Article pubs.acs.org/Macromolecules
Polyelectrolyte Molecular Weight and Salt Effects on the Phase Behavior and Coacervation of Aqueous Solutions of Poly(acrylic acid) Sodium Salt and Poly(allylamine) Hydrochloride Rungsima Chollakup,†,‡ John B. Beck,§ Klaus Dirnberger,⊥ Matthew Tirrell,*,†,§,∥ and Claus D. Eisenbach*,§,⊥ †
Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States KAPI, Kasetsart University, Bangkok, Thailand § Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States ⊥ Institute for Polymer Chemistry, University of Stuttgart, D-70569 Stuttgart, Germany ‡
ABSTRACT: Phase separation of polyelectrolyte complexes (PECs) between the polyacid (sodium salt) and polybase (hydrochloride) of poly(acrylic acid) (PAA) and poly(allylamine) (PAH), respectively, has been investigated in aqueous solution. Chain length of the PAA was varied (25 < Pw < 700) holding Pw of the PAH constant at 765. The polyacid/ polybase mixing ratio (10−90 wt %) and the ionic strength as salt concentration (0−3,000 mM) were systematically varied. Sample turbidity was utilized as an indicator of PEC formation, complemented by optical microscopy for discrimination between precipitate and coacervate. Salt-free systems always resulted in PEC precipitates; however, coacervates or polyelectrolyte solutions, respectively, were formed upon exceeding critical salt concentrations, the PEC formation also depending on the employed PAA/PAH ratio. The lower the PAA molecular weight, the lower were the critical salt concentrations required for both the precipitate/coacervate and coacervate/solution transitions. The experimental phase behavior established here is explained by molecular models of coacervate complexation, addressing effects of polyelectrolyte molecular weight and salt screening. micelle7 or protein complex8 systems. Depending on the kind of polysaccharides used, proteins and polysaccharides can form either fractal aggregates or coacervates.5 Before complex coacervation occurred, soluble protein/polysaccharides whose size increased simultaneously with the extent of charge neutralization were formed.6 Molecular models have been proposed.5 Temperature-dependent successive clustering, i.e., formation of small soluble aggregates, has been identified to be a coarcervation precursor in polyelectrolyte/micelle coacervation. 7 Molecular weight and polydispersity effects on subsequent splitting into smaller and larger aggregate sizes with temperature increase were observed.7 The structure of polyelectrolyte−protein complexes was found to depend on the chain length of the polyelectrolyte:8 samples made with short polyelectrolyte chains were fluid whereas gelled systems resulted when long chains were employed. However, due to the paucity of work on structurally well-defined polymers, there is limited understanding of the interplay of key parameters controlling the formation of the fluid complex, and what factors are decisive for the formation of a coacervate in contrast to a
1. INTRODUCTION Polyelectrolyte complexes (PEC) may result on mixing of oppositely charged polyelectrolytes in aqueous solution. The formation of PECs often leads to phase separation phenomena, i.e., the appearance of either a precipitate (insoluble solid) in a liquid−solid phase separation, or two liquid phases in a liquid− liquid phase separation. The latter case leads to one phase more concentrated in polymer, which is the coacervate (polyelectrolyte-rich dense liquid phase), and the other, more diluted phase is the equilibrium solution. Polymer coacervate complexes (PCC) are of importance in biological systems, and in various application technologies. This was emphasized in the original and first report on PECs and PCCs1 (in the gelatin/acacia gum system), and is also expressed in recent reviews on this subject.2,3 In the context of biological systems, the contribution of complex coacervation in the under-water adhesion of, for example, the mineralized tubes of the sandcastle worm has been recognized.4 Complex formation is dependent on many factors, i.e., molecular weight, polymer concentration, ratio of the two interacting polyelectrolytes and the ionic strength, pH, and temperature of the solution. This has been recognized from the beginning of the studies of PEC formation.1 Recently new insights into coacervate complex formation were reported from studies of biopolymeric polyelectrolytes5,6 and polyelectrolyte/ © 2013 American Chemical Society
