Synthesis and Characterization of Mesogen-Jacketed Liquid

Mar 16, 2012 - ... new 2-vinylbiphenyl-based mesogen-jacketed liquid crystalline polymer with a high glass transition temperature and low threshold mo...
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Synthesis and Characterization of Mesogen-Jacketed Liquid Crystalline Polymers through Hydrogen-Bonding Yiding Xu, Wei Qu, Qian Yang, Jukuan Zheng, Zhihao Shen,* Xinghe Fan,* and Qifeng Zhou Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ABSTRACT: In an attempt to construct mesogen-jacketed liquid crystalline polymers through noncovalent interactions, poly(2-vinylbenzene-1,4-dioic acid) and five series of pyridine derivatives were synthesized as hydrogen-bonding donor and acceptors, respectively. The structures of the compounds were confirmed by 1H NMR, FT-IR, and gel permeation chromatography. A series of complexes with the same degree of polymerization and polydispersity were obtained by mixing the hydrogen-bonding donor and acceptors. The hydrogen-bonds were confirmed by FT-IR spectra. We found that the rigidity and structures of pyridine derivatives and the strength of the hydrogen-bonding all influenced the liquid crystalline behaviors of the resulting complexes. Smectic A and columnar nematic phases were observed in complexes PPANCx and PPANNEC, respectively.

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INTRODUCTION Liquid crystalline polymers (LCPs) have received much attention because of their potential applications, such as nonlinear optic devices and engineering plastics.1 Mesogen-jacketed liquid crystalline polymers (MJLCPs) are side-chain LCPs with a very short spacer or no spacer between the backbone and laterally attached bulky side groups, which are often mesogens.2,3 Various liquid crystalline (LC) phases, such as columnar nematic, hexagonal columnar, rectangular columnar, smectic A, and smectic C, have been observed, resulting from a small change in chemical structures of MJLCPs.4 In addition, the molecular weight also plays an important role in the phase behavior of MJLCPs. So far, most of MJLCPs have been synthesized by polymerizing small molecules which contain vinyl groups. Thus, synthesis of monomers is one of the most important and difficult parts in researching MJLCPs. Because of the differences of polymerizability of monomers, it is difficult to obtain MJLCPs with similar DP and PDI values. To further investigate the relationship between polymer liquid crystalline phase and side-group structure, we prefer to have MJLCPs with the same degree of polymerization via noncovalent interactions between a polymer containing the basic part of MJLCPs and different small molecules. As one of the key interaction in aggregation in nature, hydrogenbonding (H-bonding) is widely used in building LCPs. Since the first report of the supramolecular side-chain polymer containing a hydrogen-bonded (H-bonded) mesogenic core,5 the design and preparation of supramolecular LCPs built through H-bonding have attracted much attention. Main-chain, sidechain, and network supramolecular LCPs have been prepared through H-bonding.5−9 In a recent research on MJLCPs, H-bonding between side groups leads to the smectic packing.10 © 2012 American Chemical Society

On the other hand, as another important type of noncovalent interactions, electrostatic interaction has been utilized to obtain lamellar structures in ionic complexes between MJLCPs and surfactants.11 In this work, a new series of MJLCPs containing H-bonding in their side groups were designed. Herein, poly(2-vinylbenzene1,4-dioic acid) (PPA) was used as the H-bonding donor and several pyridine derivatives as the acceptors. As shown in Chart 1, H-bonded mesogens were directly attached to the polymer backbone laterally. Smectic phases and a columnar nematic phase were observed in three of the complexes. The results demonstrated that H-bonding with bond energy much smaller than those of covalent bonds were also stable enough to built MJLCPs.

