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Poly[(side-on mesogen)-alt-(end-on mesogen)]: A Compromised Molecular Arrangement Meng Wang,† Wei-Wei Bao,† Wen-Ying Chang,‡ Xu-Man Chen,† Bao-Ping Lin,† Hong Yang,*,† and Er-Qiang Chen*,‡ †

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School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Jiangsu Key Laboratory for Science and Application of Molecular Ferroelectrics, Southeast University, Nanjing, Jiangsu Province 211189, China ‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics at the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: In recent years, sequence-controlled side-chain liquid crystal polymers (SCLCPs) have gained extensive interest because mesogenic units with different lengths and distributions can form various ordered sequences, which further endow LCP materials with diverse functions. In this manuscript, a side-chain side-on maleimide-containing monomer 2,5-bis-(4-butoxybenzoyloxy)-benzoic acid 6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl ester (Y1801) and a side-chain end-on styrene-containing monomer 4′-[6-(4-vinyl-phenoxy)-hexyloxy]-biphenyl-4-carbonitrile (Y1802) are combined in one single macromolecular chain and orderly polymerized in an alternative sequence to form an alternating copolymer Poly(Y1801-alt-Y1802). The chemical structure and alternating sequence of Poly(Y1801-alt-Y1802) are confirmed by GPC and NMR techniques. The combination of DSC, POM, and WAXS data indicates that, although the side-on homopolymer PY1801 and the end-on homopolymer PY1802 both exhibit the nematic phase, their alternating copolymer Poly(Y1801-alt-Y1802) shows an interdigitated smectic A phase, a compromised molecular arrangement instead. In addition, a strong fluorescence emission of Poly(Y1801-alt-Y1802) is observed, which might provide this novel alternating-structured liquid crystal polymer with potential applications in luminescent materials and devices.

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INTRODUCTION Liquid crystal polymers (LCPs) have attracted great scientific attention since they were first discovered in the middle of the 20th century.1−6 Benefited from their excellent thermal stability and mechanical property, LCPs have wide applications in many fields, such as organic photoelectric materials, engineering plastics, high-modulus fibers, tunable diffraction gratings, and thermal-isolation materials.7−11 Generally, LCPs can be divided into two main categories:12−14 (1) main-chain liquid crystal polymers, which embed the liquid crystal molecules into the polymer main chains, and (2) side-chain liquid crystal polymers (SCLCPs), which graft the liquid crystal molecules as side groups onto the polymer main chains. Among them, SCLCPs can be divided into two different parts: side-on SCLCPs and end-on SCLCPs, which depend on whether the mesogenic units are attached onto the polymer backbones laterally (side-on) or longitudinally (end-on), as © XXXX American Chemical Society

shown in Figure 1a. The difference of the relative position of the side groups and polymer backbones between side-on SCLCPs and end-on SCLCPs could markedly influence their functional properties.15 Compared with end-on SCLCPs, sideon SCLCPs with spacers between mesogens and polymer backbones tend to force the mesogenic directors parallel to the orientations of polymeric backbones, possess larger backbone anisotropy, and form the nematic phase predominantly.16−19 A typical and interesting example was that SCLCPs with chiral backbones and achiral mesogenic units usually showed the achiral nematic phase if the side groups were side-on mesogens.20 Meanwhile, they always presented the cholesteric phase if the side groups were end-on mesogens21−25 since the Received: March 25, 2019 Revised: July 9, 2019

A

DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX

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properties of these monomers and polymers were studied by nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). Polarized optical microscopy (POM) and one-dimensional/two-dimensional wide-angle X-ray scattering (1D/2D-WAXS) were performed to confirm the mesophase transition behavior and the molecular arrangements of the alternating SCLCP Poly(Y1801-alt-Y1802). The ultraviolet− visible (UV−vis) and fluorescence spectra of two monomers and three polymers were also measured to evaluate their photophysical properties.

