Synthesis and Self-Assembly of Rod–Rod Block Copolymers with

Sep 30, 2013 - The self-assembling behaviors of the coil–coil and rod–coil BCPs have been .... The scattering vector q is defined as q = 4πsinθ/...
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Synthesis and Self-Assembly of Rod−Rod Block Copolymers with Different Rod Diameters Feng Zhou,† Tieying Ye,† Lingying Shi,† Chan Xie,†,‡ Shankui Chang,† Xinghe Fan,† and Zhihao Shen†,* †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ School of Material Science and Engineering, Nanchang Hangkong University, Nanchang, Jiangxi 330063, China S Supporting Information *

ABSTRACT: We synthesized a series of rod−rod block copolymers, poly(octyl-4′-(octyloxy)-2-vinylbiphenyl-4-carboxylate)b-poly(γ-benzyl-L-glutamate) (PVBP-b-PBLG) with different rod diameters, and we focused on the effect of size disparity on their self-assembling behaviors in bulk. By using differential scanning calorimetry, wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering, and transmission electron microscopy techniques, we found that the columnar nematic (ΦN) liquid crystalline phase of PVBP and the hexagonal columnar (ΦH) phase of PBLG were disrupted to different extents depending on the volume fractions of PVBP (f PVBP’s). When f PVBP is about 60%, the ΦH phase of PBLG is retained with the appearance of the two second-order diffractions in WAXD patterns. When f PVBP is increased to above 73%, the ordered structure of PBLG is destroyed, and when f PVBP is 91% even the first-order diffraction peak disappears. At the same time, the ΦN phase of PVBP changes from poorly ordered to well ordered with increasing f PVBP. Accordingly, with f PVBP increasing from 60% to 91%, the microphase-separated structure goes through a poorly ordered structure to an interdigitated lamellar morphology and to a bilayer lamellar structure.

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INTRODUCTION Block copolymer (BCP) is a polymer that comprises two (or more) homopolymer subunits that are covalently linked. The chemical differences between the two blocks lead to the microphase separation of the BCP, which forms various periodic structures.1,2 As the sizes of the periodic structures are on the nanometer scale, about 10−100 nm, the self-assembly of BCPs has tremendous applications in nanotechnology, such as photonic band gap materials, nanostructured networks, and nanolithographic templates.3−7 These applications encourage us to study the mechanism of the self-assembly of BCPs. According to the flexibilities of the two blocks, BCPs can be divided into coil− coil, rod−coil, and rod−rod types. The self-assembling behaviors of the coil−coil and rod−coil BCPs have been widely investigated during the past few decades.8,9 For coil−coil BCPs, phase structures are determined by the volume fraction f of the individual block and χN, where χ is the Flory−Huggins interaction parameter and N is the total degree of polymerization (DP).10 Compared with coil−coil BCPs, for rod−coil BCPs, the intramolecular interaction of the rod block is also critically important in determining the phase behaviors.11−15 Combining two rod blocks forms rod−rod BCPs. Borsali and co-workers predict that the microphase separation of rod−rod and rod−coil BCPs occurs at (χN)c = 8.2 and (χN)c = 8.5, respectively, where (χN)c is the critical value of χN, while that of linear coil−coil BCPs occurs at (χN)c = 10.5.16 The differences in the (χN)c values may be attributed to the increase © XXXX American Chemical Society

of incompatibility induced by the rigid block. However, there are still unresolved problems in the mechanism of the selfassembling behaviors of rod−rod BCPs. To construct rod−rod BCPs, at least four different types of rod structures, including conjugated polymers, poly(hexyl isocyanate) (PHIC) and its derivatives, polypeptides, and liquid crystalline polymers (LCPs), have been used.17 All-conjugated rod−rod BCPs, such as poly(3-hexylthiophene)-block-poly[3-(2-ethylhexyl)thiophene] (P3HT-b-P3EHT), have been synthesized,18−22 and through detailed study of their self-assembling behaviors, Scherf et al. pointed out that based on the rigid-rod structure of such rod-like building blocks, generally low-curvatured aggregates formed during their self-assembly, i.e., rod−rod BCPs tended to assemble into vesicular (dilute solution) or lamellar (concentrated solution or solid state) nanostructures.23 PHIC forms an α-helical conformation with a persistent length of 50−60 nm and can be used as a rod block.24,25 Banno et al. found that the self-assembly of the left-handed helical polyisocyanides on highly oriented pyrolytic graphite (HOPG) upon exposure to organic solvent vapors could form smectic-like twodimensional (2D) helix bundles, which had different geometries with respect to the molecular packing, head-to-head or head-totail, parallel or antiparallel, and interdigitated or bilayer on a Received: July 12, 2013 Revised: September 10, 2013

