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Dependency of Particle Sizes and Colloidal Stability of Polyelectrolyte Complex Dispersions on Polyanion Structure and Preparation Mode Investigated b...
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J. Phys. Chem. B 2007, 111, 8668-8675

Dependency of Particle Sizes and Colloidal Stability of Polyelectrolyte Complex Dispersions on Polyanion Structure and Preparation Mode Investigated by Dynamic Light Scattering and Atomic Force Microscopy† Marcela Mihai‡ “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, RO-700487 Iasi, Romania

Ecaterina Stela Dragan* “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, RO-700487 Iasi, Romania

Simona Schwarz§ Leibniz Institute of Polymer Research, Hohe Strasse 6, 01069 Dresden, Germany

Andreas Janke| Leibniz Institute of Polymer Research, Hohe Strasse 6, 01069 Dresden, Germany ReceiVed: February 28, 2007; In Final Form: April 24, 2007

Polyelectrolyte complex (PEC) dispersions were prepared by controlled mixing of three random copolymers of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) with either t-butyl acrylamide (TBA) [P(AMPS54co-TBA46) and P(AMPS37-co-TBA63)] or methyl methacrylate (MM) [P(AMPS52-co-MM48)] with an ionenetype polycation, containing 95 mol % N,N-dimethyl-2-hydroxypropyleneammonium chloride repeat units (PCA5), with their structural characteristics being deeply investigated by dynamic light scattering (DLS) and atomic force microscopy (AFM). Shape, size, and polydispersity of the PEC dispersions were directly observed by AFM as a function of polyanion structure, the ratio between charges, n-/n+, and the titrant addition rate (TAR). The particle sizes increased and the colloidal stability decreased with the increase of the nonionic comonomer content and with the decrease of TAR. It was demonstrated that the medium particle sizes of the complex nanoparticles adsorbed on silicon wafers measured by AFM, in the dry state, were close but always lower than those measured by DLS, both before and after the complex stoichiometry.

I. Introduction Considerable interest has been dedicated to the polyelectrolyte complexes (PECs) formed by mixing synthetic polyelectrolytes, because, unlike the much more complicated biological supramolecular architectures stabilized by H-bonds and hydrophobic interactions besides the electrostatic ones, they allow separation or combination, in a predetermined way, of different types of interactions.1-13 As a function of some parameters regarding the complementary polyions, such as polyion charge density, charge strength (nature), polyion molar mass, and polyion concentration, and some parameters concerning the characteristics of the preparation environment such as pH, ionic strength, or temperature, three main types of PECs have been prepared: insoluble and amorphous precipitates (polysalts),1-10 soluble PECs,14-19 and PECs as stable colloidal dispersions.23-37 PECs responsive to the external stimuli such as pH12,21 and temperature20,22 are very promising materials for medical science, † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * Telephone number +40.2322217454; fax number +40.232211299; E-mail [email protected]. ‡ [email protected]. § [email protected]. | [email protected].

ecology, drug delivery and controlled release systems, on-off switches, and so on. PECs as colloidal dispersions, bearing charges in excess, demonstrated effectiveness in flocculation,23,29,33 surface modification of different substrates such as silica, membranes,27,28 and medical applications as nanocarriers for drugs, proteins, DNA.12,19,38 A high colloidal stability, narrow size distribution, and concentration as great as possible are characteristics which still require further investigation of the controlling parameters. Therefore, the preparation of PEC dispersions with a narrow distribution of particle sizes and a high storage colloidal stability has been a central task for the past decade.27,31,32 However, finding the formation conditions of PEC particles having a more or less constant size and a high storage stability, by mixing synthetic polyelectrolytes, is still a challenge. In this context, Mu¨ller et al. have reported on the characterization of the centrifuged nonstoichiometric PEC dispersions, prepared from poly(diallyldimethylammonium chloride) (PDADMAC) and poly(maleic acid-co-R-methylstyrene) sodium salt as oppositely charged polyelectrolytes, by atomic force microscopy (AFM) compared with dynamic light scattering (DLS), at a polymer concentration of >1 mmol/L.27,31 Monomodal PEC nanoparticles were obtained by consecutive centrifugation.31 It was recently

10.1021/jp071655q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

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CHART 1: Structures and Some Characteristics of the Polyions Used in the Complex Formationa

a Mu - mass per charge calculated as the ratio between the mean molar mass of the repeat unit of the copolymer, i.e., [MAMPS × fAMPS + MNM × (1 - fAMPS)] and fAMPS, where MAMPS ) 229, fAMPS is the molar fraction of AMPS, and MNM is the molar mass of the nonionic monomer.

