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Hierarchical nanostructures of a liquid crystalline block copolymer with a hydrogen-bonded calamitic mesogen
Polymer 182 (2019) 121835
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Hierarchical nanostructures of a liquid crystalline block copolymer with a hydrogen-bonded calamitic mesogen Hongbing Pan, Anqi Xiao, Wei Zhang, Longfei Luo, Zhihao Shen *, Xinghe Fan ** 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
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchically ordered structure Supramolecular liquid crystalline block copolymers Calamitic mesogen Hydrogen bonding
With a pyridine derivative containing a calamitic mesogen 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]4-yl)oxy)hexyl)oxy)pyridine (HEBC6) used as the hydrogen-bonding acceptor and polydimethylsiloxane-b-poly (2,5-bis(4-carboxy phenyl)styrene) (PDMS-b-PM3H) as the hydrogen-bonding donor, a series of supramolecular liquid crystalline block copolymers (SLCBCPs) were prepared through hydrogen bonding. In the supramolecular block, the calamitic mesogen was decoupled from the motion of PM3H chains by using a flexible spacer. Different microphase-separated nanostructures and liquid crystalline (LC) structures were obtained by varying the degree of polymerization of the PM3H block and the molar ratio of HEBC6 to PDMS-b-PM3H. The SLCBCPs can selfassemble into hexagonally packed cylinders (HEX), lamellae (LAM), and inverted HEX. Smectic A phase and parallel packing of the calamitic mesogens were also observed on a smaller length scale, and these two ordered structures are synergistic and promotional. Therefore, hierarchically ordered structures can be obtained from these SLCBCPs.
1. Introduction Materials functions are determined by both chemical and physical structures. Fabrication of hierarchically ordered nanostructures plays a vital role for human beings to learn from nature and prepare functional materials [1,2]. Block copolymer (BCP) is an important model to realize the functionality of materials owing to its microphase-separated nano structures at 10–100 nm [3–8]. Scientists have gained in-depth knowl edge about coil coil BCPs both experimentally and theoretically after decades of research [9–14]. There are extensive research efforts on rod coil BCPs, and some novel structures have been discovered [15–21], for example, the zig-zag lamellar phase [22], Fddd phase [23, 24], perforated lamellar phase [25], and even Frank-Kasper phases [26]. In addition, rod blocks can form smaller-scale ordered structures at sub-10 nm within the domains of microphase-separated nanostructures owning to the orientational interaction. As a consequence, various kinds of structure-within-structures have been observed [17,19,27]. These hi erarchical structures offer great possibilities for achieving complex functionality in materials [3,18,28]. Conjugated polymers, polyisocyanates, polyamino acids, and some
liquid crystalline (LC) polymers are commonly used as rod blocks in the construction of rod coil BCPs [29–32]. Mesogen-jacketed liquid crys talline polymers (MJLCPs) are a special kind of side-chain polymers having physical properties resembling those of main-chain LC polymers and often serve as rod blocks [32–35]. Our group has systematically studied the self-assembly of rod coil BCPs containing MJLCPs, and a remarkably rich variety of nanostructures including structur e-within-structures have been obtained [32,36–39]. To simplify synthe sis, in recent years, supramolecular strategies were adopted to construct MJLCPs and their BCPs [40–42]. That is, functionalized small molecules and polymeric precursors were bonded via supramolecular interactions. With poly(dimethylsiloxane)-b-poly(2-vinylterephthalic acid) (PDMS-b-PM1H) and [4-(40 -hexyloxy)-styryl]pyridine (NC6) acting as the hydrogen-bonding donor and acceptor, respectively, a series of su pramolecular LC BCPs (SLCBCPs) were prepared [42]. Owing to the high conformational asymmetry, the curved interfaces between the rod and coil blocks promote the persistent formation of the cylindrical structure. In addition, the LC phase on a sub-10 nm length scale can develop when the molar ratio of acceptor to donor exceeds a threshold value of 50%. Thus, such supramolecular strategies can be employed to construct
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Shen), [email protected] (X. Fan). https://doi.org/10.1016/j.polymer.2019.121835 Received 20 August 2019; Received in revised form 19 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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are also discussed. PDMS-b-poly(2,5-bis(4-carboxy phenyl)styrene) (PDMS-b-PM3H) was used as the hydrogen-boning donor with the consideration of the ease of control during polymerization. A pyridine-terminated small molecule 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]-4-yl)oxy) hexyl)oxy)pyridine (HEBC6) was used as the hydrogen-bonding acceptor, in which the pyridine ring and a multi-benzene mesogen was connected through a flexible spacer. A series of hydrogen-bonded complexes were prepared by grafting the acceptor to donor, with vari ation in the donor/acceptor molar ratio. With increases in the molar ratio and degree of polymerization (DP) of the PM3H block, different microphase-separated structures with periodic sizes over 25 nm were observed. At the same time, the supramolecular block develops into the LC phase at a sub-10 nm length scale. Although the acceptor is bonded to the donor through hydrogen bonding, the decoupling effect of the spacer in the acceptor makes the ordered arrangement of the multi-benzene mesogens possible. Furthermore, synergy and promotion were observed for the formation of the LC phase of the supramolecular MJLCP block and ordered packing of the multi-benzene pendant groups. The formation of these structure-within-structures on different length scales demonstrates again the feasibility of the supramolecular strategy and the combination of the “jacketing” effect and the decoupling effect, which are beneficial to the construction of functional materials with hierarchical nanostructures.
Chart 1. Two different design principles of SLCBCPs.
structure-within-structures in rod coil BCPs containing MJLCPs (Chart 1a). When the malar ratio of acceptor to donor increases, the rigidity of the supramolecular block will increase. However, the microphaseseparated nanostructure is limited to be cylindrical owing to the curved interfaces, as mentioned above [42]. Alternatively, if a func tionalized small molecule is bonded to the polymeric precursor through a flexible spacer, the conjugation in the side chain of the supramolecular block will be broken. In addition to the microphase-separated nano structure and the LC phase, the grafted small molecules may form or dered structures because of the decoupling effect (Chart 1b). Introduction of a flexible spacer may achieve three kinds of structures with different periodic sizes. Our recent work on the self-assembly of an SLCBCP with a hydrogen-bonded discotic mesogen has proven the feasibility of this idea [43]. The microphase-separated structure of the BCP, the LC phase of the supramolecular MJLCP block, and the LC phase of the grafted discotic mesogen could co-exist. As the counterpart of the discotic mesogen, the calamitic mesogen designed in this work can also be introduced to the side chain of the polymer precursor via a spacer. The difference of the structure of the grafted mesogens in the confined nanospace and its effect on the self-assembling structures of the SLCBCPs
2. Experimental 2.1. Materials and characterization The hydrogen-bonding donor samples, PDMS-b-PM3H (DmM3Hn, in which m and n are the DPs of the two blocks), are the same as those we used previously [43]. 4-Hydroxypyridine was purchased from J&K company. Other common reagents used in the synthesis, such as po tassium carbonate (K2CO3), potassium iodide (KI), methanol (CH3OH), dichloromethane (CH2Cl2), N,N-dimethylformamide (DMF), and the deuterated chloroform (CDCl3) used in the 1H NMR characterization, were provided by the Beijing Chemical Reagents Company. All the re agents were used as received. The details of the instrument used for characterization in this work, including 1H NMR, MS, differential scanning calorimetry (DSC), FT-IR, elemental analysis (EA), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS),
Scheme 1. Synthesis of the Hydrogen-Bonding Acceptor HEBC6. 2
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Chart 2. Chemical structure of the hydrogen-bonded complex.