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substrate.26 Polypeptides, such as poly(γ-benzyl-L-glutamate) (PBLG), form an α-helix or a β-sheet conformation depending on molecular weight (MW) and temperature,27,28 and a great deal of research has been reported about their use as the rod blocks.29−32 LCPs are another kind of rigid structures as rod building blocks. Among the LCPs studied in the literature, mesogen-jacketed liquid crystalline polymers (MJLCPs),33,34 in which bulky pendants are considered to be densely packed around the backbone and force the backbone to adopt an extended conformation,35,36 have attracted increasing interest in recent years. The rod length of an MJLCP can be tuned by changing the DP, while the rod diameter can be altered by varying the chemical structure of the side chain.37 MJLCPs have already been used as rod blocks in synthesizing well-defined rod−coil diblock and triblock copolymers.15,38−41 Recently, Zhou et al. synthesized a rod−rod BCP, poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene}-block-poly(γ-benzyl-L-glutamate) (PMPCS-b-PBLG), based on an MJLCP and PBLG, and investigated their phase behaviors, and hierarchical selfassembling structures on the nanometer scale were obtained.42

According to the literature, phase structures of rod−rod BCPs are determined by the volume fraction f of the individual block, χN, and the packing schemes of the rod block, which is similar to rod−coil BCPs. However, compared with coil−coil and rod−coil BCPs, both blocks of rod−rod BCPs have relatively fixed shapes, such as cylindrical or flaky, due to their rigid nature, and we can easily tune the shapes and sizes of rod−rod BCPs through carefully designing the chemical structures of the two rod blocks. For example, we can design BCPs with different rod shapes or diameters. The influences of these intrinsic structural features on the self-assembling behaviors of rod−rod BCPs have not been explored. As a result, herein we designed and synthesized a series of rod−rod BCPs, poly(octyl4′-(octyloxy)-2-vinylbiphenyl-4-carboxylate)-block-poly(γ-benzyl-L-glutamate) (PVBP-b-PBLG), with different rod diameters. We focused on the effect of size disparity on the self-assembling behavior of these BCPs in bulk. When the two rods with different diameters are covalently coupled in one BCP, there will be competitions between the arrangement of the two rods, and such competitions will cause disruptions to the ordered structures of the two blocks; i.e., the arrangement of one rod will disturb the packing of the other. Accordingly, the interplay between the two rod blocks will certainly affect the microphaseseparated nanostructure, leading to the change in the phase diagram. By using differential scanning calorimetry (DSC), polarized light microscopy (PLM), one-dimensional wide-angle X-ray diffraction (1D WAXD), 2D wide-angle X-ray diffraction (2D WAXD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) techniques, we found that the existence of the rod blocks and the difference in rod diameters influenced both the liquid crystalline (LC) properties of the two blocks and the self-assembling behaviors of these rod− rod BCPs.

Scheme 1. Synthesis of PVBP-b-PBLG BCPs

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EXPERIMENTAL SECTION

Materials. N,N,N′,N″,N″-Pentamethyl diethylenetriamine (PMDETA, TCI, 98%) and trifluoroacetic acid (TFA, Acros, 99%) were used as received. N,N-Dimethylformamide (DMF, Beijing Chemical Reagents Co., A.R.) and tetrahydrofuran (THF, Beijing Chemical Reagents Co., A.R.) were used after being distilled. Chlorobenzene was washed with H2SO4, NaHCO3, and distilled water and then dried with anhydrous calcium chloride and finally distilled. CuBr (Beijing Chemical Reagents Co., A.R.) was purified by washing with acetic acid, followed by washing with methanol, and then dried for use. All other commercially available chemicals were used as received. Synthesis of PVBP-b-PBLG Rod−Rod BCPs. The details for the synthesis of the rod−rod BCPs are shown in the Supporting Information.

