Coassembly of Linear Diblock Copolymer Chains and Homopolymer

Feb 22, 2018 - A combined computer simulation and experimental study on coassembly of poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene ...
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Article Cite This: Macromolecules 2018, 51, 1894−1904

Coassembly of Linear Diblock Copolymer Chains and Homopolymer Brushes on Silica Particles: A Combined Computer Simulation and Experimental Study Wangmeng Hou,† Yuan Feng,‡ Baohui Li,*,‡ and Hanying Zhao*,† College of Chemistry and Key Laboratory of Functional Polymer Materials of the Ministry of Education, and ‡School of Physics and Key Laboratory of Functional Polymer Materials of the Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China

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S Supporting Information *

ABSTRACT: A combined computer simulation and experimental study on coassembly of poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (PDMAEMA-b-PS) block copolymers and PS brushes on silica particles was performed. PS brushes on silica particles at two different grafting densities were prepared by the “grafting to” approach, and PDMAEMA-b-PS block copolymers with different molecular weights and compositions were synthesized by reversible addition−fragmentation chain transfer polymerization. In THF/methanol mixtures, block copolymer chains and PS brushes coassemble into surface micelles (s-micelles), with collapsed PS cores and PDMAEMA coronae. Meanwhile, block copolymer chains are able to self-assemble into block copolymer micelles (b-micelles). Computer simulation results and experimental results indicate that block copolymer concentration, PS and PDMAEMA block lengths, and PS grafting density exert significant influences on the coassembly process. In low BCP concentration regime, the average size of s-micelles increases with BCP concentration and keeps unchanged at high concentration. The PS block length has a significant influence on the size of s-micelles. The average size increases with an increase in PS block length. For a BCP with long solvophilic PDMAEMA block, it is energy favorable to self-assemble into b-micelles, but to coassemble into s-micelles. With an increase in PDMAEMA block length, the morphology of the s-micelles changes from wormlike/spherical structures to spherical structures and to smaller spherical structures. The average size of the s-micelles coassembled by PS brushes at a lower grafting density is smaller than those coassembled by PS brushes at a higher grafting density.

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self-consistent field calculations to determine the self-assembly behavior of the tethered BCP chains in a selective solvent.14 In a selective solvent, the BCP brushes associate into surface micelles (s-micelles), where the solvophobic blocks form the cores and the soluble blocks form the outer coronae of the micelles. Yin and co-workers reported a simulated annealing study on the morphology of BCP brushes in a selective solvent.15 In their study, multiple morphological transitions of the BCP brushes can be induced by varying block lengths as well as polymer grafting densities. Brittain, Zhao, and co-workers synthesized BCP brushes and studied nanopatterns formed by the tethered BCP chains.16−18 They prepared poly(styrene-block-methyl methacrylate) (PS-b-PMMA) brushes on the flat silicate substrates. Upon treatment of the BCP brushes with cyclohexane, a good solvent for PS and a precipitant for PMMA, PMMA blocks collapse from the solvent and PS blocks form shields around the PMMA aggregates.18 The average domain size of the surface-immobilized micelles is strongly dependent on the two block lengths.2 Shipp and co-workers synthesized

INTRODUCTION Polymer brushes refer to end-tethered polymer chains, which are anchored to the surfaces of materials at high grafting densities.1−8 Because of the high density, the polymer brushes are forced to stretch away from the surfaces, resulting in many new surface properties, including solvent-induced formation of nanoscale patterns, thermo- and pH-responsive surfaces, and nonbiofouling surfaces.9−13 Based on structure and chemical composition, polymer brushes can be divided into one-component homopolymer brushes and multicomponent polymer brushes including mixed homopolymer brushes, random copolymer brushes, and block copolymer (BCP) brushes.11 Multicomponent polymer brushes are able to make spontaneous chain organization under an external stimulus, creating stimuliresponsive surfaces on the matrices. During the past two decades, microphase separation and responsive properties of multicomponent polymer brushes, especially BCP brushes and mixed homopolymer brushes, have been intensively investigated both theoretically and experimentally. Previous theoretical14,15 and experimental studies16−18 on surface-tethered BCP brushes have demonstrated that BCP brushes can make nanosized phase separation under an external stimulus. Zhulina and co-workers used scaling arguments and © 2018 American Chemical Society

Received: November 21, 2017 Revised: February 6, 2018 Published: February 22, 2018 1894