demonstrated that almost monodisperse PECs nanoparticles as dispersions with a high colloidal stability, even after the isoelectric point, IEP, could be prepared, when poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS) was used as polyanion (PA), by strict control of the titrant addition rate (TAR) around the IEP.39,40 It is known that ionic-nonionic copolymers (block, alternate, or random copolymers) allow tailoring of the complex properties by their charge density and the hydrophilic/hydrophobic balance.23-27,31,41-43 Furthermore, the presence of stimuli-responsive comonomers recommend such copolymers as components in the preparation of novel “smart” materials. The aim of the present work was to investigate dependency of the particle sizes and the colloidal stability of the PEC dispersions formed by mixing three random copolymers of AMPS with either t-butyl acrylamide (TBA), P(AMPS54-coTBA46) and P(AMPS37-co-TBA63) or methyl methacrylate (MM), P(AMPS52-co-MM48), with the ionene type polycation, PCA5, containing 95 mol % N,N-dimethyl-2-hydroxypropyleneammonium chloride repeat units, on the polyanion structure and preparation mode. The correlation between the particle sizes of the PEC nanoparticles obtained by DLS and the medium diameter of the particles obtained by AFM gave valuable information on the complex morphology comparative with the results obtained with NaPAMPS as added PA. To the best of our knowledge, this is the first systematic investigation of the complex morphology and colloidal stability using ionic/nonionic random copolymers of AMPS as PA by employing two methods which comparatively evidence particle sizes in the swollen state (DLS) and in the dry state (AFM). II. Materials and Methods II.1. Polyelectrolytes. All polyions used in the preparation of PEC dispersions are displayed in Chart 1. The polycation

PCA5 was synthesized and purified according to the method previously described.45 NaPAMPS with a molar mass Mw ) 170 000 g/mol was synthesized and purified according to ref 46. Copolymers of AMPS with TBA and MM were synthesized by free radical polymerization with AIBN as initiator as was presented in ref 44. II.2. Preparation of PECs Dispersions. Variable volumes of the aqueous solution of PA, having a constant concentration of 5 mM, were continuously added to the aqueous solution of PC, having a concentration of 0.5 mM, with a constant TAR in the range 0.1-8.0 mL PA/mL PC.h, under magnetic stirring, at room temperature (about 25 °C), until a certain ratio between opposite charges was achieved. The PEC dispersions were still stirred for 60 min and were characterized after 24 h and 1 week of storage. II.3. Colloidal Titration. The anionic or cationic charge attributed to the polyelectrolyte in excess (either PC or PA), primary complex, and the PEC particles was determined by the Particle Charge Detector PCD 02 (Mu¨tek GmbH, Germany). The principle of this method is based on the compensation of the streaming potential induced by the adsorption of charged species onto a surface of a test vessel made of polytetrafluoroethylene by titration with a standard solution of a strong oppositely charged polyelectrolyte. The concentration of the charged groups in the examined solution was calculated from the amount of standard solution needed to reach the zero value of the streaming potential. All measurements were made at room temperature. II.4. Dynamic Light Scattering (DLS). The measurements were carried out at a fixed angle of 90° by means of a Zetasizer 3000 (Malvern Instruments, UK) equipped with a 10 mW HeNe laser (633 nm) as a light source. DLS was used to determine the z-average translation diffusion coefficient of the PEC particles. Automatic analysis of the autocorrelation function g(2)

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Figure 1. dh and PI of PEC dispersions as a function of ratio between charges (n-/n+) for different PA addition rates: (A) and (B) PA ) P(AMPS52co-MM48); (C) and (D) PA ) P(AMPS54-co-TBA46); (E) and (F) PA ) P(AMPS37-co-TBA63).

(τ) by CONTIN or cumulants gave the z-average diffusion coefficient of particles and then the z-average hydrodynamic particle diameter from the Stokes-Einstein equation and polydispersity index (PI). II.5. Atomic Force Microscopy (AFM). A Nanoscope IIIa Dimension 3100 SPM (Digital Instruments Veeco Metrology Group, Woodbury, NY) was used to examine the size and the shape of the PEC nanoparticles adsorbed on a surface. AFM images were acquired in tapping mode using a silicon probe Tap300, from BudgetSensors (Bulgaria). Prior to use, the silicon wafer supports were carefully cleaned in two steps: first in “piranha solution”, followed by intensive rinsing with deionized water, and second with the mixture NH4OH/H2O2/deionized water, at 70 °C, in an ultrasonic bath, intensively rinsed with water, and finally dried under a nitrogen flow. The cleaned substrates were immersed in the PEC dispersion for 20 min, washed with distilled water three times, each 1 min, to remove the excess of polyelectrolyte and PECs, and kept in air until the solvent was evaporated (about 48 h for all investigated samples). Without washing, a thick layer formed by the agglomeration of complex particles, and polyelectrolyte in excess was obtained, and the particles could not be analyzed

