POSS-Containing Jacketed Polymer: Hybrid Inclusion Complex with

Apr 7, 2015 - Dae-Yoon Kim , Dong-Gue Kang , Suyong Shin , Tae-Lim Choi , Kwang-Un Jeong. Polymer Chemistry 2016 7 (33), 5304-5311 ...
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POSS-Containing Jacketed Polymer: Hybrid Inclusion Complex with Hierarchically Ordered Structures at Sub-10 nm and Angstrom Length Scales Yu-Feng Zhu, Wei Liu, Meng-Yao Zhang, Yu Zhou, Yu-Dong Zhang, Ping-Ping Hou, Yu Pan, Zhihao Shen,* Xing-He Fan,* and Qi-Feng Zhou 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 ABSTRACT: A series of organic−inorganic hybrid polymers, PnPOSS (n = 6, 10, which is the number of carbon atoms in the flexible alkyl spacers), containing a mesogen-jacketed liquid crystalline polymer (MJLCP) as the main chain and a crystalline nanobuilding block, polyhedral oligomeric silsesquioxane (POSS), in the side chains were designed and synthesized for constructing a hybrid inclusion complex. The phase structures and phase transitions of PnPOSS were investigated by differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and high-resolution transmission electron microscopy. With the competitive self-assemblies of the two covalently connected building blocks, namely MJLCP and POSS moieties, PnPOSS shows various phase structures including a sub-10 nm hexagonal columnar (Colh) or columnar nematic liquid crystalline phase, an angstrom rhombohedral crystalline (KR) phase, and a hierarchically ordered phase with Colh and KR coexisting. This work provides a new approach for the design and synthesis of hybrid inclusion materials.

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INTRODUCTION Organic−inorganic hybrid nanomaterials with different length scales, compositions, functionalities, and morphologies offer prospects for both scientific research and innovative industrial applications owing to their remarkable new properties and multifunctional nature.1−4 One exciting area is incorporating polymers into the pores of inorganic materials, which introduces orders and confinements across different length scales and generates a significant degree of functionality and complexity.5 Although host−guest inclusion complex can be easily formed by taking advantage of supramolecular chemistry, such as the most well-known one generated by poly(ethylene glycol) and cyclodextrin,6 introducing polymers into channels of inorganic porous materials to form hybrid inclusion complexes is more difficult. In situ polymerization,7−9 polymer infiltration,10 and one-pot synthesis11 have been used to achieve this goal. However, structures of the inclusion complexes formed are not well-defined. To obtain precisely defined hybrid inclusion complexes, a bottom-up approach may be needed with a delicate design of building blocks. Owing to their distinct three-dimensional (3D) shapes, chemical compositions, functionalities, and excellent selfassembling abilities, nanobuilding blocks have been widely utilized in constructing organic−inorganic hybrid nanomaterials with hierarchical structures across different length scales.12,13 Introducing nanobuilding blocks into polymers is an emerging method for generating ordered structures across length scales. © XXXX American Chemical Society

For example, a two-dimensional (2D) planar nanobuilding block, triphenylene, was incorporated into the side chains of mesogen-jacketed liquid crystalline polymers (MJLCPs)14 to generate a hierarchically ordered structure.15,16 MJLCPs are rod-like polymers with excellent tunability,14,17 and they have also been used as rod blocks in constructing block copolymers with various self-assembled structures.18−21 Polyhedral oligomeric silsesquioxane (POSS), which is often regarded as the smallest silica nanoparticle with a diagonal length of approximately 1.0 nm, is a versatile nanobuilding block that is rigid and shape persistent with various peripheral functionalities.22−24 POSS-containing polymers are widely investigated for self-assembly, polymer light-emitting diodes, low dielectric constant materials, solid polymer electrolytes, ultrafiltration membrane coatings, and elastomers.25−29 Because of the crystalline nature of POSS, some POSS-containing block copolymers exhibit hierarchical structures consisting of crystalline POSS domains on a smaller length scale within the larger microphase-separated structures.30−32 Therefore, we hypothesize that a precisely defined inclusion complex may be obtained by combining crystalline POSS and ordered polymers together and fine-tuning the interaction between POSS moieties and the polymer chains. In this work, Received: January 22, 2015 Revised: March 27, 2015

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DOI: 10.1021/acs.macromol.5b00137 Macromolecules XXXX, XXX, XXX−XXX