90 � C for 48 h, followed by filtration and rotary evaporation. With a mixture of CH2Cl2/CH3OH (10:1, v:v) used as eluent of column chro matography, the pure hydrogen-bonding acceptor HEBC6 was obtained. 1 H NMR (400 MHz, CDCl3, δ, ppm): 7.59–7.50 (m, 6H), 7.49–7.42 (m, 4H), 7.19–7.14 (d, 2H), 6.98–6.92 (d, 2H), 6.70–6.60 (d, 2H), 4.03–3.97 (t, 2H), 3.94–3.87 (t, 2H), 2.65–2.58 (t, 2H), 2.00–1.72 (m, 4H), 1.70–1.50 (m, 4H), 1.48–1.22 (m, 8H), 0.89–0.85 (t, 3H). ESI MS: (M þ H)þ/z, Calcd 531.7. Found 532.5. EA: Calcd C, 83.58; H, 7.77; N, 2.63. Found C, 83.82; H, 7.78; N, 2.62. 2.3. Preparation of the SLCBCPs The preparation and treatment methods of the hydrogen-bonded complexes are similar to those in our previous report [42,43]. And the complexes are denoted as DmM3Hn(HEBC6)x, in which x is the molar ratio of acceptor to donor. The chemical structure of the SLCBCP is shown in Chart 2. Ambient and variable-temperature FT-IR were used to characterize the formation and the temperature dependence of hydrogen bonding (Fig. S1 in Supporting Information). The FT-IR results are similar to those in our previous work [42,43], demonstrating the reversible nature of hydrogen bonding in this system. DSC curves of the complexes do not exhibit transitions of the pure acceptor, proving that there is no free acceptor molecules (Fig. 1). 3. Results and discussion The nanostructures of the complexes were investigated in detail by means of SAXS and WAXS. The complexes are supramolecular BCPs by hydrogen bonding. Thus, microphase-separated nanostructures can be acquired owing to the incompatibility between the constituent blocks. In addition, the supramolecular block of the complex is an MJLCP which can exhibit LC properties when the molecular weight (MW) is high enough. Compared with the microphase-separated nanostructures with sizes usually over 10 nm, the length scale of the LC phase is in general several nanometers. The structures on different length scales are described separately below.
Fig. 1. DSC thermograms of the hydrogen-bonded complexes D120M3H45(HEBC6)x (x ¼ 0.50–1.0), along with those of donor (x ¼ 0) and acceptor HEBC6 during the first cooling (a) and second heating (b) under a N2 atmosphere at a rate of 10 � C/min.
two-dimensional (2D) SAXS, 2D wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM) were all described previ ously [42,44,45].
3.1. Nanostructures of the complexes with sizes over 10 nm The SAXS profiles of the complexes D120M3Hn(HEBC6)x (n ¼ 10, 31, 45; x ¼ 0.10–1.0) are shown in Fig. 2, and those of other samples (n ¼ 20; x ¼ 0.10–1.0) are presented in Fig. S2a. In comparison with the SAXS profiles of the pure donors (x ¼ 0) which only show one scattering halo, the appearance of several diffraction peaks in the profiles of the complexes clearly demonstrates the formation of ordered nano structures. For the samples D120M3H10(HEBC6)x (x ¼ 0.10, 0.20, 0.30, 0.40), the scattering vector ratio is 1:√3:√7 or 1:√3:√9, indicating the formation of a hexagonally packed cylindrical (HEX) structure. PDMS is the majority phase because the weight fraction of the supramolecular block (wM3H(HEBC6)) is between 34% and 46%. When the value of x further increases, the scattering vector ratio becomes 1:2:3 or 1:2, indicating the formation of a lamellar (LAM) structure. As a result, all these complexes (n ¼ 10, x ¼ 0.50–1.0; n ¼ 20, x ¼ 0.10–0.50; n ¼ 31,
2.2. Synthesis of the acceptor with the calamitic mesogen The synthetic route of the hydrogen-bonding acceptor is shown in Scheme 1. The detailed synthetic steps of the end product HEBC6 are as follows, and those of the intermediate products are described in the Supporting Information. Synthesis of 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]-4yl)oxy)hexyl)oxy)pyridine (HEBC6). Sample 3 (0.450 g, 0.870 mmol), 4hydroxypyridine (0.250 g, 2.63 mmol), K2CO3 (0.240 g, 1.74 mmol), and KI (0.0150 g, 0.00900 mmol) were added into a 100-mL threenecked flask. After three cycles of N2 pumping, 50 mL of DMF was injected into the reaction vessel. Afterwards, the mixture was stirred at 3
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Fig. 2. SAXS profiles of the hydrogen-bonded complexes DmM3Hn(HEBC6)x (m ¼ 120; x ¼ 0.1–1.0), and those of the corresponding donors (x ¼ 0) (a: n ¼ 10; b: n ¼ 31; c: n ¼ 45). Table 1 Composition and structural information of the complexes. Complex
wM3H(PHTC6) (%)
q (nm 1)a
d (nm)
nanostructurea
a or L (nm)
D120M3H10(HEBC6)0.10 D120M3H10(HEBC6)0.20 D120M3H10(HEBC6)0.30 D120M3H10(HEBC6)0.40 D120M3H10(HEBC6)0.50 D120M3H10(HEBC6)0.60 D120M3H10(HEBC6)0.80 D120M3H10(HEBC6)1.0 D120M3H20(HEBC6)0.10 D120M3H20(HEBC6)0.20 D120M3H20(HEBC6)0.30 D120M3H20(HEBC6)0.40 D120M3H20(HEBC6)0.50 D120M3H20(HEBC6)0.60 D120M3H20(HEBC6)0.80 D120M3H20(HEBC6)1.0 D120M3H31(HEBC6)0.10 D120M3H31(HEBC6)0.20 D120M3H31(HEBC6)0.30 D120M3H31(HEBC6)0.40 D120M3H31(HEBC6)0.50 D120M3H31(HEBC6)0.60 D120M3H31(HEBC6)0.80 D120M3H31(HEBC6)1.0 D120M3H45(HEBC6)0.10 D120M3H45(HEBC6)0.20 D120M3H45(HEBC6)0.30 D120M3H45(HEBC6)0.40 D120M3H45(HEBC6)0.50 D120M3H45(HEBC6)0.60 D120M3H45(HEBC6)0.80 D120M3H45(HEBC6)1.0
33.6 38.5 42.7 46.3 49.6 52.4 57.3 61.2 50.3 55.6 59.8 63.3 66.3 68.8 72.8 76.0 61.1 66.0 69.8 72.8 75.3 77.4 80.6 83.0 69.5 73.8 77.0 79.5 81.6 83.2 85.8 87.7
0.215 0.210 0.210 0.219 0.219 0.214 0.219 0.219 0.239 0.239 0.239 0.239 0.239 0.247 0.242 0.225 0.238 0.238 0.225 0.225 0.225 0.225 0.215 0.211 0.201 0.210 0.211 0.206 0.211 0.211 0.201 0.195
29.2 29.9 29.9 28.7 28.7 29.3 28.7 28.7 26.3 26.3 26.3 26.3 26.3 25.4 26.0 27.9 26.4 26.4 27.9 27.9 27.9 27.9 29.2 29.8 31.2 29.9 29.8 30.5 29.8 29.8 31.2 32.2
HEX HEX HEX HEX LAM LAM LAM LAM LAM LAM LAM LAM LAM LAMb LAMb LAMb LAM LAM LAM LAM HEX HEX HEXb HEXb HEX HEX HEX HEX HEX HEX HEX HEX
33.7 34.5 34.5 33.1 28.7 29.3 28.7 28.7 26.3 26.3 26.3 26.3 26.3 25.4 26.0 27.9 26.4 26.4 27.9 27.9 32.2 32.2 33.7 34.4 36.0 34.5 34.4 35.2 34.4 34.4 36.0 37.2
a b
Determined by SAXS results. Speculated from the wM3H(HEBC6) value.
