Morphology Evolution of PS-b-P2VP Diblock Copolymers via Supramolecular Assembly of Hydroxylated Gold Nanoparticles

We report on the strong segregation of core− shell Au nanoparticles, with a shell layer consisting of a random copolymer brush of styrene and vinylphenol (PS-rPVPh-SH), in poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer. Because of the formation of multiple hydrogen bonds between the hydroxyl groups within the shell of the nanoparticles and the pyridine group in PS-b-P2VP, the Au nanoparticles were strongly localized into P2VP domains with a very high volume fraction of nanoparticles (φp ∼ 0.53). The spatial distribution of Au nanoparticles, observed by transmission electron microscopy (TEM), is compared with results of previous experiments where homopolymers were blended with block copolymers. If the diameter d of the nanoparticles is much less than the width D of the P2VP lamellar domains, these nanoparticles are more uniformly distributed across the P2VP domain than if d is comparable to D, in which case the nanoparticles are pushed toward the center of the P2VP domains. This behavior is similar to that observed when homopolymers are blended with block copolymers. Novel morphological transitions from spherical to cylindrical P2VP morphologies and from lamellae to cylindrical PS morphologies were observed during coassembly of these functional nanoparticles with block copolymers. ■ INTRODUCTION Hydrogen bonding (H-bonding) has been widely used for supramolecular assembly of small molecules and polymers because of its molecularly specific and highly directional characteristics. In addition, the reversible and dynamic characteristics of H-bonding enable production of self-healing materials, which is an exciting emerging theme in materials science. In particular, block copolymers constructed using multiple H-bonding units instead of covalent linkages have been studied intensively, both theoretically and experimentally. A judicious choice of polymers, balancing polymer interactions with the strength and directionality of H-bonding groups, opens up possibilities to create complex structures with various properties derived from simple building blocks. Demonstrations of these possibilities include low temperature processing of materials with well-defined architectures, thermal modulation of microphase-separated domains, and development of nonconventional morphologies with nanometer-scale features such as lamellae-in-lamellae and square arrangement of cylinders. Recently, H-bonding has also been employed to blend block copolymers and homopolymers. The formation of multiple H-bonds between host and guest polymers significantly enhances miscibility causing swelling of the host domains of the block copolymers without macrophase separation and corresponding order−order morphology transitions. The incorporation of inorganic nanoparticles into block copolymer matrices has been intensively studied with the aim of combining the unique physical or chemical properties of nanoparticles with hierarchical nanophase-separated selfassembly of block copolymer, which is potentially useful in high performance catalysis, sensors, optics, and electronics. Tailoring the surface properties of nanoparticles can provide for precise control of nanoparticle segregation in block copolymers. This control has been demonstrated using nanoparticles with short aliphatic chains, homopolymers, mixed homopolymers, and random copolymers. It is known that nanoparticles completely covered by polymeric ligands show segregation behavior similar to that of the homopolymer in blending systems. For example, nanoparticles covered with a polystyrene (PS) homopolymer brush layer tend to segregate in PS domains of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) Received: October 26, 2011 Revised: December 20, 2011 Published: January 20, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 1553 dx.doi.org/10.1021/ma202391k | Macromolecules 2012, 45, 1553−1561 diblock copolymer at low particle volume fractions. However when higher volume fractions of such particles are added, they macrophase-separate due to an increase in the entropic