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lized from ethanol. Benzoyl peroxide (BPO) was recrystallized from chloroform and methanol. NMR experiments, mass spectroscopy, elemental analysis (EA), gel permeation chromatographic (GPC) measurements, thermogravimetric analysis (TGA), DSC, and 1D and 2D WAXD experiments were performed according to the procedures previously described.34 HRTEM bright-field images were obtained with a Tecnai T20 TEM instrument using an accelerating voltage of 120 kV. Synthesis. The synthesis of the monomers and the polymers is illustrated in Scheme 1. The experimental details are described as follows. Synthesis of Di(10-carboxydecyl) 2-Vinylterephthalate, V10DA. VTA (2.00 g, 10.4 mmol), potassium hydrogen carbonate (2.08 g, 20.8 mmol), and 20 mL of dry DMF were added to a 250 mL roundbottomed flask equipped with a magnetic stir bar. The mixture was allowed to be stirred at ambient temperature for 1 h to form potassium salt. A solution of 11-bromoundecanoic acid (5.52 g, 20.8 mmol) and hydroquinone (0.0100 g, 0.100 mmol) in DMF (100 mL) was added, and the resultant mixture was refluxed at 100 °C for another 24 h. After the reaction mixture was cooled to the ambient temperature, 500 mL of deionized water was added, followed by filtration. The filtrate was washed with water and brine, dried over Na2SO4, and evaporated under vacuum. The crude product was purified by flash column chromatography on silica gel with acetone/petroleum ether (v/v = 1/ 3) as the eluent to afford the product as a white powder (2.20 g). Yield: 38%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.24 (s, 1H), 7.89− 7.97 (q, 2H), 7.39−7.46 (q, 1H), 5.74−5.78 (d, 1H), 5.41−5.44 (d, 1H), 4.31−4.36 (q, 4H), 2.33−2.36 (t, 4H), 1.73−1.81 (m, 4H), 1.59−1.64 (m, 4H), 1.26−1.44 (m, 24H). MS (ESI): calcd (M − H)−/z, 559.3; found (M − H)−/z, 559.3. Synthesis of Di(6-carboxyhexyl) 2-Vinylterephthalate, V6DA. V6DA was prepared in the same manner as described for V10DA, except that 7-bromoheptanoic acid was used instead of 11bromoundecanoic acid. Yield: 35%. 1H NMR (d6-DMSO, 400 MHz, δ, ppm): 11.98 (s, 2H), 8.16 (s, 1H), 7.86−7.98 (q, 2H), 7.25−7.32 (q, 1H), 5.80−5.84 (d, 1H), 5.44−5.47 (d, 1H), 4.27−4.32 (q, 4H), 2.16−2.23 (t, 4H), 1.66−1.76 (m, 4H), 1.46−1.56 (m, 4H), 1.22−1.44 (m, 8H). MS (ESI): calcd (M − H)−/z, 447.2; found (M − H)−/z, 447.2. Synthesis of V10POSS. V10DA (1.00 g, 1.8 mmol), POSS-OH (3.13 g, 3.6 mmol), DMAP (0.04 g, 0.3 mmol), DCC (2.21 g, 10.7 mmol), and 25 mL of dry CH2Cl2 were added to a 250 mL roundbottomed flask equipped with a magnetic stir bar. The mixture was stirred at ambient temperature for 24 h. After the mixture was cooled to 4 °C, the floating solid was filtrated and washed with cold CH2Cl2. After the solvent was evaporated under reduced pressure, the crude product was purified by silica gel column chromatography with CH2Cl2 as the eluent and subsequent recrystallization from ethanol as a white powder. Yield: 70%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.23 (s, 1H), 7.89−7.96 (q, 2H), 7.39−7.45 (q, 1H), 5.73−5.77 (d, 1H), 5.41−5.43 (d, 1H), 4.30−4.35 (m, 4H), 4.01−4.04 (t, 4H), 2.27−2.30 (t, 4H), 1.59−1.90 (m, 26H), 1.24−1.47 (m, 24H), 0.80− 1.10 (d, 84H), 0.56−0.68 (m, 32H). 13C NMR (100 MHz, CDCl3, δ, ppm): 173.9, 167.0, 165.9, 139.4, 135.0, 133.4, 132.7, 130.2, 128.3, 128.1, 117.5, 66.2, 65.7, 65.6, 34.4, 29.5, 29.4, 29.3, 29.2, 28.7, 28.6, 26.1, 26.0, 25.7, 25.0, 23.9, 22.5, 22.4, 22.2, 8.4. 29Si NMR (80 MHz, CDCl3, δ, ppm): 67.6 (s, 3Si), 67.7 (s, 1Si), 67.9 (s, 4Si). HRMS (ESI): calcd M/z, 2273.9113; found M/z, 2273.9068. Anal. Calcd for C94H184O32Si16: C, 49.61; H, 8.15. Found: C, 49.98; H, 8.23. Synthesis of V6POSS. V6POSS was prepared in the same manner as described for V10POSS, except that V6DA was used instead of V10DA. Yield: 68%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.23 (s, 1H), 7.89−7.96 (q, 2H), 7.39−7.45 (q, 1H), 5.73−5.77 (d, 2H), 5.41−5.43 (d, 2H), 4.30−4.35 (m, 4H), 4.01−4.04 (t, 4H), 2.32 (t, 4H), 1.59−1.90 (m, 26H), 1.35−1.52 (m, 8H), 0.80−1.10 (d, 84H), 0.56−0.68 (m, 32H). 13C NMR (100 MHz, CDCl3, δ, ppm): 173.6, 166.9, 165.8, 139.4, 135.0, 133.4, 132.7, 130.2, 128.3, 128.1, 117.5, 66.3, 65.5, 65.4, 34.2, 28.8, 28.5, 25.8, 25.7, 24.8, 23.9, 22.5, 22.4, 22.2, 8.4. 29Si NMR (80 MHz, CDCl3, δ, ppm): 67.6 (s, 3Si), 67.8 (s, 1Si), 67.9 (s, 4Si). HRMS (ESI): calcd (M + H)+/z, 2161.7900; found M/z,

