;7:905–11. [16] Kulicke W-M, Kniewske R. Prog Polym Sci 1982;8:373–468. [17] Hara M. In: Hara M, editor. Polyelectrolytes: science and technology. New York: Marcel Dekker; 1993. Chapter 9. [18] Lebreton P, Boutevin B, Gramain P, Corpart JM. Polym Bull 1999; 43(1):59–66. [19] Volpert E, Selb J, Candau F. Polymer 1998;39(5):1025–33.
New approach to the molecular characterization of hydrophobically modified polyacrylamide I.V. Blagodatskikha,*, O.V. Vasil’evaa, E.M. Ivanovaa, S.V. Bykova, N.A. Churochkinaa, T.A. Pryakhinaa, V.A. Smirnovb, O.E. Philippovab, A.R. Khokhlovb a
Nesmeyanov Institute of Organoelement Compounds of RAS, Vavilova 28, 119991 Moscow, Russian Federation b Physics Department, Moscow State University, 119992, Leninskie Gory, Moscow, Russian Federation Received 22 January 2004; received in revised form 17 May 2004; accepted 16 June 2004
Abstract A new approach to the molecular characterization of hydrophobically associating copolymers of acrylamide is developed. It is based on the study of associative properties: the formation of intermolecular aggregates was followed by dynamic and static light scattering (DLS and SLS), while the formation of hydrophobic domains was detected by fluorescence spectroscopy with pyrene as a probe. In aqueous media, hydrophobic aggregation begins at concentrations much lower than the overlap concentration. The addition of co-solvent, acetonitrile, shifts the aggregation to the semi-dilute region. The dissolution of hydrophobic aggregates is controlled both by fluorescence spectroscopy with pyrene as a probe and by DLS and SLS. Absolute Mw values are measured by SLS in mixed solvent of optimal composition. Molecular weight distribution (MWD) is characterized by GPC using calibration with secondary standards characterized by SLS. This approach allowed us to follow MWD evolution during a micellar copolymerization of acrylamide, N-nonylacrylamide and acrylic acid. It is found that the molecular weight heterogeneity remarkably growths with the increase of conversion. q 2004 Elsevier Ltd. All rights reserved. Keywords: Hydrophobically associating polymers; Molecular weight characterization
1. Introduction Hydrophobically modified water-soluble polymers and polyelectrolytes have attracted increased interest over past decades due to their ability in controlling viscosity at various shear rates [1–3]. Among these polymers, hydrophobically modified polyacrylamides (HMPAAm) are especially attractive, in particular, for enhanced oil recovery. There are two main methods of synthesis of HMPAAm: (1) micellar radical polymerization and (2) chemical modification of polyacrylamide. The vast majority of investigations in the field of HMPAAm are concerned the micellar polymerization technique, which offers a real versatility in tuning such characteristic parameter of copolymers as monomer sequence distribution. A peculiar * Corresponding author. Tel.: C7-95-1358119; fax: C7-95-1355085 E-mail address: [email protected] (I.V. Blagodatskikh). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.040
feature of this method is as follows: water-soluble monomers (acrylamide or acrylamide and ionazable comonomer, e.g. sodium acrylate) are dissolved in water, while insoluble hydrophobic comonomer is solubilized in micelles of surfactant. The most detailed revue of micellar polymerization one can find in the work [4]. HMPAAm prepared by chemical modification are discussed in the work [5]. The key parameters controlling rheological behaviour of binary and charged ternary polymers are the chemical nature, the content and the blockiness in the distribution of hydrophobic monomer units and the molecular weight of polymer. Inhomogeneity in these parameters is also an important factor. To provide the synthesis of copolymers with the controllable properties, it is necessary to estimate the correlation between synthesis conditions and molecular characteristics of the prepared polymer. One can note that MW and MWD characterization of these polymers is rather difficult and, to our knowledge, no
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information about MWD is available in the papers dealing with HMPAAm. The main reason of this situation is the high ability of HMPAAm to intra- and intermolecular aggregation in aqueous media, very limited number of solvents for polyacrylamide, high ability to macroscopic segregation of dilute solutions in the presence of salts. All these reasons hamper the molecular weight characterization of binary and ternary polymers of acrylamide. The most reliable data on molar masses of HMPAAm provides SLS measurement in formamide. Firstly, this method was used in the work [6]. An alternative approach to molecular weight characterization of HMPAAm is proposed in the present work. The approach is based on the application of organic co-solvent acetonitrile (AN) that depressed hydrophobic aggregation. AN itself does not dissolve PAAm. Its role as a co-solvent consists in a partial disruption of water structure. As a result, the tendency to hydrophobic aggregation of side chains diminishes and molecular dissolution of copolymers occurs. Combination of fluorescence spectroscopy and DLS, being able to follow the aggregation into hydrophobic domains as well as into intermolecular species, provide a control of dissolution of hydrophobic aggregates. Further, SLS is used for Mw determination and GPC—for determination of molecular weight heterogeneity. The additional advantages of our co-solvent are low refractive index providing high value of dn/dc, isorefractivity with water (nH2OZ1.333, nANZ1.344) (which is important for light scattering), rather low viscosity of its aqueous mixtures and an ability to depress the reverse phase interaction in chromatographic columns (which is important for chromatographic application).