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EXPERIMENTAL SECTION

General Considerations. N-Methoxycarbonyl maleimide, 6amino-1-hexanol, 4-acetoxystyrene, toluene-4-sulfonyl chloride, 4′hydroxy-4-biphenyl carbonitrile azodiisobutyronitrile (AIBN), dicyclohexylcarbodiimide (DCC), di-tert-butyl peroxide (DTBP), and dimethylaminopyridine (DMAP) were obtained from Aladdin (Shanghai) Inc. Dimethylformamide (DMF) and tetrahydrofuran (THF) were distilled from CaH2 under nitrogen. Other chemical reagents were used without further purification. 2,5-Bis-(4-butoxybenzoyloxy)-benzoic acid (compound 10) was synthesized following our previous reports.38−40 The detailed synthetic protocols, instrumentation descriptions, and part of measured data are listed in the Supporting Information. Synthesis of Monomer Y1801. (1) A mixture of methyl chloroformate (3.40 mL, 44.0 mmol), maleimide (3.88 g, 40.0 mmol), N-methylmorpholine (4.82 mL, 44.0 mmol), and ethyl acetate (240 mL) was added into a 500 mL single necked round-bottom flask. After stirring at 0 °C for 30 min, the precipitate was separated by filtration. The filtrate was collected and purified by flash column chromatography (petroleum ether (v)/ethyl acetate (v) = 1/1) to give the intermediate 3 (6.10 g, yield: 98%) as a white powder. 6Amino-1-hexanol (397 mg, 2.56 mmol) and saturated NaHCO3 aqueous solution (20 mL) were slowly added into a 50 mL roundbottom flask and stirred at 0 °C. After slowly adding compound 3 (300 mg, 2.56 mmol), the above solution was continuously stirred at 0 °C for 2 h. The aqueous layer was adequately extracted with dichloromethane (25 mL) several times, and the organic solvent was removed by rotary evaporation to give the crude product. Flash column chromatography (dichloromethane (v)/ethyl acetate (v) = 3:1) was further used to purify the product to give the intermediate 5 (260 mg, yield: 52%) as a white powder.41 1H NMR (300 MHz, CDCl3) δ: 6.68 (s, 2H), 3.63 (t, J = 6.5 Hz, 2H), 3.52 (t, J = 7.2 Hz, 2H), 1.65−1.48 (m, 4H), 1.43−1.27 (m, 2H). (2) Compound 10 (684 mg, 1.35 mmol), anhydrous dichloromethane (25 mL), DMAP (16.5 mg, 0.135 mmol), and compound 5 (266 mg, 1.35 mmol) were carefully added into a 50 mL roundbottom flask. After slowly adding DCC (279 mg, 1.62 mmol), the above solution was continuously stirred at 25 °C for 48 h. After removing the organic solvent, the crude product was further purified by flash column chromatography (ethyl acetate (v)/petroleum ether (v) = 1/10) to give the desired monomer Y1801 (633 mg, yield: 68%) as a white powder. 1H NMR (600 MHz, CDCl3) δ: 8.15 (m, 4H), 7.89 (d, J = 2.9 Hz, 1H), 7.45 (m, 1H), 7.25 (s, 1H), 6.98 (m, 4H), 6.65 (s, 2H), 4.14 (t, J = 6.6 Hz, 2H), 4.06 (m, 4H), 3.46 (t, J = 7.2 Hz, 2H), 1.86−1.78 (m, 4H), 1.56−1.46 (m, 8H), 1.25 (m, 2H), 1.16 (m, 2H), 1.00 (m, 6H). 13C NMR (75 MHz, CDCl3) δ: 170.7, 164.8, 164.5, 164.0, 163.6, 148.1, 133.9, 132.3, 127.0, 124.9, 121.4, 121.0, 114.3, 77.4, 76.9, 76.5, 67.9, 65.3, 37.6, 31.1, 29.6, 28.2, 26.2, 25.3, 19.1, 13.7. ESI-MS m/z: 708.28 [M + Na]+. Synthesis of LC Monomer Y1802. (1) A mixture of anhydrous ethanol (200 mL) and KOH (22.8 g, 400.7 mmol) was slowly added into a 1 L round-bottom flask. 4-Acetoxystyrene (22.8 g, 140.7 mmol) was added into the above solution and stirred at 25 °C for 1 h. Sodium ethoxide (10.5 g, 154.4 mmol) was slowly added into the solution and continuously stirred for another 30 min at 80 °C. 6Bromohexanol (25.6 g, 140.9 mmol) dissolved in 200 mL of anhydrous ethanol was then added, and the above solution was

Figure 1. (a) Two traditional categories of SCLCPs; (b) alternating SCLCPs designed in this work.

