J. Phys. Chem. B 2007, 111, 8351-8359
8351
Complexes of Polyelectrolyte-Neutral Double Hydrophilic Block Copolymers with Oppositely Charged Surfactant and Polyelectrolyte† Stergios Pispas* Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou AVe., 11635 Athens, Greece ReceiVed: NoVember 10, 2006; In Final Form: February 6, 2007
Complexes between sodium (sulfamate-carboxylate)isoprene/ethylene oxide double hydrophilic diblock copolymers (SCIEO) and dodecyltrimethylammonium bromide (DTMAB), as well as quaternized poly(2vinylpyridine) (QP2VP), were studied in aqueous solutions, at pH 7. The complexes are formed due to electrostatic interactions between the anionic groups of the polyelectrolyte block of the copolymers and the cationic groups of the surfactant or the homopolyelectrolyte. The structure of the complexes was investigated as a function of the mixing ratio of the two components in solution and ionic strength by static, dynamic, and electrophoretic light scattering, atomic force microscopy, and fluorescence spectroscopy. The mass and size of the complexes depend on the mixing ratio between the components. A transition from intrachain to an interchain association was observed for block copolymer/ surfactant complexes. SCIEO/QP2VP complexes were found to respond to increasing concentrations of added salt. Spherical and ellipsoid shaped complexes with a core-shell micellar like structure were formed in the systems studied.
Introduction Complexes involving oppositely charged polyelectrolytes or polyelectrolyte and low-molecular-weight surfactants constitute a very active field of research in recent years.1-5 These systems attract scientific attention due to a number of interesting characteristics. From the academic point of view, such systems present the possibility of the formation of various self-assembled nanostructures, whose characteristics can be tuned via a large number of parameters, including total concentration and molecular structure of each component, charge ratio, ionic strength, and pH. These systems also show great similarities in structure and behavior with more complex biological macromolecular self-assembled systems, such as lipoproteins and protein/DNA complexes.1-6 From the practical point of view various applications, such as microencapsulation and drug delivery, separation of proteins, stabilization and flocculation of colloidal dispersions, separation membranes, coatings and responsive surfaces, and others can be based on polyelectrolyte complexes.1-3 More recently, complexes comprised of block polyelectrolytes, i.e., block copolymers with at least one polyelectrolyte block, and oppositely charged polyelectrolytes6-17 or surfactants18-28 have started to be investigated. Such systems include also complexes of amphiphilic block copolymer micelles, having a charged corona, with oppositively charged surfactants and polyelectrolytes.29-31 Double hydrophilic block copolymers with one ionic block and a nonionic hydrophilic block present larger opportunities for influencing formation, structure, and properties of nanoscale complexes and particles, through a judicious choice of the chemical structure of the block copolymer, as well as of the physicochemical parameters of the system, with a definite direction toward nanotechnological † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * Corresponding author. Tel: +30210-7273824. Fax: +30210-7273794. E-mail address:
[email protected].
