Jingyan
Zhang
*ab,
Sangui
Chen
a,
Zhiyuan
Zhu
b and
Shiyong
Liu
*b
aSchool of Materials and Chemical Engineering, Anhui Key Laboratory of Advanced Building Materials, Anhui Jianzhu University, Hefei, Anhui 230601, China. E-mail: zhangjy8@mail.ustc.edu.cn; Fax: +86 551 63528960; Tel: +86 551 63528960
bCAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China. E-mail: sliu@ustc.edu.cn; Fax: +86 551 63607348; Tel: +86 551 63607348
First published on 15th October 2013
The formation of soluble polyion complexes (PICs) from anionic block copolymers, poly(ethylene oxide)-b-poly(sodium 4-styrene sulfonate) (PEO-b-PSSNa) and cationic block copolymers, poly(ethylene oxide)-b-poly(quaternized 2-(dimethyl amino)ethyl methacrylate) (PEO-b-PQDMA) was investigated by fluorescence spectroscopy, laser light scattering (LLS), and stopped-flow light scattering. Colloidally stabilized dispersions could be obtained upon direct mixing of the aqueous solutions of these two block copolymers, which indicated the formation of core–shell nanostructures with the core consisting of interpolymer electrostatic complexes between PSSNa and PQDMA blocks and the corona of PEO block. Both LLS and fluorescence results revealed that the most compact complex micelles formed at the equal molar ratio of oppositely charged SSNa and QDMA residues. The kinetics of the assembly process was studied via stopped-flow upon direct mixing of the two polymer solutions. The complexation process between PEO-b-PQDMA and PEO-b-PSSNa was fast and could finish within seconds. Moreover, the relaxation process can only be detected at near equal SSNa to QDMA molar ratios. The relaxation curves can be well fitted by a double-exponential function, leading to a fast relaxation process related to the initial quasi-equilibrium complex formation and a slow process related to the pre-complex structure rearrangements to the final equilibrium complexes. Both stages are determined as second-order reactions and processed through a micelle fusion–fission mechanism. Fluorescence kinetic studies revealed that the neutralization of an oppositely charged polyion was too fast to be detected and should be completed within the stopped-flow dead-time. Thermodynamic studies revealed that spontaneous complexation is entropy driven. Upon increasing the ionic strength of the solutions, the complexation processes became slower due to the decrease of entropy driving force. The PIC dissociation process was further studied and considered to consist of two competing processes: a second-order process depending on PIC concentration and a first-order process independent of the PIC concentration.
Although the formation and properties of PICs are extensively investigated, studies of the dynamic mechanism of intermolecular complexation are really scarce. To our knowledge, there are only a few literature reports related the complexation kinetics. Morawetz et al.31,32 studied the kinetics of complexation between dansyl-labeled poly(acrylic acid) (PAA) and poly-(ethylene oxide) (PEO) via hydrogen-bond interactions from the evolution of fluorescence intensity with time. The kinetic traces are fitted with double-exponential functions, leading to two rate constants. However the assignments of these two rate constants to specific physical processes have not been not stated. Morishima et al.33 reported the dynamic interactions between a pyrene-labeled polyelectrolyte, poly(sodium 2-(acrylamido)-2-methyl propanesulfonate) (Py-PAMPS), and an oppositely charged rodlike micelle, dimethyl-oleylamine oxide (DMOAO) mixed with hexadecyltrimethylammonium chloride (CTAC), by a fluorescence quenching technique with a hydrophobic quencher residing in the micelle core. The kinetic traces were fitted with double-exponential functions, but they did not ascribe any physical processes to the two resulted rate constants.
Stuart et al.34 reported the kinetics of micelle formation with polyelectrolyte complex cores from PAA and poly-((dimethylamino)ethyl methacrylate)-b-poly(glyceryl ethacrylate) (PDMA-b-PGMA) in aqueous solution at pH 7. Upon mixing, they unexpectedly observed the decrease of scattering intensities with time, while we know that the formation of micellar complexes will indeed increase the scattering intensities relative to those of initially molecularly dissolved solutions. They concluded that the formation of final stable micellar complexes is preceded by a macroscopic phase separation, followed by subsequent rearrangement. Lindhoud and Stuart et al.35 studied the relaxation times of the complex formation with light scattering titrations and size relaxation measurements. They found the difference in system relaxation time (τ) as compared to the experimental time (τexp) in terms of the structures and composition of polyelectrolyte complexes. For system A, it consisted of two weakly charged polyelectrolytes, such as PDMA and poly(acrylic acid)-b-poly(acryl amide) (PAA-b-PAAm), τ < τexp, while system B consisted of a strongly and a weakly charged polyelectrolyte, such as PAA and poly(2-methyl vinyl pyridinium iodide)-b-poly(ethylene oxide) (P2MVP-b-PEO), τ > τexp.