DOI: 10.1021/acs.macromol.7b02461 Macromolecules 2018, 51, 1894−1904

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Macromolecules

concentration of the BCPs, and the block lengths of PDMAEMAb-PS on the morphologies of the s-micelles were investigated.

poly(styrene-block-butyl acrylate) brushes on the surfaces of clay layers. After treatment of the BCP brushes with a selective solvent, spherical s-micelles as well as wormlike aggregates were formed on the surfaces.19 Sun and co-workers prepared disulfidetethered poly(tert-butyl acrylate-block-styrene) (PtBA-b-PS) brushes on silica particles. In acetone, a good solvent for PtBA and a precipitant for PS, the BCP brushes self-assemble into s-micelles with collapsed PS in the cores and solvated PtBA in the coronae.20 Upon cleavage of the disulfides, cleaved s-micelles were obtained. Mixed homopolymer brushes, composed of two or more chemically different end-tethered homopolymers, are also able to make spontaneous chain organization in response to environmental stimulus.21 Marko and Witten theoretically predicted the formation of “rippled” phases and layered structures by symmetric mixed homopolymer brushes on a flat substrate.22,23 Wang and Müller used single-chain-in-mean-field simulations to study microphase separation of binary mixed homopolymer brushes in different solvents.24 At a large chain length asymmetry, mixed polymer brushes self-assemble into two-layered nanostructures in a nonselective solvent. Minko, Stamm, and co-workers synthesized PS/poly(2-vinylpyridine) (P2VP) mixed brushes on the surfaces of silicon wafers, and the mixed polymer brushes presented switching properties in response to environmental changes.21,25−27 Zhao and co-workers synthesized well-defined mixed brushes on Y-initiator-functionalized substrates by two-step polymerizations.28,29 Different morphologies formed by the mixed brushes, including isolated nanodomains and two-layered nanostructures, were observed.30 In this research, we are interested in the formation of nanopatterns coassembled by BCP chains and homopolymer brushes on silica particles. In a previous study, we demonstrated that “free” BCP chains and BCP brushes were able to coassemble into surface-immobilized micelles.31 Although some interesting results have been obtained, fundamental problems are left unsolved. In this research, a combined computer simulation and experimental study on the coassembly of poly(2-(dimethylamino)ethyl methacrylate)-block-polystyrene (PDMAEMA-b-PS) and PS brushes on silica particles was performed. PS brushes on silica particles (SiO2−PS) were prepared by the “grafting to” approach, and PS-b-PDMAEMA block copolymers were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization. SiO2−PS and PDMAEMA-b-PS were dispersed/ dissolved in THF, a good solvent for PS and PDMAEMA, and excess methanol, a good solvent for PDMAEMA and a precipitant for PS, was added into the solution. Driven by surface free energy minimization, PS brushes make coassembly with PDMAEMA-b-PS, resulting in the formation of s-micelles. Meanwhile, BCP micelles (b-micelles) self-assembled by BCP chains are formed in the solution. The coassembly process of SiO2−PS and PDMAEMA-b-PS is shown in Scheme 1. In this research, the effects of grafting density of PS brushes, the