separately in such images. The PEC particle sizes were determined from their lateral dimensions (diameter) on the AFM images, using the device software. AFM observations were repeated on different 10 × 10 µm2 areas of the same sample. III. Results and Discussion III.1. DLS. The influence of the content and nature of the nonionic comonomer on the particle sizes (dh) and PI of the PEC dispersions formed by mixing P(AMPS54-co-TBA46), P(AMPS37-co-TBA63), and P(AMPS52-co-MM48) with PCA5 are presented in Figure 1. The global complex stoichiometry was evident at a ratio between charges, n-/n+, of around 0.90, for P(AMPS52-co-MM48) (Figure 1A) and P(AMPS54-co-TBA46) (Figure 1C), and at around 0.95 for P(AMPS37-co-TBA63). Two TAR regimes have been used in the PEC preparation: 0.6 mL PA/mL PC.h and 3.8 mL PA/mL PC.h. It was evident for all PAs that a higher TAR led to smaller PEC nanoparticles (Figure 1A,C,E); after the stoichiometric point, the sizes ranged from 150 to 165 nm for the higher TAR and up to 245-300 nm for the lower TAR. The particle sizes before the stoichiometry have been clearly influenced only by the PA structure,

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Figure 2. dh after one week of storage as a function of the ratio between charges (n-/n+) for (A) P(AMPS52-co-MM48), (B) P(AMPS54-co-TBA46), (C) P(AMPS37-co-TBA63); TAR ) 0.6 mL PA/mL PC.h (upper curves) and 3.8 mL PA/mL PC.h (bottom curves).

increasing with the increase of molar mass per charge (Mu) (Chart 1). A common characteristic for the PECs formed with P(AMPS52-co-MM48) (Figure 1A) and P(AMPS54-co-TBA46) (Figure 1C) was the strong maximum arising just after the stoichiometry, followed by the decrease and leveling off, with the maximum value of the particle sizes being higher when TBA was used as a nonionic comonomer. A similar shape of the curves corresponding to the variation of the optical density of the complex nanoparticles at λ ) 500 nm (OD500) as a function of the ratio between charges, n-/n+, has been previously reported for these copolymers.44 The maximum in the particle sizes at the complex stoichiometry was correlated with the presence of the nonionic and hydrophobic comonomer (MM and TBA) and was attributed to a lower stability of the complex dispersion just around the IEP, similar to other ionic/nonionic copolymers as added polyions.26,42 A distinct characteristic for these PEC dispersions consists of the values of the PI, which also present a maximum at the complex stoichiometry and level off after that at around 0.05, for both TAR regimes, when TBA was the nonionic comonomer (Figure 1D), with the PI values being much higher (0.07-0.1) in the case of P(AMPS52-co-MM48) (Figure 1B). Usually, PI describes the heterogeneity of the sample; the values lower than 0.05, which are rarely seen, describe the monodisperse particles, and those ranging from 0.1 to 0.3 indicate a broad size distribution. The increase of the TBA content from 46 mol % up to 63 mol % caused distinct differences in the characteristics of the complex dispersion. Thus, the average hydrodynamic particle sizes were much higher when P(AMPS37-co-TBA63) was used as PA, both before and after the stoichiometry (Figure 1E), except for the values corresponding to the maximum present in the case of P(AMPS54-co-TBA46). To explain this difference, the decrease in the charge density of the copolymer P(AMPS37co-TBA63) must be considered, with its value being about 0.37, based on the ionic monomer content. We assume that the lower charge density and the increase of the nonionic comonomer content have two unfavorable consequences for the complex formation: (1) the increase of the PA amount requested for the charge compensation up to the complex stoichiometry, which causes the increase of the particle sizes, and (2) the formation of higher swollen aggregates. These results are in agreement with the data previously reported by other groups.26,41 These aspects are also supported by the values of PI (Figure 1F), which present a minimum just at the stoichiometry, which can be assigned to the separation of bigger particles 24 h after preparation, when the first measurements were performed. After the stoichiometry, when complex particles are somehow stabilized by the PA in excess, both the particle sizes and the PI were almost independent of the molar ratio between charges.