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EXPERIMENTAL METHODS

Materials. 2-Vinylbenzene-1,4-dioic acid, 12 4-(4′-hydroxy)styrylpyridine,13 4-alkoxyphenol,14 and 4-(4′-alkoxy)styrylpyridine15,20 were prepared according to the literature. Benzoyl peroxide (BPO) was purified by recrystallization from chloroform/methanol. Tetrahydrofuran (THF) was refluxed over sodium under argon and distilled out before use. N,N-Dimethylformamide (DMF) was refluxed over potassium hydroxide and distilled out before use. Chlorobenzene was washed by H2SO4, distilled water, NaHCO3, and distilled water separately, refluxed over CaH2, and distilled before use. All other reagents were used as received from commercial sources. Characterization Methods. All of the characterization methods, such as 1H NMR spectrometry, FT-IR spectroscopy, gel permeation chromatography (GPC), mass spectrometry (MS), thermogravimetric Received: December 20, 2011 Revised: February 29, 2012 Published: March 16, 2012 2682

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Chart 1. Chemical Structures of H-Bonded MJLCPs

2. Polymerization. The polymer, poly(di-tert-butyl-2-vinylbenzene1,4-dioate) (PPE), was obtained by conventional solution radical polymerization.21 The monomer (4.56 g, 30 mmol), 1.00 mL of chlorobenzene solution of 0.05 M BPO, 15 mL of chlorobenzene, and a magnetic stir bar were added into a polymerization tube. After three freeze−pump−thaw cycles, the tube was sealed off under vacuum. Polymerization was carried out at 90 °C for 12 h. The tube was then opened, and the reaction mixture was diluted with THF. The resultant polymer was precipitated and washed with methanol. After redissolved in THF and reprecipitated in methanol, the polymer was dried to a constant weight. Yield: 55%. 3. Synthesis of Poly(2-vinylbenzene-1,4-dioic acid). A polymer sample of 1.56 g was dissolved in 50 mL of chloroform. Then 15 mL of trifluoroacetic acid was slowly added at the temperature of an ice/ water bath. The solution was further stirred at ambient temperature for 24 h. After the removal of the solvent under reduced pressure, the residue was washed with ethyl ether for three times to obtain the product as a white powder. Yield: 95%. Synthesis of Pyridine Derivatives. The synthetic routes of five series of pyridine derivatives are depicted in Scheme 2. The experimental details are described below using ethyl isonicotinate (NE2), 4-(dodecyloxy)phenyl isonicotinate (NEC12), 4-(4′-hexanoxy)styrylpyridine (NC6), 2-N-acetylamino-4-methylpyridine (NN), and 4-(methoxy)phenyl 2-N-acetylaminoisonicotinate (NNEC) as examples. 1. Synthesis of Ethyl Isonicotinate (NE2). A mixture of isonicotinic acid (1.23 g, 10 mmol), 20 mL of ethanol, and 0.5 mL of H2SO4 were fluxed for 12 h in a 100 mL round-bottomed flask. After ethanol was evaporated under reduced pressure, about 20 mL of Na2CO3 solution (1 M) was added into the mixture. Then the mixture was extracted with ether. After evaporation of the solvent, the product was obtained as a colorless liquid. Yield: 90%. 1H NMR (300 MHz, δ, ppm, CDCl3): 1.46−1.55 (t, 3H), 4.35−4.50 (q, 2H), 7.80−7.90 (d, 2H), 8.75−8.85 (d, 2H). 2. Synthesis of 4-(Dodecyloxy)phenyl Isonicotinate (NEC12). Isonicotinic acid (1.23 g, 10 mmol), 4-dodecyloxyphenol (1.35 g, 5.0 mmol), DCC (6.20 g, 30 mmol), DMAP (0.12 g, 1.0 mmol), and 50 mL of dichloromethane were mixed in a 100 mL round-bottomed flask, and the mixture was stirred for 24 h at ambient temperature. The precipitate was filtered off and washed with dichloromethane for several times. After the evaporation of the solvent, the crude product was purified via silica gel column chromatography with ethyl acetate as the eluent to obtain 4-(dodecyloxy)phenyl isonicotinate as a white

analysis (TGA), differential scanning calorimetry (DSC), onedimensional wide-angle X-ray diffraction (1D WAXD), polarized light microscopy (PLM), and small-angle X-ray scattering (SAXS), used in this study were similar to those reported previously.16,17 Synthesis of Poly(2-vinylbenzene-1,4-dioic acid) (PPA). The synthetic routes of poly(2-vinylbenzene-1,4-dioic acid) are depicted in Scheme 1. The experimental details are described below in three steps.

