Phase Behavior of Star-Shaped Polystyrene-block-poly (methyl

Jul 22, 2014 - ABSTRACT: The phase behavior of 18-arm star-shaped polystyrene-block-poly(methyl methacrylate) copolymers. ((PS-b-PMMA)18) with ...
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Phase Behavior of Star-Shaped Polystyrene-block-poly(methyl methacrylate) Copolymers Sangshin Jang, Hong Chul Moon, Jongheon Kwak, Dusik Bae, Youngmin Lee, and Jin Kon Kim* National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea

Won Bo Lee Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Korea S Supporting Information *

ABSTRACT: The phase behavior of 18-arm star-shaped polystyrene-block-poly(methyl methacrylate) copolymers ((PS-b-PMMA)18) with various volume fractions of PMMA block ( f PMMA) was investigated by transmission electron microscopy and small-angle X-ray scattering. (PS-b-PMMA)18 was synthesized by atom transfer radical polymerization from α-cyclodextrin (α-CD) having 18 functional groups for the initiation. We also prepared the corresponding linear PS-bPMMAs by cutting the ester groups connecting α-CD and PS chains in (PS-b-PMMA)18 through the hydrolysis. The microdomains of (PS-b-PMMA)18 changed from body-centered-cubic spheres (BCC), hexagonally packed cylinders (HEX), perforated lamellae (PL), and lamellae (LAM), with increasing f PMMA from 0.3 to 0.8. Interestingly, (PS-b-PMMA)18 with f PMMA of 0.77 showed highly asymmetric lamellar microdomains, while the corresponding linear PS-b-PMMA with the same volume fraction should not have lamellar microdomains. Thus, the microdomains are highly affected by the molecular architecture of block copolymer. The experimental results are discussed with the prediction based on the self-consistent mean-field theory.

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INTRODUCTION Block copolymers have been extensively investigated due to their diverse self-assembled structures such as spheres, cylinders, gyroids, and lamellae depending on the volume fraction of one of the blocks, the degree of polymerization (N), the Flory−Huggins interaction parameter (χ), and the molecular architecture of the block copolymers.1−4 Some research groups have investigated the effect of the molecular shape, for instance, linear versus nonlinear types, on block copolymer microstructures, while the volume fractions of the blocks are maintained.5−8 Tselikas et al. showed that (polyisoprene)2-b-polystyrene (I2S), (polyisoprene)3-b-polystyrene (I3S), and polyisoprene-b-(polystyrene)2 (IS2) miktoarm copolymers exhibited bicontinuous cubic, cylindrical, and lamellar microdomains, respectively, even though all miktoarm copolymers had the same volume fraction of PS ( f PS = 0.5).5,6 It is well-known that a symmetric linear block copolymer should show lamellar microdomains. Thomas and co-workers7,8 reported the effect of number of arms (n) on the morphology of star-shaped polystyrene-b-polyisoprene copolymers ((PS-bPI)n) with a fixed volume fraction of PS (f PS = 0.3). While the hexagonally packed cylindrical microdomain was observed for the linear block copolymer (n = 1), (PS-b-PI)n with f PS = 0.3 showed the gyroid structure when n is larger than 8. Also, Hadjuk et al. reported thermoreversible transition from gyroid © 2014 American Chemical Society

to hexagonally packed cylinders of (PS-b-PI)6 with f PS = 0.27 by increasing temperature from 120 to 150 °C.9 Gido and coworkers10−12 also investigated morphologies of (polyisoprene)m(polystyrene)n miktoarm copolymers depending on the molecular architecture. Recently, star-shaped multiarm block copolymers with n > 10 have gained a strong interest because they can form stable unimolecular micelle.13−17 The stability of conventional micelles prepared by linear block copolymers strongly depends on the external conditions such as solvent, temperature, concentration, and pH.18,19 On the other hand, once the unimolecular micelles are formed from multiarm block copolymer, they are very stable even at harsh environmental conditions because all of the arms forming the micelles are covalently linked into the single core.20 Pang et al. synthesized 21-arm star-shaped poly(acrylic acid)-b-polystyrene (PAA-bPS)21. After fabricating unimolecular micelle, they prepared several monodisperse metal, magnetic, and semiconducting nanocrystals with core−shell and hollow nanostructures.21 Also, the effect of the molecular geometry of star-shaped poly(acrylic acid)-b-poly(methyl methacrylate) copolymer micelles on the Received: March 21, 2014 Revised: June 30, 2014 Published: July 22, 2014 5295

