1712
Langmuir 2005, 21, 1712-1718
Articles Chemically Tuned Amphiphilic Diblock Copolymers Dispersed in Water: From Colloids to Soluble Macromolecules Denis D. Bendejacq,*,† Virginie Ponsinet,‡ and Mathieu Joanicot† Complex Fluids Laboratory, UMR 166 CNRS/Rhodia, Cranbury, New Jersey 08512 Received April 22, 2004. In Final Form: December 17, 2004 We investigate by small-angle scattering the structural behavior in water of a family of asymmetric poly(styrene-stat-(acrylic acid))-block-poly(acrylic acid), i.e., P(S-stat-AA)-b-PAA, diblock copolymers. These diblocks are of constant block ratio and increasing molar fraction, φAA, ranging from 0 to 1, of acrylic acid in the first P(S-stat-AA) statistical block. We identify three types of structural behavior in water: (i) for φAA e 0.25, the structures found in water are out-of-equilibrium micelle-like objects, reminiscent of the macrophase separation in the solid state, with no reorganization upon dispersion; (ii) for φAA g 0.50, the diblocks dispersions in water are at equilibrium. For high φAA, the diblocks are soluble in water, demonstrating that a transition from colloid-like objects to soluble macromolecules is achieved. Close to the transition, (φAA ≈ 0.50), the diblocks form objects interpreted as comprising a water-swollen core formed by the P(S-stat-AA) block, surrounded by a swollen brush composed of the majority PAA block, above a apparent critical micelle concentration. However, these diblocks do not behave as macrosurfactants, and their self-association behavior is rather interpreted as a microphase separation which can arise from the incompatibility between two polymer blocks P(S-stat-AA) and PAA placed in a common solvent water.
Introduction The structural behavior of diblock copolymers dispersed in a solvent selective for one of the two blocks has been extensively investigated, both experimentally and theoretically. On the theoretical side, such systems are generally considered as analogous to small-surfactant molecules in water, with which they bear many resemblances.1 As surfactant molecules do, diblock copolymers placed in a selective solvent are usually found to present a critical micellar concentration (cmc) below which the diblock chains are soluble as unimers. Beyond the cmc, the diblock chains form micellar aggregates that comprise a core formed by the less soluble block, while the more soluble block forms a surrounding shell swollen with solvent. By analogy with surfactant molecules, the free energy of diblock chains AB placed in a solvent selective for block B, is described as the sum of two contributions:2 (i) an enthalpic contribution that arises from the unfavorable contacts between the solvent molecules and block A, which favors the segregation of blocks A into solvent-poor microdomains; (ii) entropic contributions, namely, entropy of mixing of the diblock chains in the solvent and conformational entropy of the chains themselves, which favor a soluble, homogeneous state where entropy is maximum. The micellization is then classically described as resulting from a minimization of the contacts between the solvent molecules and the less soluble A blocks, despite the net entropic loss that results from such association. † Present address: Centre de Recherche d’Aubervilliers, 52 Rue de la Haie Coq, 93308 Aubervilliers Ce´dex, France. ‡ Present address: Centre de Recherche Paul Pascal, Avenue Schweitzer, 33600 Pessac, France.
(1) Tuzar, Z. J. Macromol. Sci.. Pure Appl. Chem. 1992, 29, 173. (2) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998.
The molecular weights of blocks A and B, the composition of the diblock A-B,3-5 the extent of solvent selectivity, and, finally, the architecture of the copolymer6,7 are all expected to somewhat control both the occurrence and the value of the cmc. The enthalpic cost of contact between a polymer block and a solvent being proportional to the degree of polymerization of the block, the solvent selectivity is usually strong unless the degree of polymerization of the less soluble block is kept very low. When selectivity is high, micellization leads to the formation of micelles with collapsed, solvent-free cores unable to reorganize.8,9 For instance, asymmetric PS-b-PAA diblocks with a PS block containing more than ca. 10 segments form spherical water-free PS cores10,11 in water, a solvent highly selective for the polyelectrolyte PAA block, with low values of cmc.12 In highly selective conditions where solvent molecules cannot enter the core of the micelle formed, diblock aggregates with glassy cores are usually said to be “frozenin” as they are out-of-equilibrium from a thermodynamic (3) Hodrokoukes, P.; Pispas, S.; Hadjichristidis, N. Macromolecules 2002, 35, 834. (4) Pepin, M. P.; Whitmore, M. D. Macromolecules 2000, 33, 8644. (5) Izzo, D.; Marques, C. M. Macromolecules 1997, 30, 6544. (6) (a) Zukang Z. B.; Chu, B.; Peiffer, D. G. Langmuir 1995, 11, 1956. (b) Sotiriou, K. Nannou, A.; Velis, G.; Pispas, S. Macromolecules 2002, 35, 4106. (c) Kwang, H. K.; Huh, J.; Won, H. J. J. Chem. Phys. 2003, 118, 8468. (7) Chu, B.; Zhou, Z.; Wu, G. J. Noncryst. Solids 1994, 172-79, 1084. (8) Khalatur, P. G.; Kholkhlov, A. R.; Nyrkova, I. A.; Semenov, A. N. Macromol. Theory Simul. 1996, 5, 713. (9) Hurtrez G., Dumas P., Riess G. Polym. Bull. 1998, 40, 203-210. (10) Burguie`re, C.; Chassenieux, C.; Charleux, B. Polymer (Guilford) 2003, 44, 509. (11) Groenewegen, W.; Lapp, A.; Egelhaaf, S. U.; van der Maarel, J. R. C. Macromolecules 2000, 33, 3283 and 4080. (12) Khougaz, K.; Zhang, L.; Moffitt, M.; Eisenberg, A. Polym. Sci., Ser. A 1996, 38, 331.
