Persistent Formation of Self-Assembled Cylindrical Structure in a

Jul 20, 2018 - When the ratio of pyridine to carboxylic acid is 0.50 or lower, the SLCBCPs are coil–coil-like. However, when the ratio exceeds 0.50,...
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Persistent Formation of Self-Assembled Cylindrical Structure in a Liquid Crystalline Block Copolymer Constructed by Hydrogen Bonding Hongbing Pan, Wei Zhang, Anqi Xiao, Xiaolin Lyu, Zhihao Shen,* and Xinghe Fan*

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Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: A series of supramolecular liquid crystalline block copolymers (SLCBCPs) were prepared by hydrogen-bonding interaction between poly(dimethylsiloxane)-b-poly(2-vinylterephthalic acid) (PDMS-b-PM1H) and [4-(4′-hexyloxy)styryl]pyridine (NC6). PDMS-b-PM1H serves as the hydrogen-bonding donor and NC6 as the hydrogen-bonding acceptor. The SLCBCPs are obtained by mixing the hydrogen-bonding acceptor and donor in pyridine. Through increasing the molar ratio of pyridine to carboxyl, the SLCBCPs can transform from coil−coil block copolymers (BCPs) to rod−coil ones. When the ratio of pyridine to carboxylic acid is 0.50 or lower, the SLCBCPs are coil−coil-like. However, when the ratio exceeds 0.50, the SLCBCPs behave like rod−coil BCPs because the supramolecular block PM1H(NC6) exhibits liquid crystalline (LC) behavior owing to the “jacketing” effect. Small-angle X-ray scattering and transmission electron microscopy experiments were employed to characterize the microphase-separated nanostructures of the SLCBCPs. Interestingly, when the weight fraction of the supramolecular block PM1H(NC6) ranges from 51% to 92%, hexagonally packed cylinders (HEX) are always obtained. Compared to conventional BCPs, the SLCBCPs prepared can more easily self-assemble into the HEX nanostructures that may potentially serve as nanotemplates and porous materials after selective etching. In addition, the SLCBCPs can form hierarchically ordered nanostructures, including the HEX nanostructure of the SLCBCP and the LC phase of the supramolecular block.

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INTRODUCTION As soft materials, block copolymers (BCPs) have been widely studied and used in nanotechnology owing to their selfassembled periodic nanostructures.1−5 BCPs can be composed of coil and rod blocks. Through the permutation and combination of different types of blocks, coil−coil, rod−coil, rod− rod, and even more complicated ones such as triblock and multiblock copolymers can be constructed.6−9 For coil−coil BCPs, three crucial parameters influence the self-assembling behaviors, including the Flory−Huggins interaction parameter (χ), the volume fraction of the individual block (f), and the total degree of polymerization (N).2,10 Various nanostructures can be obtained by the self-assembly of coil−coil BCPs, such as lamellar (LAM), double gyroid, cylindrical, and spherical phases.4,11 Compared with coil−coil BCPs, the phase behaviors of rod−coil BCPs are significantly different because of the additional conformational asymmetry between the rod and coil blocks and the orientational interaction between the rod−like blocks themselves.6 As a result, there are some unique features for rod−coil BCPs, i.e., the stronger phase separation ability and the asymmetry of the phase diagram.6,10,12 In addition, some unusual phases such as zigzag LAM phase13,14 and Fddd structure15,16 have been observed. Recently, Bates, Dorfman, © XXXX American Chemical Society

and co-workers even found Frank−Kasper phases in poly(isoprene)-b-poly(lactide) diblock copolymers.17 Liquid crystalline (LC) BCPs are a special kind of rod−coil BCPs because the LC blocks can form ordered mesophases on a smaller length scale compared to that of the microphaseseparated nanostructures.6,18 Consequently, hierarchically ordered structures are easily formed through self-assembly. In the past decades, our research group has been studying rod−coil BCPs containing LC rod−like blocks that are mesogen-jacketed liquid crystalline polymers (MJLCPs).19−22 MJLCPs, proposed by Zhou et al., are side-chain polymers with physical properties of main-chain LC polymers because of the “jacketing” effect of the bulky side groups.23,24 Rich varieties of nanostructures have been obtained through the selfassembly of the BCPs containing MJLCPs.19−22,25,26 With the development of supramolecular chemistry, noncovalent bonds, especially hydrogen bonds, have been employed to construct side-chain LC polymers and corresponding BCPs.27−34 Several reviews focusing on the formation of hierarchically ordered structures and their tunable Received: April 16, 2018 Revised: July 4, 2018