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Scheme 1. Synthetic Routes for (PS-b-PMMA)18 by ATRP with α-CDa

a

The lower panel represents the molecular architecture of each step (the core consists of PS chains (red), while the shell consists of PMMA chains (blue)).

polarity of the solvent was investegated.22 Matsen and coworkers predicted phase behavior of star-shaped copolymers by self-consistent field theory (SCFT) simulation and showed shifted phase boundaries and expanded O70 regions of starshaped copolymer comparing with diblock copolymer.23,24 However, the experimental investigation of phase behavior or microstructures of multiarm block copolymers with n > 10 in bulk state is very limited.25,26 In this study, we synthesized star-shaped PS-b-PMMA having 18 arms ((PS-b-PMMA)18) with various volume fractions of PMMA block ( f PMMA) by atom transfer radical polymerization (ATRP) from α-cyclodextrin (α-CD) having 18 functional groups for the initiation. We also prepared the corresponding linear PS-b-PMMAs by cutting the ester groups connecting αCD and PS chains in (PS-b-PMMA)18 through the hydrolysis. The microdomains of (PS-b-PMMA)18 investigated by smallangle X-ray scattering (SAXS) and transmission electron microscopy (TEM) changed from body-centered-cubic spheres, hexagonally packed cylinders, perforated lamellae, and lamellae with increasing f PMMA from 0.3 to 0.8. Interestingly, (PS-bPMMA)18 with f PMMA of 0.77 showed highly asymmetric lamellar microdomains, while the corresponding linear PS-bPMMA with the same volume fraction should not have lamellar microdomains. The asymmetric lamellar structure (or line pattern) can extend the limitation of conventional block copolymer lithography because linear-type block copolymer gives only symmetric (or nearly symmetric) line patterns.

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sodium bicarbonate (NaHCO3) solution, and deionized (DI) water. Purified organic phase was dried with MgSO4, and the solvent was removed under reduced pressure. The product was precipitated in hexane, giving a yellow solid (1.46 g, 41% yield). We confirmed via nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) that all of the hydroxy groups in α-CD are replaced by bromine (see Figure S1 of the Supporting Information). For instance, the peaks corresponding to hydroxy groups at 4.4−4.5 and 5.6−5.8 ppm were not seen in NMR spectra, while NMR spectra near 2 ppm corresponding to the hydrogen attached to bromine were distinctly shown (Figure S1a). Also, the MALDI-TOF spectrum (Figure S1b) shows a peak at 3680 g/mol, which is the exactly same as that of Br-CD when all of 18 hydroxy groups on α-CD (M = 972 g/mol) are completely brominated. The molecular weight difference between the highest and the second highest peak is 80 g/mol, corresponding to HBr which was removed during the each ionization step in MALD-TOF experiment. Synthesis of Star-Shaped 18-Arm Polystyrene ((PS-Br)18) (3). Star-shaped polystyrene (PS-Br)18 was synthesized from the Br-CD macroinitiator. Br-CD (21.4 mg, 0.1 mmol), copper bromide(I) (CuBr) (14 mg, 0.1 mmol), and 2,2′-bipyridyl (31 mg, 0.2 mmol) were placed in an ampule. Distilled styrene (5 g, 50 mmol) was added to the ampule and degassed by three freeze−thaw cycles. The reaction was carried out at 90 °C for 12 h. The molecular weight of polystyrene (PS) block was controlled by reaction time. After the reaction, the ampule was dipped in liquid nitrogen to terminate the reaction. The reacted solution was diluted with tetrahydrofuran (THF), passed through a neutral alumina column to remove the catalyst, and then precipitated in methanol. The product was fully dried in a vacuum at 80 °C for 24 h. Synthesis of Star-Shaped 18-Arm PS-b-PMMA: (PS-bPMMA)18 (4). (PS-Br)18 was employed as a macroinitiator for (PSb-PMMA)18. The molar ratio of methyl methacrylate (MMA):(PSBr)18:CuBr:N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) was 350:1:1:2. The mixture was placed in an ampule, and anisole was added as a solvent (1 mL of MMA in 1 mL of solvent). The ampule was degassed by three freeze−thaw cycles and then placed in oil bath at 90 °C. The molecular weight of PMMA block was controlled by reaction time. The final solution was passed through a neutral alumina column to remove catalyst and precipitated in methanol. Finally, the product was dried in a vacuum at 80 °C for 24 h. Linear PS-b-PMMA. To analyze the block composition, the molecular weight of the arms consisting of (PS-b-PMMA)18, the ester groups located between cyclodextrin and PS block were cut by hydrolysis in the basic condition.28 (PS-b-PMMA)18 solution in THF (0.1 g in 10 mL) and KOH solution in methanol (0.2 g in 6 mL) were mixed and refluxed for 24 h. Hydrolyzed solution was precipitated in methanol and then neutralized by adding 10 mL of HCl. Precipitated