10.1021/la048983r CCC: $30.25 © 2005 American Chemical Society Published on Web 02/04/2005
Diblock Copolymers of Tunable Amphiphilicity
Langmuir, Vol. 21, No. 5, 2005 1713
Table 1. Chemical and Structural Data of the Diblock Copolymers NB1d sample
fa
φAAb
c
Mn (g/mol)
NS
NAA
NB2e
df (nm)
RX,Cg (nm)
A1 A2 A3 A4 A5 A6
0.164 0.161 0.161 0.150 0.144 N/A
0.000 0.140 0.242 0.517 0.737 N/A
17050 15800 16300 18500 16400 16000
20 17 16 11 7 N/A
0 3 5 12 9 N/A
208 192 198 229 200 N/A
24.5 ( 0.5 24.1 ( 0.5 22.9 ( 0.5 ∼26.7 ( 1.0 N/A N/A
8.8 ( 0.1 8.0 ( 0.1 7.4 ( 0.1 N/A N/A N/A
a First block volume fraction for all P(S-stat-AA)-block-PAA diblocks. b Μolar fraction of acrylic acid in the first statistical P(S-stat-AA) block. c Total number-average molecular weight Mn of the diblock. d NB ) NS + NAA is the total degree of polymerization of the P(S-stat-AA) first block. e Degree of polymerization of the PAA second block. f Characteristic melt structure spacing. g Radius of the spherical P(Sstat-AA) domains in the melt. All data extracted from ref 23.
point of view. In a range of weaker solvent selectivity, however, diblock copolymers can form equilibrium structures. This is the case, for instance, of PEO/PPO-based block copolymers,13-15 usually considered as true “macrosurfactants”: they form spherical micelles at thermodynamic equilibrium with a population of free amphiphiles, the micelles comprising PPO cores hydrated with a water fraction of about 50%. Micellization in such equilibrium systems is characterized by a partition of solvent inside and outside of the core of the object formed.15-19 Given a diblock chemistry, playing with the solvent selectivity to find these equilibrium situations can be done a posteriori, by adopting mixtures of different solvents, or by changing temperature, which both change the solvophobicity of the blocks. However, an easier way would be to play a priori on the chemical composition of each block. Such a study was carried out on PEO/PPO-based macrosurfactants, whose self-assembling properties can be altered by statistically copolymerizing EO in the otherwise core-forming PPO block.20 The introduction of an increasing fraction of EO greatly impacts the diblock amphiphilicity and, thus, its drive for micellization. One may wonder about the impact of such a statistical modification in the case of PS/PAA-based diblocks, which form frozen-in aggregates incapable of reorganizing. The question addressed in the present paper is whether a selective chemical modification of the hydrophobic PS block, which alters the selectivity of water toward PS/PAA-based diblocks, can turn out-of-equilibrium systems into macrosurfactants. A novel controlled radical polymerization technique, called MADIX and developed by Rhodia,21-23 gives access to block copolymers whose blocks can fairly easily be made statistical blocks of different monomers. In particular, diblock copolymers of poly(styrene-stat(acrylic acid))-block-poly(acrylic acid), i.e., P(S-stat-AA)b-PAA, can be synthesized using this technique, with a (13) Goldmints, I.; Yu, G.-E.; Booth, C.; Smith, K. A.; Hatton, T. A. Langmuir 1999, 15, 1651. (14) Goldmints, I.; Holzwarth, J. F.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 6130. (15) Goldmints, I.; Von Gottberg, F. K.; Smith, K. A.; Hatton, T. A. Langmuir 1997, 13, 3659. (16) Fukumine, Y.; Inomata, K.; Takano, A.; Nose, T. Polymer (Guilford) 2000, 41, 5367. (17) Honda, C.; Sakaki, K.; Nose, T. Polymer (Guilford) 1994, 35, 5309. (18) Lodge, T. P.; Xu, X.; Ryu, C. Y.; Hamley, I. W.; Fairclough, J. P. A.; Ryan, A. J.; Pedersen, J. S. Macromolecules 1996, 29, 5955. (19) Lodge, T. P.; Pudil, B.; Hanley, K. J. Macromolecules 2002, 35, 4707. (20) Yulin Deng, R. B., Jifeng Ding, R. B.; Stubbersfield, R. B.; Heatley, F.; Attwood, D.; Price, C.; Booth, C. Polymer (Guilford) 1992, 33, 1963. (21) Corpart, P.; Charmot, D.; Zard, S. Z.; Biadatti, T.; Michelet, D. US Patent 6,153,705 to Rhodia Chimie, 2000. (22) Destarac, M.; Charmot, D.; Franck, X.; Zard, S. Z. Macromol. Rapid Commun. 2000, 21, 1035. (23) Charmot, D.; Corpart, P.; Adam, H.; Zard, S. Z.; Biadatti, T.; Bouhadir, G. Macromol. Symp. 2000, 150, 23.