Scheme 1. Synthetic Routes of PPA

1. Synthesis of Di-tert-butyl-2-vinylbenzene-1,4-dioate. 2-Vinylbenzene-1,4-dioic acid (9.60 g, 50 mmol), 2-methylpropan-2-ol (17.50 g, 0.20 mol), N,N-dimethylpyridin-4-amine (DMAP, 0.60 g, 5.0 mmol), N,N′-dicyclohexyl carbodiimide (DCC, 30.90 g, 0.15 mol), and 200 mL of dried dichloromethane were added into a 500 mL round-bottomed flask. The mixture was stirred at ambient temperature for 24 h. After the floating solid was filtrated off and the solvent was evaporated under reduced pressure, the crude product was purified by silica gel column chromatography with dichloromethane as the eluent to obtain the monomer as a light yellow liquid. Yield: 60%. 1H NMR (300 MHz, δ, ppm, CDCl3): 1.61 (s, 18H), 5.37−5.43 (d, 1H), 5.69−5.78 (d, 1H), 7.31−7.45 (q, 1H), 7.78−7.92 (m, 2H), 8.17 (s, 1H). 2683

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Scheme 2. Synthetic Routes of Pyridine Derivatives

solid. Yield: 55%. 1H NMR (300 MHz, δ, ppm, CDCl3): 0.95−1.00 (t, 3H), 1.20−1.80 (m, 20H), 3.95−4.00 (t, 2H), 6.88−6.94 (d, 2H), 7.08−7.17 (d, 2H), 7.88−7.97 (d, 2H), 8.83−8.90 (d, 2H). EI−MS: M/z = 383. Anal. Calcd Ffor C24H33NO3: C, 75.16; H, 8.67; N, 3.65; Found: C, 75.33; H, 8.73; N, 3.71. 3. Synthesis of 4-(4′-Hexanoxy)styrylpyridine (NC6). 4-(4′Hydroxy)stilbazole (1.97 g, 10 mmol), 1-bromohexane (0.85 g, 5.2 mmol), K2CO3 (1.39 g, 10 mmol), and 50 mL of DMF were added into a round-bottomed flask, and the mixture was stirred at 65−70 °C for 7 h. After cooled, the reaction mixture was poured into excess ice/water with intense stirring. The precipitate was washed with water and purified by silica gel column chromatography with ethyl acetate as the eluent to obtain 4-(4′-hexanoxy) styrylpyridine as a light yellow solid. Yield: 65%. 1H NMR (300 MHz, δ, ppm, CDCl3): 0.86−0.90 (t, 3H), 1.20−1.60 (m, 6H), 1.75−1.84 (m, 2H), 3.92−4.00 (t, 2H), 6.80−7.50 (m, 8H), 8.54−8.59 (d, 2H). EI−MS: M/z = 281. Anal. Calcd for C19H23NO: C, 81.10; H, 8.24; N, 4.98; Found: C, 81.63; H, 8.65; N, 5.34. 4. Synthesis of 2-N-Acetylamino-4-methylpyridine (NN). 2-Amino4-methylpyridine (3.31 g, 30 mmol) and about 15 mL of acetic anhydride were added into a 100 mL round-bottomed flask, and the mixture was stirred at 80 °C for 10 min. The reaction mixture was poured into excess NaHCO3 solution with intense stirring. Then the solution was extracted with dichloromethane for three times. After evaporation of the solvent, the product was obtained as a white solid. Yield: 98%. 1H NMR (300 MHz, δ, ppm, CDCl3): 2.18 (s, 3H), 2.37 (s, 3H), 6.86−6.88 (d, 1H), 8.09 (s, 1H), 8.10−8.12 (d, 1H), 8.97 (s, 1H). EI−MS: M/z = 150. Anal. Calcd for C8H10N2O: C, 63.98; H, 6.71; N, 18.65; Found: C, 63.90; H, 6.75; N, 18.54. 5. Synthesis of 4-(Methoxy) Phenyl 2-N-Acetylaminoisonicotinate (NNEC). NN (12.10 g, 81 mmol) and 14.5 g of KMnO4 were dissolved in 500 mL of water. After stirred for 15 min, 15 mL of 98% H2SO4 was added into the solution, and the mixture was heated at