we incorporate two POSS moieties to each side chain of an MJLCP via flexible alkyl spacers. To tune the interaction of POSS moieties and the polymer main chain and facilitate the formation of the hybrid inclusion complex, spacers with six and ten methylene units were adopted in corresponding polymers P6POSS and P10POSS, respectively. The chemical structure of PnPOSS (n = 6, 10) is illustrated in Chart 1. The molecular Chart 1. Chemical Structure of PnPOSS

design is based on the following considerations. First, the precisely defined chemical structure of the hybrid is ensured by the covalent linking nature of the POSS-containing monomer from which the polymer is obtained by free radical polymerization. Then, the intriguing phase structure of the MJLCP containing the 3D POSS nanobuilding block is formed from the competitive self-assemblies of the MJLCP and the nanobuilding block. Furthermore, flexible spacers with different lengths are used to tune the self-assembled structure. Last but not the least, a high POSS content is necessary for forming the crystalline POSS matrix of the inclusion complex. Herein, we utilized differential scanning calorimetry (DSC), polarized light microscopy (PLM), one-dimensional (1D) and 2D wide-angle X-ray diffraction (WAXD), synchrotron-radiation small-angle X-ray scattering (SAXS), and high-resolution transmission electron microscopy (HR-TEM) to investigate the phase behaviors of PnPOSS.

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EXPERIMENTAL SECTION

Materials and Measurements. 2-Vinylterephthalic acid (VTA) was synthesized according to the literature.33 3-Hydroxypropylheptaisobutyl-POSS (POSS-OH, Sigma-Aldrich), 11-bromoundecanoic acid (98%, Beijing Isomersyn Technology), 7-bromoheptanoic acid (98%, Beijing Isomersyn Technology), potassium bicarbonate (KHCO3, A.R., Beijing Chemical Reagents Co.), N,N′-dicyclohexylcarbodiimide (DCC, A.R., J&K Chemical), 4-(dimethylamino)pyridine (DMAP, A.R., J&K Chemical), and N,N-dimethylformamide (DMF, HPLC, J&K Chemical) were used as received. Dichloromethane (CH2Cl2, A.R., Beijing Chemical Reagents Co.) was dried over anhydrous magnesium sulfate. Chlorobenzene (A.R., Beijing Chemical Reagents Co.) was washed with concentrated sulfuric acid, followed by washing with a 5% sodium carbonate solution and with water, and then distilled from calcium hydride. Azobis(isobutyronitrile) (AIBN) was recrystalB