Storage colloidal stability of the complex dispersions presented in Figure 1 were observed by measuring the particle sizes after one week of storage (Figure 2). For the copolymers with almost the same content in the ionic comonomer, the values of the maximum decreased but were still present after one week of storage (Figure 2A,B). After the stoichiometry, the values of the particle sizes only slightly changed when the TAR of 3.8 was used in their preparation; the differences were significant in the case of the lower TAR (0.6), compared with those measured after 24 h, with the decrease of the size particles being in the range 15-30 nm. As was mentioned in the Experimental Section, the charges detected by PCD are attributed to the polyion in excess (PC before the stoichiometry and PA after that), primary complexes, more before the stoichiometry, and to the complex particles. The complex stoichiometry was also confirmed by the colloidal titration for all polyion pairs under study (results not shown here). The values of the particle sizes, dh, and PI as a function of TAR, for a constant ratio between charges, n-/n+ ) 1.2, were plotted in Figure 3, for the complex dispersions formed between all copolymers and PCA5. The influence of TAR on the complex particle sizes was strongly manifested when TAR was lower than 3.0 mL PA/mL PC.h. Figure 3A shows significant differences between particle sizes of the PA/PCA5 complexes as a function of the nature and content of nonionic comonomer in the PA structure. Thus, the biggest particles were obtained with the PA having the lowest charge density, i.e., P(AMPS37-co-TBA63), on the entire range of TAR. PI values strongly depended on the polyion pair and the TAR (Figure 3B). In the case of P(AMPS37-co-TBA63), a monotonous increase of PI with the decrease of TAR under about 3 mL PA/mL PC.h was observed, and an almost constant value of PI was found for higher TAR. PI of the complex particles formed with P(AMPS54-co-TBA46) as PA as a function of TAR varied in a different way, with three distinct regions being observed, as follows: an almost constant value of PI at around 0.06 is evident when TAR varied in the range 0.6-3 mL PA/mL PC.h; the PI values abruptly increased for TAR lower than 0.6 mL PA/mL PC.h, and also increased for TAR higher than 3 mL PA/mL PC.h. DLS measurements show that nearly monodisperse particles were found for all TAR values higher than 0.6 mL PA/mL PC.h when P(AMPS52-co-MM48) was used as PA. III.2. AFM. AFM was used as a direct method to obtain information about the morphology of the PEC particles consisting of size, shape, and size polydispersity. The AFM amplitude images, obtained in the tapping mode, of some complex dispersions formed by mixing P(AMPS52-co-MM48) and P(AMPS37-co-TBA63) with PCA5, at different charge ratios and

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Figure 3. dh and PI of PA/PCA5 PEC particles as a function of titrant addition rate (TAR) at a constant molar ratio between charges, n-/n+, of 1.2: PA ) P(AMPS52-co-MM48) (tilted solid triangle), P(AMPS54co-TBA46) (4), and P(AMPS37-co-TBA63) (3).

at different TARs, adsorbed and dried on silicon wafers are presented in Figures 4 and 5, respectively. As Figure 4 shows, at a low ratio between charges, when PC is in excess, very small complex particles are evident (n-/n+ ) 0.5, Figure 4a). The PEC particles exhibit a condensed, compact core surrounded by a thick and fluffy coat, more clearly evidenced at low TAR, just at the complex stoichiometry (n-/ n+ ) 0.9, Figure 4b) and after the stoichiometry, at a ratio between charges, n-/n+, of 1.2 (Figure 1c,d, for TAR ) 0.6 and 0.4, respectively). The fluffy shell can be attributed to the small particles, which are not well protected against collision and therefore condensed around a scrambled-egg core, which appeared to be highly aggregated. The more uniform morphologies observed for a TAR of 2 (Figure 4e) show more homogeneous size dispersion, induced by the efficient protection with the PA in excess adsorbed on the particle surface, and confirm the DLS results presented in Figure 3. Increasing TAR (TAR ) 3.8, Figure 4f) strongly decreased the particle sizes and their polydispersity, in agreement with the PI values given by DLS for the PEC formed with P(AMPS52-co-MM48) as PA (Figure 3B). Unlike the morphologies previously observed for the complex particles formed between P(AMPS52-co-MM48) and PCA5 (Figure 4), at a ratio n-/n+ ) 1.2, only compact and highly aggregated particles were characteristic for the polyion pair P(AMPS37-co-TBA63)/PCA5, at low TAR (Figure 5a,b). Increasing the TAR, lower and hemispherical particles, and lower size dispersity, i.e., a lower aggregation level, were observed (Figure 5c,d). The highly aggregated structures found for this system are explained by the low charge density of the P(AMPS37-coTBA63), with the higher content of the hydrophobic comonomer being responsible for the lower colloidal stability of the PEC dispersion.