Received: September 26, 2011 Revised: March 1, 2013 Published: March 13, 2013 2376
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Phenomenologically similar molecular weight effects on the formation of water-soluble, nonstoichiometric solution complexes were found for high molecular weight poly(acrylic acid) sodium salt host-polyelectrolyte and comparatively low molecular weight linear poly(ethylenimine) guest-polyelectrolyte.15 Distinct effects of the length ratio of interacting polyelectrolyte chains on the formation and stability of polyelectrolyte multilayers (the poly(N-ethyl-4-vinylpyridinium bromide)/poly(methacrylic acid) pair, in particular) deriving from multiple intermolecular polymer/polymer contacts, and similarities with properties of PECs in solution, have been elucidated.16 In this context, preferential binding of high molecular weight polycation with high molecular weight polyanion was reported. For the assembly of multilayered films consisting of poly(acrylic acid) and poly(allylamine), distinct effects of the molecular weight and polydispersity of the polyanion on the film growth were observed and associated with the chain length influence on the rate of diffusion of the polyanion into the film during assembly.17 The literature examples given above illustrate the diversity of the systems that have been investigated, and the problems in deriving from these data general conclusions on molecular weight effects in PEC and coacervate formation. In this paper, we present experimental data on effects of varying the molecular weight of the poly(acrylic acid) sodium salt (PAANa) in a PAANa-PAH polyelectrolyte pair, consisting of a PAH of constant, high molecular weight, on the phase behavior of the PEC system under various experimental conditions. Our studies aimed to narrow down the stability range of the aqueous coacervate phase consisting of high molecular weight PAH of weight-average degree of polymerization Pw(PAH) = 765 and PAANa-X with Pw(PAA) (=X) between 25 and 695 against the PEC precipitate and the polyelectrolyte solution. Experimental variables were the polyanion/polycation stoichiometry and the salt concentration. In order to ensure that the investigated series of PAH/PAANa-X test specimens differing in acid/base repeat unit mixing ratio n−/n+ and NaCl salt concentration, were as close as possible to thermodynamic equilibrium, variations of n−/n+ mixing ratio and ion strength were not varied in a continuous diluting or concentrating procedure starting from a ternary PAANa-X/PAH/NaCl aqueous system, but each test specimen of a given PAANaX/PAH/NaCl mole ratio was mixed separately and further investigated after a sufficiently long incubation time. In practice, this was achieved by a protocol of subsequent addition steps of first aqueous PAANa-X into pure water (salt-free systems) or aqueous NaCl solution (systems with set ion strength), mixing, followed by the addition of aqueous PAH solution and again mixing. Details will be given in the Experimental Section. Thus, the protocol employed in this work, i.e., point-by-point analysis which quasi is a precision landing in the phase diagram coordinates, principally allows localizing the two-phase regimes of PEC precipitate/aqueous solution as well as liquid PEC/ aqueous solution, and the one-phase polyelectrolyte solution. The phase transition boundary line can then be constructed between the established phase regimes. This procedure significantly differs from a stepwise addition of salt solutions to an aqueous polyacid/polybase system of a given n−/n+ mixing ratio, or a stepwise change of the n−/n+ mixing ratio by addition of either the aqueous polyacid or polybase solution which could indicate the phase transition in a titration curvelike manner. Since it had been established in previous studies9