Table 1. Molecular Weights, f PVBP Values, and Calculated Molecular Lengths of the BCPs along with Nanostructural Characteristics from SAXS Results sample PVBP82-b-PBLG141 PVBP79-b-PBLG77 PVBP79-b-PBLG47 PVBP79-b-PBLG20

Mn(PVBP)a (×104 g/mol) Mn(PBLG)b (×104 g/mol) f PVBPc (%) LPVBPd (nm) LPBLGe (nm) 3.81 3.67 3.67 3.67

3.10 1.70 1.04 0.45

60 73 82 91

20.5 19.8 19.8 19.8

21.1 11.6 7.1 3.0

f PVBP*f (%) dg (nm) nanostructureg 61 71 80 90

60.4 41.9 32.0 53.7

poorly ordered LAMh LAM LAM

a Mn of PVBP obtained from the combination of GPC and MALDI−TOF MS. bMn of PBLG calculated on the basis of the Mn of PVBP from GPC and the ratio between the integrations of the peaks of PVBP and PBLG in 400 MHz 1H NMR. cVolume fraction of PVBP, calculated using the number-average degrees of polymerization of the PVBP segment and the PBLG block in combination with the following densities: ρPBLG = 1.28 g/cm3;42 ρPVBP = 1.02 g/cm3.43 dTheoretical molecular length of PVBP, calculated using the zigzag model. eTheoretical molecular length of PBLG, calculated on the basis of the 18/5 α-helical structure. fTheoretical volume fraction of PVBP, calculated using the theoretical molecular length multiplied by the area of the rod, VPVBP = LPVBP × (πdPVBP2/4), VPBLG = LPBLG × (πdPBLG2/4), and f PVBP = VPVBP/(VPVBP + VPBLG). gDetermined by SAXS. hLAM = lamellar structure.

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Characterization. The MWs of the BCPs were determined with the combination of gel permeation chromatography (GPC), 1H NMR, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI−TOF MS) measurements. GPC experiments were conducted on a Waters 2410 instrument equipped with a Waters 2410 RI detector, with THF or DMF in presence of LiBr (1.0 mg/mL) as eluent (1.0 mL/min). The calibration curve was obtained with linear polystyrenes as standards. 1H NMR spectra were obtained with a Bruker 400 MHz spectrometer. MALDI−TOF MS measurements were performed on a Bruker Autoflex high-resolution tandem mass spectrometer. Fourier transform infrared spectroscopy (FTIR) was carried out using a Thermo Scientific Nicolet iN10-MX spectrophotometer between 4000 and 1000 cm−1 at a spectral resolution of 4 cm−1, and the number of scans was 16. DSC examination was carried out on a TA Q100 DSC calorimeter in a nitrogen atmosphere. PLM observation was performed on a Nikon DS-Ri1 microscope with an Instec HCS302 hot stage. One-dimensional WAXD experiments were carried out on a Philips X’Pert Pro diffractometer with a 3 kW ceramic tube as the X-ray source (Cu Kα) and an X’celerator detector. Twodimensional WAXD patterns were obtained using a Bruker D8 Discover diffractometer with a GADDS as a 2D detector calibrated with silicon powder and silver behenate. The oriented films by mechanical shearing were mounted on the sample stage with the point-focused X-ray incident beam either parallel (X direction) or perpendicular (Y or Z direction) to the shear direction (X direction). SAXS experiments were carried out on a Bruker Nanostar SAXS instrument using Cu Kα radiation at a wavelength of 0.154 nm. The working voltage and current were 40 kV and 40 μA, respectively. The scattering vector q is defined as q = 4πsinθ/λ, where the scattering angle is 2θ, and the d-spacing (d) is given by 2π/q. The background scattering was recorded and subtracted from the sample patterns. TEM was used to investigate the microphase separation of samples on a Hitachi H-9000NAR electron microscope.

blocks, we failed to characterize the PDIs of the BCPs using GPC. However, from the PDIs of the PVBP and PBLG blocks, the PDI values of the BCPs should be in a reasonable range.