80 °C for 8 h. After cooled, the precipitate was filtered off. An aqueous solution of Na2CO3 was added until the pH of the reaction solution reached 10. The solution was then extracted with ethyl acetate twice. The aqueous phase was concentrated and acidized with HCl. The crude product 2-N-acetylamino isonicotinic acid was obtained as a white solid. Yield: 15%. Crude 2-N-acetylamino isonicotinic acid (2.05 g), 4-methoxyphenol (2.50 g, 20 mmol), DCC (6.25 g, 30 mmol), DMAP (0.12 g, 1.0 mmol), and 30 mL of dried dichloromethane were added into a 100 mL roundbottomed flask, and the mixture was stirred at ambient temperature for 48 h. The precipitate was filtered off and washed with dichloromethane for several times. After evaporation of the solvent, the crude product was purified via silica gel column chromatography with ethyl acetate as the eluent to obtain 4-(methoxy) phenyl 2-N-acetylamino-isonicotinate as a white solid. Yield: 30%. 1H NMR (300 MHz, δ, ppm, CDCl3): 2.36 (s, 3H), 3.83 (s, 3H), 6.91−6.96 (d, 2H), 7.11−7.16 (d, 2H), 7.72−7.76 (d, 1H), 8.21 (s, 1H), 8.44−8.47 (d, 1H), 8.89 (s, 1H). EI−MS: M/z = 286. Anal. Calcd For C15H14N2O4: C, 62.93; H, 4.93; N, 9.79; Found: C, 63.10; H, 4.88; N, 9.81. Preparation of H-Bonded Complexes. The H-bonding donor and acceptor with equal molar pyridine moieties and −COOH moieties were dissolved in DMF, and the solvent was removed by slow evaporation. The complexes were then dried under reduced pressure.

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RESULTS AND DISCUSSION

Synthesis and Characterization of the H-Bonding Donor. The structures of the monomer and PPE were confirmed by 1H NMR. As shown in parts a and b of Figure 1, the signals of the vinyl group in the monomer at 5.4, 5.7, and 7.3 ppm in Figure 1a completely disappeared in Figure 1b, indicating the 2684

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was performed under a nitrogen atmosphere at the heating and cooling rates of 10 °C/min. No thermal transitions were observed during both the heating and cooling processes. Characterizations of H-Bonded Complexes. PPANEx. The complexes of PPANEx showed poorer thermal stability than the other four series of complexes probably because all NEx were volatile liquids. The FT-IR spectrum of PPANE6 is shown in Figure 3. The absorption bands at 1940 and 2550 cm−1 were

Figure 3. FT-IR spectrum of PPANE6.

indicative of strong H-bonding between carboxylic acid of PPA and the pyridine group of NE6.5,18 No further studies were carried out due to the poor stability of these complexes. PPANECx. The second series of H-bonding acceptors NECx containing two rings were designed and synthesized in order to increase the stability of the complexes. As expected, the temperatures at 1% weight loss of PPANECx were about 180 °C (mainly due to the weight loss of the PPA moieties), which were much higher than those of PPANEx. DSC experiments were carried out to study the thermal behaviors of NECx and PPANECx. As shown in Figure 4a, a melting peak with an onset temperature of 80 °C and a crystallization peak with an onset temperature of 43 °C were observed during the heating and cooling processes of NEC4, respectively. For the PPANEC4 complex, there was an endothermic process with a peak temperature of 70 °C during the second heating process (Figure 4b), indicating that not all NEC4 complexed with PPA. The crystallization of NEC4 during first cooling was disturbed by the partially H-bonded structure, which led to the lack of an exothermic peak. In the variable-temperature 1D WAXD experiments, diffraction peaks in the high-angle region (2θ > 20 °C) also confirmed the existence of macrophase separation. PPANCx. The FT-IR spectrum of PPANC6 is shown in Figure 5. The carbonyl band of PPANC6 appeared at 1705 cm−1, which had a 8 cm−1 shift compared with that of PPA, indicating that the H-bonding between the pyridine moiety and the acid moiety was weaker than that between two acid moieties.19 The bands at 1950 and 2550 cm−1 also confirmed the existence of H-bonding. DSC experiments were also carried out to study the miscibility between the H-bonding donor and acceptors. As shown in Figure 6, there were two endothermic peaks and two exothermic peaks during the heating and cooling processes of NC6, as reported in literature.20 However, no transition peaks were observed in the DSC curves of PPA and PPANC6, indicating the formation of a H-bonded complex.