penalty of block copolymer chain stretching resulting from nanoparticle incorporation. Most characteristic features of homopolymer addition in blending systems such as swelling of domains, morphological transitions of block copolymer, and macrophase separation of additives were observed in coassembly of nanoparticles and block copolymers. In particular, a strong segregation of nanoparticles into specific domains via Hbonding has also been reported. For example, localization of nanoparticles of cadmium selenide (CdS), Au, and silicon (Si) covered with hydroxyl groups has been observed in the poly(ethylene oxide) (PEO) or poly(4-vinylpyridine) (P4VP) domains of diblock copolymers. Because of the strong enthalphic attraction of H-bonds, macrophase-separation of nanoparticles was significantly suppressed in favor of microphase separation and very high volume fraction of nanoparticle incorporation (∼36 vol %) was demonstrated. Despite the similarity between polymer blend and nanoparticle incorporation systems, according to our best knowledge, no investigation correlating these two systems in detail have been reported. Combining knowledge from both systems may allow general predictions of segregation behavior for new sets of nanoparticles or homopolymers in block copolymer matrices in order to create customized and multifunctional hybrid materials. In this paper, Au nanoparticles covered with a brush layer of thiol-terminated random copolymers of styrene and vinylphenol (PS-r-PVPh-SH) were prepared and the H-bonding attraction between the hydroxyl groups on the nanoparticle ligands and pyridine rings of PS-b-P2VP diblock copolymers studied. The PS-r-PVPh-SH ligands were synthesized by reversible addition−fragmentation transfer (RAFT) polymerization of styrene and 4-acetoxystyrene (PS-r-PAS-RAFT) followed by hydrazinolysis to deprotect the phenol groups and to generate a thiol group at the chain end. By controlling the relative number of p-vinylphenol to styrene units, solubility of Au nanoparticles in nonpolar solvents could be tuned to maximize the H-bonding strength. As a result, preparation of nanoparticle/block copolymer mixtures and solvent annealing could be carried out in nonpolar solvents without any disruption or weakening of H-bonds by solvation of the phenol units by polar solvents. The random copolymer-coated nanoparticles were strongly segregated in P2VP domains of PS-b-P2VP block copolymer of various molecular weights and 2VP mole fractions. The spatial distribution of the nanoparticles in P2VP domains of block copolymer were compared to results of previous experiments where homopolymers were blended with block copolymers. The high incorporation of homogeneously distributed nanoparticles into P2VP domains induced morphological transitions of the PS-b-P2VP diblock copolymers. ■ EXPERIMENTAL METHODS Synthesis of PS-r-PAS-RAFT by RAFT Polymerization. Thiolterminated random copolymers of styrene and 4-acetoxystyrene (PS-rPAS-RAFT) were synthesized via reversible addition−fragmentation transfer (RAFT) polymerization. Dithioester-RAFT agent was prepared by the procedure described elsewhere. Two PS-r-PASRAFTs were synthesized with different compositions. Styrene (SigmaAldrich, > 99%) and 4-acetoxystyrene (Sigma-Aldrich, 96%) were purified by passage through a basic aluminum oxide column prior to use. Styrene (6.02 g, 55.80 mmol), 4-acetoxystyrene (1.01 g, 6.20 mmol), azobis(isobutyronitrile) (AIBN, 0.01 g, 0.06 mmol), and dithioester RAFT agent (0.30 g, 0.62 mmol) were mixed and degassed by three freeze−pump−thaw cycles. The polymerization was carried out at 90 °C for 7 h under vacuum. The polymer was then precipitated in methanol and dried under vacuum. The molecular weight (Mn) and polydispersity index (PDI) of the random copolymer PS22-r-PAS3RAFT as measured by gel permeation chromatography (GPC, calibrated by PS standards), were 2.6 kg/mol and 1.1, respectively. The composition of PS-r-PAS-RAFT calculated from H NMR was found to be 12 mol % 4-acetoxystyrene (∼3 repeating units). The Mn value of PS22-r-PAS3-RAFT calculated from the ratio of the peak area of RAFT agent to that of PS (broad peaks from 6.2 to 