applications. Synthetic polymer chemistry can provide a large number of alternative block copolymers that can be used for the creation of nanosized self-assembled supramolecular structures, able to respond and restructure to different environments. Therefore, further investigations on this direction are definitely needed and will produce valuable new knowledge. In this paper results on the structure and solution behavior of complexes formed between novel sodium (sulfamate-carboxylate)isoprene/ethylene oxide (SCIEO) anionic-neutral block polyelectrolytes and dodecyltrimethylammonium bromide (DTMAB), a cationic surfactant, as well as quaternized poly(2vinylpyridine) (QP2VP), a cationic linear polyelectrolyte, in aqueous solutions at pH 7, are reported. The effects of relative concentrations of the two components and ionic strength on the structure and colloidal properties of the complexes are studied by static, dynamic, and electrophoretic light scattering, atomic force microscopy, and fluorescence spectroscopy. The effect of copolymer composition is also addressed. The copolymers employed possess (a) a high-charge-density polyelectrolyte sodium poly[(sulfamate-carboxylate)isoprene] block, (SCI), carrying two negatively charged groups, with different pH sensitivity (pH sensitive carboxylic and pH insensitive sulfonic groups) at the same monomeric unit. Due to the presence of anionic charges, this block is expected to interact electrostatically with the positively charged DTMAB and QP2VP in aqueous solutions. The particular anionic polyelectrolyte shows some hydrophobic character due to the presence of unfunctionalized isoprene segments,32 a characteristic that is expected to influence the interactions with the hydrophobic tail of DTMAB. Furthermore, SCI has structural similarities, in terms of functional groups, with the natural polysaccharide heparin and shows biocompatibility,32-34 (b) a non-ionic, water soluble poly(ethylene oxide) (PEO) block that it is not expected to interact with the positively charged (macro)molecules and ensures solubility of the complexes formed. The multifunctional character of these novel block copolymers introduces a number of
10.1021/jp067437z CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007
8352 J. Phys. Chem. B, Vol. 111, No. 29, 2007 SCHEME 1: Molecular Structures of the Double Hydrophilic Diblock Copolymers and the Quaternized Poly(2-vinylpyridine) Utilized in This Study
interesting properties to the micellar like complexes under investigation. Experimental Polymer Synthesis. The double hydrophilic block copolymers were prepared by a post polymerization functionalization reaction on the polyisoprene block of poly(isoprene-b-ethylene oxide) diblock (PI-PEO) precursors, obtained by anionic polymerization high vacuum techniques.35 The functionalization scheme involves reaction of the double bonds in the PI block with chlorosulfonyl isocyanate and subsequent basic hydrolysis of the lactam intermediate with NaOH. In this way sulfamate and carboxylate groups are introduced in the same isoprene monomeric unit, resulting in an anionic polyelectrolyte with a high charge density. Molecular characterization results show that the yield of the functionalization reaction ranges from 70% to 78%.32 This implies that, due to the presence of unreacted PI double bonds, the SCI blocks of the copolymers retain some hydrophobic character. Therefore, the SCI blocks can be regarded as random copolymers of the sodium sulfamate/ carboxylate isoprene hydrophilic units and isoprene hydrophobic units. The samples used were dialyzed against water in order to remove residual salts. More details regarding the synthesis of the copolymers are given elsewhere.32 The quaternized poly(2-vinylpyridine) sample was synthesized by reacting a poly(2-vinylpyridine) homopolymer, obtained by anionic polymerization, with a 10-fold excess of methyliodide in THF, at reflux temperature, for 48 h. The polymer was characterized by FTIR spectroscopy and found to be nearly 100% quaternized. The chemical structures of the polymers used in this study are shown in Scheme 1, while their molecular characteristics are given in Table 1. Preparation of Solutions. Stock solutions of the block copolymers were prepared by dissolving a weighed amount of the dialyzed samples in the appropriate volume of a filtered phosphate buffer solution (pH 7.0, ionic strength 0.01 M). The copolymers, DTMAB and QP2VP, were readily soluble in the buffer, but solutions where left to stand overnight for better equilibration. Solutions of the complexes were prepared by adding different amounts of the DTMAB or QP2VP solutions to copolymer solutions of the same total volume and concentration, under stirring. Where needed, additional volumes of buffer solutions were added in order to achieve a constant final volume for all solutions prepared. In this way the concentration of the copolymers was kept constant through the series of solutions while that of the surfactant and QP2VP was varied along with the mixing ratio. Mixing resulted in the development of a bluish tint indicating the formation of supramolecular complexes. The
Pispas TABLE 1: Molecular Characteristics of the Polymers Used in This Study sample
Mw
Mw,PEO
Mw/Mn
wt % SCPI
SCIEO-1 SCIEO-2 QP2VP
54 200 33 700 103 000
8 400 14 800 -
1.05 1.03 1.02
79 52 -