2. Experimental 2.1. Materials Binary and ternary polymers of the following structures
where RZ–(CH2)nCH3, nZ8, 11; XZ–O, –NH; YZ–H, –
CH3, xZ0.002–0.015, yZ0–0.2 were prepared by freeradical copolymerization of monomers in aqueous micellar media using the known [6–8] technique. Ammonium persulfate (PSA) was used as an initiator, sodium dodecyl sulfate (SDS) was used as a surfactant. Acrylamide (AAm), PSA, SDS and (acrylamido)-2methylpropanesulfonic acid (AMPS) (Aldrich) and acrylic acid (AA) (Fluka) were used as received. n-Dodecylmethacrylate (DDMA) and n-nonylmethacrylate (NMA) were obtained from the Institute of Polymer Chemistry (Dzerzhinsk, Russia). N-Nonylacrylamide (NAAm) and N-dodecylacrylamide (DDAAm) were synthesized using the method described in Ref. [7]. Copolymers and charged terpolymers of various compositions were studied. The content of hydrophobic monomer was chosen to maintain solubility in water: up to 0.3 mol% of DDAAm or DDMA in uncharged copolymers and up to 1 mol% in charged terpolymers; up to 0.5 mol% of NMA and NAAm in uncharged copolymers and up to 1.5 mol% in charged terpolymers. The content of sodium acrylate (SA) or SAMPS was up to 20 mol%. The synthesis procedure was similar to that described in a previous paper [9]. The synthesis conditions were as follows: the total monomer concentration was 2–6 g/dl, PSA concentration was 4.4!10K4–4.4!10K3 mol/l. AA monomer was used either in its acidic form (at pHZ3.5) or in the salt form (at pHZ9.5, which was adjusted before polymerization by the addition of concentrated NaOH). AMPS was converted to the salt form sodium(acrylamido)2-methylpropanesulfonate (SAMPS) before polymerization by the same manner as AA. As an example, we describe below the procedure for the synthesis of terpolymers 1.5C9(Am)-11/13SA. The synthesis was carried out into a 100–150 ml flask equipped with a reverse condenser, an attachment for bubbling argon, a thermometer, a magnetic stirrer, and a thermostating jacket. A solution of SDS (1.14 g, 4 mmol) in 20 ml of water was placed into the flask and stirred under a flow of argon for 15 min. NAAm (0.0706 g, 0.358 mmol) was added to the as-prepared micellar solution. Solution of AAm (1.4485 g, 20 mmol) and acrylic acid (0.2333 g, 3 mmol) in 30 ml of water was prepared in a separate flask and poured into the reaction flask to a constantly stirred micellar solution of NAA under a flow of argon. The reaction mixture was heated to 50 8C and stirred at this temperature in a flow of argon until complete solubilization of the hydrophobic monomer in surfactant micelles was achieved. A solution of ammonium persulfate (0.0563 g, 0.25 mmol) in 1 ml of water was then added. The total concentration of the monomers in the synthesis was 3 wt% based on the total volume of the mixture. The series of synthesis with different reaction duration at 50 8C was performed (10, 20, 30 min, 1, 5 h). The reaction mixture was cooled to room temperature and diluted with water to the polymer concentration of about 1.5–2%. Acrylic acid units were converted to sodium acrylate by the addition of NaOH solution to pHZ9.5 under