x ¼ 0.10–0.40) with a scattering vector ratio of 1:2:3 or 1:2 in SAXS profiles self-assemble into LAM structures. SAXS profiles of some com plexes (n ¼ 20, x ¼ 0.60–1.0) only have one diffraction peak because of poor ordering. For rod coil BCPs with a large conformational asym metry, the region of the LAM structure is widened compared with their coil coil counterparts [15]. In consideration of the wM3H(HEBC6) values of these samples, their self-assembled structures may reasonably be assigned as LAM. For the samples with n ¼ 31, x ¼ 0.50, 0.60 and n ¼ 45, x ¼ 0.10–1.0, the scattering vector ratio becomes 1:√7 or 1:√3 or 1:√3:√7, indicating the formation of HEX structures. For these
complexes, the values of wM3H(HEBC6) are between 75% and 88%; therefore, PDMS should be the cylinders dispersed in the matrix of the supramolecular MJLCP block. For another two samples with n ¼ 31 and x ¼ 0.80 or 1.0, the SAXS profiles lack the higher-order diffraction peaks. Because their wM3H(HEBC6) values exceed 80%, the self-assembled structures may reasonably be assumed to be HEX. The detailed composition and nanostructure information of the complexes are listed in Table 1. The values of d-spacing’s calculated from q values of the first-order peak for LAM structures (L values) and the converted cylinder-to-cylinder distances (a values) for HEX structures are all above 4
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Fig. 3. WAXS profiles of the hydrogen-bonded complexes DmM3Hn(HEBC6)x (m ¼ 120; x ¼ 0.1–1.0), and those of the corresponding donors (x ¼ 0) (a: n ¼ 10; b: n ¼ 31; c: n ¼ 45).
25 nm, which are in the normal range for microphase-separated struc tures of BCPs. As the supplementary evidence of the microphase-separated structures, the TEM micrographs of the LAM structure and the HEX structure with PDMS as dispersed cylinders are provided in Fig. S3. However, owing to the flexible nature of PDMS when it is the major component, the HEX structure with PDMS as the continuous phase was not observed by TEM. 3.2. Structures of the complexes at a sub-10 nm length scale The WAXS profiles of complexes D120M3Hn(HEBC6)x (n ¼ 10, 31, 45; x ¼ 0.10–1.0) are shown in Fig. 3, and those of other samples (n ¼ 20; x ¼ 0.10–1.0) are presented in Fig. S2b. The profiles of all these com plexes and the donors have one diffuse scattering halo at about 8.5 nm 1, and the corresponding d-spacing is 0.74 nm. It comes from the average lateral distance of the PDMS blocks [46]. For the complexes (x ¼ 0.10–1.0), D120M3H20(HEBC6)x D120M3H10(HEBC6)x (x ¼ 0.10–0.80), D120M3H31(HEBC6)x (x ¼ 0.10–0.6), and D120M3H45(HEBC6)x (x ¼ 0.10–0.50), their profiles exhibit a second diffuse scattering halo at about 14 nm 1, corresponding to a d-spacing of 0.45 nm, which represents the average distance between the pendant groups in the side chain of the supramolecular block. Lack of diffraction peaks in the WAXS profiles indicates that the samples mentioned above are not liquid crystalline. With increasing x, diffraction peaks start to appear. For the samples D120M3H20(HEBC6)1.0, D120M3H31(HEBC6)x (x ¼ 0.80–1.0), and D120M3H45(HEBC6)x (x ¼ 0.60–1.0), the sharp diffraction peaks in the low-angle region have a scattering vector ratio of 1:2:3, indicating the formation of a smectic LC phase. The q value of the first-order diffraction peak is 0.910 nm 1, and the corresponding d-spacing is 6.90 nm. Assuming that the alkyl chain takes on all-trans conformation, the size of the hydrogen-bonding acceptor is 3.29 nm, and the side-chain length of the hydrogen-bonding donor is 1.39 nm. The sum of twice the size of the acceptor and the side-chain length of the donor is 7.97 nm. With the consideration of inevitable interdigitation of the alkyl chains in the side chains of the supramolecular block, the experimental value is quite consistent with the calculated one, sug gesting that the supramolecular block may be in the smectic A (SmA) phase. At the same time, there appears a sharp diffraction peak on top of the amorphous halo at about 14 nm 1, and the exact peak position is 14.3 nm 1 (corresponding to a d-spacing of 0.44 nm), indicating that the pendant calamitic mesogenic groups in the supramolecular block are arranged in an ordered fashion facilitated by π π stacking. The simul taneous appearance of the smectic phase and π π stacking illustrates the
Fig. 4. 2D WAXD patterns of the hydrogen-bonded complexes for D120M3H20(HEBC6)1.0 (a) and D120M3H45(HEBC6)1.0 (b) with the X-ray beam along the Y direction. The insert in (a) shows the shearing geometry.
synergy and promotion between these two kinds of ordered packing. In order to determine the type of the smectic phase, 2D WAXD ex periments were conducted. All the samples were sheared along the X direction, and the X-ray incident beam was along the Y direction. For (Fig. 4a) and both complexes D120M3H20(HEBC6)1.0 D120M3H45(HEBC6)1.0 (Fig. 4b), the diffraction originating from the smectic layers of the supramolecular MJLCP block splits into two sharp arcs centered on the equator, while the amorphous halo in the high5
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Fig. 5. 2D SAXS patterns (a: D120M3H20(HEBC6)1.0; b: D120M3H45(HEBC6)1.0) with the X-ray beam along the Y direction, the schematic diagrams of molecular arrangement in the hydrogen-bonded complexes (c: D120M3H20(HEBC6)1.0; d: D120M3H45(HEBC6)1.0), the cartoons of the corresponding components in the com plexes (e), and the shearing geometry (f).
angle region splits into two arcs centered on the meridian, proving that the supramolecular block does form the SmA phase with the layer normal perpendicular to the shear direction. The appearance of the highangle diffraction arcs on the meridian indicates that the direction of the π π stacking is parallel to the shear direction. Notably, although the microphase-separated nanostructures are different for the two com plexes, the orientation of the SmA phase and π π stacking are the same in the different types of confined nanospace. 3.3. Relative orientation of the structures on different length scales For the exploration of the relative orientation among the structures on different length scales, 2D SAXS experiments were conducted after 2D WAXD measurements, and the results are illustrated in Fig. 5. In the 2D SAXS patterns of the two complexes (Fig. 5a and b), the diffraction from the microphase-separated nanostructures appears as a ring without splitting even though shearing was carried out for multiple times. However, the relative orientation among the structures on different length scales can still be deduced. Generally, the main chain of the polymer tends to be aligned along the shear direction. Thus, for D120M3H20(HEBC6)1.0, the normal of the microphase-separated LAM structure is perpendicular to that of the SmA phase (Fig. 5c). However, the situation of D120M3H45(HEBC6)1.0 is more complicated. Thomas et al. proposed four structural models of smectic layers and cylindrical microdomains with two types of boundary conditions for the LC meso gens with respect to the inter-material dividing surface, including parallel-transverse, perpendicular-parallel, parallel-parallel, and transverse-perpendicular [47], in which the first and second words indicate the orientation of the BCP cylinders and the LC layers, respec tively. They observed transverse-perpendicular orientation in their system. In a previous work on supramolecular BCP formed by blending bent-core molecules with polystyrene-b-poly(4-vinylpyridine) via hydrogen bonding, the perpendicular-parallel orientation was observed [48]. In this work, the SmA phase is well oriented whereas the microphase-separated structure is not on the basis of the 2D WAXD and SAXS results. Hence, the orientation of the hierarchical structure under shear may be dominated by the LC phase. For D120M3H45(HEBC6)1.0, the cylinders and matrix of the HEX structure are formed by the flexible PDMS and rigid supramolecular MJLCP, respectively. As mentioned above, the normal of the SmA phase is perpendicular to the shear di rection. Therefore, the long axis of the HEX structure formed by
Fig. 6. Variable-temperature SAXS (a) and WAXS (b) profiles of the hydrogenbonded complex D120M3H45(HEBC6)1.0 during heating.