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2161.7896. Anal. Calcd for C86H168O32Si16: C, 47.74; H, 7.83. Found: C, 47.46; H, 7.78. Synthesis of P10POSS. As shown in Scheme 1, the polymer P10POSS was obtained by conventional free radical polymerization in chlorobenzene. Experimental details are as follows. V10POSS (200 mg, 88.0 mmol), AIBN (0.144 mg, 0.880 mmol), and 600 mg of chlorobenzene were added to a dry glass tube equipped with a magnetic stir bar. The reaction mixture was purged with nitrogen and subjected to three freeze−pump−thaw cycles and sealed under vacuum. Polymerization was carried out at 60 °C for 48 h and then stopped by dipping the tube in ice/water. After the tube was broken, the solution was diluted with 5 mL of tetrahydrofuran (THF) and then added dropwise to vigorously stirred acetone (200 mL). To eliminate the unreacted monomer completely, we repeated the precipitation process. By filtration and drying in vacuum at 35 °C for 24 h, the target polymer, P10POSS, was obtained as a white powder. Yield: 50%. Synthesis of P6POSS. P6POSS was prepared from V6POSS in the same manner as described for P10POSS, except that BPO was used instead of AIBN, and the polymerization was conducted at 90 °C. Yield: 44%.

weights (MWs) through free radical polymerization according to the literature.27 Herein, conventional free radical polymerization in chlorobenzene was employed to afford the polymers, PnPOSS. With AIBN as the initiator, P10POSS with a high MW was prepared. For V6POSS, the monomer with shorter spacers, we did not obtain the high-MW polymer when polymerization reaction was conducted using AIBN at 60 °C, probably owing to the stronger steric hindrance of the side chains. After the reaction temperature was increased to 90 °C with BPO as the initiator, high-MW P6POSS was obtained after two precipitation processes. Figure 1 shows the 1H NMR spectra of V10POSS and P10POSS in CDCl3. The characteristic resonances corresponding to vinyl H appearing at δ of 5.41−5.77 and 7.39−7.45 ppm in the spectrum of V10POSS completely disappear after polymerization, and the peaks of P10POSS in the bottom curve of Figure 1 are rather broad and consistent with the structure of P10POSS, indicating the successful polymerization. Table 1 gives the molecular characteristics of PnPOSS. GPC analysis shows that number-average MWs (Mn’s) of P10POSS and P6POSS are 5.6 × 10 4 and 1.2 × 10 5 g/mol, with polydispersities of 1.56 and 2.42, respectively. Physically, PnPOSS is a white powder readily soluble in CH2Cl2, chloroform, THF, and toluene, but it has poor solubility in acetone and alcohol. TGA results (Table 1) indicate that PnPOSS has an excellent thermal stability with a 5% weight loss temperature of above 340 °C in nitrogen. Phase Transition and Phase Structure. The phase transition of PnPOSS was studied by DSC experiments at a

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RESULTS AND DISCUSSION Synthesis and Characterization. The monomers were facilely synthesized in two steps. Flexible spacers were first introduced to obtain the diacid intermediate, VnDA. Then Steglich esterification of VnDA and POSS-OH was carried out to prepare the monomers, VnPOSS. The chemical structure of VnPOSS was elucidated by 1H/13C/29Si NMR, HR-MS, and EA. Because of their bulkiness, POSS-containing monomers are difficult to homopolymerize into polymers with high molecular C

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Figure 2. DSC thermograms of P10POSS and P6POSS recorded during the first cooling and the subsequent heating process at a rate of 20 °C/min.

PnPOSS is lower than that of octaisobutyl-POSS (261 °C),36 which indicates that the polymer main chain of PnPOSS restricts the 3D packing of the POSS moieties to some extent. PLM was used to examine the texture of PnPOSS. Typical micrographs of P10POSS are shown in Figure 3. P10POSS

Figure 1. 1H NMR of V10POSS (top) and P10POSS (bottom).

Table 1. MWs, MW Distributions, and Thermal Properties of PnPOSS n 6 10

Td Tg Mn (×104 g/mol)a Mw/Mna (°C)b (°C)c 12.2 5.6

2.42 1.56

349 364

70 63

Ttransition (°C) (and enthalpy change, kJ/molPOSS)d 150 (13.7) 156 (17.7)

a

Determined by GPC in THF using polystyrene standards. b5% weight loss temperature obtained from TGA under a nitrogen atmosphere at a heating rate of 10 °C/min. cEvaluated by DSC during the first heating cycle at a rate of 20 °C/min. dEvaluated by DSC during the second heating cycle at a rate of 20 °C/min.