end-on mesogenic directors and the main chain orientation tended to have a tilt angle that might induce helical structures of the terminally attached mesogens under the twisting force of chiral backbones.26 Moreover, lightly cross-linked side-on SCLCPs, also known as side-on side-chain liquid crystal elastomers (side-on SCLCEs), exhibit faster stimulus-responsive rates and superior shape morphing capabilities in comparison with end-on SCLCE materials.27,28 In recent years, sequence-controlled SCLCPs have gained extensive interest because mesogenic units with different lengths and distributions can form various ordered sequences, which further endow LCP materials with diverse functions.29−35 For example, Ma and colleagues synthesized four different sequence-determined SCLCPs that could present different phase transitions and polarized optical morphologies by adjusting the specific spacing between adjacent mesogens.36 Chiellini’ group reported three series of alternating end-on SCLCPs Poly[vinylether-alt-(end-on mesogen)] and realized a random distribution of mesomorphic groups by appropriately adjusting the stoichiometry of feed mixtures.37 Inspired by these pioneering works, we are curious about if side-on mesogens and end-on mesogens were combined in one single SCLCP and orderly polymerized in an alternative sequence, what kind of mesomorphic property the corresponding alternating SCLCP Poly[(side-on mesogen)-alt-(end-on mesogen)] could have? What would be the molecular arrangement and potential functions of this alternating SCLCP? In this work, we designed and synthesized a side-chain sideon maleimide-containing monomer Y1801 and a side-chain end-on styrene-containing liquid crystal (LC) monomer Y1802 as shown in Figure 1b and further prepared a novel alternating poly[(side-on mesogen)-alt-(end-on mesogen)] SCLCP Poly(Y1801-alt-Y1802) through a classical styrene-maleimide alternative copolymerization approach, which has the advantages of operational simplicity, mild conditions, and high polymerization degrees in synthesizing alternative copolymers. In addition, two homopolymers PY1801 and PY1802 derived from these two monomers, respectively, were also prepared for comparison purposes. The structures and thermodynamic B

DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthetic Routes of (a) Monomers Y1801 and Y1802, (b) Homopolymers PY1801 and PY1802, and (c) Alternating SCLCP Poly(Y1801-alt-Y1802)

maintained at 80 °C for 48 h. The solution was extracted by dichloromethane, and the organic layer was collected. After removing the solvent by rotary evaporation, the crude product was purified by flash column chromatography (ethyl acetate (v)/petroleum ether (v) = 1/10) to give intermediate 14 as a colorless oil (20.0 g, yield: 70%). A mixture of p-tosyl chloride (2.30 g, 12.13 mmol), anhydrous dichloromethane (20 mL), and compound 14 (2.25 g, 11.03 mmol) was added into a 100 mL round-bottom flask. After dropwise adding triethylamine (2.2 g, 21.73 mmol) under a N2 atmosphere, the above solution was further stirred at 25 °C for 24 h. After removing the solvent by rotary evaporation, the crude product was thus purified by flash column chromatography (ethyl acetate (v)/petroleum ether (v) =1/20) to give compound 16 as a white powder (1.86 g, yield: 45%). 1 H NMR (600 MHz, CDCl3) δ: 7.79 (d, J = 8.2 Hz, 2H), 7.35−7.31 (m, 4H), 6.84−6.80 (m, 2H), 6.65 (m, 1H), 5.60 (d, J = 17.6 Hz, 1H), 5.12 (d, J = 10.9 Hz, 1H), 4.03 (m, 2H), 3.91 (t, J = 6.4 Hz, 2H), 2.44 (s, 3H).

(2) 4-Hydroxy-4′-cyanobiphenyl (1.06 g, 5.46 mmol), compound 16 (1.86 g, 4.96 mmol), DMF (25 mL), and K2CO3 (0.82 g, 6.00 mmol) were carefully added into a 100 mL round-bottom flask. After stirring at 80 °C under a nitrogen atmosphere for 48 h, 100 mL of water was thus added, and the reaction solution was extracted with dichloromethane three times (50 mL × 3). The resulting white product was concentrated and purified by flash column chromatography (ethyl acetate (v)/petroleum ether (v) = 1/20) to give the monomer Y1802 as a white powder (1.81 g, yield: 92%).42 1H NMR (600 MHz, CDCl3) δ: 7.69 (d, J = 8.3 Hz, 2H), 7.64 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 8.6 Hz, 2H), 7.34 (d, J = 8.5 Hz, 2H), 6.99 (d, J = 8.6 Hz, 2H), 6.85 (d, J = 8.6 Hz, 2H), 6.65 (m, 1H), 5.60 (m, 1H), 5.12 (m, 1H), 4.03(t, J = 6.4 Hz, 2H), 3.98 (t, J = 6.4 Hz, 2H), 1.84 (m, 4H), 1.56−1.58 (m, 5H). 13C NMR (75 MHz, CDCl3) δ: 159.7, 158.9, 145.3, 136.2, 132.6, 131.3, 130.3, 128.4, 127.4, 127.1, 119.2, 115.1, 114.5, 111.5, 110.0, 68.0, 29.2, 25.9. ESI-MS m/z: 397.1 [M − H]−. C