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DOI: 10.1021/acs.macromol.8b00806 Macromolecules XXXX, XXX, XXX−XXX

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BL16B1 at the Shanghai Synchrotron Radiation Facility (SSRF)44 and at Beamline 1W2A at the Beijing Synchrotron Radiation Facility (BSRF).45 The sample was packed in an aluminum foil for the SAXS measurement. Then the sample was embedded in an epoxy resin and ultramicrotomed into thin films with thicknesses of ca. 50 nm. Afterward, the thin films were transferred onto carbon-coated copper grids for TEM measurements. Wide-angle X-ray scattering (WAXS) experiments were conducted on a Micromax002+ SAXS instrument using Cu Kα radiation at a wavelength of 0.154 nm. The working current and voltage were 40 kV and 40 mA, respectively. An imaging plate was used to record the scattering patterns which extended to the high-angle range. Then, the scattering patterns were integrated to acquire the WAXS data. Synthesis of the Hydrogen-Bonding Donor and Acceptor. The synthetic route of the hydrogen-bonding donor and chemical structure of the hydrogen-bonding acceptor is shown in Scheme 1 and

properties have been published.35−39 Generally, there are two types of polymers with hydrogen-bonded side chains39,40 (A and B in Chart 1). Type A has mesogens hydrogen-bonded Chart 1. Hydrogen-Bonded Side-Chain Liquid Crystalline Polymers

Scheme 1. Synthetic Route of the Hydrogen-Bonding Donor to the polymer side chains. Type B is “grafted” polymers obtained through hydrogen-bonding interaction of backbonefunctionalized polymers with end-functionalized mesogenic molecules. The hydrogen bond acts as a linking group. In our previous work on the synthesis of MJLCPs, the hydrogenbonding strategy had also been used.41,42 With poly(2vinylterephthalic acid) used as the hydrogen-bonding donor and pyridine derivatives as the acceptors, columnar nematic and smectic phases were observed.41 The hydrogen-bonded mesogens are directly attached to the polymer backbone laterally. These kinds of hydrogen-bonded MJLCPs (type C in Chart 1) are different from the types mentioned above. In this work, poly(dimethylsiloxane)-b-poly(2-vinylterephthalic acid) (PDMS-b-PM1H) and [4-(4′-hexyloxy)styryl]pyridine (NC6) were used as the hydrogen-bonding donor and acceptor, respectively. The microphase-separated nanostructures and LC phase behaviors were adjusted by changing the degree of polymerization (DP) of the PM1H block and the molar ratio of NC6 to the carboxylic acid group in the PM1H block. The resulting BCPs always self-assemble into hexagonally packed cylindrical (HEX) nanostructures when the weight fraction of the hydrogen-bonded block PM1H(NC6) ranges from 51% to 92%, which is not frequently observed in rod−coil BCPs that tend to broaden the phase region of LAM structures. On the other hand, the hydrogen-bonded block PM1H(NC6) can form LC phases when the molar ratio of the acceptor to donor exceeds 0.50, leading to hierarchically ordered nanostructures.

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Chart 2a. According to our previous report, the tert-butyl ester group is easily removed from the benzene ring by heating.46 In order to

Chart 2. Chemical Structures of the Hydrogen-Bonding Acceptor (a) and Hydrogen-Bonded Complex (b)