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RESULTS AND DISCUSSION Synthesis and Characterization of PVBP-b-PBLG Rod− Rod BCPs. As shown in Scheme 1, the rod−rod BCPs were synthesized by combining atom transfer radical polymerization (ATRP) of octyl-4′-(octyloxy)-2-vinylbiphenyl-4-carboxylate,43 ring-opening polymerization (ROP) of γ-benzyl-L-glutamate Ncarboxyanhydride,44 and a subsequent copper-catalyzed Huisgen’s 1,3-dipolar cycloaddition (click chemistry)45,46 to connect the two rigid-rod blocks, following the method previously developed.42 The details of the synthetic process are shown in the Supporting Information. Because the solubilities of the two homopolymers were different, we could not find a good solvent for both. Therefore, we chose the mixed solvent, DMF/THF, for the coupling reaction. Thin layer chromatography (TLC) was carried out to verify the formation of the BCPs (details are shown in Supporting Information). BCPs were purified for characterization after being washed with n-hexane and DMF. The MWs of the PVBP and PBLG homopolymers were determined with GPC, MALDI−TOF MS and 1H NMR. The data are summarized in Table 1. The number-average MW (Mn) of PVBP was calculated by combining the results from GPC and MALDI−TOF MS. The calculation details are shown in Supporting Information. The Mn of PBLG was calculated on the basis of the Mn of PVBP from GPC and the ratio between the integrations of the peaks of methyl- (0.7−1.0 ppm) of PVBP and benzyl- (5.0−5.2 ppm) of PBLG in 1H NMR. The 1 H NMR spectra (CDCl3/TFA with 15% TFA) of PVBP79, PBLG77, and PVBP79-b-PBLG77 are shown in Figure 1 as an example. The polydispersity index (PDI) of PVBP obtained from GPC with THF as the eluent is about 1.1, and that of PBLG with DMF (1.0 mg/mL LiBr) as the eluent is 1.2−1.5. As mentioned above, due to the lack of a good solvent for both

Figure 1. 1H NMR spectra of PVBP79, PBLG77, and PVBP79-b-PBLG77 in a CDCl3/TFA mixture with 15% of TFA. C

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the azide band in the spectra of the BCPs evidences the completion of the coupling reactions. Moreover, for PVBP79-bPBLG20 with the lowest MW of PBLG, the existence of amide I and amide II bands at 1655 and 1550 cm−1, respectively, and the absence of the amide I at 1636−1640 cm−1 and 1622−1632 cm−1 for the parallel and antiparallel β-sheets, respectively, (Figure S4b in Supporting Information) show that the PBLG block takes the α-helical secondary structure,44,47 which is in agreement with the conclusion in literature that the α-helical structure is preferred for PBLG with a DP larger than 19.29 Liquid Crystalline Properties of the Rod−Rod BCPs. As reported in the literature, the PVBP block exhibits a columnar nematic (ΦN) LC phase,43 and the PBLG block has a hexagonal columnar (ΦH) structure.29 We first obtained the 1D WAXD patterns of a blend of the two homopolymers, PVBP79/PBLG77 (1/1, w/w) (Figure 2). The sample film was prepared by solution-casting from a THF solution and then dried at ambient temperature. In Figure 2, the diffraction peak with 2θ = 4.71° (corresponding to a d-spacing of 1.87 nm) can be attributed to the ΦN phase of PVBP, and the diffraction peaks with 2θ values of 6.57°, 11.4°, and 13.2° having a scattering vector ratio of 1:√3:√4 originate from the ΦH phase of PBLG with a d100 value of 1.58 nm. In addition, there is an amorphous halo with a 2θ of ∼20° (corresponding to a d-spacing of about 0.48 nm). The characteristic reflections from each rod are clearly discerned in the 1D WAXD patterns, which means that neither

We also used FTIR to monitor the azide vibrational band for the completion of the click coupling reaction. The FTIR spectrum of azido-PBLG clearly reveals the presence of this vibrational band at 2098 cm−1, characteristic of azide moieties (Figure S4a in Supporting Information). The disappearance of

Figure 2. 1D WAXD patterns of the blend of PVBP79/PBLG77 (1/1, w/w) during heating from 40 to 200 °C and subsequent cooling to 40 °C at the rate of 5 °C/min.

Figure 3. 1D WAXD patterns of PVBP79-b-PBLG77 (a, heating; b, cooling) and PVBP82-b-PBLG141 (c, heating; d, cooling) during the first heating and first cooling. D

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Figure 4. 1D WAXD patterns of PVBP79-b-PBLG47 (a, heating; b, cooling) and PVBP79-b-PBLG20 (c, heating; d, cooling) during the first heating and first cooling.