controlled molar fraction φAA of acrylic acid in the PS block. In the melt state, we have shown that PS-b-PAA diblock copolymers form microphase separated structures.24 On the other hand, once a high enough value of φAA of acrylic acid has been copolymerized, P(S-stat-AA)-b-PAA diblocks do not form microstructures anymore:25 the a priori chemical modification was thus successful in tuning the incompatibility between the two blocks, resulting in an order-disorder transition (ODT) in the melt state. The purpose of the present paper is to investigate the impact on the structural behavior in water of PS/PAA-based diblocks, with an increasing molar fraction φAA of acrylic acid in the PS block. Experimental Section Diblock Synthesis and Characterization. Five poly(styrene-stat-ethylacrylate)-block-poly(ethylacrylate) diblock copolymers, P(S-stat-EA)-b-PEA Rhodiblocks, were synthesized via the MADIX approach21-23 in aqueous emulsion. The synthesis, the chemical characterization of the precursor diblocks (composition of the statistical first block; molecular weight of the first block; polydispersities and compositions), their hydrolysis, and acidification to poly(styrene-stat-(acrylic acid))-block-poly(acrylic acid) diblocks have been detailed in another paper.25 The P(Sstat-AA)-b-PAA diblocks are labeled A1 to A5. These diblocks are asymmetric in terms of composition since, in all cases, the volume fractions of the first block in the diblock melt are equal to f ≈ 0.16 (weight fraction w ≈ 0.12, see Table 1 for all values). The total degree of polymerization of the diblocks is always close to N ) NB1 + NB2 ≈ 220 (constant total molecular weight of approximately 16000 g/mol), where in all cases, NB1 ≈ 20 and NB2 ≈ 200 are the degrees of polymerization of the first and second block, respectively. NB1 is defined as the sum of the numbers NS and NAA of styrene and acrylic acid units in the first block, respectively, and φAA is the molar fraction of acrylic acid in the first P(S-stat-AA) blocks: φAA ) NAA/(NS + NAA). Specimen A6 is a homopoly(acrylic acid) of degree of polymerization approximately equal to 220, directly synthesized in solution in ethanol, using the same polymerization technique. Films were slowly cast (3-4 days) from tetrahydrofuran solutions of the diblocks A1 to A5, at a polymer concentration of 15-20 wt %, in poly(tetrafluoroethylene) molds. They were placed overnight under vacuum at room temperature and heated for an hour at 60 °C to remove traces of THF. The final films were 0.2-0.4 mm thick. Small-angle X-ray scattering (SAXS) experiments were performed to determine the phase behavior of these diblocks in the melt state. All results have been reported in a previous paper25 and are summarized in Table 1, where we indicate the volume fractions f of the first block in the diblock melt, the fraction φAA of acrylic acid in the first P(S-stat-AA) block, the total molecular weight Mn of the diblock, and the different blocks degrees of polymerization. Structure Characterization. Small-angle neutron scattering (SANS) was used to identify the structures and spacings on (24) Bendejacq, D.; Ponsinet, V.; Joanicot, M.; Loo, L.; Register, R. Macromolecules 2002, 35, 6645. (25) Bendejacq, D.; Ponsinet, V.; Joanicot, M.; Vacher, A.; Airiau, M. Macromolecules 2003, 36, 7289.
1714
Langmuir, Vol. 21, No. 5, 2005
Bendejacq et al.
the dispersions of diblocks, all at room temperature. The experiments were performed on the time-of-flight small-angle neutron diffractometer (SAND) at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory, USA. Absolute intensities were obtained from calibration against a standard sample of a 50:50 volume mixture of a protonated and deuterated poly(styrene) (Mw ) 1.15 × 106 kg/mol), after subtraction of a reference cell of D2O and a normalization over the thickness of the samples.26 The intensities were found to follow, at high q, a law Aq-4 + B: the B value was obtained by fit and subtracted as a background from the whole spectra. In the following, we present the SANS results obtained on the dispersions in deuterated water of samples A1 to A6.