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Macromolecules Synthesis of Homopolymer PY1801. As shown in Scheme 1b, Y1801 (100 mg, 0.16 mmol), DMF (0.2 mL), and DTBP (0.2 mg, 1.5 × 10−3 mmol) were slowly added into a 10 mL Schlenk tube. The above mixture was adequately degassed and exchanged with highpurity nitrogen via several freeze−thaw cycles and then stirred at 130 °C for 24 h. A large amount of methanol was poured into the above solution to precipitate the polymer. The gathered product was further dissolved in dichloromethane and reprecipitated from methanol. After three cycles, the product was further dried in vacuum for 24 h to give the homopolymer PY1801 as a brown powder (20 mg, yield: 20%). 1 H NMR (600 MHz, CDCl3) δ: 7.95 (m, 4H), 7.71 (m, 1H), 7.26 (m, 1H), 7.06 (m, 1H), 6.78 (m, 4H), 3.87 (m, 8H). 13C NMR (75 MHz, CDCl3) δ: 162.6, 147.2, 131.2, 123.8, 120.4, 113.3, 66.9, 64.7, 30.1, 28.5, 27.2, 24.0, 18.1, 12.8. Synthesis of Homopolymer PY1802. AIBN (0.4 mg, 2.5 × 10−3 mmol), Y1802 (100 mg, 0.25 mmol), and anhydrous THF (0.3 mL) were carefully added into a 10 mL Schlenk tube. The above mixture was adequately degassed and exchanged with high-purity nitrogen via several freeze−thaw cycles and then stirred at 60 °C for 24 h. A large amount of methanol was poured into the above solution to precipitate the polymer. The gathered product was further dissolved in dichloromethane and reprecipitated from methanol. After three cycles, the product was further dried in vacuum for 24 h to give the homopolymer PY1802 as a white powder (80 mg, yield: 80%). 1H NMR (600 MHz, CDCl3) δ: 7.39−7.53 (m, 6H), 6.85 (m, 2H), 6.53 (m, 4H), 3.88 (m, 4H), 1.73 (m, 4H), 1.49 (m, 6H). 13C NMR (75 MHz, CDCl3) δ: 158.7, 144.7, 131.6, 130.3, 127.4, 126.0, 117.6, 114.0, 113.5, 113.0, 110.4, 109.1, 67.0, 28.2, 24.9. Synthesis of Alternating Copolymer Poly(Y1801-alt-Y1802). AIBN (0.48 mg, 3.0 × 10−3 mmol), Y1801 (100 mg, 0.16 mmol), 1,2dichloroethane (0.4 mL), and Y1802 (58 mg, 0.15 mmol) were carefully added into a 10 mL Schlenk tube. The above mixture was adequately degassed and exchanged with high-purity nitrogen via several freeze−thaw cycles and then stirred at 70 °C for 6 h. A large amount of methanol was poured into the above solution to precipitate the polymer. The gathered product was further dissolved in dichloromethane and reprecipitated from methanol. After three cycles, the product was further dried in vacuum for 30 h to give the copolymer Poly(Y1801-alt-Y1802) as a white powder (110 mg, yield: 70%). 1H NMR (600 MHz, CDCl3) δ: 8.07 (m, 4H), 7.83 (m, 1H), 7.52 (m, 4H), 7.43 (m, 2H), 7.17 (m, 2H), 6.89 (m, 8H), 3.97 (m, 12H). 13C NMR (75 MHz, CDCl3) δ: 163.7, 162.7, 158.6, 147.3, 144.0, 131.5, 131.4, 131.3, 127.2, 125.9, 123.8, 113.9, 113.3, 76.2, 76.0, 30.1, 28.2, 27.2, 24.9, 24.3, 18.1, 12.8.

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Table 1. Molecular Weight Measurements of Polymers entry

polymer

Mn (g/mol)

Mw (g/mol)

DPa

PDI

1 2 3

PY1801 PY1802 Poly(Y1801-alt-Y1802)

4600 136500 104200

7200 290800 221300

7 342 96

1.56 2.13 2.12

a

The degree of polymerization was estimated based on the GPC data.