determine the pyrolysis product and the isothermal time of degradation, isothermal TGA experiments on the PM1 homopolymer and the PDMS-b-PM1 BCP were conducted at 200 °C (Figure S1 in the Supporting Information). For the PM1 homopolymer, the weight loss is 36.5% when the sample was held at 200 °C for 60 min. The weight loss is consistent with that during the conversion of PM1 to PM1H (36.8% loss), indicating that the tert-butyl group was removed completely. On the other hand, an isothermal experiment was conducted on the PDMS-b-PM1 BCP to determine the stability of the pyrolysis product. From the TGA trace, the weight keeps constant up to 180 min after the initial thermal decomposition, indicating that the carboxylic acid-functionalized BCP would not dehydrate to yield anhydrides under this condition, although the decarboxylation phenomenon was reported in the literature.47,48 Therefore, the crude product of the hydrogen-bonding donor PDMS-b-PM1H was synthesized from its precursor PDMS-b-PM1 by holding it at 200 °C for 1 h. Then the crude product was washed with anhydrous ether three times to remove the residual isobutylene molecules.48 The synthesis of the hydrogen-bonding acceptor follows the procedure in our previous work.41 Electrospray ionization (ESI) positive MS: (M + H)+/z = 282.2. EA: calcd for C19H23NO: C, 81.10; H, 8.24; N, 4.98. Found: C, 81.25; H, 8.15; N, 4.86. Preparation of the Hydrogen-Bonded Complexes. The hydrogen-bonding acceptor and donor were dissolved in pyridine,

EXPERIMENTAL SECTION

Materials. The precursor of PDMS-b-PM1H, poly(dimethylsiloxane)-b-poly[2,5-bis(tert-butoxylcarbonyl)styrene] (PDMS-b-PM1), was synthesized following our previous work.43 Deuterated dimethyl sulfoxide (DMSO-d6, solvent used in 1H NMR for PDMS-b-PM1H and NC6) and deuterated methylene chloride (CH2Cl2-d2, solvent used in 1H NMR for PDMS-b-PM1) were purchased from the J&K Scientific LTD and used as received. Anhydrous ether (A.R.) and pyridine (HPLC) were purchased from the Beijing Chemical Reagents Company and used as received. Characterization. All of the characterization methods, including FT-IR, MS, elemental analysis (EA), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM), used in this study were described in our previous work.20 1H NMR spectra were obtained with a Bruker 400 MHz spectrometer. Small-angle X-ray scattering (SAXS) measurements were performed at Beamline B

DOI: 10.1021/acs.macromol.8b00806 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules separately. The solutions containing the acceptor and donor with different molar ratios of the pyridine derivative NC6 to the carboxylic acid group in PM1H were mixed together after complete dissolution, followed by stirring at ambient temperature for 24 h. Afterward, the solvent was removed by slow evaporation at 65 °C for about 1 week. Then the resultant complexes were annealed at 130 °C under vacuum for 48 h to facilitate assembly. The solvent molecules were removed completely under this condition according to the literature.49 In addition, the weight of complex was almost the same as the sum of those of the donor and the acceptor, indicating the absence of solvent molecules. The chemical structure of the hydrogen-bonded complex is shown in Chart 2b.

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RESULTS AND DISCUSSION Synthesis and Characterization of the HydrogenBonding Donor and Acceptor. The 1H NMR spectra of the hydrogen-bonding donor PDMS-b-PM1H, its precursor PDMS-b-PM1, and the hydrogen-bonding acceptor NC6 are shown in Figure 1. Compared with PDMS-b-PM1, the signals

Figure 2. FT-IR spectra of the hydrogen-bonding donor and its precursor.

PDMS-b-PM1H was calculated on the basis of PDMS-b-PM1 reported in our previous work.43 The calculation formula can be expressed as M = M1 − 112n, where M and M1 are the MWs of PDMS-b-PM1H and PDMS-b-PM1, respectively, and n is the DP of the PM1 block. The coefficient of n, 112, is twice the MW of an isobutylene molecule because one repeating unit of the PM1 block contains two tert-butyl groups. All the molecular characteristics and thermal properties are summarized in Table 1. Table 1. Molecular Characteristics and Thermal Properties of the Hydrogen-Bonding Donor polymer

Mna (g/mol)

ĐMa

WPM1Ha (%)

Tdb (°C)

D120M1H37 D120M1H52 D120M1H61 D120M1H73 D120M1H86 D120M1H134

15800 18700 20400 22600 25100 34300

1.09 1.15 1.15 1.15 1.15 1.21

44 53 57 61 65 74

289 306 300 299 295 287

a Determined from the molecular characteristics of PDMSm-b-PM1n. The MW is calculated according to the formula M = M1 − 112n, where M1 is the MW of PDMSm-b-PM1n and n is the DP of the PM1 block. bThe 1% weight loss temperature was evaluated by TGA under a nitrogen atmosphere at a heating rate of 10 °C/min.