Eisenberg et al.36,37 reported the kinetics of the complexation between poly(methyl methacrylate-co-4-vinylpridine) (PMMA-co-P4VP) and partially sulfonated polystyrenes (PS-co-PSSA) in DMSO-d6 by 1H NMR. They concluded that the whole process was occurring through a complex mechanism. The fast stage of the process related to the formation of cationic–anionic pairs was a second-order reaction, while the slow stage at which a “ladder”-like complex was formed could be presented as containing two opposing first-order reactions. Bakeev and Kabanov et al.38 reported the complexation kinetic studies of mixing oppositely charged linear polyelectrolytes, pyrene-labeled poly(sodium methacrylate) (PMANaPy) and poly(N-ethyl-4-vinyl pyridinium bromide) (PEVPB), by luminescence quenching and laser light scattering techniques. They pointed out the complexation process consisted of a first rapid coupling of the oppositely charged polyions with the formation of a nonequilibrium interpolyelectrolyte network, followed by a slow relaxation process, finally leading to the formation of the individual PICs. On the basis of this, Kabanov et al.39 further studied the kinetics of the formation from anionic polyion shell micelles (PSM), PS-b-PEVPB in aqueous solution, and linear polycation PMANa. They found that the coupling reactions involving PSMs were much slower as compared to those of linear polyelectrolytes.
As stated above, the complexation process involves two stages. The first fast stage is the approaching of polyions which leads to nonequilibrium complex formation. This stage proceeds within milliseconds. The second slower stage is the formation of equilibrium complexes through polyion interchange reactions. But in the experiments described by Eisenberg and Kabanov et al.,36–39 they did not observe the first stage directly. Limited by the experimental apparatus, the fast process was completed before the first kinetic data were obtained. Second, the complexation for kinetic studies often occurred between a long polyelectrolyte and a short oppositely charged polymer. And the complexes always possessed an excess charge to ensure the assemblies were stable in solutions. As reported by Kataoka et al.40 at 1999, the core–shell type supramolecular assemblies can only be formed with the charged segments possessing matched chain lengths. The above reports may not reflect the actual kinetic process of the formation of water soluble core–shell complex micelles.
Stopped flow is a useful tool for kinetic studies of processes with relaxation times down to 1–2 ms. This technique has been widely used in biophysics, such as protein folding. Recently, we have successfully employed stopped-flow to study the kinetics of interpolymer complexes from poly(4-vinylphenol-g-styrene) (PVPh-g-PS) and poly(styrene-co-4-vinylpyridine) (STVPy) via hydrogen-bonding interactions in THF.41
Herein, we investigate the complexation formed by two double hydrophilic block copolymers, poly(ethylene oxide)-b-poly-(sodium 4-styrene sulfonate) (PEO-b-PSSNa) and poly(ethylene oxide)-b-poly(quaternized 2-(dimethylamino)ethyl methacrylate) (PEO-b-PQDMA), with comparable block length and charged repeats. With soluble PEO chain, polyanion and polycation can form soluble complex even at equal SSNa to QDMA molar ratio. The complexation kinetics is investigated by stopped-flow scattering intensity while varying the cationic to anionic molar ratio, polymer concentration, added salt concentration, and temperature. Stopped-flow fluorescence intensity has also been used employing pyrene as the fluorescence probe. The PIC dissociation has also been studied.