6
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Fig. 7. Schematic diagram of the hierarchical nanostructures formed by the hydrogen-bonded complexes, with molar ratio of hydrogen-bonding acceptor to donor (x) and DP of the PM3H block (n) as the vertical and horizontal axes, respectively.
microphase separation is also perpendicular to the shear direction (Fig. 5d). In other words, the normal of the SmA phase is parallel to the long axis of the cylinders of the HEX structure. In this case, BCP cylinders are in the least favorable orientation while the smectic layers are in their most favorable orientation. The schematic diagram of the molecular chain arrangement of the two complexes are presented in Fig. 5c and d.
SLCBCPs, which is similar to the grafted discotic mesogen we studied previously [43]. However, in comparison with the discotic counterpart that tends to form a discotic nematic (ND) phase in the microphase-separated nanospace, the calamitic mesogens prefer to be arranged in a parallel fashion owing to the π π interaction among them. On the other hand, the self-assembling behavior of the SLCBCPs are different. Owing to the larger MW of the precursor BCP and the higher content of flexible alkyl chains in the hydrogen-bonded discotic meso gen, the resultant SLCBCP can only form the SmA-within-HEX hierar chically ordered nanostructure. However, the SLCBCP containing calamitic mesogen in this work can form SmA-within-LAM and SmA- within-HEX hierarchically ordered nanostructures.
3.4. Temperature dependence of the structures on different length scales Variable-temperature SAXS and WAXS experiments were carried out to study the structural transformation of the complexes. With D120M3H45(HEBC6)1.0 as an example, the scattering vector ratio of the diffraction peaks in the SAXS profiles remains unchanged during the heating process (Fig. 6a), indicating that the microphase-separated nanostructure is stable at high temperatures. On the other hand, the WAXS results are much more complicated (Fig. 6b). The diffraction peaks in the low-angle region do not vary with temperature, suggesting that the SmA phase formed by the supramolecular block is stable. However, the sharp diffraction peak representing the π π stacking in the high-angle region becomes a diffuse halo at high temperatures, implying that the ordered packing of the pendant calamitic mesogenic groups in the side chains of the supramolecular block is destroyed. Upon cooling, the π π stacking interaction is recovered at low temperatures (Fig. S4). However, the disappearance of the ordered packing of the calamitic mesogenic groups during heating is not detected in the DSC curves (Fig. 1), which may be related to the small enthalpic change of the transition.
4. Conclusions In this work, a series of SLCBCPs were prepared by grafting a small molecule with a calamitic mesogen to BCP precursors through hydrogen bonding. Different hierarchical nanostructures were obtained from the self-assembly of the hydrogen-bonded complexes. The HEX structures with PDMS as the continuous phase, the LAM structures, and the inverted HEX structures with PDMS as the dispersed cylinders all having sizes over 25 nm are formed in sequence when the molar ratio of the acceptor to donor (x) and the DP of the PM3H block (n) increase. When n � 20 and the value of x is large enough (x � 0.8), the supramolecular block can develop into the SmA phase at a sub-10 nm length scale owing to the “jacketing” effect of the grafted bulky pendant group. Meanwhile, the pendant calamitic mesogenic group is able to form an ordered arrangement attributed to π π stacking. Moreover, the SmA phase and the π π stacking are synergistic and promotional to each other when x is increased. Thus, the hierarchically ordered nanostructures, such as SmA-within-LAM and SmA-within-HEX along with the ordered packing of the pendant calamitic mesogen, can be obtained. On the one hand, this method of grafting an LC mesogen onto BCPs using a flexible spacer by non-covalent bonds can be used to construct and control hierarchically ordered nanostructures. On the other hand, such hydrogen-bonded complexes with hierarchical nanostructures may be made into func tional materials after washing away the acceptor small molecules by taking advantage of the reversibility of hydrogen bonding.
3.5. Hierarchically ordered structures of the SLCBCPs On the basis of the results mentioned above, the hydrogen-bonded complexes can self-assemble into structures on different length scales. For example, the sample D120M3H20(HEBC6)1.0 can simultaneously form a LAM structure with a larger spacing over 25 nm and an SmA phase at sub-10 nm, resulting in an SmA-within-LAM hierarchically or dered nanostructure. As a second example, D120M3H45(HEBC6)1.0 can self-assemble into an SmA-within-HEX hierarchical nanostructure. With x and n as the vertical and horizontal axes, respectively, a schematic diagram of the hierarchical nanostructures is shown in Fig. 7. In sum mary, the hydrogen-bonded calamitic mesogen can induce the hierar chically ordered self-assembled nanostructures of the resultant 7
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Acknowledgements
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The authors express their sincere gratitude to the support of the National Key R&D Program of China (Grant 2018YFB0703702) and the National Natural Science Foundation of China (Grants 51725301 and 21174006). The authors also acknowledge the assistance of the staff at beamline 1W2A in BSRF and BL16B1 in SSRF for synchrotron-radiation SAXS experiments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.polymer.2019.121835. References [1] D. Losic, G. Mitchell James, H. Voelcker Nicolas, Diatomaceous lessons in nanotechnology and advanced materials, Adv. Mater. 21 (29) (2009) 2947–2958. [2] W.-B. Zhang, X. Yu, C.-L. Wang, H.-J. Sun, I.F. Hsieh, Y. Li, X.-H. Dong, K. Yue, R. Van Horn, S.Z.D. Cheng, Molecular nanoparticles are unique elements for macromolecular science: from “nanoatoms” to giant molecules, Macromolecules 47 (4) (2014) 1221–1239. [3] M. Lazzari, M.A. L� opez-Quintela, Block copolymers as a tool for nanomaterial fabrication, Adv. Mater. 15 (19) (2003) 1583–1594. [4] A.-V. Ruzette, L. Leibler, Block copolymers in tomorrow’s plastics, Nat. Mater. 4 (1) (2005) 19–31. [5] T.P. Lodge, Block copolymers: past successes and future challenges, Macromol. Chem. Phys. 204 (2) (2003) 265–273. [6] F.S. Bates, G.H. Fredrickson, Block copolymers-designer soft materials, Phys. Today 52 (1999) 32–38. [7] F.H. Schacher, P.A. Rupar, I. Manners, Functional block copolymers: nanostructured materials with emerging applications, Angew. Chem. Int. Ed. 51 (32) (2012) 7898–7921. [8] M. Gopinadhan, P. Deshmukh, Y. Choo, P.W. Majewski, O. Bakajin, M. Elimelech, R.M. Kasi, C.O. Osuji, Thermally switchable aligned nanopores by magnetic-field directed self-assembly of block copolymers, Adv. Mater. 