Results Melt State Behavior. The study of the melt state of the P(S-stat-AA)-b-PAA diblock copolymers has been published previously.25 Let us recall that the pure PSb-PAA diblock (sample A1) presents a microphase separation of spherical domains made of the minority PS block, embedded in a continuous PAA matrix made of the majority block and presenting a liquidlike three-dimensional order. We found that the introduction of acrylic acid segments in the first PS block induces a decreases of the effective Flory-Huggins parameter χeff between the two constitutive blocks P(S-stat-AA) and PAA, resulting in a decrease of the P(S-stat-AA)-core radius and in a decrease of the chain density at the interfaces. Eventually, an order-disorder transition occurs in the melt state when the effective χ parameter becomes small enough: for acrylic acid φAA ratios in the first block equal to or larger than 50%, specimens A4 and A5 present no self-assembled structure. The melt state results are summarized in Table 1, where we indicate the structure characteristic sizes (period and object radius). The rest of the present report deals with the dispersion in water of the copolymers beforehand organized in the melt state. Frozen Dispersions. After the study of the melt structure, films of the copolymer were powdered and dispersed in deuterated water, D2O, at different diblock weight fractions wp larger than 0.01 (i.e., 1 wt %). The weight fractions wB1 and wB2 of first block P(S-stat-AA) (denoted B1) and second block PAA (denoted B2) in water are computed from wp. With the composition of each diblock known, precise amounts of deuterated sodium hydroxyde NaOD were added so as to perform a constant value for the base-to-acid molar ratio nNaOD/nAA ) 0.5, where nNaOD is the number of moles of NaOD added and nAA is the number of acrylic acid functions coming from the second PAA block only. The dispersions were then allowed to equilibrate at room temperature for several days, before undergoing SANS experiments. For all five systems, this procedure led to homogeneous, transparent solutions of quasi-neutral pH, which were studied by SANS. Let us first detail the data concerning specimen A1. SANS spectra are displayed in Figure 1 for three diblock weight fractions: wp ) 0.02, 0.05, and 0.10. In the smaller q region (0.05-0.02 nm-1) of the spectra, we observe a peak, whose position q* decreases when wp decreases. As a result, this peak is interpreted as structure factor peak. In the higher q-region (0.02-0.2 Å-1) of the spectra in Figure 1, the SANS patterns also present damped oscillations whose asymptotic envelop follows I(q) ∝ q-4, called the Porod regime, showing that copolymer A1 forms objects in water which are close to boxlike profiles regardless of the concentration. The existence of oscillations demonstrates that the objects formed by A1 in water are well-
defined in terms of density profile, size, and shape. The inset in Figure 1b displays even more clearly these welldefined, discernible damped oscillations: we show the Porod’s representation q4I(q)/wp, vs q, where I(q)/wp is the intensity normalized by the diblock weight fraction wp in the medium. For the three different diblock weight fractions presented, i.e., 0.02, 0.05, and 0.10, the oscillations superimpose in position, height, and amplitude after normalization by wp. This demonstrates that within this range of concentration, the size and shape of the elementary object formed by A1 in water have not changed upon dilution. The swelling behavior of the objects formed by specimen A1 is studied through the measured shifts in the position q* of the correlation peak presented above, which we compare with the geometrical swelling law of a three-dimensional array of colloidal objects: 2π/q* ) AR(1/φ)1/3, where φ and R are the volume fraction and radius of objects, respectively, while the prefactor A of this scaling law is a constant of the order of unity and depends only on the lattice adopted (for a BCC phase, A ) (1/x2)(8π/3)1/3 ≈ 1.43). In particular, in the absence of size change upon dilution, the theoretical law conveniently reads: 2π/q* ∝ (1/φ)1/3. Figure 2 shows the plot log(2π/q*) vs log(1/fB1) for specimen A1, where fB1 is the net volume fraction of the first block, computed from the weight fractions wB1 and wB2, using the densities of D2O (1.10 g/cm3), PS (1.05 g/cm3), and PAA (1.47 g/cm3). A linear fit of the data leads to a slope of 0.31 in good agreement with the geometrical law predicting 0.33, which shows that the objects are dispersed without reorganization or size evolution, thus confirming the form factor analysis. Finally, the SANS spectra shown in Figure 1 and analyzed in the previous paragraphs, can be compared with what is described theoretically and experimentally in the literature11,27-29 for the small-angle scattering of block
(26) Tae, G., Kornfield, J. A., Hubbell, J. A. and Lal, J. Macromolecules 2002, 35, 4448.
(27) Castelletto, V.; Hamley, I. W.; Pedersen, J. S. J. Chem. Phys. 2002, 117, 8124.
Figure 1. Plots of the absolute SANS scattered intensity I(q) vs q for dispersions of specimen A1 at different concentrations, i.e., wp ) 0.10 (A), 0.05 (B), and 0.02 (C). The lines are theoretical curves computed within the bare-core approximation (see text) with no adjustable parameters, taking the size distribution (Rs ) 88 Å, σ ) 0.18) measured in the melt state. Inset: Normalized Porod’s plot q4I(q)/wp vs q, which shows the superimposition in position and amplitude of the form factor oscillations corresponding to different concentrations.