Figure 1. 1H NMR spectra of the monomer (a, with the inset showing the partially magnified image) and PPE (b).

successful polymerization (in Figure 1b, the signals of benzene ring were partially shielded by the large tert-butyl moieties). Because of the poor solubility of PPA in THF, its molecular weight (91 × 103 g/mol) and polydispersity index (1.46) were calculated from those of PPE. FT-IR experiments were carried out to confirm the deprotection reaction. As shown in Figure 2,

Figure 2. FT-IR spectra of PPE and PPA.

the CO band of the polymer at 1720 cm−1 shifted to 1695 cm−1, indicating that −COOC(CH3)3 changed to −COOH. In addition, the broad band between 3000 and 3700 cm−1 also confirmed the existence of −COOH groups. TGA measurements showed that the temperature of 1% weight loss of PPA was about 180 °C under a nitrogen atmosphere, which was lower than those of some similar MJLCPs reported previously,4 mainly because of the high density of carboxylic acid in the side groups of the polymer. The DSC experiment of PPA 2685

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Figure 4. DSC thermograms of NEC4 (a) and PPANEC4 (b) at the heating and cooling rates of 10 °C/min under a nitrogen atmosphere.

Figure 6. DSC thermograms of NC6 (a), PPA (b), and PPANC6 (c) at the heating and cooling rates of 10 °C/min under a nitrogen atmosphere. Figure 5. FT-IR spectra of PPA and PPANC6.

Therefore, the ordered structure could be attributed to the H-bonded complex. Because the d-spacing (2.22 nm) was close to half of the calculated length (4.60 nm) of the side group of the complex, we presumed that there might be a diffraction peak at an even lower angle. After the cooling process of 1D WAXD, SAXS experiments were employed to confirm this conjecture. As shown in Figure 9, two peaks with q values of 1.36 and 2.88 nm−1 were observed. The ratio of the scattering vectors of these two peaks was approximately 1:2, indicating a smectic packing of PPANC6. The d-spacing of the first diffraction peak was 4.62 nm, which was close to the calculated side-group length of the H-bonded complex. Thus, the structure of PPANC6 could be smectic A (Figure 10). Similar to PPANC6, two peaks with a scattering vector ratio of 1:2 were also observed in the SAXS experiment of PPANC12 (Figure 9), and the structure of

PLM experiments during heating and cooling were carried out to study the LC behavior of PPANC6 (Figure 7). The sample did not show birefringence during the heating progress. When the temperature reached 90 °C in the cooling process, birefringence was observed, although there were no endothermic peaks in the DSC curve. One-dimensional WAXD was used to further study the phase structure of the complex. Only an amorphous high-angle halo was observed during the heating process (Figure 8a), indicating that no nanoscale ordered structure was formed. A diffraction peak at a 2θ of 3.98° (d = 2.22 nm) appeared when the sample was cooled to 130 °C (Figure 8b). There were no diffraction peaks during the cooling process of NC6, and 130 °C was much higher than the isotropic-LC transition temperature of NC6. 2686

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Figure 9. SAXS patterns of PPANC6 and PPANC12 after the cooling process of 1D WAXD.