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EXPERIMENTAL PART

Materials. Styrene (98%) was purchased from Tianjin Chemical Reagent Company. Before use, it was washed with 5% sodium hydroxide aqueous solution, dried over anhydrous magnesium sulfate, and distilled under reduced pressure. DMAEMA (99%) was purchased from Acros. Before use, it was purified by passing through basic alumina column (dichloromethane was used as the eluent), removing solvent on a rotary evaporator, and distilling under reduced pressure. 4-Cyanopentanoic acid dithiobenzoate (CPADB) was synthesized in this laboratory by using a method reported previously.32 2,2′-Azoisobutyronitrile (AIBN), purchased from Guo Yao Chemical Company, was purified by recrystallization from ethanol. Hexylamine (Aladdin, 99%), (3-mercaptopropyl)trimethoxysilane (Sigma-Aldrich, 95%), 2,2′-dipyridyl disulfide (Alfa Aesar, 98%), and tri-n-butylphosphine (Strem Chemicals Inc., 99%) were used as received. Spherical silica particles with an average size of 170 nm were prepared by using the Stöber method.33 Synthesis of PDMAEMA. PDMAEMA was synthesized by RAFT polymerization. A typical polymerization was described as follows. DMAEMA (6.45 mL, 38.2 mmol), AIBN (8.4 mg, 0.051 mmol), and chain transfer agent CPADB (100.0 mg, 0.358 mmol) were dissolved in 12 mL of 1,4-dioxane in a 50 mL Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 70 °C for 8 h and stopped by quenching the Schlenk flask in ice water. After concentrated on a rotary evaporator, PDMAEMA was precipitated in n-hexane and dried under reduced pressure. Synthesis of PDMAEMA-b-PS. PDMAEMA-b-PS block copolymers were prepared by RAFT polymerization using PDMAEMA as macromolecular chain transfer agents (macro-CTA). A typical polymerization was described as follows. Styrene (1.92 mL, 16.8 mmol), AIBN (0.49 mg, 0.0030 mmol), macro-CTA (0.2 g), and toluene (1.4 mL) were added into a Schlenk flask. The solution was degassed by three freeze−pump−thaw cycles. The polymerization was conducted at 90 °C for 15 h and stopped by quenching the solution in ice water. After being concentrated on a rotary evaporator, the BCP was precipitated in n-hexane and dried under reduced pressure. Synthesis of PS. PS was synthesized by RAFT polymerization. Styrene (15.2 mL, 133 mmol), AIBN (5.80 mg, 0.0354 mmol), CPADB (70.0 mg, 0.250 mmol), and toluene (10.0 mL) were mixed in a Schlenk flask. After three freeze−pump−thaw cycles, the polymerization was performed at 90 °C for 12 h. After the polymerization, PS was precipitated in methanol and dried under reduced pressure. Synthesis of Silica Particles with Pyridyl Disulfide Groups on the Surfaces (SiO2-ss-py). The method for the preparation of SiO2ss-py was the same as our previous research.34 (3-Mercaptopropyl)trimethoxysilane (1.0 mL, 5.1 mmol) and pyridine (1.0 mL, 12.3 mmol) were added dropwise to a suspension of silica particles (1.0 g) in anhydrous toluene (30.0 mL) under an argon atmosphere. The reaction was refluxed at 115 °C for 24 h. After centrifugation (10 000 rpm, 5 min), the silica particles were collected and washed with methanol. After drying at reduced pressure, thiol-modified silica particles (SiO2-SH) were collected. To prepare SiO2-ss-py, SiO2-SH were dispersed in methanol (10.0 mL) ultrasonically in a 25 mL Schlenk flask, and excess 2,2′-dipyridyl disulfide (1.12 g, 5.10 mmol) was added into the solution. After being degassed by two freeze−pump−thaw cycles, the disulfide exchange reaction was performed overnight. The pyridyl disulfide-functionalized silica particles were collected by centrifugation (10 000 rpm, 5 min), washed with methanol, and dried under reduced pressure. Synthesis of PS Brushes on Silica Particles (SiO2-PS). PS brushes on silica particles were synthesized by the “grafting to” method. SiO2-ss-py (0.30 g) and PS (0.15 g) were dispersed/dissolved in 3.0 mL of THF in a 10 mL Schlenk flask. Hexylamine (30 μL, 0.23 mmol) was added into the solution under an argon atmosphere. The mixture was degassed by two freeze−pump−thaw cycles, and the reaction was allowed to perform for 24 h. The silica particles with PS brushes were

Scheme 1. Schematic Representation for the Coassembly of PDMAEMA-b-PS Chains and PS Brushes on Silica Particles into Surface Micelles (s-Micelles) and Block Copolymer Micelles (b-Micelles)

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DOI: 10.1021/acs.macromol.7b02461 Macromolecules 2018, 51, 1894−1904