Laser light scattering was used to investigate the complexation between opposite polyelectrolytes. Fig. 1 shows the dynamic LLS results of PIC micelles prepared at varying [QDMA]/[SSNa] molar ratios. Apparently, aqueous solution of PIC micelles formed at [QDMA]/[SSNa] = 1.0 exhibits the strongest characteristic bluish scattering tinge. We can clearly see that in the [QDMA]/[SSNa] range of 0.5 to 1.6 the intensity-average hydrodynamic radii, 〈Rh〉, are ca. 15–44 nm, exhibiting a maximum value of 44 nm and a relatively narrow size polydispersity (μ2/Γ2) of 0.10 at [QDMA]/[SSNa] = 1:
1. Mean-while, scattered light intensity also exhibits a maximum at [QDMA]/[SSNa] = 1
:
1, indicating that polyelectrolyte complexation between oppositely charged PMAA and PQDMA backbones proceeds stoichiometrically. Stuart et al.46 had concluded that PIC micelles only exist in a small window with respect to the mixing ratios of the two components and the most compact PIC micelles with the highest aggregation number only form at the mixing ratio where the excess charge of the polyelectrolyte mixture is close to zero. This also proves to be true for the current case, in which PIC micellar solution exhibits the largest scattered light intensity and the biggest size at [QDMA]/[SSNa] = 1
:
1. Beyond the [QDMA]/[SSNa] range of 0.5 to 1.6, the mixed solutions are optically clear and exhibit very low scattered intensity, suggesting the formation of soluble complexes with quite loose structures. It may be due to the presence of electrostatic repulsion which prevents the formation of compact structures from excess charges. Thus, in subsequent studies, we mainly concentrate on PIC micelles prepared at a fixed stoichiometric mixing ratio of [QDMA]/[SSNa] = 1
:
1.
It should be noted that the PIC micelles for LLS studies were all collected from stopped-flow conditions. Fig. S1 (ESI†) shows a comparison of the hydrodynamic radius distributions, f(Rh), of the PIC micelles via two different mixing methods, stopped-flow mixing and dropwise addition under stirring, respectively. We can see clearly there are apparent differences between them. The obtained PIC micelles are smaller with a broader distribution by fast mixing with stopped-flow than those obtained by drop addition under stirring. Mezei et al.47 recently investigated the effects of different mixing protocols on the charged nature and size distribution of hyperbranched poly(ethylene imine) (PEI)–sodium dodecyl sulfate (SDS) complexes formed in aqueous solution. It was found that colloidal dispersion of isolated PEI–SDS nanoparticles formed on a stopped-flow apparatus, while large clusters formed typically if a less efficient mixing protocol was chosen. We also presented recently that there is difference between stopped-flow mixing and less efficient mixing on pH-induced micellization. The formed micelles of PEO-b-PDEA block copolymers by stopped-flow mixing were smaller than those via usual methods.48
The formation of PIC micelles was also studied by fluorescence spectroscopy using pyrene as the fluorescence probe. It is well known that the fine structure of the emission spectrum of pyrene, especially the ratio between the intensities of the first and third vibrational bands, I1/I3, is rather sensitive to the polarity of its surrounding medium: this ratio was demonstrated to decrease if the environment of the fluorescent probe becomes less polar.49,50 Fig. S2a (ESI†) shows the fluorescence spectrum of pyrene in solutions of pure PEO-b-PQDMA, PEO-b-PSSNa and PIC micelles. It is clear that the fluorescence intensities of pyrene in PIC micelles are weaker than those in pure polyions. The calculated values of I1/I3 of the component at different QDMA to SSNa molar ratio are shown in Fig. S2b (ESI†). The evaluated value of I1/I3 of pyrene in PIC is lower than the value in pure polyelectrolyte solutions (I1/I3 ≈ 1.88) and reaches a minimum value at [QDMA]/[SSNa] = 1:
1. This indicates that the formed PIC micelle core possesses less-polar microenvironment than aqueous solutions, which is similar to the literature reports.46 In addition, the I1/I3 value was relatively high beyond the equal QDMA to SSNa molar ratio, which most probably indicates that the complexes formed in this region possess a swollen micellar core with lower water insolubility.
Compared to the reports by Eisenberg and Kabanov et al.,36–39 the complexation process observed here is much faster. It most probably is because the complexes formed from block polyions possess core–shell micellar structures with a small size, while the assemblies from oppositely charged polyions with unmatched chain lengths possess complicated structures. Bakeev and Kabanov et al.38 have also studied the complexation kinetics of mixing oppositely PMANaPy and PEVPB at [cation]/[anion] = 0.2 using stopped-flow technique. They failed to observe the kinetics process of the fast stage even within 5 ms. This can point out to the relaxation process being quite fast when one polyion is in excess. In the present case, beyond the [QDMA]/[SSNa] range of 0.65 to 1.25, the complexation processes of the formation of loose structural complexes induced by coupling of oppositely charged polyions are completed within 2.6 ms. The complexation process can only be detected from the formation of compact PIC micelles.