mixed solutions were left for equilibration overnight and they were filtered through 0.45 µm hydrophilic Teflon Millipore filters before measurements. For the investigation of the effect of added salt concentration on the structure of the complexes, different amounts of analytical grade NaCl solutions were added to the solutions of the complexes. Colloidal stability of the complex solutions was found to depend on the composition of the block copolymers ranging from a few days to several months. Concentrations in the range 1 × 10-4 to 5 × 10-4 g/mL were utilized for the copolymers. At copolymer concentrations higher than 5 × 10-4 g/mL, usually turbid solutions were obtained, depending also on the concentration of the second component, and light-scattering measurements were not possible. Additionally, light-scattering measurements on dilutions of the initial mixed solutions did not show any disruption of the complexes in the concentration range where the method was applicable. Surfactant concentrations were kept lower than the nominal cmc of the surfactant (5.4 × 10-3 g/mL) in its aqueous solutions, in order to avoid direct surfactant micelle/block copolymer interactions in the system. The presence of surfactant micelles may introduce complications in the interpretation of the results and the state of aggregation in the system, since not only free surfactant molecules but also surfactant micelles would have been able to complex with the SCI block of the copolymers. Furthermore, neutralization of the negative charges on the copolymers by surfactant was carried out up to 85% neutralization of the charged groups of the copolymer, in order to avoid accumulation of uncomplexed surfactant on the polymeric chains (especially in the form of micelles). In the case of QP2VP, micelle formation cannot take place, so neutralization degrees higher than 100% were utilized. Methods. Molecular weights and molecular weight distributions of the precursor block copolymers were determined by size exclusion chromatography (SEC) using a Waters system, with THF as the eluent, and a PS calibration curve. A small amount (3%v/v) of triethylamine was added to the eluent for the characterization of the P2VP precursor. Composition of the precursor block copolymers and microstructure of the PI block were determined by 1H NMR spectroscopy using a Bruker AC 300 instrument in CDCl3 at 30 °C. Infrared spectra of the precursors and the final double hydrophilic block copolymers and 2-vinylpyridine-based homopolymers were taken, using a Bruker Equinox 55 Fourier transform instrument, equipped with an attenuated total reflectance (ATR) diamond accessory. Dialyzed double hydrophilic block copolymers were investigated. More details about the molecular characterization of the diblock copolymers are given elsewhere.32 Static and dynamic light-scattering measurements on aqueous solutions of the complexes were performed with an AXIOS150/EX (Triton Hellas) light-scattering photometer, equipped with a 30 mW laser source, operating at 658 nm, and an Avalanche photodiode detector. Measurements were made at a scattering angle of 90° at 25 °C, on solutions of the complexes at different mixing ratios and salt concentrations. Due to the high turbidity of the solutions of the complexes at the concentrations needed for differential refractive index (dn/dc) determination, dn/dc values at different mixing ratios could not be determined experimentally. Alternatively a dn/dc value could
Double Hydrophilic Block Copolymers
J. Phys. Chem. B, Vol. 111, No. 29, 2007 8353
be calculated by using the additivity rule for mixed solutions, utilizing the dn/dc values for the pure components.36 However, this would result in errors in the combined dn/dc values for the mixed solutions and consequently to the final calculated mass of the aggregates, since during the complexation process exchange of counterions takes place and the constituting species alter their chemical composition in the complexed state, compared to the uncomplexed one. Additionally, the exact stoichiometry of the complexes cannot be estimated with reasonable accuracy. Furthermore, the dependence of the dn/ dc values for the systems under investigation on salt concentration and after complexation cannot be estimated or calculated a priori. Therefore, the static light scattering results were evaluated in a semiquantitive manner, taking into account the proportional relation between light scattering intensity and mass of the species in solution.36,37 Attention has been paid to the relative changes in the values of excess scattering intensity as a function of the mixing ratio of the components. A rough estimation of dn/dc variation with the amount of the added component, on the basis of refractive index increments of the individual components, shows that dn/dc varies within 2% for the case of block copolymer/surfactant mixtures and less than 7% for the case of block copolymer/homopolyelectrolyte mixtures in the concentration ranges investigated. Autocorrelation functions, from DLS measurements, were analyzed by the cumulants method and the CONTIN routine. One population of diffusing species was resolved in all cases. Polydispersities were evaluated from cumulants analysis, through the second cumulant and are given as values of the ratio µ2/Γ2, where µ2 is the second cumulant and Γ the decay rate of the correlation function. Apparent hydrodynamic radii, Rh,app, at different polymer concentrations were calculated by aid of the Stokes-Einstein equation:
Rh,app ) kT/6πηoDapp
(1)