PBLG block shows that in this BCP PBLG still has the ΦH structure, while the PVBP block is still in the ΦN LC phase. The 1D WAXD data of PVBP79-b-PBLG47 and PVBP79-bPBLG20, with f PVBP values of 82% and 91%, respectively, are shown in Figure 4. In these two BCPs, with decreasing DP of the PBLG block, the ΦH phase of PBLG becomes less ordered. For PVBP79-b-PBLG47, the peak from the PBLG segment appears at 2θ = 6.33°, and it is difficult to recognize the higher-order Bragg reflections. Moreover, for PVBP79-b-PBLG20 with the lowest DP of the PBLG block among all BCPs, even the first-order diffraction peak can hardly be recognized, which indicates that the ΦH phase of PBLG is more or less destroyed. According to the literature,27,28 usually PBLG with a low molar mass (DP ≤ 20) partially takes the β-sheet structure with increasing temperature, and the corresponding peak should be at ∼4.9°. However, the peak from the β-sheet structure of PBLG may be overlapped with the diffraction of PVBP (2θ at ∼4.7°), making it indiscernible. And for the PVBP block, with its volume fraction increasing, the ΦN phase of PVBP can gradually develop as evidenced by the sharp diffraction with a 2θ of 4.71° at high temperatures. In order to corroborate with the above result, we used PLM to observe the textures of the PVBP homopolymer and PVBP79-b-PBLG20. Sample films were cast from THF solutions, dried at ambient temperature, and then heated to 200 °C. The PLM micrographs taken at 200 °C are shown in Figure 5. Compared with the texture of the PVBP homopolymer, that of PVBP79-b-PBLG20 is quite different.

rod in its respective macrophase-separated domain interferes with the packing of the other in the blend. To study the effect of the rod diameter disparity, we examined the LC properties of the BCP samples. The 1D WAXD results of PVBP79-b-PBLG77 are shown in parts a and b of Figure 3. The diffraction peaks from the PVBP and PBLG blocks are not as sharp as those in Figure 2, which means that the ordered structures of the two blocks are disrupted to a certain degree. For the PVBP block, the sharp diffraction peak disappears, leaving an amorphous halo in the low-angle region. On the other hand, the ΦH phase of PBLG becomes less ordered, and it is difficult to recognize the higher-order Bragg reflections. DSC was also used to probe the thermal transitions of this BCP. Data are shown in Figure S5 of Supporting Information. For the PVBP79 homopolymer, the isotropic-toLC transition temperature (TLC) is about 170 °C, while in PVBP79-b-PBLG77, this transition disappears, consistent with the 1D WAXD results. On the other hand, for PBLG77, there is a thermal transition at 82 °C, which can be attributed to the 7/2 to 18/5 α-helical conformational transition of PBLG, and the transition shifts to 92 °C in the BCP, indicating that the thermal transition of the PBLG block is also affected by the PVBP block. The 1D WAXD profiles of PVBP82-b-PBLG141 with a f PVBP value of 60%, which is lower than that of PVBP79-bPBLG77, are displayed in parts c and d of Figure 3. The appearance of the two second-order Bragg reflections from the E

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However, we can still observe birefringence although the strength is weaker. This result proves that in PVBP79-b-PBLG20, the PVBP segment retains its ΦN LC phase because the lowMW PBLG does not have strong enough impact to disrupt its phase structure. The above results demonstrate that the competitions between the LC phases of the two covalently coupled rods with different diameters cause great disturbances to the ordered packing of the two blocks especially when they have high MWs. Because the above effect on the LC properties of the two segments is not significant in the rod−rod BCP with similar rod diameters, such as PMPCS-b-PBLG,42 in which the parallel alignment of the two rods does not cause packing problems for the LC phase, it suggests that the effect may be due to the difference in rod diameters. For the self-assembly of BCPs, the areas of the interfaces between the two segments greatly influence the phase diagrams.15 As for rod−rod BCPs containing rods with similar diameters, the interfacial areas between the two rods are similar, and the influence can be neglected. However, for rod−rod BCPs with different rod diameters, there is an imbalance between the interfacial areas of the two rods. This imbalance induces the disruption of the ordered structures, as shown by the results of the PVBP-bPBLG BCP samples. Nanostructures of the PVBP-b-PBLG Rod−Rod BCPs. The appearance of the reflections arising from both PVBP and PBLG blocks in 1D WAXD profiles indicates the microphase separation of the rod−rod BCPs. It will be interesting to know how the self-assembling structure changes when the two rods with different diameters are combined in one rod−rod BCP. The interplay between the two rod blocks will certainly affect the microphase-separated nanostructure. We first used 2D WAXD