Diblock Copolymers of Tunable Amphiphilicity
Langmuir, Vol. 21, No. 5, 2005 1715
Figure 2. Dilution laws in the form of log(2π/q*) vs log(1/fB1) for specimens A1 (filled squares), A2 (open triangles), A3 (crosses), and A4 (open circles). The two straight lines are leastsquares power-law fits to the experimental data for specimens A1 and A4, of slopes 0.31 and 0.135, respectively.
copolymer based micellar structures. A full model, which takes into account the structure factor on the one hand and the form factor combining the core-core, core-brush, and brush-brush correlation factors on the other hand, can be developed and efficiently used to fit the data in some cases. However, it is sometimes enough, in the case of SANS data on amphiphilic copolymers in heavy water,30-32 to use the so-called “bare core approximation” for the analysis of the damped form factor oscillations. Using this approximation, we construct the intensity of an assembly of spherical domains of PS (of scattering length density FPS ) 1.42 × 1010 cm-2), of the same mean size RS ) 8.8 nm and same log-normal size distribution D(R,RS,σ) of width σ ) 0.18 as those extracted from the X-ray data on the melt systems, surrounded by heavy water (FD2O ) 6.38 × 1010 cm-2)33,34
I(q) ) n(wp)
(
)
∫
∞ 4π 3 2 〈RS 〉 (FD2O - FPS)2 0 D(R,Rs,σ) 3 sin qR - qR cos qR 3 (qR)3
[
]
2
dR
with n(wp) ) wpNA/Mng the number density of scattering objects at diblock weight fraction wp, where NA is the Avogadro number, Mn is the molecular weight of the diblock chains, and g ) (4/3π〈RS3〉)/(vstyNB1) is the core aggregation number (i.e., the number of diblock chains involved in a single scattering object), vsty ) 0.166 nm3 is the volume of a melt styrene segment, 4/3π〈RS3〉 is the volume of a single scattering core, where 〈RS3〉 is the third moment of the log-normal radius distribution. This constructed function is compared, with no adjustable parameter, to the experimental curves in Figure 1. The model superimposes very well with the experimental data, showing that the bare-core approximation is valid in this q window and that the PAA outer shell may be neglected at first. More importantly, this confirms the absence of (28) Pedersen, J. S.; Gerstenberg, M. C.; Macromolecules 1996, 29, 1363. (29) Pedersen, J. S. J. Chem. Phys. 2001, 114, 2839. (30) Shusharina, N. P.; Linse, P.; Kholkhlov, A. R. Macromolecules 2000, 33, 8488. (31) Moffitt, M.; Yisong Yu, V.; Diep Nguyen, V.; Graziano, V.; Scheider, D. K.; Eisenberg, A. Macromolecules 1998, 31, 2190. (32) Svaneborg, C.; Pedersen, J. S. Macromolecules 2002, 35, 1028. (33) Small-Angle Scattering of X-Rays; Guinier, A., Fournet, G., Eds.; Wiley & Sons: New York, 1955. (34) Small-Angle X-ray Scattering; Porod, G, Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982.
reorganization in the object size and shape. Finally, our current understanding of these systems is that they consist of spherical core-shell objects, reminiscent of those formed in the melt state by the diblock through self-assembly: they simply dispersed in water without shape or size reorganization. These dispersions can be shown to be out of thermodynamic equilibrium, due to the glassiness of the PS core: when the present dispersions are annealed above the glass transition temperature of the cores (ca. Tg(PS) ) 50 °C measured by differential scanning calorimetry, in agreement with ref 35), their SANS spectra present a significant shift of the form factor oscillations toward larger q values,36 showing a decrease in the radius of the scattering object has occurred. Therefore, while the objects appear frozen as long as no annealing is applied, they immediately reorganize when the dispersions are heated above Tg(PS). Moreover, we found that the annealed core radius, always smaller than the nonannealed one, decreases with increasing charge fraction of the PAA brush, as was already measured by van der Maarel et al.37 The reorganization and size decrease upon annealing are directly related to the relaxation of the object curvature frustration which arises from the selective swelling of the PAA brush as water invades the PAA domains of the melt systems.38 At room temperature, the glassiness of the PS core prevents such relaxations and causes the objects to be out of thermodynamic equilibrium. The study of P(S-stat-AA)-block-PAA samples A2 and A3 shows similar results: Figure 3 displays the concentration-normalized spectra for samples A1, A2, and A3, each at two different concentrations. For each diblock, the normalized intensities superimpose at large wavevector, showing that the objects do not evolve as the systems are diluted. We observe in Figure 2 that the swelling behavior of both A2 and A3 obeys as well the threedimensional law 2π/q* ∝ (1/φ)1/3 expected for an assembly of non-reorganizing spherical objects. In the inset of Figure 3, the Porod’s plots of the SANS data show that the successive minima and maxima of the form factor oscillations shift to larger q-values as φAA increases, indicating the decrease of the core radius as also shown in the previous melt state study.25 In short, the differences between the spectra obtained for A1, A2, and A3 solely reflect the differences in the cores radii formed in the melt state by these three different copolymers. In conclusion, copolymers having a fraction φAA of acrylic acid in the first block, smaller than or equal to 0.242, form self-assembled objects in the melt state which disperse as colloidal objects when water is added. No organization occurs upon dispersion, unless a specific procedure (such as annealing above Tg(PS)) is carried out during the preparation process. Such a dispersion state proceeds down to a very low concentration, meaning that these systems have nosor a very lowscmc. These systems are referred to as frozen. Equilibrium Systems. In contrast to the three previous systems, specimens A4 and A5 do not microphase separate in the melt state.23 These homogeneous (disordered) copolymer films are dispersed in heavy water as previously described. Figure 4 displays the concentration normalized SANS intensity I(q)/wp vs q for specimen A4 (35) Polymer Handbook, 3rd ed.; Brandrupt, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1989. (36) Bendejacq, D. PhD Thesis, Universite´ de Paris 6, 2002. (37) van der Maarel, J. R. C.; Groenewegen, W. Langmuir 2000, 16, 7510. (38) Bendejacq, D.; Santagelo, C. Ponsinet, V.; Joanicot, M. To be submitted for publication.