where k is the Boltzmann constant, T the absolute temperature, ηo the solvent viscosity, and Dapp the diffusion coefficient calculated from the analysis of the correlation function at the particular polymer concentration. ζ potential measurements were performed at 25 °C with a ZetaPlus Analyzer (Brookhaven Instruments Corporation) equipped with a 35 mW solid-state laser, operating at λ ) 660 nm. ζ-potential values determined, using the Smolukowski equation relating the ionic mobilities with surface charge, are the average of ten repeated measurements. AFM measurements were performed on a Quesant Q-Scope 250 instrument. Samples for imaging were prepared by dipping fresh, dried silicon wafers, pre-cleaned with isopropanol, in aqueous solutions of the complexes, for typically 5 to 10 min. After withdrawing the wafer from the solution, excess water was blotted carefully by filter paper and samples were left to dry in air. In this way supramolecular structures were absorbed on the wafer surface from the same solutions investigated by light scattering for direct comparison. Steady-state fluorescence spectra of pyrene probe in the aqueous solutions were recorded with a double-grating excitation and a single-grating emission spectrofluorometer (Fluorolog-3, model FL3-21, Jobin Yvon-Spex) at room temperature (ca 25 °C) using air-equilibrated solutions. Excitation wavelength was λ)335 nm and emission spectra were recorded in the region 350-500 nm, with an increment of 1 nm, using an integration time of 0.5 s. Slit openings of 1 nm were used for both the excitation and the emitted beam. The I1/I3 were determined as the average of three measurements (where I1, I3 are the
intensities of the first and the third peaks of the pyrene fluorescence spectra at 372 and 383 nm, respectively). Measurements were performed in the same solutions prepared and investigated with other techniques. Results and Discussion Complexes with DTMAB. The conditions employed in solution preparation facilitate the formation of block copolymer/ surfactant complexes, with surfactant molecules bound to the copolymer chains. Static and dynamic light scattering were employed for characterizing the changes in mass and size of the complexes formed by the SCIEO block copolymers, which carry a negatively charged block, with the cationic DTMAB, as a function of the concentration of added surfactant (i.e., at different mixing ratios). Figure 1 shows results from the system employing SCIEO-2 and DTMAB. Excess light-scattering intensity, I90, from the solutions changes little compared to the intensity measured from the pure diblock copolymer solution, at low surfactant concentrations (low neutralization levels, i.e., at an excess of negative charges in solution). However, at neutralization degrees higher than 30%, intensity increases rapidly, reaching a plateau at ∼60% neutralization. On the other hand hydrodynamic radius of the species initially shows a gradual decrease reaching a well-defined minimum at close to 30% neutralization. Above that point an increase in Rh is observed toward a constant value at ∼60% neutralization. In parallel, polydispersity values are large at low concentrations of surfactant, while they fall below 0.3 at larger DTMAB concentrations. It seems that the point of 30% neutralization of the anionic charges in the copolymer signifies a transition point. The combined static and dynamic light-scattering results suggest that at the low surfactant concentration region an intramolecular process, in respect to the copolymer chains, takes place since the mass of the species in solution remains almost constant (based on the small change in I90) and the Rh decreases reaching a minimum. Presumably, incorporation of surfactant on the SCI blocks and partial neutralization of their negative charges results in a collapse of this block. This transition is reflected on the overall size of the species in solution (most probably unimolecularly dissolved diblock chains with a small content of bound surfactant exist in solution). The presence of a minimum in the hydrodynamic size of block copolymer/surfactant complexes, at a certain copolymer/surfactant mixing ratio, has been observed before,18,21 but it was not attributed to an intramolecular collapse, since interchain association was observed at all mixing ratios in those cases. Furthermore, the minimum was observed at higher neutralization degrees (from 65 to 100%) although more hydrophobic surfactants (C16PyBr18 and CTAB,21 with a chain of sixteen carbon atoms) were utilized. The intramolecular block copolymer/surfactant association in the present case may be enhanced by the presence of hydrophobic isoprene units in the SCI block. Apparently, intramolecular hydrophobic domains, containing the isoprene units and the hydrocarbon chains of the bound surfactant, can be formed at a lower neutralization degree in comparison to refs 18 and 21, despite the fact that in the present case a surfactant with a shorter hydrophobic chain is utilized. Due to the rapid increase of I90 at higher neutralization levels, an intermolecular association process must be taking place in this region, resulting in the formation of aggregates (complexes) incorporating many diblock chains with complexed surfactant. However, due to the collapse of the SCI block, as more surfactant gets complexed with it, and the fact that the unimolecularly dissolved SCIEO chains in the absence of surfactant show a large size (because of the polyelectrolyte
8354 J. Phys. Chem. B, Vol. 111, No. 29, 2007
Pispas
Figure 2. ζ potential vs surfactant concentration for SCIEO-2/DTMAB complexes at pH 7 and 0.01 M salt (cSCIEO-2 ) 4.95 × 10-4 g/mL).