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Macromolecules collected by centrifugation (10 000 rpm, 5 min), washed with THF, and dried under reduced pressure. Coassembly of PDMAEMA-b-PS and SiO2-PS. A typical procedure for the coassembly process was described as follows. SiO2-PS (30.0 mg) and PDMAEMA-b-PS (4.0 mg) were dispersed/dissolved in 2 mL of THF under sonication. Methanol (14.0 mL) was added into the solution, and a stable emulsion was obtained. After centrifugation (7000 rpm, 2 min), silica particles were collected, and the BCP micelles self-assembled by the BCP chains were left in the solution. Characterization. 1H NMR spectra were collected in CDCl3 at 400 MHz with a Varian UNITY-plus 400 M apparatus at room temperature. Size exclusion chromatography (SEC) was carried out with degassed DMF as eluent at a flow rate of 1.0 mL/min on a system consisting of a Hitachi L-2130 HPLC pump, a Hitachi L-2350 column oven operated at 50 °C, three Shodex columns with 5000−5K, 400−0.5K, and 5−0.15K molecular ranges, and a Hitachi L-2490 refractive index detector. The apparent molecular weight was calculated based on poly(methyl methacrylate) (PMMA) standards. For transmission electron microscopy (TEM) imaging, a Tecnai G2 20 S-TWIN electron microscope equipped with a Model 794 CCD camera (512 × 512) was operated at an operating voltage of 200 kV. Scanning electron microscope (SEM) observations were carried on a Verios 460L ultrahighresolution SEM apparatus. UV−vis spectra were recorded on a Shimadzu 2450 UV−vis spectrophotometer at room temperature. The hydrodynamic diameters of the colloidal particles were measured on a Malvern Zetasizer Nano-ZS equipped with a 10 mW HeNe laser at a wavelength of 633 nm.

and the substrate prefers B-segments rather than A-segments; we hence set the pair interactions as εAB = 4.0, εAS = 2.2, εBS = −1.0, εAW = 2.0, and εBW = 0.0, and the others are εij = 0.0 with i, j = A, B, W, T, and S. Starting from the initial configuration, the ground state of the system is obtained by running a set of Monte Carlo simulations at decreasing temperatures. The trial moves used in the simulation is the exchanging movement between chain monomers and solvent molecules.40 At least 4.86 × 105 Monte Carlo steps (MCS) are performed at each annealing step, where one MCS is defined as the time taken for on average and all the lattice sites are visited for one trial move. Furthermore, for some systems, 9 × 105 MCS are performed at each annealing step for testing, and quantitatively identical results are obtained, indicating that the MCS used are adequate. When a HP/BCP blend system reaches an equilibrium, a part of BCP chains take part in the formation of s-micelles with HP chains on the substrate, while other self-assemble to form b-micelles in solution. The s-micelles are hemispherical or hemicylindrical, whereas the free micelles are spherical. When the s-micelles are hemispherical, the average radius of s-micelles, RSM, can be estimated by the equation R SM =

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⎛ 3 ⎞1/3 ⎜ ⎟ a{2[nHPNHP + nC(NA + NB)]/τ }1/3 ⎝ 4π ⎠

where nHP is the number of HP chains in the brush, nC is the average number of BCP chains involved in the formation of s-micelles, NHP, NA, and NB are the lengths of HP chains, A-block, and B-block of the BCP chains, respectively, τ is the number of hemispherical s-micelles on the substrate, and a is the unit length of the simulation box space, and a = 1 in our model.

SIMULATION DETAILS Our simulation was carried out in a simple cubic lattice box with volume V = Lx × Ly × Lz (Lx = Ly = Lz = L= 60) via the simulated annealing method.35,36 We used the single-bond fluctuation model.37−39 To be consistent with experiment, our model system consists of surface-grafted homopolymer A, “free” AB BCP chains, and selective solvent (S). The selective solvent S is good for B-block but poor for A-block and A-homopolymer. The repeating unit numbers of each homopolymer chain, A-block, and B-block of BCP chains are NHP, NA, and NB, respectively, and NHP is fixed at 11 in our simulations. The total repeating unit number of a BCP chain is NA + NB. The bond length of the polymers is set to be 1 and √2 so that each site has 18 nearest-neighbor sites. Each homopolymer chain is tethered on an impenetrable planar surface by its one end segment. The substrate occupies the z = 0 surface, and the lattice sites of the surface cannot be occupied by solvent or other chain segments except the first segment on each homopolymer chain. The distance between two neighboring grafted points is d, which is the “spacing” in both x and y directions. The grafting density is defined as σ = 1/d2. For simplicity, another impenetrable planar surface is placed on the z = Lz − 1 surface, and the lattice sites of this surface cannot be occupied by any solvent molecules or chain segments. Periodic boundary conditions are applied along the x and y directions. By defining the nBCP as the number of BCP chains introduced in solution, the concentration of BCPs can be calculated by CP = nBCP (NA + NB)/(L − 2)L2. The energy of the system is calculated by taking account of the pair interactions between different species on two nearestneighbor sites. An energy Eij = εijkBTref is assigned for the pair of components i and j, where εij is the reduced interaction energy, kB is the Boltzmann constant, Tref is a reference temperature, and i, j represent A, B, W, T, and S corresponding to A-segment, B-segment, grafting substrate, the top surface, and solvent, respectively. Because the selective solvent is poor for A-segments, but good for B-segments, A and B are incompatible,