1716
Langmuir, Vol. 21, No. 5, 2005
Figure 3. Absolute, concentration-normalized intensity I(q)/ wp vs q, for specimens A1 (A), A2 (B), and A3 (C) at two different concentrations, i.e., wp ) 0.10 (open circles) and 0.10 (open triangles). For the sake of clarity, the A, B, and C curves, have been vertically shifted by multiplying factors 100, 10, and 1, respectively. Inset: Corresponding Porod’s representation q4I(q)/wp vs q, which shows the evolution of the form factor oscillations in position and amplitude between the three systems. All curves have been vertically shifted for the sake of clarity.
at wp ) 0.05, 0.10, 0.20, and 0.30 (vertically shifted by multiplying factors 1, 10, 100, and 1000, for the sake of clarity). For the highest concentration investigated, i.e., wp ) 0.30, the spectrum presents features that strongly resemble those of specimen A1 at the same concentration and which have been detailed previously, namely: (i) a structure factor peak at position q* in the small q region, and (ii) form factor oscillations (even better evidenced in the Porod’s representation in the inset of Figure 4) along with a Porod asymptotic envelop I(q) ∝ q-4, in the higher q region. These features demonstrate that for a high enough concentration, an object of sharp interface, welldefined in terms of size and shape, is formed by specimen A4 in water, whereas this diblock did not form any structure in the initial dry state. Let us analyze the evolution of both features as the concentration is varied. The form factor oscillations, on one hand, are significantly shifted to higher q values as wp is decreased. This is confirmed in the inset of Figure 4, where we plot the corresponding Porod’s representation q4I(q)/wp vs q, displaying more clearly the successive maxima and minima in the oscillations. This demonstrates that the object formed by A4 in water is not frozen but changes as dilution proceeds. The structure factor peak position q*, on the other hand, moves toward lower q values when wp is decreased. This corresponds to an increase of the characteristic distance between objects and should be compared to the swelling law of a three-dimensional array of colloidal objects presented before. Figure 2 shows the plot log(2π/q*) vs log(1/fB1) for specimen A4, where fB1 is computed as explained before, assuming the density of the first P(S-stat-AA) block to be the composition average between those of PS and PAA. In contrast to what was found for the frozen systems, the apparent power law of exponent 0.135 shows an anomalous swelling law likely due to an evolution of R along the dilution path, in the
Bendejacq et al.
Figure 4. Absolute intensity I(q) vs q, for specimen A4 at four different concentrations, i.e., wp ) 0.30 (A), 0.20 (B), 0.10 (C), and 0.05 (D). The lines are the fits using the theoretical curves computed with a bare core approximation assuming a hydrated core (see text), from which we extract the values given in Table 2. For the sake of clarity, the A, B, C, and D curves, as well as the corresponding fits, have been vertically shifted by multiplying factors 1000, 100, 10, and 1, respectively. Inset: Corresponding Porod’s plot q4I(q) vs q, showing the evolution of the form factor oscillations in position and amplitude as the concentration is varied. All curves have been vertically shifted for the sake of clarity.