calibration using polystyrene standards was performed to analyzed molecular weight measurements of those polymers. The homopolymer PY1801 was synthesized by using di-tertbutyl peroxide (DTBP) as the radical initiator in anhydrous DMF at 130 °C for 24 h. The number average molecular weight (Mn) of PY1801 was 4600 g/mol, and the weight average molecular weight (Mw) was 7200 g/mol, which indicated a low degree of polymerization (DP) of maleimidecontaining molecule Y1801. Based on the literature, maleimide is an electron-deficient molecule with a weak electron-donating ability and could only form oligomers. The styrene-containing molecule Y1802 was polymerized by using AIBN as the initiator in anhydrous THF at 60 °C for 24 h to give the homopolymer PY1802 as a white solid with a yield of 80%. The Mn and Mw of PY1802 were measured as 136500 and 290800 g/mol, respectively. As a typical electron-rich molecule, styrene tended to carry out free radical polymerization efficiently under mild reaction conditions to give homopolymers with high DP and molecular weights. The styrene-maleimide alternative copolymerization reaction between Y1801 and Y1802 was carried out using AIBN as the radical initiator in dry dichloroethane at 70 °C to give the desired copolymer Poly(Y1801-alt-Y1802) as a white solid with a yield of 70%. The Mn and Mw of Poly(Y1801-alt-Y1802) were measured as 104200 and 221300 g/mol. Moreover, from the GPC chromatogram (Figure S14), only a single sharp peak of Poly(Y1801-alt-Y1802) was observed, which implied that side reactions were neglectable during the alternative copolymerization process. 1 H NMR spectra of monomers Y1801 and Y1802 and their corresponding homopolymers PY1801 and PY1802 are shown in Figure 2a,b. After the free radical polymerization, the peak at ∼6.65 ppm originally belonging to the olefin protons of the side-on maleimide group of Y1801 disappeared completely in the 1H NMR spectrum of PY1801. Similarly, as presented in Figure 2b, the peaks at ∼5.03 and ∼5.53 ppm ascribed to the resonance signals of the three terminal olefin protons of the vinyl group of Y1802 vanished in the 1H NMR spectrum of PY1802. The disappearances presented solid evidence for the successful homopolymerization of Y1801 and Y1802. Furthermore, the spectra of homopolymers PY1801 and PY1802 showed broad peaks, which were typical characteristics of polymers due to the slower motions of the protons. The alternating structure of Poly(Y1801-alt-Y1802) was also confirmed by 1H NMR and 13C NMR techniques. As shown in Figure 2c, based on the integral ratio of the aromatic protons (He,Hf ∼8.07 ppm) and the aromatic proton (Hh,Hi,Hj,Hk ∼6.89 ppm), the molar ratio of Y1801 and Y1802 could be roughly determine by solving the following equation: Y1801/Y1802 = (I8.07/4)/(I6.89/8), where I8.07 and I6.89 represent the integral values of peaks at ∼8.07 and ∼6.89 ppm. It was further confirmed that the molar ratio of two monomers in Poly(Y1801-alt-Y1802) was approximately 1:1. Following literature protocols,43,44 the alternating maleimide-

RESULTS AND DISCUSSION

As shown in Scheme 1a, we designed and synthesized a sidechain side-on monomer Y1801 and a side-chain end-on monomer Y1802. Maleimide (1) was first reacted with methyl chloroformate (2) to give the intermediate 3, which was further coupled with 6-amino-1-hexanol (4) to provide the alcohol 5. Meanwhile, 2,5-dihydroxy-benzoic acid (6) was transformed to 2,5-dihydroxy-benzoic acid benzyl ester (7), which went through an esterification reaction with 4-butoxybenzoic acid (8) and a following reduction reaction with Pd/C to give the key intermediate acid 10. After a DCC coupling reaction between compounds 5 and 10, the side-chain side-on monomer Y1801 was successfully prepared. The synthesis of the end-on mesogen Y1802 started with a basic hydrolysis of acetic acid 4-vinyl-phenyl ester (11) and then went through two etherification reactions with 6-bromo-hexan-1-ol (13) and 4′-hydroxy-biphenyl-4-carbonitrile (17) successively to provide the desired monomer. As shown in Scheme 1b,c, the homopolymers PY1801 and PY1802 and the copolymer Poly(Y1801-alt-Y1802) were prepared by free radical polymerization, respectively. As listed in Table 1, gel permeation chromatography (GPC) based on D

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Figure 2. 1H NMR spectra of (a) Y1801, PY1801, (b) Y1802, PY1802, and (c) Poly(Y1801-alt-Y1802). 13C NMR spectra of (d) Poly(Y1801-altY1802), and the expanded regions at (e) 125−155 and (f) 50−70 ppm.