Similarly, the structure of the hydrogen-bonding acceptor was verified by 1H NMR, MS, and EA. Characterization of the Hydrogen-Bonded Complexes. A series of hydrogen-bonded complexes with different molar ratios of NC6 to the carboxylic acid group were prepared. The hydrogen-bonded complexes are denoted as PDMSm-b-PM1Hn(NC6)x, abbreviated as DmM1Hn(NC6)x (where m and n represent the DPs of the PDMS and PM1H blocks, respectively, and x refers to the molar ratio of NC6 to carboxylic acid). The hydrogen-bonded complexes were characterized by FT-IR and DSC. With D120M1H73(NC6)x as an example, Figure 3a shows the normalized FT-IR spectra of the complexes. With increasing molar ratio, the intensities of the bands at 1930 and 2550 cm−1 increase, which confirms the formation of hydrogen bond between the pyridine derivative and the carboxylic acid group.50,51 For the hydrogen-bonded complex (A−H···B), where the proton is not transferred, with increasing hydrogen-bonding strength, the A−H band splits into three characteristic intensive broad bands (usually denoted A, B, and C) at about 2800, 2500, and 1800 cm−1. The two broad bands of the complex at 2550 and 1930 cm−1 correspond to the “B” and “C” bands discussed above, indicative of strong hydrogen bonds.51 Moreover, the new broad band above 3000 cm−1

1

Figure 1. H NMR spectra (400 MHz) of the hydrogen-bonding donor PDMS-b-PM1H and its precursor PDMS-b-PM1 (a) and that of the hydrogen-bonding acceptor NC6 (b).

of the tert-butyl group disappear, and those of the carboxylic acid hydrogen appear in the 1H NMR spectrum of PDMS-bPM1H. To further confirm the existence of the carboxylic acid group, FT-IR experiments of PDMS-b-PM1 and PDMS-bPM1H were conducted, with results presented in Figure 2. The stretching vibration of the carbonyl group shifts from 1720 to 1697 cm−1 (inset graph in Figure 2), indicating that the chemical environment of the carbonyl group has changed.41 According to the report in the literature, the characteristic carbonyl group would split into two peaks at about 1806 and 1762 cm−1 if anhydrides are formed.48 However, the stretching vibration peak of the carbonyl group still remains a single peak at 1697 cm−1, which is another evidence for the absence of anhydrides. On the other hand, the appearance of the broad acid band between 2500 and 3500 cm−1 proves the existence of the carboxylic acid group. Thus, the chemical structure correctness of the hydrogen-bonding donor is confirmed by the 1H NMR and FT-IR data. The molecular weight (MW) of C

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Figure 4. DSC curves of D120M1H73(NC6)x along with those of D120M1H73 and NC6 during the first cooling (a) and the second heating (b) at a rate of 10 °C/min under a N2 atmosphere.

processes, indicating that no free acceptor is present. In other words, the acceptor remains as hydrogen bonded to the donor. Microphase-Separated Nanostructures of the Hydrogen-Bonded Complexes. SAXS and TEM were used to characterize the nanostructures formed by the hydrogen-bonded complexes. Figure 5 shows the SAXS profiles of the representative hydrogen-bonded complexes, and those of other samples are presented in Figure S4. As shown by the SAXS profiles, the samples have the scattering vector ratio of 1:√3:2:√7, 1:√3:√7, or 1:2:√7, characteristic of HEX structures. The weight fractions of the supramolecular block (wPM1H(NC6)’s), the q values of the first-order diffraction peaks, corresponding d-spacing values, the nanostructures, and the calculated values of the lattice parameter a are shown in Table 2. It is interesting that all the samples with the wPM1H(NC6) values ranging from 51% to 92% self-assemble into HEX structures. In order to further verify the nanostructures of the hydrogenbonded complexes, TEM experiments were conducted on samples after SAXS measurements were performed. The representative TEM micrographs of the hydrogen-bonded complexes are shown in Figure 6, and those of other samples are presented in Figure S5. Because PDMS blocks possess a higher electron density, the dark parts are the PDMS microdomains and the bright ones are the PM1H(NC6) microdomains in the TEM micrographs. With D120M1H52(NC6)0.10 as an example, the a value measured from the TEM micrograph is 27.5 nm, which is consistent with the SAXS result of 26.1 nm (Table 2) within the range of experimental errors. The combination of SAXS and TEM results together proves the HEX structures formed by the hydrogen-bonded complexes with the PDMS block as the dispersed phase. In order to study the effect of temperature on the nanostructures, variable-temperature SAXS experiments were conducted from ambient temperature to 150 °C. The representative variable-temperature SAXS profiles of D120M1H52 (NC6)0.50 and D120M1H52(NC6)1.0 are shown in Figure 7,