rate of 20 °C/min, as shown in Figure 2, and the results are summarized in Table 1. For P10POSS, an exothermic transition peak at 121 °C with a latent heat of 16.3 kJ/molPOSS is observed in the first cooling thermogram. The subsequent heating shows an endothermic transition peak at 156 °C with a latent heat of 17.7 kJ/molPOSS. Considering the large value of the enthalpic changes of transitions, these two peaks may be attributed to first-order transitions in the condensed state associated with ordered structures, namely the crystallization and melting of POSS moieties in the side chains. P6POSS has similar DSC thermograms to those of P10POSS, except a lower melting temperature (150 °C) and a smaller latent heat (13.7 kJ/ molPOSS). This result indicates that the crystal of POSS moieties has a better stability and a higher degree of order in P10POSS than in P6POSS. Compared to the melting temperature (112 °C)35 of the POSS-containing polymer with one POSS moiety in one repeating unit, PnPOSS has a higher melting temperature, indicating that PnPOSS has much a higher degree of order. On the other hand, the melting temperature of

Figure 3. PLM textures of P10POSS at 150 °C (a) and 25 °C (b).

shows strong birefringence in the whole temperature range before degradation, and the nontypical texture does not change much during cooling. P10POSS and P6POSS have similar textures under PLM observation. To verify the phase structures of PnPOSS, 1D WAXD experiments were carried out on the as-precipitated and the thermally annealed P10POSS samples. The as-precipitated sample was obtained with precipitation in acetone by dripping from a THF solution. For the thermally annealed sample, ∼50 mg of P10POSS was thermally treated from 170 to 25 °C at a rate of 1 °C/min. The same GPC trace as that of the asprepared sample was obtained, indicating that P10POSS is stable during such a process. As shown in Figure 4, seven broad diffraction peaks are observed at q = 4.28, 5.88, 7.86, 8.65, 9.70, D

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Figure 4. 1D WAXD profiles of the as-precipitated (a) and the thermally treated (b) P10POSS.

13.64, and 17.62 nm−1 in the WAXD profile of the asprecipitated sample. After thermal annealing, however, these diffraction peaks become sharper and more peaks appear in the similar q regions, with the halo at q = 4.28 nm−1 nearly unchanged. The corresponding d-spacing values of the characteristic sharp peaks are 1.08, 0.81, 0.72, and 0.46 nm, which can be assigned as (101), (110), (012), and (113) diffractions of a rhombohedral structure (KR) with a = b = 1.61 nm, c = 1.71 nm, and γ = 120°, which are essentially the same as those of the octaisobutyl-POSS.37 Particularly, the broad halo with a q value of 4.28 nm−1 (d = 1.47 nm) is not included in the rhombohedral structure. Hence, this halo may be derived by the introduction of the MJLCP main chain. Therefore, WAXD results suggest that POSS moieties in P10POSS self-assemble into a rhombohedral crystalline structure exclusively, and some other less ordered, larger structures associated with the polymer main chain may coexist in the self-assembled structure of P10POSS in the bulk. To further identify the phase structures and phase transitions at different length scales and temperatures, temperaturedependent WAXD experiments on a P10POSS sample were then performed. As shown in Figure 5a, the pattern of the ascast sample in the q range of less than 10 nm−1 renders four diffraction peaks with the d-spacing values of 1.47, 1.08, 0.80, and 0.73 nm, which is consistent with the 1D WAXD results at ambient temperature. After the sample was heated to 120 °C, more diffraction peaks appear, and the diffraction peaks which exist at ambient temperature become stronger and sharper. Besides, five peaks (indicated by the black arrows in Figure 5a) with the q values of 1.51, 2.62, 3.05, 3.99, and 4.59 nm−1 with a q ratio of 1:31/2:41/2:71/2:91/2 develop, suggesting a 2D hexagonal ordered structure with a = b = 4.80 nm, γ = 120°. Considering that the calculated length of the side chain with all alkyl spacers adopting all-trans conformation is about 5.0 nm, we speculate that this 2D hexagonal columnar (Colh) phase is developed by the rod-like supramolecular mesogenthe MJLCP chain as a whole at 120 °C. In addition, the diffraction peaks with q values of 5.75, 7.71, and 8.53 nm−1 indicate the crystalline structure formed by the POSS moieties in the side chains. Hence, a 2D liquid crystalline (LC) structure (Colh) and a crystalline structure (KR) coexist in a hierarchically ordered structure covering the length scale from sub-10 nm to a few angstroms. The structure can be regarded as an ordered inclusion complex with polymer main chains included in between POSS crystals. The scattering profile becomes less complicated at 250 °C. The crystalline peaks with q values of