EXPERIMENTAL SECTION

Styrene (Aldrich, 99%) and methyl methacrylate (Aldrich, 99%) were distilled under reduced pressure. Copper(I) bromide (Aldrich, 98+%) was purified by washing with acetic acid, ethanol, and diethyl ether sequentially. After filtering, copper(I) bromide was stored in a vacuum. 2,2′-Bipyridyl, N,N,N′,N″,N″-pentamethyldiethylenetriamine, α-bromoisobutyryl bromide, α-cyclodextrin, and 4-(N,N-dimethylamino)pyridine were used as received from Aldrich. The synthetic scheme of (PS-b-PMMA)18 is given in Scheme 1. Synthesis of Br-CD (2). For ATRP, all of the hydroxy groups on α-CD (1) were replaced by bromine groups through the esterification with 2-bromoisobutyryl bromide (BIBB).27 First, α-CD (0.972 g, 1 mmol) was completely dried in a vacuum at 80 °C for 2 h and suspended in the mixture of pyridine (8 mL) and chloroform (14 mL), in which 4-(N,N-dimethylamino)pyridine was also added as a catalyst. Then, BIBB (6.2 mL, 50 mmol) was added dropwise to the suspension in an ice bath. The mixture was stirred for 24 h at room temperature. The reacted mixture was filtered to remove pyridinium bromide and then sequentially washed with 1 N hydrochloric acid (HCl), saturated 5296

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product was filtered and dried in a vacuum at 80 °C for 24 h to remove solvent. To check whether all of PMMA chains were connected to PS chains, the dried product (50 mg) was put into cyclohexane (10 mL) to remove any homo PS blocks without initiating for the polymerization of MMA. This solution was stirred for 12 h, filtered, and dried in a vacuum at 80 °C for 24 h. Molecular Characterization. The absolute molecular weight of Br-CD was measured by MALDI-TOF mass spectrometer (Bruker REFLEX III) operating at an accelerating voltage of 20 kV. The matrix used for all samples was dithranol without adding any salt, and the standards kit (calibration mixture 2: Applied Biosystems) was used for the calibration. Molecular weight and polydispersity index (PDI) of all polymers were also measured by size exclusion chromatography (SEC: Waters 2414 refractive index detector) with two 300 mm (length) × 7.5 mm (inner diameter) columns including particle size of 5 μm (PLgel 5 μm MIXED-C: Polymer Laboratories) with THF as an eluent and a flow rate of 1 mL/min at 30 °C. Volume fraction and molecular weight of (PS-b-PMMA)18 and the corresponding linear PS-b-PMMA prepared by cutting the ester groups in (PS-b-PMMA)18 were obtained by 1H NMR (Bruker digital Avance III). Chloroform-d (CDCl3) was used as solvent. The molecular characteristics of all the samples are summarized in Table 1.

RuO4 vapor for 5 min at room temperature. The micrographs were taken at room temperature by bright-field TEM (S-7600 Hitachi Ltd.) at 80 kV.