styrene-maleimide structure could be confirmed by 13C NMR spectra. Two expanded ranges around 50−70 (carbons in the polymer main chain) and 125−155 ppm (carbons attached to the main chain) are shown in Figure 2e,f. A main subpeak of styrene quaternary carbon (C5) was observed at ∼131 ppm (Figure 2e), which was the most typical characteristic peak of the alternating maleimide-styrene-maleimide triad based on the previous work.43 The resonance signals observed in the region of 50−70 ppm could be ascribed to the carbons (C1, C2, C3, C4) of the alternating maleimide-styrene-maleimide triad in the polymer backbone, which presented another evidence for the successful alternative copolymerization of Y1801 and Y1802. As a conclusion, the alternating structure of SCLCP Poly(Y1801-alt-Y1802) has been established through a classical styrene-maleimide alternative copolymerization approach. The mesomorphic behaviors and thermal properties of two monomers and three polymers were adequately investigated by DSC and POM. The crystallization of monomer Y1801 was quite difficult, and no obvious exothermic peak was observed during the first cooling process (Figure 3a). During the second heating process, an exothermic process prior to the melting peak (around 99 °C) was observed. Differently, its

corresponding homopolymer PY1801 showed an enantiotropic LC phase in the temperature range of 61−87 °C (Figure 3c). Monomer Y1802 showed complicated phase transitions during the heating process and has two phase transition peaks in the cooling process implying that a mesophase possibly existed between the crystal state and the isotropic state (Figure 3b), while its corresponding homopolymer PY1802 only exhibited one phase transition peak around 122 °C (Figure 3d), which was the LC-to-isotropic transition based on POM observation. The DSC curve of the alternating copolymer Poly(Y1801-altY1802) (Figure 3e) presented a glass transition temperature at 68 °C and a clearing point temperature at 129 °C, which was ascribed to the LC-to-isotropic transition. In good agreement with DSC results, monomer Y1801 with a crystalline spherulite texture shown in Figure 4a possessed no mesophases. In contrast, as indicated in Figure 4b, the POM image of monomer Y1802 presented the Schlieren texture during the cooling process, which was the characteristic of the nematic phase. As shown in Figure 4c−f, PY1801, PY1802, and Poly(Y1801-alt-Y1802) all showed ambiguous birefringent textures, which were often observed in polymer samples because of their high viscosities. E

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Figure 3. DSC curves of (a) Y1801, (b) Y1802, (c) PY1801, (d) PY1802, and (e) Poly(Y1801-alt-Y1802) recorded in the first cooling and second heating processes.

Figure 5. 1D-WAXS patterns of (a) Y1801, (b) Y1802, (c) PY1801, (d) PY1802 during heating process, and Poly(Y1801-alt-Y1802) during (e) heating process and (f) cooling process.

typical nematic phase. In agreement with the DSC data, the 1D-WAXS patterns of monomer Y1802 recorded at different temperatures all presented sharp crystallization peaks during the heating process, which implied that Y1802 might have at least two crystal forms on heating. However, during the cooling from the isotropic state, two diffuse peaks in both small-angle and wide-angle regions were observed at the temperatures ranging from 125 to 100 °C, which demonstrated that monomer Y1802 has a monotropic nematic phase on cooling. Differently, its homopolymer PY1802 showed one diffuse peak in the wide-angle region and one diffuse peak in the smallangle region, which were characteristics of the nematic phase. The combination of DSC, POM, and 1D-WAXS data indicated that PY1802 has a nematic phase. As shown in Figure 5e,f, the 1D-WAXS pattern of Poly(Y1801-alt-Y1802) presented two scattering peaks at ∼30.2 and ∼14.7 Å in the small-angle region with a reciprocal d-spacing ratio of 1:2 and a diffuse peak at ∼4.19 Å in the wide-angle region, which indicated that a smectic layer structure possibly existed in this copolymer system. As the temperature increased, the sharp peak at small angles (q = 2.08 nm−1) gradually weakened and disappeared completely at 160 °C. This peak should be associated with the lengths of two monomers Y1801 and Y1802. However, the d spacing (∼30.2 Å) was larger than the fully extended lengths of two monomers Y1801 (l = 22.0 Å) and Y1802 (l = 25.1 Å), estimated by Dreiding models, but less than the sum of the lengths of the two monomers Y1801 and Y1802. Thus, we considered that the alternating SCLCP Poly(Y1801-alt-Y1802) might have an interdigitated smectic phase. Meanwhile, it is noticed that the peak at the small-angle region was slightly

Figure 4. POM images of (a) Y1801 recorded at 50 °C, (b) Y1802 recorded at 110 °C, (c) PY1801 recorded at 60 °C, (d) PY1802 recorded at 120 °C, and (e,f) Poly(Y1801-alt-Y1802) recorded at 70 °C on heating and cooling processes.