Figure 5. Temperature-dependent WAXD profiles of P10POSS during the first heating (a) and the subsequent cooling (b) processes.

more than 5 nm−1 disappear, and a broad halo at q ≈ 5.6 nm−1 develops, indicating the melting of the crystalline structure formed by the POSS moieties in the side chains at high temperatures. However, the intensities of the peaks corresponding to the 2D Colh structure remarkably increase, and the peaks also become sharper, suggesting that the Colh structure becomes more ordered during heating. Consequently, P10POSS exhibits a 2D Colh order (with a = b = 4.49 nm, γ = 120°) together with the crystalline POSS domains at low temperatures and loses its crystalline order of POSS on the angstrom length scale at high temperatures. During the cooling process, the diffraction pattern of P10POSS remains the same at temperatures higher than the crystallization temperature of the POSS crystal in the side chains, indicating the 2D Colh phase persists when POSS moieties are molten at high temperatures. When the temperature is 120 °C, however, the crystalline peaks reemerge at q values of more than 5 nm−1, indicating that the crystalline structure of POSS moieties in the side chains forms again at 120 °C. Meanwhile, intensities of the peaks in the low q region E

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(Coln) phase with the whole polymer chain as a supramolecular mesogen. Its intensity decreases gradually and disappears when the crystalline peak reappears at low temperatures upon cooling. Compared with the scattering profile for the as-cast sample, the crystalline peaks become weaker and broader after cooling, indicating that the crystalline structure becomes less ordered after cooling. This unusual phenomenon should be owing to the stronger disrupting effect of the shorter spacers in the side chains on the POSS crystal compared to those for P10POSS. To further verify the ordered structure of PnPOSS at the sub10 nm length scale, temperature-dependent synchrotronradiation SAXS experiments were performed with film samples at beamline 1W2A of the Beijing Synchrotron Radiation Facility (BSRF), China.38 Figure 7 shows the SAXS profiles of

decrease dramatically. Therefore, the crystallization of POSS moieties in the side chains greatly influences the degree of order of the 2D Colh structure developed by the rod-like MJLCP supramolecular mesogen. The diffraction pattern at ambient temperature is similar to that at 120 °C, except for thermal contraction. Note that, besides the two series of diffraction peaks corresponding to the Colh phase and the KR structure, there is another diffraction peak at the q value of ∼4.28 nm−1 (dspacing value of ∼1.47 nm), which cannot be indexed. As shown in Figure 5, this peak disappears at 250 °C upon heating and reappears at 120 °C during cooling, in coincidence with the melting and crystallization of the POSS moieties in the side chains. Therefore, it may be related to another ordered structure at low temperatures after crystallization. This will be discussed later. Figure 6 shows the temperature-dependent WAXD profiles of P6POSS, which follows the similar transition pathways as

Figure 7. Synchrotron-radiation SAXS profiles of P6POSS (a) and P10POSS (b) during cooling. Figure 6. Temperature-dependent WAXD profiles of P6POSS during the first heating (a) and the subsequent cooling (b) process.

PnPOSS during cooling. For P6POSS, there is only one diffraction peak generated at high temperatures, which is consistent with WAXD results and is associated with Coln structure. During cooling this peak decreases in intensity and disappears at ambient temperature, indicating that the Coln structure cannot retain during cooling. On the other hand, P10POSS shows three diffraction peaks with the scattering vector ratio of 1:31/2:2 at high temperatures, indicating a 2D long-range ordered Colh structure, which is also in agreement

those of P10POSS. For the as-cast P6POSS, three diffraction peaks are observed, indicating the KR structure of POSS moieties. Upon heating to high temperatures, the crystalline peaks diffuse to become broad halos after melting, and a new peak with a q value of 1.49 nm−1 (d-spacing of 4.21 nm) appears. This peak may be assigned as a columnar nematic F