Figure 3. FT-IR spectra of the hydrogen-bonded complexes D120M1H73(NC6)x with different molar ratios along with those of D120M1H73 and NC6 (a); variable-temperature FT-IR spectra of D120M1H73(NC6)1.0 at different temperatures during heating (b) and cooling (c).

when x ≥ 0.75 corresponds to the “A” band mentioned above, further demonstrating the increase in the hydrogen-bonding strength. In order to study the temperature dependence of the hydrogen bond, variable-temperature FT-IR experiments were conducted. Parts b and c of Figure 3 are the normalized FT-IR spectra of D120M1H73(NC6)1.0 during heating and cooling, respectively, in the range 30−210 °C. The intensities of the bands at 1930 and 2550 cm−1 decrease with increasing temperature, and vice versa, indicating that the hydrogen bond is partly broken at high temperatures and recovered during the cooling process.51 With the consideration of the thermal decomposition temperature of the hydrogen-bonding acceptor (about 220 °C as shown in Figure S2) and donor (about 300 °C as shown in Figure S3), all the subsequent variable-temperature measurements were conducted below 150 °C to avoid the decomposition of the samples and the breaking of the hydrogen bond. The first step of the 8% weight loss of the hydrogenbonding donor (Figure S2) may correspond to the formation of anhydrides. However, the temperature exceeds 300 °C when the side reaction occurs. The DSC thermograms of the hydrogenbonded complexes are shown in Figure 4. Except that of the hydrogen-bonding acceptor NC6, the thermograms do not show any endothermic or exothermic peaks in the heating and cooling D

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Figure 5. SAXS profiles of the hydrogen-bonded complexes with different PM1H blocks and different molar ratios of NC6 to carboxylic acid (a: D120M1H52(NC6)x; b: D120M1H61(NC6)x; c: D120M1H73(NC6)x).

Table 2. Nanostructure Information of the Hydrogen-Bonded Complexes polymer

wPM1H(NC6) (%)

q (nm−1)

d (nm)

nanostructure

a (nm)

D120M1H37(NC6)0.10 D120M1H37(NC6)0.20 D120M1H37(NC6)0.30 D120M1H37(NC6)0.40 D120M1H37(NC6)0.50 D120M1H37(NC6)0.75 D120M1H37(NC6)1.0 D120M1H52(NC6)0.10 D120M1H52(NC6)0.20 D120M1H52(NC6)0.30 D120M1H52(NC6)0.40 D120M1H52(NC6)0.50 D120M1H52(NC6)1.0 D120M1H61(NC6)0.10 D120M1H61(NC6)0.20 D120M1H61(NC6)0.30 D120M1H61(NC6)0.40 D120M1H61(NC6)0.50 D120M1H61(NC6)1.0 D120M1H73(NC6)0.25 D120M1H73(NC6)0.50 D120M1H73(NC6)0.75 D120M1H73(NC6)1.0 D120M1H86(NC6)0.50 D120M1H86(NC6)1.0 D120M1H134(NC6)0.50 D120M1H134(NC6)1.0

50.8 55.9 60.0 63.4 66.3 71.8 75.8 59.2 64.0 67.8 70.9 73.4 81.5 63.0 67.6 71.2 74.1 76.4 83.8 73.2 79.5 83.4 86.1 82.0 87.9 87.7 91.9

0.257 0.257 0.246 0.245 0.245 0.242 0.241 0.278 0.273 0.278 0.268 0.257 0.257 0.262 0.262 0.262 0.257 0.251 0.244 0.242 0.242 0.226 0.221 0.259 0.259 0.223 0.186