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RESULTS AND DISCUSSION Except BH5, all (PS-b-PMMA)18 (BH1−BH4) were synthesized from the same macroinitiator (PS-Br)18-A whose PDI is 1.07. Thus, the molecular weight of PS core for BH1−BH4 is the same. Since the absolute molecular weight of star-shaped polymers is much different from that measured by SEC based on PS standards,29,30 we measured the absolute molecular weight of (PS-Br)18-A by SEC combined with multiangle light scattering (SEC-MALS), and it was determined to be 213 000 (see Figure S2 of Supporting Information). As expected, this is much higher than that (85 000) measured by SEC based on PS standard. When we cut the ester groups linking α-CD and PS chains, the corresponding linear PS polymer (Cut PS-A) was 12 000. This is very similar to 1/18 of the molecular weight of (PS-Br)18-A measured by SEC-MALS. To check the number of PMMA chains in (PS-b-PMMA)18, the ester group linking α-CD and PS chains was hydrolyzed by KOH (see section 3 of the Supporting Information). Figure S3b gives the SEC trace of the cut arm (Cut BH3), from which two peaks are clearly seen. Since the peak position corresponding to the lower molecular weight was the same as that of the Cut PS-A macroinitiator, we concluded that unreacted homo PS arms existed in (PS-b-PMMA)18. The peak ratio of the higher and lower molecular weight in the SEC trace was 0.75:0.25. When we removed this unreacted PS homopolymer by cyclohexane, SEC trace showed only one peak. From NMR spectra of Cut BH3 before and after cyclohexane treatment (Figure S3c,d), the unreacted PS homopolymer in Cut BH3 was 25 vol %, which is consistent with SEC results. We also obtained other samples and found that the unreacted homo PS arms exist ∼25 vol % in all (PS-bPMMA)18 samples. The reason why the PMMA chains are not attached to all of the PS arms might be due to steric effect because of 18 polymer arms which are fixed within a short distance (less than 1 nm). Thus, all the initiation sites for MMA polymerization could not be fully exposed. Figure 1 gives SAXS profile and TEM image of BH1 with f PMMA = 0.38. Sample was annealed at 220 °C for 24 h in a vacuum, followed by rapid quenching to liquid nitrogen. SAXS profile shows higher order peaks at √4q* and √7q*, indicating that BH1 has hexagonally packed (HEX) cylindrical microdomains. TEM image also shows HEX cylinders of PMMA chains. Dark and bright regions in the TEM image correspond to PS and PMMA phases, respectively, because PS phase was selectively stained by RuO4. Although the crosssectional view of the cylinders shows good HEX ordering, longitudinal cylinders are poorly ordered (inset of Figure 1b). This suggests that the formation of HEX cylinders by starshaped copolymers might be different from that by linear block copolymers. We found that linear PS-b-PMMA (Cut BH1) from cutting BH1 does not show any microdomain because of low molecular weight of Cut BH1. This suggests that when linear block copolymers without showing any microdomain are combined and become star-shaped copolymer, they could form microdomains. Figure 2 gives the SAXS profile and TEM image of BH2 with f PMMA = 0.40 and Cut BH2 and Cut BH2*. Figure 2a clearly shows that BH2 has perforated lamellar microdomains due to the appearance of a peak at 1.08q* (inset of Figure 2a) in

Table 1. Molecular Characteristics of (PS-b-PMMA)18 Employed in This Study sample code

Mn,SECa

Mn,NMRb

Mw/Mna

f PMMAb

morphologyc

(PS-Br)18-A (PS-Br)18-B Cut PS-A Cut PS-B BH1

85 000 190 000 12 000 18 500 146 000

315 000

1.07 1.07 1.22 1.27 1.17

0.38

BH2

152 000

324 000

1.18

0.40

BH3 BH4

183 000 260 000

365 000 758 000

1.17 1.37

0.48 0.77

BH5

274 000

452 000

1.11

0.30

18 000 20 000 25 000 68 000 26 000

20 000 22 000 24 000 67 000 26 000

1.40 1.42 1.51 1.53 1.31

0.44 0.46 0.52 0.82 0.34

PMMA HEX cylinders perforated lamellae lamellae asymmetric lamellae PMMA BCC spheres disordered state lamellae lamellae PS spheres disordered state