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an additional helix-like structure originated from the polymer main chain may exist. The phase structure of P10POSS deserves more discussion. The WAXD profile in Figure 4 shows that the POSS moieties in P10POSS form a 3D ordered crystalline structure at ambient temperature. From a crystallographic viewpoint, the polymer main chain of P10POSS should be excluded from the crystalline structure of POSS moieties. However, the main chain and POSS moieties are chemically linked together with a relatively short spacer (calculated length of ∼1.26 nm with all methylene units adopting all-trans conformation). Therefore, the polymer main chains have to coassemble with side-chain POSS moieties in the 3D space. In the rhombohedral lattice (a = b = 1.61 nm, c = 1.71 nm, and γ = 120°) of the POSS crystal, POSS moieties have a hexagonal packing in one layer. Structurally, the distance between the adjacent layers is c/3 = 0.57 nm, and the distance between the two repeating units of a polystyrene main chain is 0.20−0.25 nm. Because six POSS moieties in a hexagon correspond to three repeating units in one layer, the polymer main chain with an average length of 0.60−0.75 nm should accommodate in-between layers, indicating that it may have a somewhat bended conformation. Moreover, taking account of the 2D WAXD results and the equal offsets between layers within the ABCA sequence in the rhombohedral lattice, the polymer main chain may generate a pseudohelix structure, as shown in Figure 9.

with WAXD results. Although the intensities of these peaks decrease during cooling to ambient temperature, the Colh structure retains with the degree of order decreasing to some extent. The comparison of the phase behaviors of P6POSS and P10POSS suggests that the spacer length strongly influences the ordered structure. With shorter spacers, the POSS crystal disrupts the ordered packing of main chains. However, longer spacers decouple the interactions between the POSS crystal and the main chains, which facilitates the formation of the ordered Colh structure. Introducing a spacer with a suitable length is significant for the self-assembly and the degree of order of these POSS-containing polymers. Molecular Arrangement of P10POSS. To investigate the hierarchically ordered structure of P10POSS at ambient temperature, 2D WAXD experiments on P10POSS were performed. The oriented sample was prepared by mechanically shearing a film with a thickness of ∼1 mm at 200 °C followed by annealing at 100 °C overnight. The 2D WAXD pattern is shown in Figure 8 with the X-ray incident beam perpendicular

Figure 9. Proposed structural model of P10POSS at ambient temperature (the polymer main chain may slightly deviate from the dotted position due to its flexible nature). Figure 8. 2D WAXD pattern of P10POSS with the X-ray beam perpendicular to the shear direction at ambient temperature (a), shearing geometry (b), and azimuthal scan (c) of peaks 2−4 in (a).

Taking advantage of the synchrotron-radiation SAXS profiles of P10POSS, we reconstructed the electron density map, as shown in Figure 10a. A hexagonal structure is clearly observed. Considering the molecular dimension of P10POSS, we speculate that one hexagon represents the cross section of a polymer chain. The blue and green zones with the highest electron density, the red zone with the lowest electron density, and the yellow dot in the center with a medium electron density should be POSS moieties, flexible spacers, and the polystyrene main chain, respectively. This molecular arrangement is highly consistent with the inclusion structure we proposed. We also investigated the morphology of P10POSS with HR-TEM. Microtome-sliced thin film samples after SAXS experiments were used. The polystyrene main chain was stained with ruthenium tetroxide for 15 min to increase the electron density contrast. As shown in Figure 10b, a dotted structure is observed with an average distance of 4.50 nm among dots. The

to the shear direction. In general, the polymer chain will be oriented parallel to the direction of the shear force (X direction in Figure 8). We found all the diffractions appearing in the 1D WAXD and SAXS profiles. Particularly, the six diffractions in the low-angle region which are labeled by numbers in Figure 8a offer more information. Peaks 1 and 2 are (100) and (200) diffractions, respectively, of the Colh structure on the sub-10 nm scale. Peaks 4, 5, and 6 are the (101), (110), and (012) diffractions, respectively, of the KR structure of POSS at the angstrom level. Peak 3 is the additional peak (d-spacing ∼1.47 nm) which cannot be assigned to either the Colh or the KR structure. Figure 8c shows the azimuthal scan of peaks 2−4 in the 2D WAXD pattern. All of the three diffractions show four maxima in the same direction in the quadrants, indicating that G

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Figure 11. Schematic drawing of the phase behavior and the molecular packing of PnPOSS at different temperatures.

structure (KR) coexist in a hierarchically ordered structure of P10POSS, forming an ordered inclusion complex. The driving force for the formation of the hierarchical structure of P10POSS at ambient temperature can be attributed to minimization of the free energy guided by the competition of the self-assemblies of the MJLCP and the POSS moieties. However, P6POSS loses its sub-10 nm length-scale order, and the KR structure is also depressed to a large extent at ambient temperature, mainly due to the stronger mutual interference of the building blocks in P6POSS with a shorter spacer. From a structural viewpoint, P10POSS is a molecularly precise inclusion polymer prepared from a monomer with the polymerizable group and the inorganic matrix covalently linking together. From a self-assembly point of view, such a hybrid system containing a nanobuilding block (POSS) and a polymer is quite similar to a block copolymer, although the dimensions of the building blocks decrease to the nanometer scale from tens of nanometers. This may open a new avenue for synthesizing polymer materials with hierarchically ordered structures on the sub-10 nm scale owing to the large variety of functional polymers and nanobuilding blocks.