The observed temporal variations in light-scattering intensity, Is(t), are analyzed assuming a series of N first-order decays between structures spanning the initial and final states. The use of this elementary model to describe the complex changes occurring during the formation of PIC micelles is clearly an oversimplification. However, in the absence of direct structural information on the nature of the intermediate states, it provides a self-consistent way of analyzing the scattering data. Thus, dynamics data from these experiments are fit to a multi-exponential function of the following form
![]() | (1) |
![]() | (2) |
![]() | (3) |
![]() | ||
Fig. 3 Overall kinetic coefficients (kf,obs) obtained during the formation of PIC micelles as a function of [QDMA]/[SSNa] mixing molar ratios. The experimental conditions were the same as those described in Fig. 2. |
Double-exponential function fittings lead to two characteristic kinetics coefficients, k1,obs and k2,obs, relating to two continuous processes. Compared the two pre-exponential constants A1 and A2, the first fast process related to k1,obs possesses scattered light intensity increasing with a large amplitude (ca. 0.7–0.8) reflecting most of the polyelectrolyte unimers coupling together accompanied by a great change in the aggregation size or the aggregation number densities, which mostly leads to quasi-equilibrium state formation. The second slower process related to k2,obs with a relatively small scattered light intensity increasing in amplitude is mostly related to the formation of final equilibrium complexes through arrangement of the aggregates in the first stage. The detailed investigation of these two processes will be discussed in upcoming studies. Since most of the relaxation process can be observed in the equal [QDMA]/[SSNa]molar ratio, thus, in subsequent studies, we fix the PIC micelles prepared at [QDMA]/[SSNa] = 1:
1 for further detailed investigation.
We then studied the copolymer concentration dependence of complexation kinetics. The dynamic curves of PIC formation at different total polymer concentrations are shown in Fig. 4, and the QDMA to PSSNa molar ratio is fixed at an equal ratio. Although the LLS and stopped-flow experiment detections have different time scales, the LLS experiments are carried out after the polyions have been mixed for hours while stopped-flow was recorded for the changes in the initial stage after the mixing, the final equilibrium scattering intensities obtained from the stopped-flow traces accord with the trend from LLS experiments, which increases almost linearly with concentration. It further indicates that the complexation process is fast, and the whole relaxation process reaches the final equilibrium state in 0.4 s. Thus, it is very suitable for investigating the complexation kinetics using a stopped-flow technique.
All the dynamic curves in Fig. 4 can be well fitted with a double-exponential function (N = 2). Fig. S5 (ESI†) shows the burst phase scattering intensities as a function of total polymer concentration, which is obtained by reading the value after extrapolating the double exponential fitting curve to −2.6 ms, the dead time of the stopped flow. The burst phase scattered intensities increase linearly with the total polymer concentration. This implies that the first fast process is only associated with a very simple process, i.e., the formation of quasi-equilibrium mixed aggregates. The increase of the total concentration only increases the number density of these initially formed aggregates.
Double exponential-function fitting results versus the polymer concentration are shown in Fig. 5. Both characteristic kinetics coefficients, k1,obs and k2,obs, increase with polymer concentration. Bakeev and Kabanov et al.38 reported complexation kinetics studies of mixing oppositely charged linear PMANaPy and PEVPB, by luminescence quenching and laser light scattering techniques. They pointed out the complexation process consisting first of the rapid coupling of the oppositely charged polyions with the formation of a nonequilibrium interpolyelectrolyte network followed by a slow relaxation process, leading finally to the formation of the individual PICs. But they did not ascribe any relaxation mechanism to the two processes. Dormidontova et al.52–54 have theoretically studied the kinetics of micelle evolution of block copolymers from unimers toward the final equilibrium state; they proposed a micelle fusion–fission-unimer expulsion/entry joint mechanism for block copolymer micelle evolution.
![]() | ||
Fig. 5 Double-exponential fitting results of kinetic traces obtained during the formation of PIC micelles as a function of PEO-b-QDMA concentrations. The experimental conditions were the same as those described in Fig. 4. |
The kinetics coefficient ki,obs can be translated to a time constant form via:
![]() | (4) |
![]() | (5) |
In the first process, polyion unimers quickly associate into large amounts of a small complex and then translate to quasi-equilibrium complexes. Unimer aggregating into small complexes is driven by electrostatic interactions and excess amounts of unimers; and the formation of small complexes with a low aggregation number practically does not involve any stretching of the soluble PEO blocks. Thus, association of unimers into small complexes is expected to be very quick, considering that the diffusion coefficient of unimer chains in aqueous solution is very high. The growth of small complexes into quasi-equilibrium complexes then proceeds via the micelle fusion–fission mechanism.54 The concentration dependence of k1,obs can then be easily understood as a second-order reaction.