Figure 5. PLM micrographs of PVBP79 (a) and PVBP79-b-PBLG20 (b) at 200 °C.

Figure 6. 2D WAXD patterns of sheared film samples of PVBP82-b-PBLG141 with the X-ray incident beam along Z (a), Y (b), and X (c) directions, azimuthal scans of the diffraction arcs for the two blocks (d from a; e from b), and schematic drawing of the shearing geometry with the X-axis as the shear direction (f). F

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Figure 7. 2D WAXD pattern of a sheared film sample with the X-ray incident beam along the Z direction (a), the corresponding azimuthal scans of the two diffraction arcs from the two blocks (b), and the packing schemes of the two blocks (c) for PVBP79-b-PBLG77; 2D WAXD pattern of a sheared film sample with the X-ray incident beam along the Y direction (d), the corresponding azimuthal scans of the two diffraction arcs from the two blocks (e), and the packing schemes of the two blocks (f) for PVBP79-b-PBLG47.

shown in Figure 8, and the calculated d-spacing values of the diffraction peaks are listed in Table 1. As shown in Figure 8, dspacing values at 200 °C are a little larger than those at room temperature because q decreases at the higher temperature, which can be attributed to the fact that both blocks are in their LC phases at 200 °C and the macromolecular chains tend to be more extended. For PVBP82-b-PBLG141 (Figure 8a), there is only one diffraction peak at q* = 0.104 nm−1, and the lack of higher-order Bragg reflections indicates that this sample may adopt a poorly ordered structure. This suggests that the high MWs of the rod blocks make the BCP less ordered possibly due to kinetic reasons. The peaks in the SAXS profiles of PVBP79-bPBLG77 (Figure 8b) have a scattering vector ratio of 1:2, which is the characteristic of a lamellar structure, and the primary reflection is at q* = 0.150 nm−1 corresponding to a d-spacing of 41.9 nm. For PVBP79-b-PBLG47 (Figure 8c), the scattering vector ratio is 1:2:3, and the primary reflection is at q* = 0.196 nm−1 corresponding to a d-spacing of 32.0 nm. And for PVBP79-bPBLG20 (Figure 8d), the diffraction peaks have a scattering vector ratio of 1:2:3:4, and the primary reflection is at q* = 0.117 nm−1 corresponding to a d-spacing of 53.7 nm. These latter two BCPs also possess lamellar structures, more ordered with even higherorder reflections. For all the four BCPs, annealing at 200 °C leads to more ordered structures. To figure out the packing schemes in the microphaseseparated nanostructures, we calculated the molecular lengths of the two segments. As both of the segments in rod−rod BCPs are assumed to be rigid, the zigzag model was used for the PVBP segment, and the 18/5 α-helical structure was applied for the PBLG block for the calculation. The results are shown in Table 1. And we also used the calculated molecular lengths to calculate f PVBP*, which is close to the f PVBP value calculated

