Macromolecules 2008, 41, 8895-8902
8895
Depletion Interactions: Effects of Added Homopolymer on Ordered Phases Formed by Spherical Block Copolymer Micelles Sayeed Abbas† and Timothy P. Lodge*,†,‡ Departments of Chemical Engineering & Materials Science and Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed June 19, 2008; ReVised Manuscript ReceiVed September 9, 2008
ABSTRACT: Three distinct poly(styrene-b-isoprene) (SI) diblock copolymers with molecular weights of 16-16, 38-14, and 50-13 kDa for styrene and isoprene, respectively, formed spherical micelles when dissolved in diethyl phthalate (DEP). Since DEP is a styrene-selective solvent, micelles with polyisoprene in the core and polystyrene in the corona were formed. At block copolymer concentrations of 20%, 16%, and 14% in DEP, the spherical micelles of SI(16-16), SI(38-14), and SI(50-13) pack onto a face-centered cubic (FCC) lattice, a mixture of FCC and body-centered cubic (BCC) lattices, and a BCC lattice, respectively. Polystyrene homopolymers with molecular weights of 4, 48, and 180 kDa were added to these ordered solutions. The following general trends were observed: the FCC phase tended to disorder, and samples that originally behaved like soft solids exhibited liquidlike flow behavior. The effect increased strongly with both the molecular weight and concentration of homopolymer in the solution. Furthermore, the BCC lattice tended to be displaced by the FCC lattice, or to disorder, when homopolymer was added. These results can be explained by invoking depletion interactions, which have been studied extensively in colloid/polymer mixtures. However, the phenomenon differs in certain details from colloidal systems because the addition of homopolymer can also influence the aggregation number of the micelles, which in turn affects the lattice packing of the micelles.
Introduction Self-assembly of diblock copolymers in selective solvents offers a rich array of nanostructures with diverse applications. In dilute solutions an AB diblock copolymer self-assembles in an A-selective solvent to form a spherical micelle with A in the corona and B in the core. Other micelle morphologies are also possible, and by proper choice of block copolymer composition, solvent, and temperature, one can control the micelle morphology. Detailed reviews on this subject are available.1,2 When the concentration of spherical micelles is increased in solution to approximately 10-20%, the micelles pack onto either a face-centered cubic (FCC) or body-centered cubic (BCC) lattice.3-12 In some cases the A1513,14 and LS13 superlattice15,16 structures have also been reported. The primary factors controlling the phase behavior of a diblock copolymer in a selective solvent include temperature, polymer concentration, and volumetric composition of the diblock.4 The aim of this work is to examine how the introduction of homopolymer affects the behavior of cubic phases formed by spherical micelles and thereby to introduce a new control parameter in the selfassembly of diblock copolymers. Qualitatively, the addition of homopolymer to micelle solutions is similar to depletion interactions in colloidal systems.17-20 When homopolymer is added to a solution of colloidal particles, the polymer coils are excluded from a region close to the surface of the sphere due to the high entropic penalty for conformational distortion. Thus, a depletion layer as thick as the radius of gyration (Rg) of the polymer coils in the solution develops around each sphere. When the spheres are far apart from each other, a uniform concentration gradient of the polymer exists from the bulk to the surface of the spheres, resulting in an isotropic osmotic force of the polymer coils on the particle. If two spheres approach one another and the depletion layers overlap, the polymer coils are excluded from the overlapping * Author for correspondence: E-mail
[email protected]. † Department of Chemical Engineering & Materials Science. ‡ Department of Chemistry.