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stirring, then the stirring were continued for 1 h. After that, reaction solution was added dropwise to a fivefold excess (by volume) of a methanol–acetone (1:1) mixture, filtered, washed with methanol, and dried under vacuum (2– 5 mmHg) over 5–7 h at 60 8C. Usually, the duration of synthesis was 5 h and the yield was about 90%. The following designations are assumed in this work: 1.5C9(Am)-11/13SA denotes copolymer with 1.5 mol% of NAAm and 13 mol% of SA units, 0.5C9(MA)-67/5SA denotes terpolymer with 0.5 mol% of NMA and 5 mol% of SA units according to the reaction feed composition. The surfactant to monomer ratio, SMRZ[SDS]/[hydrophobe] according to McCormick et al. [10], is shown through the hyphen. 2.2. Dynamic and static light scattering DLS and SLS experiments were performed on PhotoCor Complex (PhotoCor, Russia) setup equipped with the real time multiple tau correlator PhotoCorFC and an automatic goniometer. A Uniphase 1135P He–Ne laser (lZ633 nm, 20 mW power) was used as a light source. Milli-Q water was used for the preparation of solutions. Solvents were filtered through Durapore (Millipore) 0.22 mm membrane, polymer solutions were filtered through Durapore 0.45 or 0.65 mm membrane. The homodyne intensity correlation function G(2)(t) was measured within the range of delay times from 2!10K8 to 5!103 s. The normalized homodyne intensity correlation function g(2)(t) is related to the normalized electric field time correlation function g(1)(t) by the Siegert relation [11]: gð2Þ ðtÞ Z 1 C bjgð1Þ ðtÞj2 ; where g(2)(t)ZG(2)(t)/G(2)(N), G(2)(N) is an experimentally determined baseline, b is a coherence factor. In dilute solutions of monodisperse particles, field correlation function is connected with the translation diffusion as follows: gð1Þ ðtÞ Z expðKt=tÞ Z expðKGtÞ Z expðKDq2 tÞ;
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spectra of decay times, efficient diffusion coefficient D and hydrodynamic radius Rh were calculated using relations DZ1/q2t, DZkT/6phRh, where h is a solvent viscosity. The true values of D and Rh were obtained by extrapolation to qZ0 and cZ0. SLS study was performed at the same instrument in the range of scattering angles from 30 to 1408. Weight average molar mass Mw, z-average root mean square radius of gyration Rg and second virial coefficient A2 were determined by double extrapolation using Zimm method, based on the following relations: 1 1 Kc=RQ y 1 C hR2g iq2 C 2A2 c; Mw 3 where K Z 4p2 ðdn=dcÞ2 n2o =No l4 : Refractive index increment dn/dc was measured using Chromatix KMX-16 (Milton Roy) differential refractometer. dn/dcZ0.169G0.002 were determined for PAAm and HMPAAm in water and dn/dcZ0.165G0.004—in 0.1 M NaNO3–AN 8:2 (v/v). 2.3. GPC study GPC study was performed with Waters liquid chromatograph equipped with M501 pump, UK6 injector and M2410 DRI detector. The purge with He was applied to degas the eluent. Millenium software was used for data processing. For GPC/MALLS experiments, Waters M501 pump, UK6 injector with M410 DRI coupled with Dawn-F light scattering photometer (Wyatt Technology) was used. The data obtained were processed with ASTRA 2.01 program. Ultrahydrogel 2000 and 1000 (Waters Ass.) were used as hydrophilic columns. 0.05–0.1 M NaNO3–AN (8:2 or 7:3) mixture was used as an eluent at flow rate 1 ml/min. Eluent was filtered through 0.22 mm Durapore membrane (Millipore). Samples were dissolved in eluent, concentration of probes was 1 mg/ml and the volume was 100 ml. Sample solutions were filtered through 0.45 or 0.65 mm Durapore membrane (Millipore).