24.4 24.4 25.5 25.6 25.6 26.0 26.1 22.6 23.0 22.6 23.4 24.4 24.4 24.0 24.0 24.0 24.4 25.0 25.7 26.0 26.0 27.8 28.4 24.2 24.2 28.2 33.8

HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX

28.2 28.2 29.4 29.6 29.6 30.0 30.1 26.1 26.6 26.1 27.0 28.2 28.2 27.2 27.2 27.2 28.2 28.9 29.7 30.0 30.0 32.1 32.8 27.9 27.9 32.6 39.0

representative 1D WAXS profiles of the samples, and those of others are presented in Figure S7. For the pure hydrogenbonding donor (x = 0), there are no diffraction peaks in the low-angle region, indicating that the PM1H block is disordered. When the hydrogen-bonding acceptor is introduced (x > 0), new scattering halos appear in the low-angle region. With increasing x, the scattering halos gradually become sharp diffraction peaks, indicating the formation of LC phases. The main peak at q = 1.42 nm−1 in the WAXS profiles is attributed to the supramolecular LC phase. It has a d-spacing of 4.42 nm, which is close to the length (4.60 nm, assuming all-transconformation of alkyl chains) of the hydrogen-bonded side group in the complexes, with the consideration of possible

and those of other samples are shown in Figure S6. All the hydrogen-bonded complexes maintain the HEX nanostructures during heating. The first-order peaks slightly move toward the lower angle region during heating due to thermal expansion. LC Phase Behaviors of the Hydrogen-Bonded Complexes. The LC phase behaviors of the hydrogenbonded complexes were first characterized by DSC (Figure 4). The DSC thermograms do not show any peaks during heating and cooling processes, not only indicating that the acceptor is completely hydrogen-bonded to the donor but also suggesting the lack of any phase transitions. Furthermore, the LC phase behaviors of the hydrogen-bonded complexes were studied using 1D WAXS experiments. Figure 8 shows the E

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Figure 6. TEM micrographs of the hydrogen-bonded complexes with different PM1H blocks and different molar ratios of NC6 to carboxylic acid (a: D 120 M1H 52 (NC6) 0.10 ; b: D 120 M1H 52 (NC6) 0.30 ; c: D 120 M1H 52 (NC6) 0.50 ; d: D 120 M1H 52 (NC6) 1.0 ; e: D 120 M1H 73 (NC6) 0.25 ; f: D120M1H73(NC6)0.50; g: D120M1H73(NC6)0.75; h: D120M1H73(NC6)1. 0).

values of FWHM decline quickly with increasing x, indicating increased order. The slope suddenly decreases at x ≈ 0.5, suggesting a transition from the disordered state to an ordered (LC) phase. The FWHM value levels off when x > 0.5, indicating that the molecular arrangement of the supramolecular block tends to be stabilized. For D120M1Hn(NC6)x (n = 73), the situation is similar. However, the change at x ≈ 0.5 is not as sharp as those of other samples. This indicates that the formation of the LC phase is related to both x and the DP of the PM1H block, and increasing x and DP of the PM1H block is beneficial to the development of the LC phase. The hydrogenbonded complexes can self-assemble into hierarchically ordered nanostructures, including the microphase-separated HEX structure on a larger length scale and the LC phase on a smaller length scale. The variable-temperature WAXS experiments were also conducted from ambient temperature to 150 °C. The representative variable-temperature WAXS profiles are shown in Figure S8. The LC phase remains unchanged during the heating process. Discussion on the Persistent Formation of the HEX Structure. For typical rod−coil BCPs, the phase diagram is asymmetric because of the conformational asymmetry between the rod and coil blocks. Owing to the preferred parallel arrangement of the rod−like blocks, the LAM structure enjoys a wide region in the phase diagram compared with the case of coil−coil BCPs.6 Chen et al. reported the self-assembling behavior of a series of discotic LC BCPs PEG−poly(TPm) with the weight fraction of the discotic LC block (f w,DLC) ranging from 37% to 90%.53 When f w,DLC = 67%−80%, the HEX structure was obtained. When f w,DLC = 62%, the HEX structure was formed at high temperatures, whereas the LAM structure was obtained at low temperatures because of crystallization of PEG. In fact, the authors did not determine the threshold f w,DLC value when the HEX structure started to appear. However, in the system studied here, PDMS is noncrystalline, and the HEX structure is always obtained with the weight fraction of the supramolecular block PM1H(NC6) ranging from 51% to 92%. The situation is different from the above-mentioned one in the literature, and the reason is similar to the case of bottlebrush block copolymers (BBCPs).54,55 In the bottlebrush architecture, the length of the backbone and the brush side chains could both affect its morphology. The most important factor for controlling