Cut Cut Cut Cut Cut

BH1*d BH2*d BH3*d BH4*d BH5*d

a Determined by SEC based on PS standards. bMeasured by 1H NMR spectra (density of PS (dPS) = 1.05 g/cm3, density of PMMA (dPMMA) = 1.18 g/cm3). cDetermined by SAXS and TEM. dCut linear PS-bPMMAs after the removal of unreacted PS homopolymer by cyclohexane. The f PMMA in Cut block copolymer is larger than that in corresponding (PS-b-PMMA)18.

Small-Angle X-ray Scattering (SAXS). SAXS experiments were performed at room temperature on beamline 4C at the Pohang Accelerator Laboratory (PAL) (Korea), where a W/B4C double multilayer delivered monochromatic X-rays on the samples with a wavelength of 0.1608 nm. The sample-to-detector distance was 3 m. A 2-D CCD camera (Princeton Instruments, SCX-TE/CCD-1242) was used to collect the scattered X-rays. Sample was annealed at 220 °C for 24 h in a vacuum, followed by rapid quenching to liquid nitrogen. Thickness of the sample was 1.0 mm, and the exposure time was 10 s. Transmission Electron Microscopy (TEM). To observe the TEM image, a sample was first annealed at 220 °C for 24 h in a vacuum followed by rapid quenching to liquid nitrogen. Then, an ultrathin section of the sample was prepared by using a Leica Ultracut Microtome (EM UC6 Leica Ltd.) at room temperature with thickness of ∼40 nm. The PS domains of specimens were stained by exposure to 5297

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Figure 1. (a) SAXS profile at room temperature and (b) TEM image of cross-sectional view of cylinders in BH1 with f PMMA = 0.38. The inset is TEM image corresponding to longitudinal cylinders.

Figure 2. (a) SAXS profile of BH2 with f PMMA = 0.40. The inset is the enlarged first SAXS peak. (b) TEM images of BH2. The upper and lower images in the left panel show the microdomains parallel and normal to the perforated layer, respectively. The right image shows microdomains parallel to the layer without having perforations. (c) SAXS profile and TEM image (inset) of Cut BH2. (d) SAXS profile and TEM image of Cut BH2* after removing unreacted PS homopolymer by cyclohexane treatment.

addition to 2q*.31 Two TEM images as shown in the left panel of Figure 2b show the microdomains parallel and normal to the perforated layer, and parallel to the layer without having perforation is given in the right panel. To investigate the effect of molecular architecture on microdomain morphologies, SAXS profile and TEM image of Cut BH2 are obtained (Figure 2c). SAXS profile of Cut BH2 shows a weak peak at 3q*, indicating that it has lamellar microdomains. BH2 has 25 vol % of unreacted PS chains; thus it is mixture of 75:25 (v/v) linear PS-b-PMMA and PS homopolymer. When the unreacted PS homopolymers are completely removed by cyclohexane (Cut BH2*), lamellar microdomains are clearly observed as shown in Figure 2d. Cut BH2* without having PS homopolymer has symmetric volume fraction (f PMMA = 0.46). From the results, the molecular architecture in block copolymer significantly affects the microdomain formation. Perforated lamellar microdomains

were prepared by star-shaped copolymer whose arm block copolymer generates lamellar microdomains. Figures 3a,b gives SAXS profile and TEM image of BH3 with f PMMA = 0.48. A peak at 3q* in SAXS profile was observed, indicating that BH3 has lamellar microdomains. Lamellar microdomains are also observed in TEM image (Figure 3b). Lamellar microdomains were also observed for Cut BH3 as well as Cut BH3* because volume fraction of arms were very symmetric (f PMMA = 0.52) (Figure 3c,d). Figure 4a,b gives SAXS profile and TEM image of BH4 with f PMMA = 0.77. From SAXS profile having 2q*, BH4 shows lamellar microdomains. This result is very interesting because BH4 is highly asymmetric (f PMMA = 0.77). Also, the lamellar width corresponding to PMMA (white region) is much larger than that corresponding to PS (dark region). Namely, highly asymmetric lamellar microdomains are formed. This kind of highly asymmetric lamellae could not be expected for 5298

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Figure 3. (a) SAXS profile (the inset is the enlarged SAXS profile near 3q*) and (b) TEM image of BH3 with f PMMA = 0.48. (c) SAXS profile and TEM image of Cut BH3. (d) SAXS profile and TEM image of Cut BH3* after cyclohexane treatment.