D Z limq/0 ðG=q2 Þ:
2.4. Fluorescence spectroscopy
Here, t is the decay time, GZ1/t is the decay rate, D is the diffusion coefficient, qZ ð4p=lÞsinðq=2Þ is the wave vector, q is the scattering angle. If there is a large number of independent decay processes in the system, g(1)(t) is a weighed sum of individual contributions. In continuous form, one can write ðN gð1Þ ðtÞ Z AðtÞexpðKt=tÞdt;
The fluorescence spectroscopy measurements were performed with the Hitachi MPF-3 spectrofluorimeter using 5 and 1.5 nm bandpass settings for excitation and emission, respectively. The excitation wavelength was 338 nm. Pyrene was used as fluorescent probe. Pyrene obtained from Fluka was recrystallized three times from absolute ethanol. Solutions for fluorescence measurements were prepared by first pipetting 0.012 ml of pyrene stock solutions (2!10K4 mol/l in ethanol). Then, 3 ml of polymer solution of a given concentration was added to the flask and stirred for 1 day before the fluorescence measurements were made. Polarity parameter of pyrene I1/I3 equal to the ratio of the first to the third emission peak
0
where A(t) is a distribution of scattered light over decay times. CONTIN program [12] based on the inverse Laplace transform was used for determination of this distribution function. For every relaxation maximum found in the
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intensities (I1Z371 nm, I3Z383 nm) was measured. In the mixtures water–AN, I1/I3 was determined in a similar way. It was previously found that I1/I3Z2 in these mixtures. The limiting solubility of pyrene in the mixture water–AN (8:2) was shown to be equal to 2!10K4 mol/l. 2.5. Rheological study Rheological experiments were performed with the Haake RS150L rheometer equipped with a cone and plate measuring unit. The dynamic measurements were conducted over the frequency range 5!10K3–10 Hz. All the measurements were made within the linear viscoelastic region. The temperature TZ20G0.1 8C was adjusted by an external thermostat.
3. Results and discussion 3.1. Fluorescence spectroscopy Aggregation of surfactants and associating polymers can be studied on the molecular level using fluorescence spectroscopy with pyrene as a probe due to its sensitivity to the polarity of microenvironment [13–15]. The formation of hydrophobic microregions (domains) in aqueous media and the penetration of pyrene molecules into these domains lead to the drop of I1/I3 value from 2 to appr. 1.1–1.2. Usually, the critical aggregation concentration (cac) is determined as an inflexion point of the curve I1/I3 as a function of concentration. Characteristic features of aqueous solutions of polymers under study are the low cac values and the broad transition range, where I1/I3 value decreases. In some cases, we did not follow the complete transition region of concentration (in particular, in mixed solvents that will be discussed below). Therefore, we determined critical aggregation concentration (cac) as a polymer concentration corresponding to the onset of the I1/ I3 decrease. Cac values determined in such a way were of appr. 1!10K3–1!10K2 g/dl. These values are much lower than the overlap concentrations of the parent PAAm (here and in what follows, overlap concentrations were estimated as c*Z1/[h], where [h]Z1!10K2M0.755 according to w Kulicke et al. [16]). Fig. 1 shows concentration curves of the polarity parameter of pyrene for terpolymers of various compositions. The higher is the content of hydrophobic units, the lower is the cac value. The low cac and the high sensitivity to salts leading to phase separation in dilute region make the molecular weight characterization of these polymers in aqueous solutions virtually impossible. In general, the hydrophobic aggregation is due to the specific structure of water. Therefore, the addition of a cosolvent with smaller ability to the formation of H-bonds leads to the partial disruption of water structure and weakens the hydrophobic aggregation. In the present
Fig. 1. Dependencies of the polarity parameter of pyrene I1/I3 on the concentration of aqueous solutions of terpolymers 0.5C12(Am)41/10SAMPS (1), 0.5C12(Am)-48/20SAMPS (2) and 1C12(Am)28/20SAMPS (3). Synthesis conditions: total monomer concentration cmonZ3.8%, initiator concentration cPSAZ4.4!10K3 mol/l.
work, AN was chosen as a co-solvent. At the addition of 20–30% of AN, the solubility of copolymers essentially improves. Solubility of a series of copolymers in water–AN mixtures was studied using the method of clouding points. It is found that the partial precipitation begins upon addition of 40% or more of AN depending on polymer composition and Mw. An optimal dissolving capacity demonstrates water– AN mixture 7:3. Fig. 2 shows the evolution of concentration dependencies of I1/I3 after the addition of AN to the solution of terpolymer 1C12-28/20SAMPS. One can see that the addition of 20% of AN to water shifts cac value from 1!10K3 up to appr. 1!10K2 g/dl, further increase of AN content up to 30%
Fig. 2. Dependencies of the polarity parameter of pyrene I1/I3 on the concentration of solutions of terpolymer 1C12(MA)-28/20SAMPS in water (1), in water–AN (8:2) (2) and in water–AN (7:3) (3).
