Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Evolution of Structure in a Comb Copolymer−Surfactant Coacervate Anastasiia Fanova,† Miroslav Janata,‡ Sergey K. Filippov,‡ Miroslav Š louf,‡ Miloš Netopilík,‡ Alessandro Mariani,§,∥ and Miroslav Š těpánek*,†
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†
Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 2030, 12840 Prague 2, Czech Republic ‡ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 16206 Prague 6, Czech Republic § Helmholtz Institute Ulm, Helmholtzstraße 11, 89081 Ulm, Germany ∥ European Synchrotron Radiation Facility, 71, avenue des Martyrs, 38043 Cedex 9 Grenoble, France S Supporting Information *
ABSTRACT: The interaction between a double-hydrophilic comb copolymer with the polyanionic backbone poly[methacrylic acid-statpoly(ethylene glycol) methyl ether methacrylate] (PMAA−PEGMA) and the cationic surfactant N-dodecylpyridinium chloride (DPCl) was studied in alkaline aqueous solutions by using a combination of light and X-ray scattering techniques, covering 5 orders of magnitude in space (the q vector range from 10−5 to 5 nm−1) and time (from milliseconds to several hours). The results showed that the polyelectrolyte−surfactant (PE−S) complex of PMAA−PEGMA and DPCl forms micrometer-sized coacervate particles containing collapsed PMAA−PEGMA chains with attached and densely packed DPCl micelles. Time-resolved SAXS measurements coupled with a stopped-flow apparatus revealed that the phase separation of the PE−S complex into a coacervate phase occurred in 0.7 markedly differed from each other: While the PMAA/DPCl system underwent macroscopic phase separation, leading to the instantaneous precipitation of the solid PE−S complex, the PMAA−PEGMA/DPCl system formed polymer-rich microscopic phase domains, as indicated by the increased turbidity. Dynamic Light Scattering (DLS). The apparent diffusion coefficients, Dapp = 1/τq2, where τ is the mean relaxation time of the given relaxation mode, observed by DLS were independent of scattering angle (Figure S12), with the exception of the low q region at Z > 0.7 where the steep increase in Dapp is caused by multiple scattering in turbid PMAA−PEGMA/DPCl solutions. (The increase was not observed in dilute dispersions with PMAA−PEGMA concentrations below 0.2 mg mL−1.) Hence, all relaxation modes corresponded to fluctuations caused by translational diffusion, and DLS results could be expressed as distributions of hydrodynamic radii. The DLS results are summarized in Figures 1 and 2. The distributions of apparent hydrodynamic radii for both PMAA and PMAA−PEGMA copolymers were bimodal (Figure 1), indicating that individual PMAA−PEGMA chains coexist with large loose aggregates. The condensation of DPCl cations on the polyelectrolyte chains supported aggregation; however, in the case of PMAA−PEGMA, the RH distributions remained bimodal up to Z = 0.7 as the PEG chains increased the solubility of the formed PE−S complex. In the Z range 0.8−2.0 of the PMAA−PEGMA/DPCl system, the narrow monomodal distributions showed a mean hydrodynamic radius of ∼300−2500 nm, confirming that
Figure 2. DLS CONTIN distributions (scattering angle, θ = 90°) of PMAA−PEGMA/DPCl aqueous solutions at (a) Z = 1 and (b) Z = 2 and at various times after mixing the polyelectrolyte and the surfactant, as shown in individual curves.