Figure 4. (a) SAXS profile and (b) TEM image of BH4 with f PMMA = 0.77. (c) SAXS profile and TEM image of Cut BH4. (d) SAXS profile and TEM image of Cut BH4* after cyclohexane treatment.

conventional linear block copolymers at f PMMA = 0.77. Han et al.32 reported that highly asymmetric lamellar microdomains were formed in binary blends of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) and polystyrene-b-poly(4-hydroxystyrene) (PS-b-PHS). In this case, hydrogen bonding interaction between P2VP and PHS is crucial role for the formation of

asymmetric lamellar structure. In this study, the molecular architecture of star-shaped PS-b-PMMA caused asymmetric lamellar microdomains. To investigate the morphologies of the arms consisting of star-shaped copolymers, linear PS-b-PMMA was obtained by cutting (Figure 4c). After removing unreacted PS chains, Cut BH4* shows spherical microdomains (Figure 5299

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Figure 5. (a) SAXS profile and (b) TEM image of BH5 with f PMMA = 0.30.

4d). This is quite expected because of the highly asymmetric volume fraction (f PMMA = 0.82) in Cut BH4*. Finally, we investigated the microdomain structure of BH5. When we used the same (PS-Br)18-A, (PS-b-PMMA)18 with f PMMA = 0.30 did not show any microdomains (see Figure S4 of Supporting Information). Thus, to obtain the microdomain structures, we increased the molecular weight of PS macroinitiator to 190 000 (namely, (PS-Br)18-B). Figure 5a,b gives SAXS profile and TEM image of BH5. Because of the existence of SAXS peaks at √3q* and √6q*, BH5 shows BCC spherical microdomains. From the TEM image, PMMA chains become spherical microdomains. Figure 6 shows schematic drawings of the formation of various microdomains in star-shaped copolymer based on the molecular point of view. Because all the arms are covalently linked to the core, building unit of morphologies is one star molecule instead of copolymer chain. If the length of shell chains of star molecule is very short compared with that of core chains, aggregation of the former chains in one star molecule does not easily occur. On the other hand, the strong aggregation of shell chains is expected for very long shell chains. The formation of spherical microdomains in BH5 with f PMMA = 0.30 could be explained by Figure 6a. In this case, we consider that the number of PMMA aggregation per one star molecule is 6 (±x, ±y, ±z axis) because the length of PMMA chain is too short. Then, one PMMA sphere in BCC lattice is formed by the assembly of aggregated parts originating from adjacent six star molecules. Next, we consider the formation of PMMA HEX cylinders for BH1 with f PMMA = 0.38. Because the PMMA chain length in the shells is larger than that in BH5, aggregation power is increased. Thus, the number of PMMA aggregation per one star molecule is decreased to 3, as shown in Figure 6b. One cylinder is composed of neighboring three star molecules coming from three different directions. However, we observed poor array of cylinders along the longitudinal direction (inset of Figure 1b). This suggests that the formation of cylindrical microdomains in star-shaped copolymer is different from that in linear block copolymer. A relatively poor ordering of cylinders along the longitudinal direction is because adjacent layers are not perfectly matched each other. When green circles in Figure 6b are the centers of star molecules at the first layer, the centers of star molecules at the second layer should be located at red circles, that is, the interstitial site of neighboring three star molecules in the first layer. This is because the interstitial site could not be occupied entirely by PS chains. However, different locations of the centers of star molecules do

Figure 6. Mechanisms of the formation of the microdomains in (PS-bPMMA)18: (a) spheres (BH5), (b) cylinders (BH1), (c) lamellae (BH3), and (d) asymmetric lamellae (BH4).