26 (2014) 5148–5154. [9] F.S. Bates, G.H. Fredrickson, Block copolymer thermodynamics: theory and experiment, Annu. Rev. Phys. Chem. 41 (1) (1990) 525–557. [10] S.B. Darling, Directing the self-assembly of block copolymers, Prog. Polym. Sci. 32 (10) (2007) 1152–1204. [11] F.S. Bates, Polymer-polymer phase behavior, Science 251 (4996) (1991) 898–905. [12] M.W. Matsen, Self-consistent field theory for melts of low-molecular-weight diblock copolymer, Macromolecules 45 (20) (2012) 8502–8509. [13] L. Leibler, Theory of microphase separation in block copolymers, Macromolecules 13 (6) (1980) 1602–1617. [14] M.W. Matsen, F.S. Bates, Unifying weak- and strong-segregation block copolymer theories, Macromolecules 29 (4) (1996) 1091–1098. [15] M.W. Matsen, M. Schick, Microphases of a diblock copolymer with conformational asymmetry, Macromolecules 27 (14) (1994) 4014–4015. [16] B.D. Olsen, R.A. Segalman, Nonlamellar phases in asymmetric Rod Coil block copolymers at increased segregation strengths, Macromolecules 40 (19) (2007) 6922–6929. [17] A. Halperin, Rod-coil copolymers: their aggregation behavior, Macromolecules 23 (10) (1990) 2724–2731. [18] B.D. Olsen, R.A. Segalman, Self-assembly of rod–coil block copolymers, Mater. Sci. Eng. R Rep. R 62 (2) (2008) 37–66. [19] M. Lee, B.-K. Cho, W.-C. Zin, Supramolecular structures from Rod Coil block copolymers, Chem. Rev. 101 (12) (2001) 3869–3892. [20] Q.A. Wang, Theory and simulation of the self-assembly of rod-coil block copolymer melts: recent progress, Soft Matter 7 (8) (2011) 3711–3716. [21] B.D. Olsen, M. Shah, V. Ganesan, R.A. Segalman, Universalization of the phase diagram for a model Rod Coil diblock copolymer, Macromolecules 41 (18) (2008) 6809–6817. [22] J.T. Chen, E.L. Thomas, C.K. Ober, S.S. Hwang, Zigzag morphology of a poly (styrene-b-hexyl isocyanate) rod-coil block-copolymer, Macromolecules 28 (5) (1995) 1688–1697. [23] M.I. Kim, T. Wakada, S. Akasaka, S. Nishitsuji, K. Saijo, H. Hasegawa, K. Ito, M. Takenaka, Determination of the Fddd phase boundary in polystyrene-blockpolyisoprene diblock copolymer melts, Macromolecules 42 (14) (2009) 5266–5271.
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Hierarchical nanostructures of a liquid crystalline block copolymer with a hydrogen-bonded calamitic mesogen Hongbing Pan, Anqi Xiao, Wei Zhang, Longfei Luo, Zhihao Shen *, Xinghe Fan ** 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
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchically ordered structure Supramolecular liquid crystalline block copolymers Calamitic mesogen Hydrogen bonding
With a pyridine derivative containing a calamitic mesogen 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]4-yl)oxy)hexyl)oxy)pyridine (HEBC6) used as the hydrogen-bonding acceptor and polydimethylsiloxane-b-poly (2,5-bis(4-carboxy phenyl)styrene) (PDMS-b-PM3H) as the hydrogen-bonding donor, a series of supramolecular liquid crystalline block copolymers (SLCBCPs) were prepared through hydrogen bonding. In the supramolecular block, the calamitic mesogen was decoupled from the motion of PM3H chains by using a flexible spacer. Different microphase-separated nanostructures and liquid crystalline (LC) structures were obtained by varying the degree of polymerization of the PM3H block and the molar ratio of HEBC6 to PDMS-b-PM3H. The SLCBCPs can selfassemble into hexagonally packed cylinders (HEX), lamellae (LAM), and inverted HEX. Smectic A phase and parallel packing of the calamitic mesogens were also observed on a smaller length scale, and these two ordered structures are synergistic and promotional. Therefore, hierarchically ordered structures can be obtained from these SLCBCPs.
1. Introduction Materials functions are determined by both chemical and physical structures. Fabrication of hierarchically ordered nanostructures plays a vital role for human beings to learn from nature and prepare functional materials [1,2]. Block copolymer (BCP) is an important model to realize the functionality of materials owing to its microphase-separated nano structures at 10–100 nm [3–8]. Scientists have gained in-depth knowl edge about coil coil BCPs both experimentally and theoretically after decades of research [9–14]. There are extensive research efforts on rod coil BCPs, and some novel structures have been discovered [15–21], for example, the zig-zag lamellar phase [22], Fddd phase [23, 24], perforated lamellar phase [25], and even Frank-Kasper phases [26]. In addition, rod blocks can form smaller-scale ordered structures at sub-10 nm within the domains of microphase-separated nanostructures owning to the orientational interaction. As a consequence, various kinds of structure-within-structures have been observed [17,19,27]. These hi erarchical structures offer great possibilities for achieving complex functionality in materials [3,18,28]. Conjugated polymers, polyisocyanates, polyamino acids, and some
liquid crystalline (LC) polymers are commonly used as rod blocks in the construction of rod coil BCPs [29–32]. Mesogen-jacketed liquid crys talline polymers (MJLCPs) are a special kind of side-chain polymers having physical properties resembling those of main-chain LC polymers and often serve as rod blocks [32–35]. Our group has systematically studied the self-assembly of rod coil BCPs containing MJLCPs, and a remarkably rich variety of nanostructures including structur e-within-structures have been obtained [32,36–39]. To simplify synthe sis, in recent years, supramolecular strategies were adopted to construct MJLCPs and their BCPs [40–42]. That is, functionalized small molecules and polymeric precursors were bonded via supramolecular interactions. With poly(dimethylsiloxane)-b-poly(2-vinylterephthalic acid) (PDMS-b-PM1H) and [4-(40 -hexyloxy)-styryl]pyridine (NC6) acting as the hydrogen-bonding donor and acceptor, respectively, a series of su pramolecular LC BCPs (SLCBCPs) were prepared [42]. Owing to the high conformational asymmetry, the curved interfaces between the rod and coil blocks promote the persistent formation of the cylindrical structure. In addition, the LC phase on a sub-10 nm length scale can develop when the molar ratio of acceptor to donor exceeds a threshold value of 50%. Thus, such supramolecular strategies can be employed to construct
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Shen), [email protected] (X. Fan). https://doi.org/10.1016/j.polymer.2019.121835 Received 20 August 2019; Received in revised form 19 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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are also discussed. PDMS-b-poly(2,5-bis(4-carboxy phenyl)styrene) (PDMS-b-PM3H) was used as the hydrogen-boning donor with the consideration of the ease of control during polymerization. A pyridine-terminated small molecule 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]-4-yl)oxy) hexyl)oxy)pyridine (HEBC6) was used as the hydrogen-bonding acceptor, in which the pyridine ring and a multi-benzene mesogen was connected through a flexible spacer. A series of hydrogen-bonded complexes were prepared by grafting the acceptor to donor, with vari ation in the donor/acceptor molar ratio. With increases in the molar ratio and degree of polymerization (DP) of the PM3H block, different microphase-separated structures with periodic sizes over 25 nm were observed. At the same time, the supramolecular block develops into the LC phase at a sub-10 nm length scale. Although the acceptor is bonded to the donor through hydrogen bonding, the decoupling effect of the spacer in the acceptor makes the ordered arrangement of the multi-benzene mesogens possible. Furthermore, synergy and promotion were observed for the formation of the LC phase of the supramolecular MJLCP block and ordered packing of the multi-benzene pendant groups. The formation of these structure-within-structures on different length scales demonstrates again the feasibility of the supramolecular strategy and the combination of the “jacketing” effect and the decoupling effect, which are beneficial to the construction of functional materials with hierarchical nanostructures.