Figure 1. Light-scattering intensity at 90°, I90 (upper plot), hydrodynamic radius, Rh (middle plot), and polydispersity (lower plot) vs concentration of the surfactant, for SCIEO-2/DTMAB complexes at pH 7 and 0.01 M ionic strength (concentration of copolymer is constant at 4.95 × 10-4 g/mL). The vertical dashed line in all three graphs corresponds to the DTMAB concentration at 35% neutralization of the negative charges of the SCIEO-2 copolymer.
character of the SCI block, Figure 1) the observed increase in Rh is not so large. Taking into account the block copolymer structure the resulting complexes at high neutralization degrees should have a core-shell micellar structure, with a hydrophobic core consisting of SCI chains partially neutralized by bound surfactant and a corona comprised of the hydrophilic PEO chains.18,21 Of course, the detailed structure and the water swelling of the core should depend on the neutralization degree. This is a key parameter for controlling the properties of the core in these systems and should be taken into consideration for practical applications of double hydrophilic block copolymer/ surfactant complexes. At low neutralization degrees a structure
resembling that of a unimolecular micelle of an amphiphilic block copolymer should be formed. ζ potential measurements on the same solutions show that the net charge of the species in solution, at both regimes discussed above, is changing from a highly negative value of the SCIEO-2 chains in the absence of surfactant to progressively lower negative values (Figure 2). This is a result of neutralization of the negative charges on the SCI chains and follows the calculated stoichiometry in the mixed solutions. A zero value for the ζ potential is not attained since 100% neutralization is not expected by the stoichiometry used. Furthermore, it seems that all negatively charged groups on the copolymer are accessible for surfactant complexation, despite the fact that they are closely placed in the chain. Of course this may be also attributed to the fact that the unreacted isoprene units may provide some extra spacing between the interacting anionic groups along the chain. In any case, ζ potential values imply that surfactant binding to diblock chains takes place even at very low concentrations of the surfactant in solution. This conclusion is corroborated by the results from fluorescence spectroscopy (Figure 3a). The I1/I3 values for pyrene change abruptly from a high value (I1/I3 ) 1.66) for the solution of the diblock copolymer to values in the range 1.3 to 1.35 for the solutions of the complexes independent of the mixing ratio. The high I1/I3 value for the diblock copolymer is expected from the highly hydrophilic nature of the two blocks, however it is lower than the 1.7-1.8 values usually obtained for aqueous environments18,21 and can be correlated to the presence of unreacted hydrophobic isoprene units in the SCI block. The situation changes remarkably when DTMAB is added to the solution. Binding of the surfactant introduces the creation of highly hydrophobic domains in the complexes. The fluorescence measurements additionally suggest that the critical aggregation concentration (cac) for this system lies below a DTMAB concentration of 1 × 10-5 g/mL. Dilution of the complexes at a constant mixing ratio always results in an increase of the I1/I3 ratio at a certain concentration as shown in Figure 3b. The data imply the existence of a critical concentration, resembling that of a critical micellar concentration, where presumably dissociation of the complexes takes place, since at lower concentrations there are no hydrophobic domains able to accommodate the pyrene probe. This observation gives additional substantiation to the core-shell micellar structure of the complexes as discussed previously. Visualization of the complexes by atomic force microscopy in the region