theoretical law 2π/q* ∝ R(1/φ)1/3. Sample A4 therefore presents a concentration-dependent self-association in water, it forms a structure in water while it did not in the melt state, and reorganizes by itself at room temperature upon dilution. It appears as an equilibrium system. At wp ) 0.01, no discernible pattern was observed by SANS, indicating a threshold concentration of association between 1 and 2%. Quantitative analysis of the SANS spectra of sample A4 was attempted as presented for sample A1 in the previous paragraph. A simple bare core model assuming a dense P(S-stat-AA)-core largely fails in describing the experimental scattering: whatever the parameters chosen for the size distribution, such a model produces a scattering curve 1 to 2 orders of magnitude stronger than what is measured experimentally. This discrepancy between the experimental intensity and that predicted by the dense core model leads us to conclude that the scattering object, formed by specimen A4 in water, has a weaker scattering length density contrast with the solvent than that considered for a bare dense P(S-stat-AA)-core. We propose that this is due to a D2O swelling of the P(S-stat-AA) cores. Following Goldmints et al.,15 who analyzed the scattering from PEO-block-PPO micelles in terms of a hydrated PPO core, we modify the previous construction to allow some D2O inside the P(S-stat-AA) cores. Strong approximations are made in this model: (i) the brush signal is negligeable in the chosen q-range, and (ii) the hydrated core is homogeneous. However, we argue that this simple approach can satisfactorily fit the experimental data when adjusting the core mean radius RS, the object polydis-
Diblock Copolymers of Tunable Amphiphilicity
Langmuir, Vol. 21, No. 5, 2005 1717
Figure 6. Surface tension at the interface between air and a solution of polymer as a function of the polymer weight fraction: copolymer A1 (open circles); copolymer A4 (open triangles); and homopolymer A6 (filled triangles). Figure 5. Absolute, concentration-normalized SANS scattered intensity I(q)/wp for specimens A5 and A6, from top to bottom, at two different concentrations, i.e., wp ) 0.10 (open circles) and 0.70 (filled circles). The experimental curves have been vertically shifted for clarity. Table 2. Structural Parameters for Sample A4 Extracted from the Fits of the SANS Data wp
RS (nm)
σ
g
φD2Oc
0.30 0.20 0.10 0.05 0.02
5.2 4.7 4.1 3.5 2.8
0.18 0.20 0.22 0.26 >0.30
111 96 76 48 55
0.544 0.484 0.447 0.45 0.573
persity σ, and the water fraction φD2Oc inside the core. The results of the fitting procedure at different concentrations are given in Table 2, as well as the aggregation number extracted from these data. The model may then produce a qualitative description of the object formed in water by A4 and its behavior upon dilution: we find that as the concentration is decreased, the mean radius of the core decreases and its size polydispersity increases. The water fraction φD2Oc inside the core is in all cases close to 50%, which is comparable to the amount found by Goldmints et al. for PEO/PPO-based diblocks.15 Specimen A5 has also been dispersed in water and studied by SANS. Figure 5 displays the concentrationnormalized intensity I(q)/wp vs q, where I(q) is the normalized, absolute scattered intensity, obtained on specimen A5 at two different diblock fractions (wp ) 0.02 and 0.10) as well as an homopolymer of acrylic acid A6 (wp ) 0.10 and 0.70). A detailed analysis of these spectra is beyond the purpose of this paper, but what is interesting to note is that the spectra are very similar and in particular both samples produce a concentration-independent signal once normalized, presenting neither a structure factor nor a spherelike form factor. Finally, we conclude that copolymer A5 is fully soluble in water at this pH, very much like a pure homopolymer of acrylic acid. As a conclusion, copolymers having a ratio φAA of acrylic acid in the first block larger than 50% do not form selfassembled objects in the melt state. When φAA is close to 50% (A4, φAA ) 0.517), the copolymer self-assembles above a threshold concentration of 1-2% in water, forming micelle-like objects, which comprise a water-swollen core of concentration-dependent radius, self-reorganizes, and appears at thermodynamic equilibrium. Unlike the frozen systems described before, these copolymers thus present an apparent cmc in an accessible concentration range. When φAA is larger (sample A5), the copolymer is fully
soluble and does not self-assemble in the concentrations and pHsranges investigated. Discussion and Conclusion. We have shown that the introduction of acrylic acid in the first block of P(S-statAA)-block-PAA copolymers, which induces a orderdisorder transition in the melt state, greatly influences the behavior of these amphiphilic copolymers upon dispersion in water. Frozen, out-of-equilibrium systems, which disperse as colloidal objects when φAA is low enough, turn to self-reorganizing equilibrium systems when φAA approaches 50% (A4, φAA ) 0.517): we thus see evidence of a drastic change in the phase behavior of amphiphilic diblocks in water, obtained via a chemical tuning of the composition of the core-forming block and, subsequently, of its hydrophobicity. The behavior of the specimen close to the boundary (A4, φAA ) 0.517) shows features of surfactant-type systems, including an apparent cmc and a micellar growth upon concentration in water. However, two experimental facts lead us to discuss the “macrosurfactant” nature of these new diblocks and propose a different interpretation of the SANS results detailed before. The first experimental evidence is that the first P(Sstat-AA) blocks of diblocks A1 to A3 are found by themselves to be completely