Chart 1. Two different design principles of SLCBCPs.
structure-within-structures in rod coil BCPs containing MJLCPs (Chart 1a). When the malar ratio of acceptor to donor increases, the rigidity of the supramolecular block will increase. However, the microphaseseparated nanostructure is limited to be cylindrical owing to the curved interfaces, as mentioned above [42]. Alternatively, if a func tionalized small molecule is bonded to the polymeric precursor through a flexible spacer, the conjugation in the side chain of the supramolecular block will be broken. In addition to the microphase-separated nano structure and the LC phase, the grafted small molecules may form or dered structures because of the decoupling effect (Chart 1b). Introduction of a flexible spacer may achieve three kinds of structures with different periodic sizes. Our recent work on the self-assembly of an SLCBCP with a hydrogen-bonded discotic mesogen has proven the feasibility of this idea [43]. The microphase-separated structure of the BCP, the LC phase of the supramolecular MJLCP block, and the LC phase of the grafted discotic mesogen could co-exist. As the counterpart of the discotic mesogen, the calamitic mesogen designed in this work can also be introduced to the side chain of the polymer precursor via a spacer. The difference of the structure of the grafted mesogens in the confined nanospace and its effect on the self-assembling structures of the SLCBCPs
2. Experimental 2.1. Materials and characterization The hydrogen-bonding donor samples, PDMS-b-PM3H (DmM3Hn, in which m and n are the DPs of the two blocks), are the same as those we used previously [43]. 4-Hydroxypyridine was purchased from J&K company. Other common reagents used in the synthesis, such as po tassium carbonate (K2CO3), potassium iodide (KI), methanol (CH3OH), dichloromethane (CH2Cl2), N,N-dimethylformamide (DMF), and the deuterated chloroform (CDCl3) used in the 1H NMR characterization, were provided by the Beijing Chemical Reagents Company. All the re agents were used as received. The details of the instrument used for characterization in this work, including 1H NMR, MS, differential scanning calorimetry (DSC), FT-IR, elemental analysis (EA), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS),
Scheme 1. Synthesis of the Hydrogen-Bonding Acceptor HEBC6. 2
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Chart 2. Chemical structure of the hydrogen-bonded complex.
90 � C for 48 h, followed by filtration and rotary evaporation. With a mixture of CH2Cl2/CH3OH (10:1, v:v) used as eluent of column chro matography, the pure hydrogen-bonding acceptor HEBC6 was obtained. 1 H NMR (400 MHz, CDCl3, δ, ppm): 7.59–7.50 (m, 6H), 7.49–7.42 (m, 4H), 7.19–7.14 (d, 2H), 6.98–6.92 (d, 2H), 6.70–6.60 (d, 2H), 4.03–3.97 (t, 2H), 3.94–3.87 (t, 2H), 2.65–2.58 (t, 2H), 2.00–1.72 (m, 4H), 1.70–1.50 (m, 4H), 1.48–1.22 (m, 8H), 0.89–0.85 (t, 3H). ESI MS: (M þ H)þ/z, Calcd 531.7. Found 532.5. EA: Calcd C, 83.58; H, 7.77; N, 2.63. Found C, 83.82; H, 7.78; N, 2.62. 2.3. Preparation of the SLCBCPs The preparation and treatment methods of the hydrogen-bonded complexes are similar to those in our previous report [42,43]. And the complexes are denoted as DmM3Hn(HEBC6)x, in which x is the molar ratio of acceptor to donor. The chemical structure of the SLCBCP is shown in Chart 2. Ambient and variable-temperature FT-IR were used to characterize the formation and the temperature dependence of hydrogen bonding (Fig. S1 in Supporting Information). The FT-IR results are similar to those in our previous work [42,43], demonstrating the reversible nature of hydrogen bonding in this system. DSC curves of the complexes do not exhibit transitions of the pure acceptor, proving that there is no free acceptor molecules (Fig. 1). 3. Results and discussion The nanostructures of the complexes were investigated in detail by means of SAXS and WAXS. The complexes are supramolecular BCPs by hydrogen bonding. Thus, microphase-separated nanostructures can be acquired owing to the incompatibility between the constituent blocks. In addition, the supramolecular block of the complex is an MJLCP which can exhibit LC properties when the molecular weight (MW) is high enough. Compared with the microphase-separated nanostructures with sizes usually over 10 nm, the length scale of the LC phase is in general several nanometers. The structures on different length scales are described separately below.
Fig. 1. DSC thermograms of the hydrogen-bonded complexes D120M3H45(HEBC6)x (x ¼ 0.50–1.0), along with those of donor (x ¼ 0) and acceptor HEBC6 during the first cooling (a) and second heating (b) under a N2 atmosphere at a rate of 10 � C/min.
two-dimensional (2D) SAXS, 2D wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM) were all described previ ously [42,44,45].
3.1. Nanostructures of the complexes with sizes over 10 nm The SAXS profiles of the complexes D120M3Hn(HEBC6)x (n ¼ 10, 31, 45; x ¼ 0.10–1.0) are shown in Fig. 2, and those of other samples (n ¼ 20; x ¼ 0.10–1.0) are presented in Fig. S2a. In comparison with the SAXS profiles of the pure donors (x ¼ 0) which only show one scattering halo, the appearance of several diffraction peaks in the profiles of the complexes clearly demonstrates the formation of ordered nano structures. For the samples D120M3H10(HEBC6)x (x ¼ 0.10, 0.20, 0.30, 0.40), the scattering vector ratio is 1:√3:√7 or 1:√3:√9, indicating the formation of a hexagonally packed cylindrical (HEX) structure. PDMS is the majority phase because the weight fraction of the supramolecular block (wM3H(HEBC6)) is between 34% and 46%. When the value of x further increases, the scattering vector ratio becomes 1:2:3 or 1:2, indicating the formation of a lamellar (LAM) structure. As a result, all these complexes (n ¼ 10, x ¼ 0.50–1.0; n ¼ 20, x ¼ 0.10–0.50; n ¼ 31,
2.2. Synthesis of the acceptor with the calamitic mesogen The synthetic route of the hydrogen-bonding acceptor is shown in Scheme 1. The detailed synthetic steps of the end product HEBC6 are as follows, and those of the intermediate products are described in the Supporting Information. Synthesis of 4-((6-((4’-((4-hexylphenyl)ethynyl)-[1,10 -biphenyl]-4yl)oxy)hexyl)oxy)pyridine (HEBC6). Sample 3 (0.450 g, 0.870 mmol), 4hydroxypyridine (0.250 g, 2.63 mmol), K2CO3 (0.240 g, 1.74 mmol), and KI (0.0150 g, 0.00900 mmol) were added into a 100-mL threenecked flask. After three cycles of N2 pumping, 50 mL of DMF was injected into the reaction vessel. Afterwards, the mixture was stirred at 3