Figure 7. SAXS profiles of D 120 M1H 52 (NC6) 0.50 (a) and D120M1H52(NC6)1.0 (b) during heating.

interdigitation of alkyl tails. Therefore, this distance is almost the diameter of the supramolecular MJLCP chain.41 On the other hand, the amorphous halo at around q = 8.5 nm−1 corresponding to a d-spacing of 0.74 nm is the mean distance between the PDMS chains, i.e., the diameter of the PDMS chains.52 In order to determine the threshold molar ratio of NC6 to carboxylic acid for the formation of the LC phases, the full width at half-maximum (FWHM) of the low-angle diffraction peak as a function of x is shown in Figure 9. For D120M1Hn(NC6)x (n = 37, 52, and 61), when x < 0.5, the F

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Figure 8. WAXS profiles of the hydrogen-bonded complexes with different PM1H blocks and different molar ratios of NC6 to carboxylic acid (a: D120M1H37(NC6)x; b: D120M1H61(NC6)x; c: D120M1H73(NC6)x).

Figure 9. FWHM of the low-angle diffraction peak as a function of the molar ratio of NC6 to carboxylic acid (x). Figure 10. Schematic illustration of the formation mechanism of the HEX structure.

molecular packing is the cross-sectional area. Increasing the size of the bottlebrush backbone does not have a significant effect on its cross-sectional area. On the other hand, increasing the size of side-chain branches would dramatically change the cross-sectional area of the cylindrical bottlebrush. As a result, highly asymmetric BBCPs containing shorter branches on one side and longer branches on the other side would favor the formation of building blocks with curved interfaces which further assemble into cylindrical structures (Figure 10). For the SLCBCPs prepared, because of the “jacketing” effect, the bulky supramolecular side chains force the backbone to be highly extended. Consequently, the architecture of the SLCBCPs is similar to that of BBCPs, and the HEX morphology exists in a wide range in the phase diagram. The formation mechanism of the HEX morphology is schematically illustrated in Figure 10.

through hydrogen bonding makes the rod−coil BCP highly asymmetric. Therefore, the SLCBCPs are easy to form the HEX morphology with curved interfaces. This kind of highly asymmetric BCPs may be potential nanotemplates and porous materials after etching the central PDMS cylinders or removing the hydrogen-bonding acceptor molecules. On the other hand, the supramolecular block PM1Hn(NC6)x can develop into the LC phase when the molar ratio of acceptor to donor is large enough (x > 0.50), resulting in hierarchically ordered nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00806. TGA traces of the hydrogen-bonding acceptor and donors; SAXS and WAXS profiles of the hydrogenbonded complexes at ambient temperature and during heating; TEM micrographs of the hydrogen-bonded complexes (PDF)

CONCLUSIONS We prepared a series of SLCBCPs DmM1Hn(NC6)x through hydrogen bonding with different DPs of the PM1H block and different molar ratios of NC6 to carboxylic acid. The microphase-separated nanostructures and LC phase behaviors were systematically studied by SAXS and WAXS. By regulation of the molar ratio of NC6 to carboxylic acid, the SLCBCPs can transform from coil−coil BCPs (x < 0.50) to rod−coil ones (x > 0.50). All the hydrogen-bonded BCPs can self-assemble into HEX nanostructures with the weight fraction of the supramolecular block PM1Hn(NC6)x ranging from 51% to 92%. The introduction of the bulky side group to one block

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Z.S.). *E-mail [email protected] (X.F.). G

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Zhihao Shen: 0000-0003-2858-555X Xinghe Fan: 0000-0002-0585-2558 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21174006 and 51725301). The authors gratefully acknowledge the scientists at Beamline 1W2A at BSRF and at Beamline BL16B1 at SSRF for their assistance on the synchrotron-radiation SAXS experiments.

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DOI: 10.1021/acs.macromol.8b00806 Macromolecules XXXX, XXX, XXX−XXX