precipitate, particularly for structurally simple synthetic polymers. In previous work,9 we investigated PEC formation by employing poly(acrylic acid) (PAA) of various degree of neutralization (with NaOH) and poly(allylamine hydrochloride) (PAH) as model polyelectrolytes of a weak polyelectrolyte pair in order to generate a complete and complementary set of data on coacervate phenomena. The complex formation of this symmetrical polyelectrolyte pair of similar chain length (as given by the weight-average degree of polymerization (Pw) being in the range of about 700 (PAH) ≤ Pw ≤ 750 (PAA)) was studied by varying the concentration and mixing ratio of the oppositely charged polymers, the ionic strength and pH of the aqueous solution, and the temperature. Among other findings, it was experimentally determined that the critical salt concentration required to attain coacervate or solution phases of a given polyacid/polybase mixing ratio is related to the number of possible anion−cation interactions between PAA carboxylate and PAH ammonium groups as varied by the degree of neutralization of PAA and pH. Molecular models explaining the effects of the polyelectrolyte mixing ratio and ionic characteristics of the system as well as temperature effects on the formation of PECs and the redissolution of PECs have been established.9 These model studies, employing a well-defined weak polyanion−polycation pair, led to a conclusive picture of PEC formation as functions of a variety of experimental parameters, with the identical polyelectrolyte pair, thus clarifying earlier findings reported in the literature that had been obtained with a variety of fundamentally incomparable systems. Considering the sparse available literature data on how the formation of PECs is affected if the molecular weight of either the polyacid or polybase component is decreased while keeping the molecular weight of the complementary component unchanged, we have complemented our model studies in this regard. We have also been motivated in this experimental work by some earlier theoretical work addressing specifically the complexation of polymers of unequal molecular weight.10 Recently the effect of chain length and salt concentration of polyelectrolyte complexes (coacervates) of PAA and poly(N,Ndimethylaminomethyl methacrylate) (PDMAEMA) has been studied for stoichiometrical acid/base unit polyacid/polybase pairs where both polyelectrolytes had almost similar chain length.11 It was found that longer polyelectrolytes tend to have a broader coexisting phase regime, and that the critical salt concentration for which phase separation was no longer observed to increase with increasing chain length. The sensitivity of stable PEC formation to the chain length of one of the two polyelectrolytes of the stoichiometric polyacidpolybase pair has been first described for complexes of poly(Lglutamic acid) with oligo(ethyleneinimne)s.12 A critical chain length of 5 ethylenimine repeat units has to be exceeded in order to form complexes. A minimum chain length for polymers to form a stable interpolymer complex was also described for poly(methacrylic acid)/poly(oxyethylene) mixtures.13 Chain length effects on the stability of PECs on the concentration of added NaCl were also reported for a series of nonstoichiometric mixtures of divinyl ether/maleic anhydride copolymer based polyanions (hydrolyzed anhydride groups) and poly(N-ethyl-4-vinylpyridine) polycations of different degree of polymerization:14 the amount of added salt required for dissolution of the PEC is less, the shorter the polycation. 2377
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and GPC. The tert-butyl protecting group was removed by trifluoroacetic acid (18.7 mg) catalyzed ester cleavage in 1,4-dioxane (25 mL) solution (4 h reflux). The isolated polymer was dissolved in water, washed with dichloromethane (3x), and the water was removed by rotary evaporation followed by drying under high vacuum. About 2.7 g of colorless, solid PAA were obtained. NMR analysis revealed complete deprotection. GPC analysis (THF eluent; polystyrene (PS) calibration) of the (PtBA) precursor polymer gave an apparent molecular weight of Mw = 25 000 g/mol and PDI = 1.18, i.e., apparent Mn(app) = 21 190 g/mol. On the basis of literature data19 which had shown that the number-average molecular weight Mn of PtBA is about 1.3 times higher than obtained by a PS calibration based SEC, Mn = 25 