To further elucidate the mesomorphic behaviors of these monomers and polymers, one- dimensional wide-angle X-ray scattering (1D-WAXS) experiments were performed. As presented in Figure 5a, the 1D-WAXS pattern of monomer Y1801 has many sharp crystallization peaks, which once again denied the possibility of liquid crystallinity. On the contrary, its corresponding homopolymer PY1801 showed one diffuse peak in the wide-angle region (Figure 5c), which demonstrated a F

DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX

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axis. A uniform diffuse ring was observed at 140 °C, which implied that the sample became isotropic. The smectic layer thickness (∼30.3 Å) was consistent with the datum calculated from the 1D-WAXS pattern of the untreated sample, which indicated that mechanical shearing force did not change the molecular arrangement (Figure 6c). The mesomorphic phase of Poly(Y1801-alt-Y1802) was identified as interdigitated smectic A (SmAd) because the quadruple scattering halo, characteristic of the smectic C phase, was not observed in the 2D-WAXS patterns. This compromised molecular arrangement was also proven by the POM image of the Poly(Y1801-altY1802) fiber. The alternating copolymer was heated up to the isotropic phase, and then a capillary pipette was attached to the melted polymer and pulled quickly to form the Poly(Y1801alt-Y1802) fiber. As shown in Figure 6b, the birefringence was extinct when the long axis of the Poly(Y1801-alt-Y1802) fiber was parallel or perpendicular to one polarizer. The transmission was maximized by rotating the fiber by 45°. It represented that the uniformity of the alignment in the alternating copolymer Poly(Y1801-alt-Y1802) was qualitatively excellent. In general, after mechanical shearing, the randomly oriented main chains of the alternating copolymer Poly(Y1801-alt-Y1802) were forced to align along the sheared direction; meanwhile, the mesogens’ molecular directions were not only perpendicular to the normal direction of the smectic layer but also perpendicular to the sheared direction (Figure 6c). This compromised molecular arrangement of Poly(Y1801alt-Y1802) should be associated with the styrene-maleimide alternative structure, which might harden the polymer main chain, aggravate the steric hindrance, and force the attached mesogens to form interdigitated structures with ordered sequences. This hypothesis might explain the interdigitated smectic A phase of the alternating SCLCP Poly(Y1801-altY1802). UV−vis and fluorescence spectra of Y1801, Y1802, PY1801, PY1802, and Poly(Y1801-alt-Y1802) were further carried out to study their optical properties. As demonstrated in Figure 7, the UV−vis spectra of two monomers and three polymers dispersed in anhydrous THF and DMF with a monomeric concentration of approximately 1.0 × 10−4 mol/L were obtained. Here, the concentrations of monomers and polymers were all calculated based on the moles of the monomeric units. The two monomers and three polymers all presented two strong absorption peaks in the UV region centered at around 225 and 280 nm in THF solution. These peaks were associated with the π−π* electronic transitions of inner carbonyl groups and phenyl rings. The absorption wavelengths of PY1801, Poly(Y1801-alt-Y1802), and PY1802 increased (red-shift) gradually, which were associated with the increased conjugated structures of phenyl rings. Meanwhile, in DMF solution, the absorption peaks centered at 225 nm became much weaker. The main reason was that the vibration effect in polar organic solvent might lead to the disappearance of the fine structures and further weaken the absorption intensity. The fluorescence spectra of two monomers and three polymers dissolved in THF (Figure 7c) or DMF (Figure 7d) solution with a monomeric concentration of approximately 1.0 × 10−4 mol/L were also recorded at 25 °C. The maximum UV absorption wavelengths of all the compounds were set as fluorescence excitation wavelengths. A maximum emission wavelength belonging to Poly(Y1801-alt-Y1802) was found at 435 nm in THF solution, which was appreciably longer than those of monomers and homopolymers (around 365 nm in THF).