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shifts the cac to the value of 1.5!10K1 g/dl, which is appr. equal to the overlap concentration of the parent PAAm. The value of cac and the amount of AN, which is necessary to shift the cac to semidilute region, depend mainly on the hydrophobicity of copolymer, i.e. on the nature of hydrophobic units, its content and blockiness of their distribution along the chain. In particular, we can compare a pair of polymers of one and the same composition and Mw differing only in their blockiness: 1.5C9(Am)-11/13SA and 1.5C9(Am)-5.5/13SA with MwZ 1.4!106 and 1.6!106 correspondingly. We have found that the higher is the blockiness (i.e. the lower is SMR), the lower is cac determined from I1/I3 (0.03 and 0.007 g/dl, correspondingly). The content and the nature of charged units as well as the polymer molar mass are additional factors influencing cac and the width of the transition region. Therefore, the combined effect of all these factors is rather complicated. 3.2. Dynamic and static light scattering It is known that the combination of fluorescence spectroscopy with DLS is a very sensitive tool to the study of hydrophobic association of polymers. It is clear from the above consideration that we did not observe hydrophobic domains in mixed solvent within the rather wide concentration region. To be sure that we can reach a molecular dissolution at these conditions, we performed DLS and SLS study of a series of binary and ternary polymers. Fig. 3 shows distributions on decay times obtained by DLS for dilute solutions of terpolymer 0.5C9(MA)-67/5SA in aqueous 0.05 M NaNO3 and in 0.05 M NaNO3–AN (7:3). In mixed solvent, we obtained from these data the value of hydrodynamic radius Rh(cZ0, qZ0)Z35 nm, which is in agreement with our supposition about molecular dissolution.
Fig. 3. Distribution over decay times of 0.5C9(MA)-67/5SA terpolymer in aqueous 0.05 M NaNO3 (1) (cZ0.01 g/dl) and in the mixture 0.05 M NaNO3–AN (7:3) (2) (cZ0.04 g/dl). Synthesis conditions: cmonZ3%, cPSAZ4.4!10K3 mol/l.
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MwZ5.6!105 and RgZ70 nm were calculated from the regular Zimm plot obtained in this solvent (Fig. 4). Rg/Rh ratio is known to depend on polymer architecture, chain conformation and polydispersity. Obtained Rg/Rhz2 and A2Z3.8!10K4 cm3 gK2 mol do not contradict to the assumption about polydisperse coils in a good solvent. By contrast, in aqueous solvent we observed much larger species with broad distribution on the sizes centred at RhZ500 nm, which indicates the formation of supramolecular aggregates. Therefore, on the basis of DLS and SLS data, one can infer that the addition of co-solvent disrupts supramolecular species formed due to hydrophobic association. It should be noted that at conditions of LS measurements the solutions of copolymer 0.5C9(MA)-67/5SA have quite different values of polarity parameters of pyrene in water and in the mixed solvent. In the mixed solvent the I1/I3 value did not manifest hydrophobic domains (I1/I3Z2), while in water I1/I3 is much lower (I1/I3Z1.6) indicating the presence of hydrophobic domains. Prevailing of an intermolecular aggregation over intramolecular one, even at very high dilution, is a specific feature of HMPAAm under study. As a consequence, the formation of two-phase gel—supernatant system is often observed in aqueous systems in the presence of salt. In particular, this was observed for terpolymers 0.5C12(MA)/20SAMPS and 1C12(MA)/20SAMPS presented on Fig. 1. This is the reason why we could not compare light scattering of these charged copolymers in aqueous salt containing solutions before and after addition of co-solvent. In salt free solutions both with and without co-solvent, charged terpolymers demonstrated typical polyelectrolyte behaviour [17]. Fig. 5 illustrates this fact. One can see that distribution over decay times is significantly shifted toward higher t in salt free system as compared with the solution containing NaNO3. Apparent Rh values estimated from these data for terpolymer
Fig. 4. Zimm plot of 0.5C9(MA)-67/5SA terpolymer in 0.1 M NaNO3–AN (7:3) mixture.