Figure 10. Reconstructed electron density map (a) and HR-TEM micrograph (b) of P10POSS.

dark dots are likely stained polystyrene main chains, and light domains are the flexible spacers and POSS moieties. This morphology deviates from an ordered rhombohedral structure due to the staggering and offset of the main chain in the ABCA layer sequence of the POSS crystal. The average distance of 4.50 nm among dots is very close to the a value of the Colh structure (4.49 nm) shown in the WAXD pattern at ambient temperature. Self-Assembling Behavior of PnPOSS. PnPOSS shows a columnar phase at high temperatures with the whole polymer chain serving as the supramolecular mesogen. At low temperatures, POSS moieties crystallize into a rhombohedral lattice, same as that of octaisobutyl-POSS. The formation of the ordered structure is highly dependent on the length of the spacer. An appropriate spacer length is vital for balancing the interactions between the two building blocks and forming the long-range hierarchically ordered structure. Figure 11 depicts the schematic drawing of the phase behavior and the molecular packing of PnPOSS. As mentioned above, the self-assembled structures of the polymer main chain and POSS moieties greatly influence each other. Owing to the chemical connection of the POSS moieties and the polymer main chain, the rearrangement of the POSS moieties inevitably changes the conformation of the polymer main chain and then changes the final self-assembled structure of PnPOSS. At high temperatures, POSS moieties are molten, and the whole polymer chain behaves as a rod-like supramolecular mesogen, leading to the formation of the Colh or Coln structure on the sub-10 nm scale. At low temperatures, however, the crystallization of the POSS moieties in the side chains dominates the self-assembling process. The degree of order of the 2D Colh structure decreases, and the LC structure (Colh) and the crystalline

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CONCLUSIONS We have demonstrated the synthesis and phase behaviors of a series of organic−inorganic hybrid polymers, PnPOSS (n = 6, 10), with various techniques including DSC, PLM, WAXD, SAXS, and HR-TEM. PnPOSS with a sufficiently high MW (Mn > 104 g/mol) was prepared by conventional free radical polymerization of POSS-containing vinyl monomers. PnPOSS has an excellent thermal stability with a 5% weight loss temperature of ∼350 °C in nitrogen. The length of the flexible spacer greatly influences the phase behavior of the polymer. At high temperatures, P10POSS behaves as a rod-like supramolecular mesogen, and a 2D Colh structure on the sub-10 nm scale forms. And at low temperatures, crystallization of the POSS moieties in the side chains of P10POSS decreases the degree of order of the 2D Colh phase to some extent, and the LC structure (Colh) and the crystalline (KR) structure originating from the self-assemblies of the MJLCP and the POSS moieties, respectively, coexist in a hierarchically ordered structure covering the length scale from sub-10 nm to a few angstroms, leading to an ordered inclusion complex. However, P6POSS forms a Coln structure at high temperatures and the KR structure at ambient temperature. This work offers a new way of preparing organic−inorganic hybrid inclusion materials H

DOI: 10.1021/acs.macromol.5b00137 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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and has further implications in the understanding of the selfassembling behaviors of nanohybrid polymers in general.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Lei Zhu at Case Western Reserve University for his helpful advices on TEM characterization and the scientists at 1W2A SAXS station at BSRF for their assistance on the SAXS experiments. We greatly appreciate the help offered by our colleague Prof. Shuang Yang in generating the electron density map. The authors also thank Dr. Bernard Lotz at Institut Charles Sadron and Prof. Stephen Z. D. Cheng at the University of Akron for their enlightening discussions and suggestions on phase structure identification. This work was supported by the National Natural Science Foundation of China (Grants 21134001 and 20990232).

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DOI: 10.1021/acs.macromol.5b00137 Macromolecules XXXX, XXX, XXX−XXX