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Fig. 2. SAXS profiles of the hydrogen-bonded complexes DmM3Hn(HEBC6)x (m ¼ 120; x ¼ 0.1–1.0), and those of the corresponding donors (x ¼ 0) (a: n ¼ 10; b: n ¼ 31; c: n ¼ 45). Table 1 Composition and structural information of the complexes. Complex
wM3H(PHTC6) (%)
q (nm 1)a
d (nm)
nanostructurea
a or L (nm)
D120M3H10(HEBC6)0.10 D120M3H10(HEBC6)0.20 D120M3H10(HEBC6)0.30 D120M3H10(HEBC6)0.40 D120M3H10(HEBC6)0.50 D120M3H10(HEBC6)0.60 D120M3H10(HEBC6)0.80 D120M3H10(HEBC6)1.0 D120M3H20(HEBC6)0.10 D120M3H20(HEBC6)0.20 D120M3H20(HEBC6)0.30 D120M3H20(HEBC6)0.40 D120M3H20(HEBC6)0.50 D120M3H20(HEBC6)0.60 D120M3H20(HEBC6)0.80 D120M3H20(HEBC6)1.0 D120M3H31(HEBC6)0.10 D120M3H31(HEBC6)0.20 D120M3H31(HEBC6)0.30 D120M3H31(HEBC6)0.40 D120M3H31(HEBC6)0.50 D120M3H31(HEBC6)0.60 D120M3H31(HEBC6)0.80 D120M3H31(HEBC6)1.0 D120M3H45(HEBC6)0.10 D120M3H45(HEBC6)0.20 D120M3H45(HEBC6)0.30 D120M3H45(HEBC6)0.40 D120M3H45(HEBC6)0.50 D120M3H45(HEBC6)0.60 D120M3H45(HEBC6)0.80 D120M3H45(HEBC6)1.0
33.6 38.5 42.7 46.3 49.6 52.4 57.3 61.2 50.3 55.6 59.8 63.3 66.3 68.8 72.8 76.0 61.1 66.0 69.8 72.8 75.3 77.4 80.6 83.0 69.5 73.8 77.0 79.5 81.6 83.2 85.8 87.7
0.215 0.210 0.210 0.219 0.219 0.214 0.219 0.219 0.239 0.239 0.239 0.239 0.239 0.247 0.242 0.225 0.238 0.238 0.225 0.225 0.225 0.225 0.215 0.211 0.201 0.210 0.211 0.206 0.211 0.211 0.201 0.195
29.2 29.9 29.9 28.7 28.7 29.3 28.7 28.7 26.3 26.3 26.3 26.3 26.3 25.4 26.0 27.9 26.4 26.4 27.9 27.9 27.9 27.9 29.2 29.8 31.2 29.9 29.8 30.5 29.8 29.8 31.2 32.2
HEX HEX HEX HEX LAM LAM LAM LAM LAM LAM LAM LAM LAM LAMb LAMb LAMb LAM LAM LAM LAM HEX HEX HEXb HEXb HEX HEX HEX HEX HEX HEX HEX HEX
33.7 34.5 34.5 33.1 28.7 29.3 28.7 28.7 26.3 26.3 26.3 26.3 26.3 25.4 26.0 27.9 26.4 26.4 27.9 27.9 32.2 32.2 33.7 34.4 36.0 34.5 34.4 35.2 34.4 34.4 36.0 37.2
a b
Determined by SAXS results. Speculated from the wM3H(HEBC6) value.
x ¼ 0.10–0.40) with a scattering vector ratio of 1:2:3 or 1:2 in SAXS profiles self-assemble into LAM structures. SAXS profiles of some com plexes (n ¼ 20, x ¼ 0.60–1.0) only have one diffraction peak because of poor ordering. For rod coil BCPs with a large conformational asym metry, the region of the LAM structure is widened compared with their coil coil counterparts [15]. In consideration of the wM3H(HEBC6) values of these samples, their self-assembled structures may reasonably be assigned as LAM. For the samples with n ¼ 31, x ¼ 0.50, 0.60 and n ¼ 45, x ¼ 0.10–1.0, the scattering vector ratio becomes 1:√7 or 1:√3 or 1:√3:√7, indicating the formation of HEX structures. For these
complexes, the values of wM3H(HEBC6) are between 75% and 88%; therefore, PDMS should be the cylinders dispersed in the matrix of the supramolecular MJLCP block. For another two samples with n ¼ 31 and x ¼ 0.80 or 1.0, the SAXS profiles lack the higher-order diffraction peaks. Because their wM3H(HEBC6) values exceed 80%, the self-assembled structures may reasonably be assumed to be HEX. The detailed composition and nanostructure information of the complexes are listed in Table 1. The values of d-spacing’s calculated from q values of the first-order peak for LAM structures (L values) and the converted cylinder-to-cylinder distances (a values) for HEX structures are all above 4
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Fig. 3. WAXS profiles of the hydrogen-bonded complexes DmM3Hn(HEBC6)x (m ¼ 120; x ¼ 0.1–1.0), and those of the corresponding donors (x ¼ 0) (a: n ¼ 10; b: n ¼ 31; c: n ¼ 45).
25 nm, which are in the normal range for microphase-separated struc tures of BCPs. As the supplementary evidence of the microphase-separated structures, the TEM micrographs of the LAM structure and the HEX structure with PDMS as dispersed cylinders are provided in Fig. S3. However, owing to the flexible nature of PDMS when it is the major component, the HEX structure with PDMS as the continuous phase was not observed by TEM. 3.2. Structures of the complexes at a sub-10 nm length scale The WAXS profiles of complexes D120M3Hn(HEBC6)x (n ¼ 10, 31, 45; x ¼ 0.10–1.0) are shown in Fig. 3, and those of other samples (n ¼ 20; x ¼ 0.10–1.0) are presented in Fig. S2b. The profiles of all these com plexes and the donors have one diffuse scattering halo at about 8.5 nm 1, and the corresponding d-spacing is 0.74 nm. It comes from the average lateral distance of the PDMS blocks [46]. For the complexes (x ¼ 0.10–1.0), D120M3H20(HEBC6)x D120M3H10(HEBC6)x (x ¼ 0.10–0.80), D120M3H31(HEBC6)x (x ¼ 0.10–0.6), and D120M3H45(HEBC6)x (x ¼ 0.10–0.50), their profiles exhibit a second diffuse scattering halo at about 14 nm 1, corresponding to a d-spacing of 0.45 nm, which represents the average distance between the pendant groups in the side chain of the supramolecular block. Lack of diffraction peaks in the WAXS profiles indicates that the samples mentioned above are not liquid crystalline. With increasing x, diffraction peaks start to appear. For the samples D120M3H20(HEBC6)1.0, D120M3H31(HEBC6)x (x ¼ 0.80–1.0), and D120M3H45(HEBC6)x (x ¼ 0.60–1.0), the sharp diffraction peaks in the low-angle region have a scattering vector ratio of 1:2:3, indicating the formation of a smectic LC phase. The q value of the first-order diffraction peak is 0.910 nm 1, and the corresponding d-spacing is 6.90 nm. Assuming that the alkyl chain takes on all-trans conformation, the size of the hydrogen-bonding acceptor is 3.29 nm, and the side-chain length of the hydrogen-bonding donor is 1.39 nm. The sum of twice the size of the acceptor and the side-chain length of the donor is 7.97 nm. With the consideration of inevitable interdigitation of the alkyl chains in the side chains of the supramolecular block, the experimental value is quite consistent with the calculated one, sug gesting that the supramolecular block may be in the smectic A (SmA) phase. At the same time, there appears a sharp diffraction peak on top of the amorphous halo at about 14 nm 1, and the exact peak position is 14.3 nm 1 (corresponding to a d-spacing of 0.44 nm), indicating that the pendant calamitic mesogenic groups in the supramolecular block are arranged in an ordered fashion facilitated by π π stacking. The simul taneous appearance of the smectic phase and π π stacking illustrates the
Fig. 4. 2D WAXD patterns of the hydrogen-bonded complexes for D120M3H20(HEBC6)1.0 (a) and D120M3H45(HEBC6)1.0 (b) with the X-ray beam along the Y direction. The insert in (a) shows the shearing geometry.