500 g/mol (Pn = 230) is calculated. With PDI = 1.18, this gives a correct weight-average molecular weight Mw = 30 000, and Pw = 417. Assuming a polymer analogous ester cleavage reaction, the same figure was taken for the obtained PAA. Since end group effects cannot be neglected in short polymers and PAA-25 is a relatively short polyelectrolyte chain compared with the other PAAs employed in this study, this polymer was further analyzed by 1H NMR analysis (D2O solvent; trimethylsilyl-1-propane-3-sulfonic acid sodium salt as internal standard for signal to chemical shift assignment). The NMR spectrum employed for end group analysis, and peak assignments, are shown in Figure 1. In addition to the characteristic signals of the −CH2−CH(COOH)(CH2)− proton at about 2.1−2.5 ppm, and the signal of the −(HOOC)−CH−CH2− CH(COOH)− protons at about 1.2−2.1 ppm, additional signals at about 1.0−1.3 and 2.5 ppm, and 7.3−7.9 ppm are seen. The former two signals could be assigned to alkyl (CH3(CH2)n‑1CH2S−) end groups originating from mercaptan (most probably octyl mercaptan (CH3(CH2)nSH; n = 7) chain transfer agent, and the latter signal is characteristic for the phenyl group originating from benzoyl peroxide initiator. By relating the 1H-signal integrals of the phenyl and mercapto end groups to the 1H-signal integral of the acrylate backbone unit a number-average degree of polymerization Pn, i.e., the average number of AA repeat units per chain can be calculated. With the assumption that PAA macromolecules (repeat unit 3H; integral 174.099), contained one end group either originating from the initiator (5H; integral 2.485) or from the chain transfer agent (integral 36.804), and assuming octyl mercaptan (15H) as chain transfer agent, a numberaverage degree of polymerization Pn ≈ 20 (average number of AA repeat units per chain) is calculated. Considering the end group contribution, this corresponds to Mn ≈ 1500 g/mol. Given the 4.9/1 mol ratio of octyl/phenyl end groups, the polymer would consist of an about 5/1 mixture of PAA chains carrying an octyl or phenyl end group, respectively. 2.1.2. Poly(allylamine hydrochloride). The cationic polyelectrolyte, poly(allylamine hydrochloride) (PAH-765) was obtained from SigmaAldrich (product no. 283223−25G). According to the catalogue specification, this polymer has a nominal molecular weight Mw = 70 000 g/mol (based on poly(ethylene oxide) (PEO) standard calibrated GPC). Since the above molecular weight specification is relative to PEO, for comparison, and in order to check the validity of this figure, this PAH was further characterized by viscosity measurements. The weight-average molecular weight Mw was calculated from viscosity measurements in aqueous 1 M NaCl solution at 25 °C according to the viscosity − molecular weight relationship20 [η] = 13.9Mw0.714 (concentration in g/mL). With the measured intrinsic viscosity [η] = 40.645 mL/g, a weight-average molecular weight Mw = 71 500 g/mol (Pw = 765) is obtained, which is good agreement with the PEO based value specified in the catalogue. Assuming a normal distribution of PAH (PDI = 2), our data correspond to Mn = 35 700 g/mol (Pn = 382). 2.2. Preparation and Analysis of Polyelectrolyte Complex Systems. In all experiments, PAH was employed as polybase, and fully neutralized poly(acrylic acid), i.e., the sodium salt PAANa, was used. The experimental protocol for the preparation of the polyelectrolyte complexes (PECs) between PAH and PAH of different molecular weights, and the characterization and analysis of the various PEC systems were the same as described in detail previously.9
of PAH-765/PAANA-695 mixtures that total polyelectrolyte concentration >0.02 wt % is necessary for obtaining an experimentally observable PCC phase, all experiments were carried out with 0.05 wt % total polyelectrolyte concentration; this is still much smaller than the critical overlap concentration c*, which is approximately 2.5 wt % for both the high molecular weight polyelectrolytes. In conclusion, it has to be emphasized that the chosen protocol allows clear discrimination between the phase regimes of stable liquid−liquid polymer coacervate complexes (PCC), solid polyacid−polybase precipitate, and true polyelectrolyte solution.