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RESULTS AND DISCUSSION Synthesis of Block Copolymers and PS Brushes. PS homopolymer and PDMAEMA-b-PS BCPs were synthesized by RAFT polymerization. A summary of molecular parameters including number-average polymerization degrees, apparent molecular weights, and dispersities of the polymers is shown in Table 1. SEC curves of the polymers are shown in Figure S1. Table 1. Summary of Number-Average Degrees of Polymerization, Number-Average Molecular Weights, and Dispersities of PS and PDMAEMA-b-PS Block Copolymers sample PS112 PDMAEMA62-b-PS58 PDMAEMA35-b-PS106 PDMAEMA62-b-PS107 PDMAEMA62-b-PS164 PDMAEMA115-b-PS114

DPPDMAEMAa DPPSa 62 35 62 62 115

112 58 106 107 164 114

Mn (×103 g mol−1)b

PDIb

11.6 15.7 16.9 20.8 26.8 29.2

1.11 1.21 1.21 1.20 1.23 1.28

a

The number-average degrees of polymerization of PS and PDMAEMA blocks were determined by 1H NMR. bThe numberaverage molecular weights (Mns) and molecular weight dispersities (PDIs) were measured by SEC on PMMA standards.

Narrowly dispersed silica particles with an average size of around 170 nm were reacted with (3-mercaptopropyl)trimethoxysilane, and thiol-modified silica particles, SiO2-SH, were obtained. SiO2-ss-py were prepared by thiol−disulfide exchange reaction between SiO2-SH and 2,2′-dipyridyl disulfide. Upon cleavage of disulfides on SiO2-ss-py with tri-n-butylphosphine, pyridine2-thione, which has absorption at 375 nm in DMF solution 1896

DOI: 10.1021/acs.macromol.7b02461 Macromolecules 2018, 51, 1894−1904

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solutions (Scheme 1). After centrifugation, silica particles with s-micelles were collected, and b-micelles were left in the solutions. Silica particles with s-micelles were dispersed in deuterated chloroform, a good solvent for both of PS brushes and BCPs, and BCP chains in the s-micelles were redissolved in the solvent. After centrifugation, 1H NMR was used to determine the amounts of the BCPs in s-micelles. Based on 1H NMR results, BCPs on silica particles and in bulk solutions can be calculated. Figure 1a shows 1H NMR spectra of PDMAEMA62-b-PS107 released from the s-micelles formed at different BCP concentrations. In the 1H NMR measurements, 0.05% CH2Cl2 in deuterated chloroform was used as an internal standard, and all the spectra were normalized at 5.30 ppm, a peak corresponding to the protons on CH2Cl2. In the spectra, the peaks at 4.0 ppm (a) and 2.6 ppm (b) are assigned to the methylene protons neighboring ester group and nitrogen atom in DMAEMA repeating units, respectively, and the peaks at 2.3 ppm (c) are attributed to the methyl protons connecting to the nitrogen atoms. We can see that in the low polymer concentration regime the intensity of peak a increases with the concentration; however, the intensity keeps unchanged when the BCP concentration is above 0.26 mg/mL. This result indicates that at low concentrations BCP chains are involved in the formation of s-micelles; however, at high concentrations self-assembly of the BCP chains into b-micelles is dominant. The 1H NMR spectrum of PDMAEMA62-b-PS107 at a concentration of 4.2 mg/mL, as shown in Figure 1a, was used as a reference, and the amounts of PDMAEMA62-b-PS107 that participated in the coassembly with PS brushes were determined. Figure 1b shows the amounts of PDMAEMA62-b-PS107 in s-micelles and in bulk solutions at different BCP concentrations. Herein, we must point out that there is equilibrium between unimers and b-micelles in a bulk solution. The amount of unimer, which is dependent on the value of CAC, is kept unchanged at a polymer concentration above CAC. As determined by light scattering, the CAC of PDMAEMA62-b-PS107 in THF/methanol is about 0.065 mg/mL (Figure S4). When the concentration of the BCP is above CAC, BCP chains self-assemble into b-micelles. Figure 1b indicates that in the low polymer concentration regime (