not affect the HEX ordering in one plane, which gives a good lateral ordering of cylinders (see Figure 1b). Finally, we consider the lamellae formation which was observed for 0.48 < f PMMA < 0.77. The self-assembly for symmetric composition could be explained by the scheme as shown in Figure 6c. Because of the further increased PMMA length in the shell, the aggregation power is stronger compared with BH1. Thus, we consider that star molecules are aggregated into two directions (upper and lower directions). Then, side by side assembly is only allowed. For highly asymmetric composition ( f PMMA = 0.77), there might be two possibilities: highly asymmetric lamellar microdomains and conventionally observed microdomains (cylinders or spheres) consisting of PS 5300

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chains (Figure 6d). As shown in Figure 4, asymmetric lamellar microdomains were observed for BH4 (f PMMA = 0.77). This is caused by stretching effect of core PS chains because the core is very crowded by high number (n = 18) of arms and unreacted PMMA part. In this situation, in order for PS cylindrical (or spherical) microdomains to form in star-shaped copolymer, PMMA chains in the shell should be greatly stretched to cover stretched PS chains. However, 25% of arms do not have the PMMA chains, which limits the covering of PS chains. Because of a significant entropy loss in PMMA chains arising from the huge stretching of PMMA chains, asymmetric lamellar microdomains formed by the self-assembling of each molecule side by side would be more favorable than the formation of PS cylindrical (or spherical) microdomains. In this situation, the homo PS arms without having PMMA chains could act as the linkage of neighboring star molecules by covering vacant space between neighboring star molecules. Experimentally, we could not synthesize fully initiated (100%) 18-arm PS-b-PMMA without having any PS homopolymer chains in this system for the polymerization sequence of PS and PMMA. Thus, to investigate the difference of phase boundary depending on f PMMA between fully initiated (100%) and partially initiated (75%) arms, we used self-consistent field theory (SCFT), and the results are shown in Figure S5. As a result, volume fraction corresponding to the lamellar microdomains was expanded for (PS-b-PMMA)18 with partially initiated (75%) arms comparing with that with fully initiated (100%) arms.

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CONCLUSION

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax (+) 82-54-279-8298 (J.K.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (2013R1A3A2042196). SAXS experiment was performed at the beam line 4C of the Pohang Accelerator Laboratory (PAL). W.B.L. acknowledges the support by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by by the Ministry of Education (2012R1A1A1042214).

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REFERENCES

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In this study, we synthesized (PS-b-PMMA)18 by ATRP with fixed molecular weight of PS block. We measured actual efficiency of second polymerization of star-shaped copolymer synthesized by ATRP. A shift of the phase diagram of (PS-bPMMA)18 was observed compared with that of linear diblock copolymer. We observed spherical microdomains at f PMMA = 0.3 and cylindrical microdomains at f PMMA = 0.38. For f PMMA between 0.48 and 0.77, lamellar microdomains were observed. Interestingly, we observed highly asymmetric lamellar microdomains at f PMMA = 0.77, which is a very asymmetric volume fraction. Also, we found different microdomains between starshaped copolymer (BH4) and its arm (linear block copolymer) (Cut BH4). The effect of molecular architecture of constituting polymer caused different morphology between (PS-bPMMA)18 and its arms (linear PS-b-PMMA).

S Supporting Information *

NMR spectra and MALDI-TOF result of brominated αcyclodextrin; SEC-MALS chromatogram for the (PS-Br)18-A; SEC chromatograms for Cut (PS-Br)18, Cut (PS-b-PMMA)18 before and after cyclohexane treatment; comparison of NMR spectra of (PS-b-PMMA)18 before and after cyclohexane treatment; SAXS and TEM data of (PS-Br)18-A-based (PS-bPMMA)18 with f PMMA = 0.30; phase diagrams of (PS-bPMMA)18 depending on PMMA volume fraction having fully initiated (100%) and partially initiated (75%) arms by SCFT. This material is available free of charge via the Internet at http://pubs.acs.org. 5301

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Macromolecules

Article

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