2. EXPERIMENTAL SECTION 2.1. Polyacid and Polybase Materials. Three commercially available and one specially synthesized poly(acrylic acid) (PAA) were employed as polyacid. The polybase was commercially available poly(allylamine hydrochloride) (PAH) in all experiments. The polyelectrolytes, and their molecular characteristics, are compiled in Table 1. When referring further on in the text to poly(acrylic acid) and
Table 1. Molecular Weight characteristics of the Employed Polyelectrolytes Poly(acrylic acid) (PAA) and Poly(allyl amine) Hydrochloride (PAH) polymer d
PAA-25 PAA-70e PAA-417f PAA-695e PAH-765d
Mwa g/mol
Pwb
PDIc
1800 5000 30 000f 50 000 71 500
25 70 417 695 765
− 2.4a 1,18f 2.9a −
a
Supplier’s data. bWeight-average degree of polymerization Pw = Mw/ MRU, with weight-average molecular weight Mw of the polyelectrolyte, and repeat unit molecular weight MRU; acrylic acid (AA): MRU = 72 g/ mol; allyl amine hydrochloride: MRU = 93.55 g/mol. cPolydispersity index PDI = MW/Mn. dPurchased from Sigma-Aldrich. ePurchased from Polyscience. fSynthesized and characterized in our laboratories. the polyacid as PAA-X or PAANa-X, respectively, and to the polybase as PAH-Y, the numbers X or Y mean the weight-average degree of polymerization Pw of the respective polymer. 2.1.1. Poly(acrylic acid). Several poly(acrylic acid)s (PAA) of different molecular weight were used as the anionic polyelectrolyte. PAA-25 was purchased from Sigma-Aldrich (product no. 323667) in powder form. According to the manufacturers data sheet, this polymer has Mw = 1800 g/mol, i.e., Pw = 25. PAA-70 was purchased from Polyscience (catalog no. 06519−250) as 50 wt % solution in water). According to the catalog specifications Mw = 5000 g/mol and PDI = 2.4, this polymer has Pw = 70, and a number-average molecular weight Mn = 2083 g/mol (Pn = 30) is calculated. PAA-695 was purchased from Polyscience (catalog no. 00627−250) as 25 wt % solution in water. According to the catalog specifications Mw = 50 000 g/mol and PDI = 2.9, this polymer has Pw = 695, and a number-average molecular weight Mn = 17 240 g/mol (Pn = 240) is calculated. PAA-417 of molecular weight Mw = 30 000 g/mol was synthesized in our laboratory by atom transfer radical polymerization (ATRP) of tert-butyl acrylate,18 and subsequent removal of the tert-butyl protecting group from the poly(tert-butyl acrylate) (PtBA) precursor polymer by ester cleavage. 8.75 g (68.25 mmol) of tBA was polymerized in toluene (5 mL) with methyl-2-propanoate (23 μL, 34.5 mg, 0.207 mmol) as initiator with CuIBr (24 mg, 0.167 mols) and pentamethyldiethylenetriamine (100 μL, 0.479 mmol) as catalyst. The polymerization was carried out at 80 °C for 18 h. The greenish reaction solution was purified by passing through alumina and washing with toluene. The combined organics were concentrated by rotary evaporation followed by 18 h drying at high vacuum. The colorless, transparent polymeric residue (about 7 g) was characterized by NMR 2378
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Figure 1. 1H NMR spectrum of poly(acrylic acid) PAA-25 (D2O solvent). that passes through a volume of 1 cm length and I0 is the incident light intensity (cf. ref 22). The turbidity data shown in the Figures is the average of three measurements, error bars displaying the standard deviations; in some cases the error bars are small enough to be obscured by the symbol for the data point. All samples were again treated in a vortex mixer prior to the UV absorbance measurement and microscopic analysis. A classification of the phase characteristics as precipitate, coacervate, or solution was unambiguously done by further investigation of the turbid system with light microscopy. The precipitate appears as an aggregated solid structure (Figure 2a, left) whereas the coacervate phase which is the immediate result of the experimental protocol (formation at salt concentrations where the precipitate does not exist) is indicated by an emulsion-like appearance (Figure 2b, left) (cf. ref 9). As verified by varying the depth of focus and focal plane, the droplets, which significantly varied in size, are transparent and did not show any sign of