Mihai et al. The AFM images of the PEC particles formed between the NaPAMPS and PCA5, at two TAR (0.6 and 3.8) and at a constant ratio between charges, n-/n+ ) 1.2, were collected for comparison in Figure 6. A deep DLS study on the influence of TAR on the characteristics of the NaPAMPS/PC particles has already been published.40 The higher the TAR was around the IEP, the lower the particle sizes, and nearly monodisperse complex nanoparticles resulted when the PA addition rate was in the range 0.280.88 mL PA/mL PC.h. As the AFM images of Figure 6 show, at TAR of 0.6 mL PA/mL PC.h, spherical particles with a nearly uniform size distribution were obtained. By increasing the TAR up to 3.8 mL PA/mL PC.h, smaller particles with a higher size distribution were formed (Figure 6b). It is thus demonstrated that an optimum TAR is necessary to tailor spherical and uniform particles, with the TAR ) 3.8 mL PA/mL PC.h being adequate when the ionic/nonionic copolymers were used as PA, and out of the optimum when NaPAMPS has been added to PCs. The images from Figure 6 were compared with those corresponding to the complex dispersions formed with two ionic/ nonionic copolymers, at the same n-/n+ of 1.2, as follows: with P(AMPS52-co-MM48) as PA, with TAR ) 0.6, Figure 4c; and with TAR ) 3.8, Figure 4f; and with P(AMPS37-co-TBA63), at a TAR 3.8, Figure 5d. The particle analysis function of the device software is designed to detect and measure the lateral dimensions of isolated particles on the sample surfaces and to determine the minimum, maximum, and medium diameters for the analyzed particles. The values of the medium diameter obtained by particle analysis were compared with the values obtained by DLS for the same PEC dispersion, for the polyion pairs P(AMPS52-co-MM48)/ PCA5 (Table 1), P(AMPS37-co-TBA63)/PCA5 (Table 2), and NaPAMPS/ PCA5 (Table 3). As Tables 1-3 show, the average sizes determined from AFM images were always slightly smaller than those obtained by the DLS measurements. Such differences have been recently reported by Hartig et al.38 when the particle sizes measured by transmission electron microscopy were compared with those found by DLS, and also by Mu¨ller et al.,31,47 who measured the particle sizes by AFM and compared them with the results obtained by DLS. These differences are attributed to the specificity of the DLS measurements, where the hydrodynamic volumes of the nanoparticles, corresponding to a more expanded structure of the nanoparticles in a suspension, are determined, unlike the sizes measured by AFM on a dried sample. Furthermore, for DLS, each experiment measures the size of an ensemble of particles, and fitting the data provides the measured sample average and polydispersity,48 unlike the AFM which measures each particle individually. By comparison of the medium particle sizes of the PECs prepared either with P(AMPS52-co-MM48) (Table 1) or with NaPAMPS (Table 3), at a ratio between charges of 1.2, at a TAR of 0.6, determined by both methods, a small difference can be observed. On the other side, Figure 4c shows the presence of very big particles (dmax) 582 nm, Table 1) mixed with many small particles, while Figure 6a shows a narrow range of particle sizes (dmax) 406 nm, Table 3). Concerning the particle medium diameters of the complexes prepared at a TAR of 3.8 (n-/n+ ) 1.2) as a function of PA structure, the smallest sizes were found when P(AMPS52co-MM48) was used as PA (Table 1) and the biggest when P(AMPS37-co-TBA63) was the PA (Table 2). The particle sizes prepared with NaPAMPS were in between (Table 3). As for the TAR of 0.6, the maximum diameter and the micrographs must be examined. The highest values of the maximum diameter

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Figure 4. Tapping-mode AFM images of the P(AMPS52-co-MM48)/PCA5 complex particles at different molar ratio between charges and different TARs, adsorbed on the silicon wafers. Scan size was 10 µm in all images.