to determine the relative orientation of the two rods in the BCPs. The sample films were prepared by solution-casting from THF solutions and then dried at ambient temperature. After annealing at 200 °C in vacuum, the samples were mechanically sheared at 160 °C to obtain the oriented films. Parts a, b, and c of Figure 6 show the 2D WAXD patterns of PVBP82-b-PBLG141 with the X-ray beam along Z, Y, and X directions, respectively. There are two pairs of strong diffraction arcs on the equator in the low-angle region, which are attributed to the ΦN phase of PVBP and the ΦH phase of PBLG. Parts d and e of Figure 6 show the azimuthal scans of the two diffraction arcs. The peaks in the two curves for the two rods have an offset of about 5°, which means that on average the two rods are oriented at an angle of 5°. The 2D WAXD results of PVBP79-b-PBLG77 and PVBP79-b-PBLG47 also reveal inclined angles of 7° and 18°, respectively, as shown in Figure 7. For PVBP79-b-PBLG20, the diffraction from PBLG is too weak to be detected, as shown in Figure 4, and it is difficult to find the diffraction arc from PBLG. The existence of the inclined angle evidences the effect of the difference in rod diameters. Because the cross-sectional areas of the two rods do not match, the “thinner” PBLG rods tend to tilt with respect to the “thicker” PVBP rods. The differences of the angles may be due to the chain length differences of the two rods. Because in these BCPs the PVBP rod length is fixed, with decreasing rod length, PBLG may tilt to a larger angle to satisfy the packing requirement. The packing schemes of the two blocks are shown in parts c and f of Figure 7. The nanostructures of the BCPs were determined by SAXS experiments. Sample films were prepared by solution-casting from THF solutions and then dried at ambient temperature. All samples were annealed in vacuum at 200 °C and then cooled to ambient temperature for measurements. The SAXS profiles are G

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Figure 8. SAXS profiles of PVBP82-b-PBLG141 (a), PVBP79-b-PBLG77 (b), PVBP79-b-PBLG47 (c), and PVBP79-b-PBLG20 (d) at room temperature (R. T.) and 200 °C.

concentration of 0.5 mg/mL. The film samples were annealed at 170 °C under a N2 flow for 3 days before measurements. Figure 9 shows the TEM micrographs. Judging from the electron densities and contents of the two segments, the dark parts are PBLG layers and the light domains are PVBP layers. For PVBP82-b-PBLG141, the structure is poorly ordered, consistent with the SAXS results. However, there are lamellae in some areas. Therefore, the structure may be poorly ordered lamellae. For the other three BCPs, TEM micrographs all show lamellar structures. The spacing of the lamellar structure is about 43 nm for PVBP79-b-PBLG77 with PVBP layers of about 32 nm and PBLG layers of about 11 nm. PVBP79-b-PBLG47 has a lamellar spacing of 38 nm, with PVBP layers of about 31 nm and PBLG layers of about 7 nm. Comparison of the layer spacings of the two blocks from TEM with the calculated molecular lengths of the two rods indicates that the PVBP segment is packed in a bilayer structure while the PBLG block takes the interdigitated single-layer arrangement for PVBP79-bPBLG77 and PVBP79-b-PBLG47, which indicates interdigitated lamellar structures, consistent with the SAXS results. Combined with 2D WAXD results, the packing schemes of the two blocks are shown in parts c and f of Figure 7. The interdigitated structure of the polypeptide block has been reported in the selfassembling behaviors of rod−coil systems.48−54 As reported by Schlaad and co-workers, the α-helix polypeptide chains usually take the interdigitated or folded up structure.48,53 For the rod− rod BCPs in this work, due to the mismatch of rod diameters,

from the MWs and the polymer densities (see Table 1), evidencing that the molecular calculation results are reasonable. Comparing the calculated molecular lengths of the two blocks with the d-spacing values from SAXS results, the lamellar spacing first decreases with decreasing length of the PBLG segment for PVBP82-b-PBLG141, PVBP79-b-PBLG77, and PVBP79-b-PBLG47, and then increases for the shorter PBLG length in PVBP79-b-PBLG20, which indicates that the two segments may arrange differently in these BCPs. As reported in literature, rod−rod BCPs can self-assemble into single-layer lamellar, interdigitated lamellar, or bilayer lamellar structures.22,26 We propose that PVBP79-b-PBLG77 and PVBP79-bPBLG47 may form interdigitated lamellae because the experimental lamellar spacings are between the calculated values of single-layer and bilayer lamellae. As for PVBP79-bPBLG20, a bilayer structure appears reasonable because twice of the molecular length of the BCP (46 nm) on the basis of the molecular length calculation is close to the d-spacing value from SAXS results. Moreover, compared with PVBP79b-PBLG77 and PVBP79-b-PBLG47, the calculated molecular length of the PBLG block is the shortest, but the spacing of the lamellar structure from the SAXS data is the largest. Therefore, PVBP79-b-PBLG20 may adopt a stacked bilayer structure. We also used TEM to directly observe the morphologies of the self-assembled structures of the BCPs. Observations were performed on solution-cast films from THF solutions at a H