unlike the case of double-hydrophilic diblock copolymers with polyelectrolyte blocks, no core/shell particles are formed, and PMAA−PEGMA/DPCl particles are domains of the phaseseparated PE−S complex. At Z > 2, RH of the particles decreased to 15−30 nm, indicating that PE−S complex is dispersed as single polymer chains or small aggregates rather than phase-separated microscopic domains The size of the phase domains increased with time, reaching stable values within ca. 3 days after mixing (Figure 2). Figure 3 shows the ζ-potential and mean hydrodynamic radius of PMAA−PEGMA/DPCl particles as functions of Z. The ζ-potential of the phase domains at Z > 0.7 became more negative than that of individual PMAA−PEGMA chains, indicating that the domains were electrostatically stabilized by the excess negative charge of PMAA−PEGMA at the interface. The Z value at which the excess of DPCl compensated the negative surface coincided with the transition from a two-phase system to a single-phase system, so that the PE−S complex became soluble again instead of forming a macroscopic phase after losing the surface charge.26,33 Small-Angle X-ray Scattering (SAXS). SAXS measurements (Figure 4) were used to study structure of PMAA− C
DOI: 10.1021/acs.macromol.9b00332 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
The behavior in the q range 0.1−1 nm−1 depends on the PMAA−PEGMA chain conformation. The stretched conformation of a pure PMAA−PEGMA comb copolymer can be treated with the wormlike chain model,34 which yields the forward scattering intensity I(0) = 0.0112 cm−1, the Kuhn length 10.2 nm, the contour length 51.2 nm, and the crosssection radius 1.1 nm. Taking into account the excess scattering lengths per unit mass of the polymer, ΔbPMAA = 1.57 × 1010 cm g−1 and ΔbPEGMA = 1.14 × 1010 cm g−1 (calculated by using the densities ρPMAA= 1.29 g cm−3 and ρPEGMA= 1.10 g cm−3), the excess scattering length of PMAA− PEGMA is Δb = 1.40 × 1010 cm g−1 and I(0) yields the molar mass of the copolymer,35 Mw = I(0)NA/cΔb2 = 32.2 kg mol−1, which is in good accordance both with the MALLS and the contour length of the chain. At Z = 0.5, the extended conformation of the PMAA− PEGMA chain with condensed surfactant ions can be treated by using the form factor for a cylinder with the length 21.6 nm and the radius 3 nm. Further surfactant aggregation on the backbone is accompanied by the collapse of the PMAA− PEGMA chain, and at Z = 0.8 and 1.0, the scattering in this region can be treated by using the form factor for a sphere with the radius R = 5.1 nm. The scattering from the PMAA−PEGMA/DPCl complex was thus fitted by using the following models:
Figure 3. The ζ-potential (curve 1) and mean hydrodynamic radius (curve 2) of PMAA−PEGMA/DPCl particles as functions of Z.
I(q) = 16IPE + Imic
∫0
1i jj
2 yz jj J1(qR 1 − x ) sin(qLx /2) zzz dx jj zz j z q2R 1 − x 2 Lx k { 2
σ2 + I0 (q − q0)2 + σ 2
(1)
ÄÅ É ÅÅ sin(qR ) − qR cos(qR ) ÑÑÑ2 Å ÑÑ I(q) = Iaggq + 9IPEÅÅÅ ÑÑ 3 3 ÅÅÅ ÑÑÑ q R Ç Ö 2 σ + Imic + I0 (q − q0)2 + σ 2
at Z = 0.5 and
−α
Figure 4. SAXS curves of the PMAA−PEGMA/DPCl system at Z = 0, 0.5, 0.8, and 1.0. Intensities of the data are incrementally shifted by a factor 20 for better readability; data for Z = 0 are directly at scale.
(2)
at Z = 0.8 and 1.0. Here, I0 is the background scattering, J1(x) is the first-order Bessel function of the first kind, IPE is the forward scattering of the PMAA−PEGMA chain, Iagg is the prefactor for the power-law contribution, L is the length of the cylinder, R is the radius of either the cylinder (eq 1) or the sphere (eq 2), and Imic, qmax, and σ are the maximum, center, and half-width at the half-maximum of the Lorentzian function, respectively. The qmax and σ are related to the distance between the centers of the micelles, d, and to the correlation length of the periodic structure, ξ, as qmax = 2π/d and σ = 1/ξ. The parameters of the fits are outlined in Table 1. The qmax, corresponding to the intermicellar distance of 3.6 nm, is in accordance with the previously reported value for the PMAA/ DPCl complex.27,28 The σ value yields the correlation length of
PEGMA/DPCl particles on the length scale 0.025−3.9 nm−1. SAXS measurements showed several structural features of the PMAA−PEGMA/DPCl system: (i) At q < 0.1 nm−1, the power law regime, I(q) ∼ q−α, with α = ∼4.9 at Z = 0.8 and 1.0 indicates scattering at the interface of the phase-separated domains of the PMAA−PEGMA/DPCl complex. The value of the power law exponent exceeding the Porod limit is discussed in the next section. (ii) The q range 0.1−1 nm−1 is dominated by the scattering from individual PMAA−PEGMA chains. (iii) At Z > 0, a correlation peak centered at qmax = ∼1.7 nm−1 appears due to scattering from densely packed DPCl micelles aggregated on the PMAA−PEGMA backbone. Table 1. Parameters of Fits of SAXS Data by Eqs 1 and 2 Z
Iagg × 108, cm−1
IPE, cm−1
R, nm
l, nm
0.5 0.8 1.0
0.075 0.029 0.011
2.97 5.07 5.06
21.56
9.56 5.04
α
Imic × 103, cm−1
σ, nm−1
qmax, nm−1
χ2a
4.91 4.93
1.2 3.1 2.2
0.577 0.269 0.205
1.52 1.76 1.75
2.13 2.59 4.73
Reduced χ2 values of the fits.