Figure 5. Tapping-mode AFM images of the P(AMPS37-co-TBA63)/PCA5 complex particles, with a molar ratio between charges of 1.2 and different TARs, adsorbed on the silicon wafers. Scan size was 10 µm in all images. (See Supporting Information for 3D image of part D.)

were found in the case of P(AMPS52-co-MM48) and the smallest in the case of NaPAMPS, as the Figures 4f, 5d, and 6b show. This is an argument for using at least two methods to

characterize the PEC dispersions to decide on conditions when complex particles with a narrow distribution of sizes can be prepared.

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Figure 6. Tapping-mode AFM images of the NaPAMPS/PCA5 complex particles, with a molar ratio between charges of 1.2 and two different TARs, adsorbed on the silicon wafers. Scan size was 10 µm in both images.

TABLE 1: Minimum (dmin), Medium (dmed), and Maximum (dmax) Diameter Determined by AFM and dh Measured by DLS for the P(AMPS52-co-MM48)/PCA5 Complex Particles, with Different Molar Ratio between Charges and Different TAR TAR mL PA/ mL PC.h 0.6 0.6 0.4 2.0 3.8

AFM

DLS

n-/n+

particle counted

dmin nm

dmed nm

dmax nm

dh nm

see figures

0.5 1.2 1.2 1.2 1.2

29 56 64 272 138

22 42 53 32 22

90 258 302 159 144

251 582 722 544 484

98 265 316 165 153

1A, 3a 1A, 3c 2A, 3d 2A, 3e 1A, 3f

TABLE 2: Minimum (dmin), Medium (dmed), and Maximum (dmax) Diameter Determined by AFM and dh Measured by DLS for the P(AMPS37-co-TBA63)/ PCA5 Complex Particles, with a Molar Ratio between Charges of 1.2 and Different TAR TAR

AFM

DLS

mL PA/ mL PC.h

n-/n+

particle counted

dmin nm

dmed nm

dmax nm

dh nm

0.4 1.1 2.0 3.8

29 34 79 204

52 78 22 58

355 232 204 168

842 748 539 451

368 240 211 174

2A, 4a 2A, 4b 2A, 4c 2A, 4d

TABLE 3: Minimum (dmin), Medium (dmed), and Maximum (dmax) Diameter Determined by AFM and dh Measured by DLS for the NaPAMPS/PCA5 Complex Particles, with a Molar Ratio between Charges of 1.2 and Two Different TAR TAR mL PA/ mL PC.h 0.6 3.8

AFM

TAR, the particle sizes increased and the colloidal stability decreased with the increase of the nonionic comonomer content, i.e., with the increase of the molar mass per charge (see Figure 1C,E), with a higher amount of PA being necessary to compensate the charges of PC, when P(AMPS37-co-TBA63) was added to PCA5 compared with P(AMPS54-co-TBA46) as PA. The average sizes determined from AFM images, in the dry state, were slightly smaller than those obtained by the DLS measurements, because the more expanded structures of the nanoparticles in suspension are determined by DLS. The novelty of the work consists in the evidence of the benefits to use DLS and AFM together to describe the morphology of the PEC dispersions. Advantages of AFM are the direct evidence of the shape, size, and polydispersity, which is not only an average value given by DLS. Normally, PEC particles mean a mixture of big and small aggregates controlled by the ratio between polyions, the polyion structure, and TAR. However, it was demonstrated that almost monodisperse PEC dispersion can be obtained by the adequate selection of polyion pair and at an optimum TAR. Acknowledgment. The authors are grateful for the financial support provided by the Saxon Ministry of Sciences and Fine Arts and the European Project “Romanian Action for Integrating, Networking and Strengthening the ERA (RAINS)”. Supporting Information Available: 3D version of Figure 5D. This material is available free of charge via the Internet at http://pubs.acs.org.

DLS

n-/n+

particle counted

dmin nm

dmed nm

dmax nm

dh nm

57 92

50 48

262 160

406 254

269 162

5a 5b

IV. Conclusion Three ionic/nonionic random copolymers of AMPS, differing by either the nonionic comonomer structure (TBA or MM) or the charge density, and the ionene-type polycation PCA5 were used to form PEC dispersions in saltfree aqueous solution, in a wide range of ratios between negative and positive charges. DLS and AFM have been used as complementary methods to describe the morphology and the colloidal stability of the PEC dispersions as a function of the polyanion structure and the TAR. Both DLS and AFM measurements, on the whole complex dispersion, showed that the complex particles were smaller at higher TAR, at the same ratio between charges (see Figures 1A and 4 for the polyion pair P(AMPS52-co-MM48)/PCA5 and Figures 1E and 5, for the polyion pair P(AMPS37-co-TBA63)/PCA5). At the same

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