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layers of about 7 nm and PVBP layers of about 46 nm, which means that both of the PBLG and PVBP rods adopt the bilayer structures. Figure 10 summarizes the proposed models for the selfassembled nanostructures of the rod−rod BCPs at 200 °C. When f PVBP is about 60%, it microphase separates into a poorly ordered lamellar structure with the PBLG segment in the ΦH phase and the PVBP block taking the poorly ordered ΦN phase. When f PVBP is between 73% and 82%, the microphaseseparated nanostructure is the interdigitated lamellar structure with the PVBP segment taking the bilayer structure and PBLG adopting the single-layer structure, and both rods are in relatively poorly ordered LC phases. Finally, when f PVBP is about 91%, both PVBP and PBLG segments have the bilayer structures, with PVBP in the ΦN phase and PBLG not liquid crystalline. The proposed structures of the BCP with f PVBP in the range of 73%∼91% are similar to cylinder-in-lamella morphologies reported in some other rod−coil48−54 and rod− rod42 BCPs possessing one polypeptide block. Although PVPBb-PBLG similarly adopts the lamellar structure, there are two differences in this BCP system. First, the PBLG block in these rod−rod BCPs can take the interdigitated or the stacked arrangement at proper volume fractions, while in other systems it is usually packed in one way. Second, the ΦH phase of PBLG is partially destroyed; i.e., the α-helix chains are in a disordered state in the lamellae, due to the mismatch of the rod diameters.

Figure 9. TEM micrographs of PVBP82-b-PBLG141 (a), PVBP79-bPBLG77 (b), PVBP79-b-PBLG47 (c), and PVBP79-b-PBLG20 (d).

the packing of PBLG is also affected by the arrangement of PVBP in addition to the intramolecular interaction between the α-helix chains. The PBLG rods tend to be packed in the interdigitated way in these BCPs because they are thinner than the PVBP rods. From the 1D WAXD results, the ΦN phase of PVBP is partially destroyed in these samples, and it is not retained at ambient temperature for the TEM experiments. Therefore, the length of the PVBP block should be shorter than the calculated value. Because the TEM samples were prepared by solution-casting, the lamellar sizes are a little different from those obtained by SAXS. As for PVBP79-bPBLG20, the lamellar spacing is about 53 nm with PBLG

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CONCLUSIONS In this study, a series of rod−rod BCPs, PVBP-b-PBLG, with different rod diameters was synthesized, and the LC properties of the two rod-like components and the microphase separation of the BCPs were determined. By using DSC, PLM, 1D WAXD, 2D WAXD, SAXS, and TEM, we have found that the existence of the rod blocks and the difference in rod diameters greatly impact both the LC phases of the individual blocks and

Figure 10. Schematic representation of the proposed models for the self-assembly of PVBP-b-PBLG BCPs. I

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the microphase-separated nanostructures of the rod−rod BCPs. Depending on f PVBP, the ΦN phase of PVBP and the ΦH phase of PBLG are disrupted to different extents. When f PVBP is about 60%, the ΦH structure of PBLG is well retained with the appearance of the two second-order diffractions in WAXD patterns. With f PVBP increasing to above 73%, the ordered structure of PBLG is destroyed, and even the first-order diffraction peak disappears when f PVBP is 91%. At the same time, the ΦN phase of PVBP goes through partially destroyed to well retained with increasing f PVBP. In addition, the microphase-separated structures are also affected. With f PVBP increasing from 60% to 91%, the d-spacing first decreases then increases. Accordingly, the microphase-separated structure goes through poorly ordered lamellar, to interdigitated lamellar, and to bilayer lamellar structures. The existence of the interdigitated and bilayer lamellae reflects the influence of the difference in rod diameters. Finally, the existence of the lamellar structure with f PVBP of 91% indicates that the rod−rod BCPs tend to self-assemble into lowcurvatured structures, consistent with the literature report.55

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ASSOCIATED CONTENT

S Supporting Information *

The details for the synthesis of the PVBP-b-PBLG BCPs, TLC experiments, calculations of the MWs of BCPs, texts discussing and figures showing the FTIR spectra and DSC curves about the BCPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Z.S.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 20990232 and 21174006). The authors also thank Dr. Y. S. Liu at Zhejiang Sci-Tech University for the assistance with the X-ray diffraction measurements.

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