region. This leads to an anisotropic osmotic pressure, and an effective attraction develops between the colloid particles. In contrast to colloidal particles, micelles offer an additional level of self-assembly: the process of micellization itself. Hence, the effect of depletion interactions can be even more subtle. Relatively little work has been done to study the effect of addition of homopolymers to diblock copolymers in a selective solvent. The earliest study to investigate this effect was performed by Watanabe and Kotaka on poly(styrene-b-butadiene) (SB) in tetradecane,21 where the diblock copolymer selfassembles to form spherical micelles with polystyrene (S) in the core and polybutadiene (PB) in the corona. Above a critical concentration of SB, an ordered phase exhibiting plastic flow under steady shear was formed. When PB was added, the solution lost its plasticity. Similar results were reported when homopolymer poly(ethylene oxide) (PEO) was added to soft solids formed by pluronic polymers in water.22 It was observed that with moderate molecular weight PEO the gel melted, whereas with high molecular weight PEO phase separation occurred. In both these papers, no scattering experiments were reported to characterize the morphology of the soft solids. Homopolymer PEO was also added to ordered phases formed by dissolving hydrophobically end-capped PEO chains in water,23-25 and similar results were obtained. By performing SAXS, the micelles were shown to pack on BCC lattice, which disordered upon addition of homopolymer.23 In addition, Yamazaki et al. performed self-consistent field theory calculations, which showed that upon increasing PEO molecular weight the homopolymer was excluded from the corona of the micelles.24 The behavior of multiarmed stars is also similar to micelles, and several recent studies have focused on multiarm star polymers with added homopolymer in solution. It was observed that with addition of homopolymer the soft solid melted.26 The dynamics of the system was investigated by rheology27 and photon correlation spectroscopy;28 the results revealed that addition of homopolymer can significantly alter the phase behavior of soft spheres. We have previously reported results from small-angle neutron scattering experiments on micelle/
10.1021/ma8013838 CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
8896 Abbas and Lodge
Macromolecules, Vol. 41, No. 22, 2008
Table 1. Polymer Characterization polymer
MN (g/mol)a
(first block MN-second block MW)
Table 2. Homopolymer:Micelle Size Ratio (r) PDI (MW/MN)a,b
SI(16-16) 32 000 16 000-16 000 1.01 SI(38-14) 52 000 38 000-14 000 1.02 SI(50-13) 63 000 50 000-13 000 1.01 S(4) 4 000 1.02 S(48) 48 000 1.01 S(180) 178 000 1.01 a MN and MW represent number-average and weight-average molecular weight, respectively, as measured by SEC and 1H NMR. b Polydispersity index (MW/MN) of the synthesized block copolymer as determined by SEC using the light scattering detector.
homopolymer mixtures, showing the presence of effective attractive forces between micelles.29 We further showed that due to the addition of homopolymer, the aggregation number (Nagg) of the micelles increased slightly. These experiments focused on disordered dilute or semidilute solutions. Here we consider the effect of addition of homopolymer on ordered phases, particularly on the face-centered cubic (FCC) and bodycentered cubic (BCC) lattices. The results are interpreted in terms of depletion interactions. Experimental Section Materials. Poly(styrene-b-isoprene) (SI) was synthesized by living anionic polymerization using standard procedures.3 Monomers styrene and isoprene were obtained from Sigma Aldrich and further purified. Styrene was stirred over calcium hydride overnight and further stirred over dibutylmagnesium for 8 h. Isoprene was stirred over calcium hydride overnight and then subsequently stirred for 4 h over n-butyllithium and dibutylmagnesium. After purification the monomers were vacuum-distilled into burets. The polymerization was initiated by injecting sec-butyllithium into a mixture of styrene and cyclohexane at 40 °C. Subsequently, the solution was stirred for 4 h at 40 °C, after which isoprene was added, and the reaction was allowed to run for an additional 4 h at 40 °C. Degassed methanol was used for termination. The reaction mixture was then washed with water, and the product was collected by precipitating the polymer in a 50/50 mixture of isopropanol and methanol. Size exclusion chromatography (SEC) with both refractive index (Wyatt Optilab) and light scattering (Wyatt Dawn) detectors was used to determine the molecular weight of the first block (polystyrene). Using 1H NMR spectroscopy, the composition of the diblock was determined, from which the molecular weight of the second block (isoprene) was also calculated. Homopolymer polystyrene (S) with different molecular weights was also synthesized via the same protocol. Hereafter, the poly(styrene-b-isoprene) diblocks and polystyrene homopolymer will be abbreviated as SI(x-y) and S(x), where x and y represents the molecular weight of polystyrene and polyisoprene, respectively, in kDa. Table 1 shows the characterization results for the synthesized polymers. The solvent diethyl phthalate (DEP) and cosolvent methylene chloride were obtained from Sigma-Aldrich and used without further purification. Sample Preparation. All samples were prepared gravimetrically with the aid of methylene chloride as the cosolvent. Typically, the polymer + cosolvent mixture was stirred for 24 h, after which the cosolvent was removed by evaporation until a constant weight of the vial was achieved. The concentration of polymer was converted to volume percent by taking the densities of polystyrene, polyisoprene, and DEP to be 1.05, 0.913, and 1.118 g/mL, respectively. A sample denoted as x% SI + y% S contains x vol % diblock SI and y vol % homopolymer S. Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) was performed on beamline 5 ID-D in the Du PontNorthwestern-Dow (DND-CAT) station at the Advanced Photon Source, Argonne National Laboratory. The energy of the radiation was either 17 keV (λ ) 0.73 Å) or 12 keV (λ ) 1.033 Å). An undulator was used as an insertion device. The energy was selected by a double crystal monochromator, and further beam collimation
SI(16-16) SI(38-14) SI(50-13)
Rh (nm)
S(4)
S(48)
S(180)
20 20 27