a
D
DOI: 10.1021/acs.macromol.9b00332 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Light Microscopy. To support our observations from light and X-ray scattering measurements, we performed LM measurements. The micrometer size of the particles allows for using LM but precludes their reliable embedding into a thin layer of vitreous ice necessary for cryo-TEM measurements. Therefore, we used LM for observation of the particles in solution. The light micrograph (Figure 6) shows droplets of the coacervate phase with the average diameter 3.1 ± 0.8 μm, which is in good accordance with both static and dynamic LS results.
ca. 7 nm, which is similar to the size of the PE−S particle formed by the single PMAA−PEGMA macromolecule with attached DPCl micelles. Combined X-ray and Light Scattering. Static LS measurements showed that in the q range 0.006−0.025 nm−1 the scattering from PMAA−PEGMA/DPCl particles was in the power-law regime with the power law exponent α = ∼5. Therefore, small-angle-light scattering measurements are necessary to reach the Guinier regime of the PE−S particles and to determine their radius of gyration. Accordingly, Figure 5
Figure 5. Combined SAXS (green triangles), SLS (red circles), and SALS (blue squares) curve with the data fitted to eq 3 (solid red line). Black lines show the components of the model, the Fisher−Burford term (dash-and-dotted line), the sphere term (dotted line), and the Lorentzian term (dashed line).
Figure 6. LM micrograph of the PMAA−PEGMA/DPCl system at Z = 1.
(3)
Kinetics of the Coacervate Phase Formation. Light scattering measurements show that above the critical aggregation concentration of DPCl, corresponding to Z = 0.7, the coacervate phase formed instantaneously as indicated by the increase in light scattering intensity. Then the size of the scatterers increased with time while the scattering intensity measured at scattering angle θ = 90°, I90°, decreased. Figure 7 shows the hydrodynamic radius, RH (curve 2), as functions of time after mixing PMAA−PEGMA with DPCl at Z = 1. In ca. 1 min after mixing the components, RH increased proportionally to t1/3 for 4 h from 200 to 800 nm. The linear growth of RH with t1/3 suggests that the coacervate droplets undergo Ostwald
in which the power-law term is replaced with the Fisher− Burford term treating both the Guinier and the power-law regime of PE−S particles. The fit yielded the following values of the parameters: Rg = 1676 nm, α = 4.34, R = 4.72 nm, qmax = 1.73 nm−1, and σ = 0.143 nm−1. The gyration radius of the PE−S particles matches the apparent hydrodynamic radius of the particles, 1.3 μm. The average value of the power-law exponent α in the q range 2 × 10−3−7 × 10−2 nm−1 is above the Porod limit. Power law exponents between 4 and 5 were reported for scattering at porous surfaces.36 A more detailed analysis of the power law regime showed that the exponent decreased with decreasing q, being 3.98 in the q range 2× 10−3−2 × 10−2 nm−1 and 4.73 in the q range 2 × 10−2−7 × 10−2 nm−1. This result indicates that at the length scale the PE−S particles are homogeneous and that scattering occurs at their smooth surface, thus confirming that the PE−S particles consist of a liquid coacervate phase. At a shorter length scale, however, the structure of the interface may resemble that of porous solids which explains the high value of the exponent.