7.3 ppm) and acetoxy groups (broad peak centered at 2.3 ppm) was 3.3 kg/mol. Following the same procedure, a PS-r-PAS-RAFT with 26 mol % 4acetoxystyrene (∼7 repeating units, PS20-r-PAS7-RAFT) was synthesized. The Mn and PDI for this copolymer were 2.7 kg/mol (3.7 kg/ mol by H NMR) and 1.1, respectively. Deprotection of PS-r-PAS-RAFT. The dithioester-terminus and acetoxy groups of the PS-r-PAS-RAFT were converted to thiol and hydroxyl groups, respectively, in a one step reaction by hydrazinolysis as reported by Lee et al. PS-r-PAS-RAFT (1.0 g) was placed in a round-bottom flask with a magnetic stirrer. Dry THF (30 mL) was transferred into the flask via a cannula after three vacuum and argon purging cycles. Under a dry argon atmosphere, 15 mmol of hydrazine dissolved in THF (Sigma-Aldrich, 1M, 50 equiv. to dithioesterterminus) was injected by syringe (Caution! hydrazine is highly toxic and should be handled with extreme care). The solution color changed from pink to yellow immediately. After the reaction was stirred overnight, the solvent and hydrazine were evaporated and the polymers were dissolved in dichloromethane, filtered with a syringe filter (Whatman, 200 nm pore size, PTFE) to remove insoluble species, and precipitated in cold hexane. The precipitated white polymers were dried under vacuum at room temperature for a day. Synthesis of PS-r-PVPh-SH-Coated Au Nanoparticles (PS-rPVPh−S-Au). Au nanoparticles coated with PS-r-PVPh-SH were synthesized using the THF one-phase method. Au precursor (HAuCl4·3H2O, Sigma-Aldrich, > 99.9%, 0.8 mmol) and 0.2 mmol of PS22-r-PVPh3-SH or PS20-r-PVPh7-SH polymers were placed in a round-bottom flask with a magnetic stirrer. Dry THF (20 mL) was transferred into the flask via a cannula under agitation after three vacuum and argon purging cycles. After stirring for 30 min, Au nanoparticles were synthesized by adding 2.3 mmol of the reducing agent, superhydride (Li(C2H5)3BH, Sigma-Aldrich, 1 M in THF), dropwise under dry argon. The unbound polymer ligands were separated from the polymer brush coated Au nanoparticles by filtering, at least 5 times, with membrane centrifugal filters (Centricon-Plus 70, MWCO 100 000 Da, Millipore Inc.) using THF as the solvent. The washed Au nanoparticles in THF were filtered with a syringe filter (Whatman, 200 nm, PTFE) and precipitated in hexane. Preparation of PS-b-P2VP/Au Nanoparticle Composites. A certain weight of Au nanoparticles was dissolved in a freshly prepared 1 wt % PS-b-P2VP diblock copolymer solution in dichloromethane (DCM) to obtain a volume fraction of nanoparticles in the range of 0.04−0.53. This volume fraction includes the volume of the polymer shell estimated from the density of the ligand (∼1.05 g/cm) and Au (∼19.3 g/cm) coupled with thermal gravimetric analysis (TGA) of the Au nanoparticles. P2VP sphere-forming (107 kg/mol with a mole fraction of 2VP, f P2VP ∼ 0.11) and lamellae-forming (72 kg/mol with f P2VP ∼ 0.40 and 199 kg/mol with f P2VP ∼ 0.48) PS-b-P2VP block copolymers were used for the preparation of composites. Thick films of block copolymers and block copolymer/nanoparticle composites were prepared by drop casting the solution of nanoparticles and PS-bP2VP block copolymer in DCM onto a thick Au-coated (∼100 nm) sodium chloride crystal window (Sigma-Aldrich, 2 mm thick). The composites were annealed in saturated dichloromethane (DCM) vapor at room temperature for at least 2 days. After drying the composite film overnight under vacuum, a thick Au layer (∼100 nm) was deposited on the sample to inhibit infiltration of the epoxy resin (Embed-812, Electron Microscopy Sciences) into the sample during TEM sample preparation. Samples of the composite film were Macromolecules Article dx.doi.org/10.1021/ma202391k | Macromolecules 2012, 45, 1553−1561 1554 embedded into epoxy resin and sliced to a thickness of about 50−70 nm by ultramicrotoming (Leica). The sliced composite samples were exposed to iodine vapor to selectively stain the P2VP domains. Characterization. The nanoparticles and cross-sections of the pure block copolymers and composite films were characterized by transmission electron microscopy (TEM, FEI Tecnai G2 