synergy and promotion between these two kinds of ordered packing. In order to determine the type of the smectic phase, 2D WAXD ex periments were conducted. All the samples were sheared along the X direction, and the X-ray incident beam was along the Y direction. For (Fig. 4a) and both complexes D120M3H20(HEBC6)1.0 D120M3H45(HEBC6)1.0 (Fig. 4b), the diffraction originating from the smectic layers of the supramolecular MJLCP block splits into two sharp arcs centered on the equator, while the amorphous halo in the high5
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Fig. 5. 2D SAXS patterns (a: D120M3H20(HEBC6)1.0; b: D120M3H45(HEBC6)1.0) with the X-ray beam along the Y direction, the schematic diagrams of molecular arrangement in the hydrogen-bonded complexes (c: D120M3H20(HEBC6)1.0; d: D120M3H45(HEBC6)1.0), the cartoons of the corresponding components in the com plexes (e), and the shearing geometry (f).
angle region splits into two arcs centered on the meridian, proving that the supramolecular block does form the SmA phase with the layer normal perpendicular to the shear direction. The appearance of the highangle diffraction arcs on the meridian indicates that the direction of the π π stacking is parallel to the shear direction. Notably, although the microphase-separated nanostructures are different for the two com plexes, the orientation of the SmA phase and π π stacking are the same in the different types of confined nanospace. 3.3. Relative orientation of the structures on different length scales For the exploration of the relative orientation among the structures on different length scales, 2D SAXS experiments were conducted after 2D WAXD measurements, and the results are illustrated in Fig. 5. In the 2D SAXS patterns of the two complexes (Fig. 5a and b), the diffraction from the microphase-separated nanostructures appears as a ring without splitting even though shearing was carried out for multiple times. However, the relative orientation among the structures on different length scales can still be deduced. Generally, the main chain of the polymer tends to be aligned along the shear direction. Thus, for D120M3H20(HEBC6)1.0, the normal of the microphase-separated LAM structure is perpendicular to that of the SmA phase (Fig. 5c). However, the situation of D120M3H45(HEBC6)1.0 is more complicated. Thomas et al. proposed four structural models of smectic layers and cylindrical microdomains with two types of boundary conditions for the LC meso gens with respect to the inter-material dividing surface, including parallel-transverse, perpendicular-parallel, parallel-parallel, and transverse-perpendicular [47], in which the first and second words indicate the orientation of the BCP cylinders and the LC layers, respec tively. They observed transverse-perpendicular orientation in their system. In a previous work on supramolecular BCP formed by blending bent-core molecules with polystyrene-b-poly(4-vinylpyridine) via hydrogen bonding, the perpendicular-parallel orientation was observed [48]. In this work, the SmA phase is well oriented whereas the microphase-separated structure is not on the basis of the 2D WAXD and SAXS results. Hence, the orientation of the hierarchical structure under shear may be dominated by the LC phase. For D120M3H45(HEBC6)1.0, the cylinders and matrix of the HEX structure are formed by the flexible PDMS and rigid supramolecular MJLCP, respectively. As mentioned above, the normal of the SmA phase is perpendicular to the shear di rection. Therefore, the long axis of the HEX structure formed by
Fig. 6. Variable-temperature SAXS (a) and WAXS (b) profiles of the hydrogenbonded complex D120M3H45(HEBC6)1.0 during heating.
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Fig. 7. Schematic diagram of the hierarchical nanostructures formed by the hydrogen-bonded complexes, with molar ratio of hydrogen-bonding acceptor to donor (x) and DP of the PM3H block (n) as the vertical and horizontal axes, respectively.
microphase separation is also perpendicular to the shear direction (Fig. 5d). In other words, the normal of the SmA phase is parallel to the long axis of the cylinders of the HEX structure. In this case, BCP cylinders are in the least favorable orientation while the smectic layers are in their most favorable orientation. The schematic diagram of the molecular chain arrangement of the two complexes are presented in Fig. 5c and d.
SLCBCPs, which is similar to the grafted discotic mesogen we studied previously [43]. However, in comparison with the discotic counterpart that tends to form a discotic nematic (ND) phase in the microphase-separated nanospace, the calamitic mesogens prefer to be arranged in a parallel fashion owing to the π π interaction among them. On the other hand, the self-assembling behavior of the SLCBCPs are different. Owing to the larger MW of the precursor BCP and the higher content of flexible alkyl chains in the hydrogen-bonded discotic meso gen, the resultant SLCBCP can only form the SmA-within-HEX hierar chically ordered nanostructure. However, the SLCBCP containing calamitic mesogen in this work can form SmA-within-LAM and SmA- within-HEX hierarchically ordered nanostructures.
3.4. Temperature dependence of the structures on different length scales Variable-temperature SAXS and WAXS experiments were carried out to study the structural transformation of the complexes. With D120M3H45(HEBC6)1.0 as an example, the scattering vector ratio of the diffraction peaks in the SAXS profiles remains unchanged during the heating process (Fig. 6a), indicating that the microphase-separated nanostructure is stable at high temperatures. On the other hand, the WAXS results are much more complicated (Fig. 6b). The diffraction peaks in the low-angle region do not vary with temperature, suggesting that the SmA phase formed by the supramolecular block is stable. However, the sharp diffraction peak representing the π π stacking in the high-angle region becomes a diffuse halo at high temperatures, implying that the ordered packing of the pendant calamitic mesogenic groups in the side chains of the supramolecular block is destroyed. Upon cooling, the π π stacking interaction is recovered at low temperatures (Fig. S4). However, the disappearance of the ordered packing of the calamitic mesogenic groups during heating is not detected in the DSC curves (Fig. 1), which may be related to the small enthalpic change of the transition.
4. Conclusions In this work, a series of SLCBCPs were prepared by grafting a small molecule with a calamitic mesogen to BCP precursors through hydrogen bonding. Different hierarchical nanostructures were obtained from the self-assembly of the hydrogen-bonded complexes. The HEX structures with PDMS as the continuous phase, the LAM structures, and the inverted HEX structures with PDMS as the dispersed cylinders all having sizes over 25 nm are formed in sequence when the molar ratio of the acceptor to donor (x) and the DP of the PM3H block (n) increase. When n � 20 and the value of x is large enough (x � 0.8), the supramolecular block can develop into the SmA phase at a sub-10 nm length scale owing to the “jacketing” effect of the grafted bulky pendant group. Meanwhile, the pendant calamitic mesogenic group is able to form an ordered arrangement attributed to π π stacking. Moreover, the SmA phase and the π π stacking are synergistic and promotional to each other when x is increased. Thus, the hierarchically ordered nanostructures, such as SmA-within-LAM and SmA-within-HEX along with the ordered packing of the pendant calamitic mesogen, can be obtained. On the one hand, this method of grafting an LC mesogen onto BCPs using a flexible spacer by non-covalent bonds can be used to construct and control hierarchically ordered nanostructures. On the other hand, such hydrogen-bonded complexes with hierarchical nanostructures may be made into func tional materials after washing away the acceptor small molecules by taking advantage of the reversibility of hydrogen bonding.
3.5. Hierarchically ordered structures of the SLCBCPs On the basis of the results mentioned above, the hydrogen-bonded complexes can self-assemble into structures on different length scales. For example, the sample D120M3H20(HEBC6)1.0 can simultaneously form a LAM structure with a larger spacing over 25 nm and an SmA phase at sub-10 nm, resulting in an SmA-within-LAM hierarchically or dered nanostructure. As a second example, D120M3H45(HEBC6)1.0 can self-assemble into an SmA-within-HEX hierarchical nanostructure. With x and n as the vertical and horizontal axes, respectively, a schematic diagram of the hierarchical nanostructures is shown in Fig. 7. In sum mary, the hydrogen-bonded calamitic mesogen can induce the hierar chically ordered self-assembled nanostructures of the resultant 7
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Acknowledgements
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