precipitate. These observations are in agreement with precipitate (“curdy”) and coacervate (droplet) appearances described in literature for other polyanion/polycation pairs.23 It has to be noted that the variation in droplet size may be different if the samples were investigated shortly after preparation (e.g., within 1 h, cf. ref 21), but our observations were the result of our experimental protocol which gave reproducible results after stabilization of the respective system for about 12 h. With time, coacervate droplets tend to coalesce; upon centrifugation, a macroscopic two-phase system is obtained. The degree of phase separation, i.e., the relative phase volumes were determined by centrifugation. For the phase-separated systems, the collected phase at the bottom of the vial was obtained as a white solid in case of the precipitate (Figure 2a, right), and as a whitish viscous liquid or transparent gel in case of the coacervate (Figure 2b, right: for better visualization, the liquid coacervate phase which had settled at the bottom of the vial was swirled). The distinctively different appearance of the centrifugatesolid particulate sediment vs viscous liquidfurther confirms the discrimination of solid precipitate and liquid dense, i.e., coacervate phase by means of light microscopy. The polyelectrolyte composition of the supernatant equilibrium sol phase was determined by means of colloidal titration.24,25 The total polyelectrolyte amount as well as the polyelectrolyte composition in
PEC systems of different polyion mixing ratios (from 1/9 to 9/1, w/w) and 0.05 wt % total polyelectrolyte concentration were prepared by sequentially pipetting the corresponding amount of 2% (w/w) aqueous PAANa stock solution (pH ∼ 8.5) and 2% (w/w) aqueous PAH stock solution (pH ∼ 3.0) into a vial containing the required amount of pure Milli-Q water or aqueous NaCl solution to give the final volume of 1.5 mL. The fully neutralized aqueous PAA (poly(acrylic acid) sodium salt; PAANa) stock solution was prepared by titration of aqueous PAA solution (initial pH ∼ 2.3) with 0.1 NaOH until pH ∼ 8.5 was reached. The PAH-765 stock solution was prepared by diluting the commercially available 25 wt % aqueous solution with Milli-Q water. In practice, first the adjusted amount of Milli-Q water or milli Q water plus 5 M aqueous NaCl stock solution (thus the aqueous NaCl solution placed into the vial containing the amount of salt required for the given salt concentration in the final 1.5 mL sample volume, cf. ref 9) was placed into the vial. Then the calculated amount of the aqueous PAANa stock solution was added, and finally the calculated amount of the aqueous PAH stock solution. After each step of adding the respective stock solution, the system was vigorously mixed by using a vortex mixer (Vortex-Genie, Scientific Industries Inc., speed setting 9−10 (2,800 rpm), 30 s). After preparation, the samples sat overnight, i.e. for about 12 h to equilibrate. In all experiments, the total volume of the PEC system was 1.5 mL. The chosen conditions of PEC preparation have been proved to guarantee a good reproducibility. We have further analyzed each sample for the type of PEC formed In this context it has to be clearly stated that this protocol, i.e., to prepare a fresh PEC system for each experiment, was followed for each sample. This means that in each experiment the salt concentration was fixed from the beginning and not varied by stepwise addition of NaCl solution to the PEC system as, e.g., in ref 21. Thus, our procedure allowed preparing PECs at conditions under which either only one type of PEC is stable, i.e. precipitate or coacervate, or under which neither a precipitate nor a coacervate but only a polyelectrolyte solution results. Turbidity measurements were carried out employing a Perkin-Elmer Lambda 2−40 UV/vis spectrophotometer at a wavelength of 500 nm. The turbidity is defined by τ = −ln(I/I0) where I is the light intensity 2379