Figure 7. PLM images of PPNC6 taken at 140 (a) and 60 °C (b) during cooling.

Figure 10. Schematic drawing of the smectic packing of PPANC6.

PPANC12 could also be smectic A also because the d-spacing of the first diffraction peak was close to the calculated length of the side group in this complex. PPANN. 2-N-Acetylamino-4-methylpyridine, which was able to form a double H-bonding with the pyridine moiety, was designed to study the relationship between the self-assembled structure and the H-bonding strength. Similar to PPANC6, PPANN did not show any transition peaks during the heating and cooling processes in DSC curves (Figure 11). PLM and 1D WAXD were also carried out. However, no LC behavior was observed. The lack of liquid crystallinity of this complex might be because the volume of the H-bonded side group was not large enough to induce the “jacketing” effect. PPANNEC. NNEC, which had a larger volume than NN, was designed to prove our conjecture. Similar to NC6 and NN, NNEC showed good miscibility with the H-bonding donor. 1D WAXD experiments were employed to study the LC behavior of the complex. As shown in Figure 12, there was only a scattering halo in the low-angle region (2θ < 5°) at 40 °C. A diffraction peak at a 2θ of 2.53° developed, and the intensity increased during the heating process, indicating that a nanoscale ordered structure was formed. The diffraction peak retained its intensity during the cooling process similar to some other MJLCPs reported previously. No higher-order diffraction peaks were observed during the heating and cooling processes.

Figure 8. 1D WAXD patterns of PPANC6 during the first heating (a) and the subsequent cooling (b) processes. 2687

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Figure 13. Schematic drawing of the columnar nematic packing of PPANNEC.

Table 1. LC Behaviors of Pyridine Derivatives and Their Complexes with PPA

Figure 11. DSC thermograms of NN (a) and PPANN (b) at the heating and cooling rates of 10 °C/min under a nitrogen atmosphere.

a

pyridine derivative

LC behavior

complex

LC behavior

NEx NECx NCx NN NNEC

no no yes no no

PPANEx PPANECx PPANCx PPANN PPANNEC

a a smectic A no columnar nematic

The complex was not stable or showed macrophase separation.

that NNEC had a similar H-bonding acceptor moiety as that of NN and similar rigidity as that of NEC).

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CONCLUSION In summary, we obtained MJLCPs with noncovalent side groups. Poly(2-vinylbenzene-1,4-dioic acid) and five series of pyridine derivatives were synthesized as H-bonding donor and acceptors, respectively. The H-bonded complexes, which had the same DP and PDI values, were studied using FT-IR, DSC, PLM, 1D WAXD, and SAXS techniques. PPANEx showed weaker stability than other four series of complexes. Macrophase separation was observed in PPANECx. PPANN did not exhibit LC phase behavior upon heating and cooling. Smectic A and columnar nematic structures were observed for PPANCx and PPCNNEC. Both the rigidity and the structure of the pyridine derivatives, as well as the strength of H-bonding, affected the liquid crystalline behavior of the complexes. This work paved a way to synthesizing MJLCPs with H-bonding. Currently using other noncovalent interactions to build MJLCPs is also in progress.

Figure 12. 1D WAXD patterns of PPANNEC during the first heating and the subsequent cooling processes.

On the basis of the position of the diffraction peak, the dspacing was calculated to be 3.48 nm, which was close to the calculated length (3.30 nm) of the side group of the H-bonded complex. Therefore, the phase structure of PPANNEC could be columnar nematic (Figure 13). The LC behaviors of the pyridine derivatives and their H-bonded complexes are concluded in Table 1. NEx, NECx, and NCx were able to form a single H-bonding with −COOH. However, only PPANCx exhibited liquid crystallinity, indicating that the rigidity of pyridine derivatives played an important role in the packing of the H-bonded complexes. Comparing the LC behaviors of PPANECx, PPANN, and PPANNEC, only PPANNEC exhibited liquid crystallinity, which indicated that the H-bonding strength was also important in the packing of the complexes (noting

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 21134001, 20974002, and 20990232). 2688

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