After the formation of quasi-equilibrium complexes in the first fast process, unimer concentration is close to the cmc. The quasi-equilibrium complexes then translate to the final equilibrium complexes. Winnik and Tenhu et al.55 have reported the chain exchange kinetics between PIC micelles formed by mixing solutions of pyrene or naphthalene labeled PEO-b-PMANa block copolymers and PQDMA homo-polymer. The chain exchange was observed to occur via two mechanisms, unimer insertion–expulsion mechanism and PIC micelle fusion–fission mechanism. Based on the above analysis, the second slow process also proceed as a second-order reaction. We can conclude that rearrangement from quasi-equilibrium complexes to the final equilibrium complexes also proceeds through the micelle fusion–fission mechanism.
Winnik et al.56 reported the micellar exchange kinetics of aqueous micelles of Triton X-100 containing a pyrenebutyrate ester using stopped-flow fluorescence time-scan measurements. With a similar analysis approach, they found that kobs was linearly dependent on the concentration but did not pass through the origin, which led to a competitive first-order mechanism. In our cases, the nonzero intercept in the concentration dependent kobs cannot be found. Thus, the relaxation processes are purely controlled as second-order reactions and followed by a micelle fusion–fission mechanism.
The PIC micelles formation is explained as spontaneous, it is believed that the process is entropy drived. Schlenoff et al.59 have pointed out the idea that mixing polyelectrolyte complexes and multilayers is an entropy driven assembly. Before PIC micelles formation, polyelectrolyte unimer is solvated; and the formation of PIC micelles should destroy the ordered configuration of the solvent molecule. The reversion of a free water molecule causes entropy increase. On the other hand, unimer mobility is restricted by the water molecular network lattice in aqueous solution, and inside the PIC micelles core the polymer chain can move more freely, which should make a small contribution to the entropy change.
The effects of ionic strength on the complexation kinetics were also studied. Fig. 7a shows the time dependence of the scattered intensity for the formation of PIC micelles in the presence of different NaCl concentrations. It reveals that the final equilibrium scattered intensities obtained decrease along with the added salt concentration. From the dynamic laser light scattering result (Fig. S6, ESI†), the PIC micelle size remains almost unchanged when NaCl concentration is below 0.2 M; the observed equilibrium scattered intensity decrease here can be explained in the following way. Detailedly examining the dynamic curves in Fig. 7a, it can be found that salt adding causes the relaxation process to slow down. When no small molecular electrolytes exist in the polymer aqueous solutions, the relaxation process quickly reaches its equilibrium within 0.4 s, while when adding salt to the polyelectrolyte aqueous solution, we can see a slightly increase trend even the dynamic curves have recorded for a 0.8 s time range. Once the salt concentration is higher than 0.2 M, the increased scattering intensity amplitudes are relatively smaller, indicating a loose and less aggregates PIC micelles formation. Moreover, we can hardly observe the amplitude increasing when the salt concentration reaches 1.0 M.
The dynamic curves in Fig. 7a are fitted by double-exponential functions (N = 2). The resulted ki,obs are shown in Fig. 7b. The observed rate constants of the two processes both decrease in the presence of salt as compared to those without salt. This is probably due to the desolvation of the charged units. As mentioned above, the complexation is the entropy-driven process, thus, ΔS during the complexation would surely decrease if salination weakens. The driven force decreases, the relaxation processes should undoubtedly get slower. Interestingly, there is a transition of rate constants when the salt concentrations continue to increase. One can attribute the latter increasing trend mainly to the PIC micelles formed under high ionic strength possessing loose structures which would be formed much faster than the compact structures.
To further comprehend the PIC micelles dissociation process, we also studied the concentration dependence. Fig. 9a shows the dissociation dynamic traces at varying PIC micelle concentrations. The kinetics curves were well fitted by a single-exponential function (N = 1) and the fitting results are shown in Fig. 9b. The kinetics coefficient kobs were linearly dependent to the concentrations, but did not pass through the origin. Thus, it is clear from these plots that the dissociation mechanism described by kobs consists of two competing processes: a second-order process (k2,d) with linear dependence on PIC micelle concentrations, and a first-order process (k1,d) with a rate independent of the PIC micelle concentrations. Different from the formation process of the PIC micelles which is followed by a second-order reaction, the dissociation process is followed by a joint mechanism consisting of both second-order and first-order reactions. Besides the micelle fusion–fission mechanism, the unimer insertion–expulsion mechanism plays an important role in the dissociation process.
Footnote |
† Electronic supplementary information (ESI) available: Additional figures as described in the text. See DOI: 10.1039/c3cp53608d |
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