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obtained if the sample is prepared with a salt concentration, which is above the critical salt concentration for the precipitate−coacervate transition. In this context, it has to be mentioned that, in the present study, the concentration of either polyelectrolyte (as well as the total polymer concentration c = 0,05 wt %) is below the critical overlap concentration c*, which was calculated from c* = 1/[η] by using the corresponding intrinsic viscosity−molecular weight relationships20 (1.0 M aqueous NaCl; 25 °C; [η] in mL/g) for PAH ([η] = 13.9·Mw0.714) and the PAANa ([η] = 41.5·Mw0.63). For both the high molecular weight polyelectrolytes (PAH-765 and PAANa-695), c* is about 2.5 wt %; c* of PAANa increases with decreasing molecular weight, e.g., a value of about 11 wt % is calculated for of PAANa-70. 3.2. Effect of Polybase/Polyacid Mixing Ratio on Polyelectrolyte Complex Formation in Salt-Free Systems. The measured turbidities of mixtures of poly(allylamine hydrochloride) (PAH-765) with poly(acrylic acid) sodium salt (PAANA-X with X = Pw) of differing chain length as a function of the polyacid/polybase mixing ratio, expressed in wt % AA units, or the acid/base unit mole ratio (mol % AA units) and the n−/n+ AA/AH units mixing ratio, respectively, are depicted in Figure 3a. Except for the system with PAANa-25, the maximum turbidity is observed for 1/1 acid/base unit stoichiometry (n−/n+ = 1), corresponding to about 43 wt % PAA in the employed polyacid/polybase mixture. The finding for PAANa-70, PAANa-417, and PAANa-695 is in accordance with literature that the highest yield of polyelectrolyte complex is observed for acid/base unit stoichiometry.16 The anomalous result observed for the mixture with PAANa-25 is attributed to chain architectural irregularities as compared to the higher molecular weight PAAs, and will be discussed further below. Phenomenologically, the yield of precipitate as related to the polyacid/polybase mixing ratio (Figure 3b) is proportional to the observed turbidity (Figure 3a). The amount of precipitate was established by colloidal titration of the sol phase after centrifugation with o-toluidine blue (o-Tb) as indicator; this method takes advantage of the color change from blue (free oTb) to pink (o-Tb bound to anionic polyelectrolyte), i.e. residual poly(allylamine) in the sol phase is directly titrated with PAA, whereas residual PAA is determined by back-titration after an aliquot of poly(allylamine) has been added to the sol phase; see ref 9 for experimental details. The precipitate obtained with polyacid/polybase mixtures that gave the highest turbidity always comprises almost all of the mass of the polyelectrolyte macromolecules in the mixtures. Furthermore, it is noticed (Figure 3b) in all systems that, as long as the maximum turbidity is not yet attained with increasing PAANa concentration, the yield of precipitate is the lowest for the system with the low molecular weight PAANa-25, and increases with increasing PAA molecular weight. The largest amount of precipitate is obtained for the high molecular weight polyacidpolybase pair of almost the same polymer chain length (PAANa-695 and PAH-765). This is a clear indication of the pronounced molecular weight dependency of PEC formation. As already established for the PAH-765/PAANa-695 pair,9 in case of the PAH-765/PAANa-417 system, for mixtures with surplus of PAH ( 1, for fixed ion strength, the stability of the complex coacervate phase is decreased the more the shorter the polyelectrolyte partner gets. This is a consequence of the interrupting of the
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under grant DMR-05-20415 (MRSEC) and DMR-07-10521 (Materials World Network). The authors acknowledge the access to the facilities of the UC Santa Barbara Materials Research Laboratory (MRL) for executing all experiments. Funding by the UC Santa Barbara MRL, and Kasetsart University (Thailand) is also gratefully acknowledged. The work on this project at Berkeley was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under the Department of Energy Contract No. DE-AC02-05CH11231. Special thanks are given to Dr. Wirasak Smitthipong for helpful technical assistance with this research.
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dx.doi.org/10.1021/ma202172q | Macromolecules 2013, 46, 2376−2390