Figure 7. Hydrodynamic radius, RH, of PMAA−PEGMA/DPCl coacervate particles at Z = 1, RH, vs t1/3. Inset: light scattering intensity at the scattering angle θ = 90°, I90°(t)/I90°(0), vs t1/3, measured values (solid circles) and values calculated from RH via eq 4 (open circles).
shows the combined scattering curve from SAXS, SLS, and SALS measurements at Z = 1. The curve was fitted by using the following model: i 2 2 2yz I(q) = Iagg jjj1 + R g q zz 3 α { k ÑÉ2 ÅÄÅ Å sin(qR ) − qR cos(qR ) ÑÑÑ σ2 ÑÑ + Imic + 9IPEÅÅÅÅ Ñ 3 3 ÑÑ (q − q0)2 + σ 2 qR ÅÅÅÇ ÑÖ −α /2
E
DOI: 10.1021/acs.macromol.9b00332 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules ripening.37 At later stages of the aging of the dispersion, the growth rate of the droplets decreased, likely because there were no smaller particles feeding the growth of larger ones and coalescence was hindered by electrostatic repulsion. It is worth mentioning that the aging was accompanied by the decrease in scattering intensity, which can be in part ascribed to the decrease of the scattering function with increasing R. Assuming that the molar mass of coacervate droplets and thus the forward scattering intensity are proportional to RH3(t), Rg2(t) = 3RH2(t)/5 for spherical particles, and taking eq 2 into account, the time dependence of I90° can be approximated as
1 i y I90°(t ) ∼ RH 3(t )jjj1 + RH 2(t )q90°2zzz (4) 10 k { I90° values calculated from RH via eq 3 are plotted in the Figure 7 inset (open circles) showing a good agreement with the measured I90° values (solid circles). As light scattering measurements indicate that large particles of the PMAA−PEGMA/DPCl complexes are formed very fast, we used time-resolved SAXS measurements. TR SAXS allowed us to study the coacervate droplets formation and evolution of their internal structure in the range of seconds after mixing PMAA−PEGMA and DPCl solutions using a stopped-flow device. Figure 8 shows time-resolved SAXS curves fitted by using eq 2. The prefactor of the power law regime, Iagg, and the area of −2
Figure 9. Parameters I1 (curve 1) and s = πσI3/2 (curve 2) after fitting time-resolved SAXS curves (Figure 8) to eq 2 as a functions of time after mixing.
means that in early times after mixing the surfactant in the coacervate phase is disordered. The observed behavior of the PMAA−PEGMA/DPCl system strongly differs from that reported for PE−S complexes of a PMAA−PEO diblock copolymer and DPCl in which a highly ordered structure of the surfactant (Pm3n cubic packing) appeared immediately after mixing with the block copolymer.28 The main difference between the PMAA−PEGMA/DPCl and PMAA−PEO/DPCl systems stems in the fact in the case of PMAA−PEO, the formation of the PE−S complex is not affected by the PEO block which is segregated from the complex, while in the case of PMAA−PEGMA, the short PEO grafts along the polyelectrolyte backbone interact with the aggregated surfactant and slow down the self-assembly process.
■
CONCLUSIONS
We have shown that PMAA−PEGMA, a comb copolymer with a polyelectrolyte backbone of PMAA and with neutral hydrophilic grafts of PEO, markedly differs from both homopolymer polyelectrolytes and double-hydrophilic diblock polyelectrolytes in the way it forms a complex with an oppositely charged surfactant: (i) Hydrated PEO combs decrease the charge density of the PMAA/DPCl complex, preventing its phase separation as a solid phase, while sterical hindrances prevent the formation of core/shell particles, which occurs in the case of diblock copolymers. Therefore, the interaction between PMAA−PEGMA and DPCl results in the formation of a coacervate phase. (ii) Time-resolved SAXS measurements showed that although the liquid−liquid interface appears immediately after mixing PMAA−PEGMA with DPCl, the formation of densely packed DPCl micelles in the coacervate phase is a much slower process, in contrast to the very fast self-assembly of DPCl in PE−S core/shell nanoparticles.28 This delayed self-assembly process likely results from the PMAA−PEGMA morphology because PEGMA chains do not allow aggregated surfactant ions to form micelles immediately after binding and micellization of DPCl micelles requires additional slow rearrangement of the polyelectrolyte backbone in the dense coacervate phase.
Figure 8. Time-resolved SAXS curves of the PMAA−PEGMA/DPCl system at Z = 1 and at various times after mixing the polyelectrolyte with the surfactant: (1) 25, (2) 165, (3) 305, (4) 655, (5) 1355, and (6) 2825 ms. Intensities of the data are incrementally shifted by a factor 10 for better readability.
the Lorentzian peak, s = πσI3/2, are plotted in Figure 9 as a function of the time after mixing the polyelectrolyte with the surfactant. Although the power law regime appears in