broader than traditional smectic mesogens’ XRD spectra, which might be derived from the coexistence of end-on and side-on mesogens. Since Y1801 and Y1802 have different mesogenic lengths, the alternating polymer Poly(Y1801-altY1802) might have a compromised molecular arrangement, which resulted in broad peak width. The two-dimensional wide-angle X-ray scattering (2DWAXS) experiments were further carried out to elucidate the mesophase of Poly(Y1801-alt-Y1802). The copolymer powder was mechanically sheared along the Y axis at 150 °C followed by annealing at 70 °C for 5 h. The 2D-WAXS patterns of a uniaxial oriented Poly(Y1801-alt-Y1802) sample were recorded at 60, 100, 120, 130, and 140 °C with an incident Xray beam (Z axis) perpendicular to the sample surfaces. As illustrated in Figure 6a, during the heating process, a pair of sharp arcs in the small-angle region was gathered on the equator, and a pair of crescents in the wide-angle region was gathered on the meridian. It was indicated that the uniaxial oriented Poly(Y1801-alt-Y1802) has a smectic mesophase with layer structures and the mesogens were arranged along the X

Figure 6. (a) 2D-WAXS patterns of Poly(Y1801-alt-Y1802) recorded at 60, 100, 120, 130, and 140 °C during the heating process. (b) POM images of Poly(Y1801-alt-Y1802) fiber. (c) Schematic illustration of the proposed molecular arrangement of Poly(Y1801-alt-Y1802). G

DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 7. UV−vis spectra of polymers and monomers dispersed in (a) THF and (b) DMF with a monomeric concentration of approximately 1.0 × 10−4 mol/L. Fluorescence spectra of polymers and monomers dispersed in (c) THF and (d) DMF with a monomeric concentration of approximately 1.0 × 10−4 mol/L.

Based on the literature,45 the π−π interaction between the phenyl rings and the carbonyl group in the polymer induced photoluminescence. This conclusion was also applied in the fluorescence spectra of these monomers and polymers dissolved in DMF. In addition, compared to the emission wavelengths in THF, the emission shifting to the longwavelength region was observed in DMF. Based on the previous report,46 the red-shift of emission wavelength in electron-rich solvents was associated with the formation of polymer/solvent complexes. A handheld UV lamp was used to irradiate the polymers dissolved in DMF with a concentration of approximately 1.0 × 10−4 mol/L for further study of photophysical properties. As shown in Figure 8, the emission of the alternating copolymer Poly(Y1801-alt-Y1802) was much stronger than those of other two homopolymers. Obviously, these covalent linkages of the styryl and maleimide units brought a sufficient close distance

for interaction among phenyl rings, carbonyl groups, and solvents. Besides, PY1801 showed stronger emission phenomena in comparison with PY1802, which might be ascribed to the rigid conformation of PY1801 in DMF. This rigid conformation may limit the free rotation of the backbones along the C−C single bonds, which were beneficial for the collection of many carbonyl groups and the formation of polymer/solvent complexes, according to the literature.46

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CONCLUSIONS

In summary, a new alternating SCLCP Poly(Y1801-alt-Y1802) comprising a side-on mesogen Y1801 and an end-on mesogen Y1802 was designed and synthesized via the styrene-maleimide alternative copolymerization method. Two homopolymers PY1801 and PY1802 were also prepared, respectively, for comparison purposes. The alternating molecular structure of Poly(Y1801-alt-Y1802) was proven by GPC and NMR experiments. The mesomorphic properties of PY1801, PY1802, and Poly(Y1801-alt-Y1802) were characterized by DSC, POM, and WAXS techniques. Both the side-on SCLCP PY1801 and the end-on SCLCP PY1802 exhibited the nematic phase, while their alternating copolymer Poly(Y1801-altY1802) showed an interdigitated SmA phase, a compromised molecular arrangement instead. In addition, UV−vis and fluorescence spectra of the alternating SCLCP Poly(Y1801-altY1802) were measured, and an obvious fluorescence emission of Poly(Y1801-alt-Y1802) in DMF solution was observed, which endowed this novel alternating-structured liquid crystal polymer with potential applications in luminescent materials and devices.

Figure 8. Photo images of irradiating (a) Y1801, (b) Y1802, (c) PY1801, (d) PY1802, and (e) Poly(Y1801-alt-Y1802) in DMF with a concentration of approximately 1.0 × 10−4 mol/L under UV (365 nm) light. H

DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00607.

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Instrumentation descriptions, synthetic protocols, NMR spectra, and GPC chromatogram (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Y.). *E-mail: [email protected] (E.-Q.C.). ORCID

Hong Yang: 0000-0003-4647-1388 Er-Qiang Chen: 0000-0002-0408-5326 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Jiangsu Provincial Natural Science Foundation of China (Nos. BK20170024, BK20180406), National Natural Science Foundation of China (Grant No. 21634001), the Fundamental Research Funds for the Central Universities (No. 2242018K40048), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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DOI: 10.1021/acs.macromol.9b00607 Macromolecules XXXX, XXX, XXX−XXX