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Fig. 5. Distributions over decay times of 1C12(MA)/20SAMPS terpolymer in the mixtures water–AN (7:3) (1) and 0.1 M NaNO3–AN (7:3) (2), cZ 0.1 g/dl.
1C12/20SAMPS were RhZ450 and 67 nm, correspondingly. At the same time, the intensity of the scattered light increased approximately 10-fold after the addition of salt. From SLS measurements performed in the mixture 0.1 M NaNO3–AN (7:3), regular Zimm plot was obtained and MwZ2.8!106 was determined. For the majority of polymers under study, one can obtain molecular solutions in an appropriate mixture containing AN. The value of thermodynamical parameter A2 essentially depended on the polymer composition. For example, small negative or almost zero A2 in water–AN (7:3) were characteristic for uncharged copolymers 0.2C12(MA) with SMRZ40–80 and MwZ0.3!106–1.0!106. In the case of C9 copolymers, positive A2 values were obtained. In the previous work [9], Mw values of a series of samples measured by conventional SLS method were compared with the results obtained by GPC/MALLS. Consistent data were obtained in the most cases. However, some copolymers of the highest hydrophobicity turned out to demonstrate great discrepancies: Mw measured by SLS were much higher than those measured by GPC/MALLS. These discrepancies were attributed to the disruption of aggregates existed in solutions in the course of chromatography.
different, but they diverge greatly in the high MW region. At the same time, peak positions of HMPAAm are situated between PSS and PEO. For this reason, we have used high MW HMPAAm’s (MwZ3!105–3!106) together with PSS and PEO in the range of Mpeak between 1.5!103 and 2! 105. Either linear or 3-d order fit was used. We have shown that the choice of the eluent composition (20 or 30% of AN) does not affect the results of analysis. Moreover, we were able to analyse MWD of polymers that could not be molecularly dissolved in eluent. For example, terpolymer 1C12(Am)-28/20SAMPS turned out to undergo phase separation into gel and supernatant in the mixture 0.1 M NaNO 3–AN (8:2) at concentrations cZ0.08– 0.1 mg/ml. We were able to study this sample by DLS and SLS only in 0.1 M NaNO3–AN (7:3). We prepared three solutions for GPC in various solvents: in 0.1 M NaNO3–AN (7:3), in water–AN (7:3) and in water–AN (8:2), and verified the results of GPC with mixtures 0.1 M NaNO3–AN (7:3 and 8:2) as eluents. The results were in good agreement with each other. One can infer that GPC being dynamic method promote dissociation of aggregates formed in solution. This circumstance can explain the mentioned above discrepancies is the results of conventional SLS and GPC/MALLS. Due to this fact, one can say that practically all of binary and ternary HMPAAm can be analysed by GPC in mixtures containing 20–30% of AN. The contribution of AN to the altering of associative properties of HMPAAm illustrates the following rheological experiment. Two solutions of terpolymer 1.5C9(Am)11/13SA (conversionZ45%,) were prepared in water and water–AN (7:3) at concentration cZ1.5 mg/ml which is close to overlap concentration for parent PAAm (c*z1/[h]Z2 mg/ml). Their rheological behaviour was examined by the method of low-amplitude oscillations. One can see from Fig. 6 that the crossing point G 0 ZG 00 shifts in mixed solvent to the higher frequency value (from appr. 0.7 up to about 15 rad/s). This shift indicates essential decrease of the effective lifetime of network junctions after
3.3. Study of molecular weight heterogeneity using GPC In this work, we have used GPC with water–AN mixtures (8:2 and 7:3) containing NaNO3 as eluents. A series of HMPAAm characterized with SLS and GPC/MALLS (MwZ3!105–3!106) were used for calibration as secondary standards together with sulfonated polystyrene standards (Polymers Laboratories) (PSS) and poly(ethylene oxide) standards (Polymer Standards Service-USA, Inc.) (PEO). In the range of M!2!105, calibration curves for these two types of standards (PSS and PEO) are not so
Fig. 6. Frequency dependencies of dynamic moduli of solutions of 1.5-C911/13SA (conversion 45%) in different solvents, cZ0.15 g/dl.