insoluble in water regardless of the pH, while those of samples A4 and A5 are found soluble at pH ) 7. Evidently, the first blocks of A4 and A5 bear enough acrylic acid segments to make them soluble in water at a high enough pH, while it is not the case for those of A1, A2, and A3. Although we did not perform a comprehensive study of the structure in water of these water-soluble first blocks, our observations are consistent with the known behavior of charged hydrophobic polymers, such as poly(styrene-stat-(styrene sulfonate)), which are soluble in water when more than 30% of the segments bear a charge.39 This observation leads us to conclude that, in the case of diblocks A4 and A5, water can solubilize both P(S-stat-AA) and PAA blocks individually and is thus only a slightly selective solvent for the first blocks. The second experimental fact was obtained by measuring the surface tension of solutions of polymers A1, A2, A4, and A6 (pure PAA) at different concentrations, using a Wilhelmy plate method (Kruss tensiometer). Results are displayed in Figure 6. For all four samples, the measured surface tension is found equal to that of pure water (72 ( 2 dyn/cm) up to a threshold polymer (39) Essafi, W.; Lafuma, F.; Williams, C. E. J. Phys. II Fr. 1995, 5, 1269
1718
Langmuir, Vol. 21, No. 5, 2005
concentration of 0.01 wt % for sample A6 (homopolymer of acrylic acid) and of 0.1 wt % for samples A1, A2, and A4 (copolymers). The surface tension decreases only above this threshold. These results show that the copolymers do not have a surfactant behavior, rather behaving toward interfaces as simple polyelectrolytes do.40 In the absence of an enthalpic driving force for the apparent micellization, as shown by the facts that the first block is soluble in water and the diblock is not surface active, a different thermodynamic effect leading to the self-assembly can be found in the incompatibility between the two separate polymers forming the diblock. Indeed, we propose that the phenomenon may be regarded as the microscopic version of the macroscopic phase separation generally induced between two homopolymers B1 and B2 in a common solvent S. Scott’s regular theory of the thermodynamics of polymer solutions41 treats the case of such a ternary system. The approach predicts that due to the repulsive contacts between B1 and B2, the mixture should present a macrophase separation between B1- and B2-rich phases (both containing solvent partitioned between the two phases), when the total polymer concentration is high enough. If one now considers the case of a diblock made of such blocks, i.e., B1-b-B2, and placed in a solvent S common for the two blocks, a macrophase separation between B1 and B2 becomes forbidden due to the chemical link between the blocks. Thus, phase separation occurs at a microscopic scale, leading to the formation of a core-shell object comprising a B1-rich phase (i.e., the swollen core, made of blocks B1 and solvent S), and a B2-rich phase (i.e., the swollen shell, made of B2 blocks and solvent S). This feature is well-known for segregative diblock copolymers cast from a solution in a nonpreferential solvent, as they often form a structure way before the solvent has completely evaporated, when its fraction becomes too low to screen the repulsive contacts between the two blocks. This is expected to occur at, or close to, the polymer overlapping concentration, which can be estimated in our system to be of the order of 1 wt %.42 (40) Theodoly, O.; Ober, R.; Williams, C. E. Eur. Phys. J. E 2001, 5, 51. (41) Scott, R. L. J. Chem. Phys. 1949, 17, 279.
Bendejacq et al.
Although the enthalpically driven macrophase separation has been verified experimentally for mixtures of neutral homopolymers,43 it has been argued that the phenomenon should not occur with mixtures of charged homopolymers, because of the counterion entropy of mixing that largely dominates over the enthalpic cost of the contacts between the polymer segments.44 However, it was experimentally demonstrated recently45 that mixtures of poly(acrylic acid) and poly(sodium styrenesulfonate) do present a macrophase separation. For a diblock made of such charged blocks, the microscopic version of the phenomenon (i.e., a microphase separation leading to an object formation) may also be expected, as in the case of a neutral diblock, although the thermodynamics greatly differ because of the counterion contribution. As a conclusion, we propose that the formation of concentration-dependent core-shell structures by the P(Sstat-AA)-b-PAA diblock A4 corresponds to a microphase separation due to the incompatibility between the polymer blocks P(S-stat-AA) and PAA placed in a common solvent, water. We expect the morphology of this microphase separation to depend on the volumes effectively occupied by each of the two swollen blocks and, therefore, on the respective degrees of polymerization and charge fractions of these blocks: as we found that a very asymmetric P(Sstat-AA)-b-PAA diblock presents a microphase separation of spherical symmetry, it should be possible to find microphase separations of lamellar or cylindrical structures. This question will be investigated in a future paper. Acknowledgment. We greatly acknowledge Mathias Destarac and Gilda Lizarraga, Rhodia, for guidance and latex synthesis, Jyotsana Lal, IPNS, for helpful support in the scattering experiments, Roya Farhoosh, Anish Parikh, and Sahar Iskandar for the additional surface tension measurements. LA048983R (42) Dobrynin, A. V.; Colby, R. H. Macromolecules 1995, 28, 1859. (43) Dobry, A.; Boyer-Kawenoki, F. J. Polym. Sci. 1947, 2, 90. (44) Roland, C. M. Macromolecules 1987, 20, 2557. (45) Hellebust, S.; Nilsson, S.; Blockhus, A. M. Macromolecules 2003, 36, 5372.