microscope, 200 kV). The size histograms of the Au nanoparticles were determined from at least 300 nanoparticles by image analysis (Image Pro) of TEM micrographs. The mean areal chain density of polymer ligands on the Au nanoparticles was calculated from the total surface area and the weight fraction of Au and polymer ligands determined by TGA. Fourier transform infrared (FT-IR, Bruker) spectra of PS-b-P2VP diblock copolymer and composite samples were taken in the wavenumber range of 450−4000 cm−1. ■ RESULTS AND DISCUSSION Scheme 1 illustrates the strategy for synthesis of Au nanoparticles coated with random copolymer ligands containing phenolic groups and their supramolecular assembly with the pyridine groups of a poly(styrene-b-2-vinylpyridine) (PS-bP2VP) diblock copolymer. Thiol-terminated poly(styrene-rvinylphenol) (PS-r-PVPh-SH) ligands are obtained from the random copolymer of poly(styrene-r-acetoxystyrene) synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization (PS-r-PAS-RAFT). The hydrolysis of acetoxy groups and the reduction of thioester groups to a secondary thiol were carried out simultaneously by addition of hydrazine. Then, Au nanoparticles covered with PS-r-PVPhSH ligands were synthesized by reduction of a Au precursor (HAuCl4·3H2O) with superhydride (Li(C2H5)3BH) dissolved in THF. A key to the design of the nanoparticles is tailoring the surface properties of the polymeric shell by controlling the number of phenolic groups per polymeric ligand with enough hydroxyl groups being present to induce a strong H-bonding interaction. At the same time, the number of hydroxyl groups needs to be low enough to maintain reasonable hydrophobicity, enabling dissolution of the nanoparticles in nonpolar solvents such as dichloromethane. This avoids the need to use polar solvents, which weaken the H-bonding interaction with the P2VP by solvation of the PVPh. The enthalphic gain from Hbond formation attracts the nanoparticles to the P2VP phase of microphase-separated PS-b-P2VP diblock copolymer domains. However the enthalphic penalty due to mixing of PS components of the polymer shell of the nanoparticles with the P2VP phase drives the nanoparticles to the PS phase. Therefore, the number ratio of styrene and phenol repeating units on the ligand determines the segregation location of nanoparticles in PS-b-P2VP domains as well as the solubility of nanoparticles in nonpolar solvents. To broadly define an optimum ratio, we synthesized two PS-r-PAS-RAFT polymers with different numbers of repeating units; PS22-r-PAS3-RAFT and PS20-r-PAS7-RAFT as shown in Table 1 (see Supporting Information, Figure S1, for H NMR spectra of each polymer). The hydrazinolysis of the random copolymers was monitored by H NMR (Figure 1). Figure 1a shows the H NMR spectrum of the PS22-r-PAS3-RAFT (CDCl3) with the aromatic protons of the dithioester end group at a chemical shift of 7.8 ppm and methyl protons of the acetoxy groups at 2.3 ppm (marked by the inverse triangle), respectively. After hydrazinolysis, these two peaks disappeared and a new broad peak at 9.0 ppm, corresponding to the phenolic protons, was observed (marked by the inverse triangle, Figure 1b, DMSOd6). This observation indicates that both the reduction of the dithioester group to a secondary thiol and hydrolysis of Scheme 1. Schematic Illustration of Synthesis of Thiol-Terminated Poly(styrene-ran-vinyl phenol) Copolymer Ligand This was performed by hydrazinolysis of poly(styrene-ran-acetoxystyrene) obtained by reversible addition fragmentation chain transfer (RAFT) polymerization and hydrogen bond formation between hydroxyl groups in ligands coated on Au nanoparticles and pyridine groups on poly(styreneb-2-vinylpyridine) diblock copolymer. The hydrolysis of acetoxy groups and the reduction of dithio-ester groups were carried out simultaneously by addition of hydrazine with tetrahydrofurane (THF) as a solvent. Gold nanoparticles were synthesized by reduction of chloroauric acid trihydrate with superhydride (Li(C2H5)3BH) in the presence of random copolymer ligands dissolved in THF. Table 1. Characterization of Poly(styrene-r-acetoxy styrene) Synthesized by RAFT Polymerization