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the addition of AN. Simultaneously dynamic moduli get an order of magnitude lower. Both these facts evidence the weakening of network junctions. It is known that hydrophobical domains play a role of network junctions in HMPAAm solutions and gels. The observed changes of rheological properties in the presence of AN as compared with aqueous systems correlate with the disruption of hydrophobic aggregates, if still exist, in the course of chromatography. One can mention the work [18], where hydrophobically end-modified telechelic PAAm (Mwz3!104) was studied by GPC in water and its mixtures with AN. In water and water–AN (9:1), high molecular weight peaks on GPC traces reflected the presence of large aggregates, while in the mixture water–AN (7:3) only peaks attributed to molecular species were detected. These data are consistent with our conclusion about the high ability of water–AN mixture containing 30% of AN to disrupt hydrophobic aggregates of HMPAAm. As an example of the study of molecular weight heterogeneity of HMPAAm, MWD evolution in the course of micellar copolymerization of AAm, NAAm and AA can be presented (Fig. 7). The specific feature of MWD evolution was found to be a remarkable growth of molecular weight heterogeneity with conversion. The growth of conversion was also accompanied with the change of polymer composition as can be seen from Table 1. The observed evolutions are in agreement with those reported earlier by Candau et al. [4] and McCormick et al. [10]. At the same time, we have found that rheological properties of terpolymers deteriorate with conversion. Fig. 8 illustrates the evolutions of dynamic moduli of solutions of cZ1.5 mg/ ml. It is seen that the value of storage modulus essentially decreases with conversion and crossing point of G 0 and G 00
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Fig. 8. Effect of conversion on frequency dependence of dynamic moduli of aqueous solutions of 1.5-C9-11/13SA, cZ0.15 g/dl. Conversion: 45% (1), 64% (2), 86% (3).
frequency dependencies shifts to higher frequencies. The similar changes in the viscoelastic properties with conversion were reported earlier in the work of Candau et al. [19]. These data were obtained for copolymers containing N,Ndihexylacrylamide as hydrophobic comonomer, i.e. for copolymers which did not demonstrate a drift in composition with conversion. Therefore, evolutions in rheological behaviour could not be connected with the compositional heterogeneity and were ascribed to the variations in the length of hydrophobic blocks. In our case, the observed evolutions in composition and the growth of polymolecularity of terpolymers allow us to suppose that the low MW fraction forming at the late stage of reaction consists mainly of macromolecules with small amount of hydrophobe or even of pure PAAm. Therefore, one should say that all types of molecular heterogeneity: compositional, structural, and molecular weight ones contribute to the deterioration of rheological properties.
4. Conclusions
Fig. 7. Evolutions of MWD in the course of synthesis of terpolymer 1.5-C911/13SA. 1—conversionZ20%, 2—conversionZ54%, 3—conversionZ 64%, 4—conversionZ86%. Curves are normalized for the values of conversion. Synthesis conditions are described in the experimental part and in Table 1.
It is shown by fluorescence spectroscopy, DLS, SLS and GPC that the addition of AN as a co-solvent can shift hydrophobic association of hydrophobically modified polyacrylamides to semi-dilute region. These copolymers can be successfully characterized by SLS, DLS and GPC in aqueous solutions containing optimal fraction of AN (about 30%). Hydrophobic aggregates, if still exist, can be disrupted during the chromatography in this eluent due to the weakening of hydrophobic junctions. The elaborated technique allowed us to reveal the broadening of MWD with the growth of conversion in the course of micellar terpolymerization. This is an additional factor along with the compositional and structural heterogeneity, which leads to the deterioration of thickening properties of the polymers prepared at high conversion.
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Table 1 Time evolutions of 1.5C9-11/13SA terpolymer composition and molecular weights Time, min
Conversion, wt%
NAAm,a mol%
SA,b mol%
Mw !10K6 (GPC)
Mw/Mn (GPC)
10 20 30 60 300
19.8 45.3 54.1 64.3 86.2
2.0 3.4 – – 1.3
14.94 13.88 13.71 13.59 12.53
1.8 1.8 1.6 1.6 1.2
3.1 4.3 4.4 4.6 5.8
a b
Determined by titration. Determined by 1H NMR.
Acknowledgements This work was supported by CRDF (RP0-13011) and by Russian Foundation for Basic Research (project no.03-0332683).
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