Synthesis and characterization of liquid-crystalline ionomers with pendant cholesteryl pyridinium salt mesogens

Abstract

Novel chiral liquid-crystalline (LC) bromo-polysiloxanes (IP, IIP and IIIP) were graft copolymerized in a one-step hydrosilylation reaction by use of polymethylhydrogenosiloxane, cholest-5-en-3-ol(3β)-10-undecenoate and 4-bromobut-1-ene. A pyridyl-containing LC monomer cholesteryl isonicotinate (CIN) was synthesized and characterized. Some LC ionomers [IP–IIIP-Py][X] (X = Br) were synthesized by use of CIN and the bromo-polysiloxanes. Other LC ionomers [IP–IIIP-Py][X] (X = BF₄, PF₆, Tf₂N) were prepared by metathesis of the pyridinium bromides. The chemical structure and liquid-crystalline properties were investigated by various experimental techniques. All the bromo-polysiloxanes display a smectic A (S_A) mesophase when they are heated and cooled, while all these LC ionomers show a chiral smectic C (S^*_C) phase on heating, and exhibit S_A and S^*_C phases on cooling. The LC ionomers show a narrower range of LC temperature than the corresponding bromo-polysiloxanes. For the LC ionomers bearing homologous pyridinium cations and different anions, the glass transition temperature, mesophase–isotropic phase transition temperature and LC temperature range tend to reduce, and the spontaneous polarization value increases slightly in the sequence of Br⁻, BF₄⁻, PF₆⁻, and Tf₂N⁻. Rearrangement of smectic layers due to electrostatic attraction and ion aggregation of cholesteryl pyridinium salt mesogens lead to the formation of the S^*_C phase for these LC ionomers.

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Introduction

Liquid-crystalline ionomers (LCIs) containing organic ions or metal ions constitute a subclass of functionalized liquid-crystalline polymers (LCPs), which are of interest because they show characteristic properties of both liquid crystals and ion-containing polymers.^1–5 This type of ionic LCPs deserves interest from both the academic and applied points of view due to an interesting characteristic feature of both LCPs and ionomers in the same material. LCIs combine specific structural organization of ionomers appearing as the development of ionic aggregates with molecular ordering of side mesogenic fragments which is responsible for the formation of liquid-crystalline phases.⁶ The incorporation of ion groups into ordered polymer matrix opens a way to novel generations of functional materials with some valuable properties such as optical anisotropy, orientation in external fields, magnetic activity and etc.⁷ For instance, Zentel et al.⁸ utilized LCIs in conjunction with amorphous polyelectrolytes to prepare multilayers. Zhao et al.⁹ used LCIs as a model system to investigate the effect of chain mobility on the magnetic field-induced orientation of mesogens. Barmatov et al.¹⁰ introduced some LCIs showing antiferromagnetic properties.

Chirality signifies the lack of inversion symmetry of molecules and the appearance of a stereochemical property. Chiral liquid crystals continue to fascinate researchers ever since their discovery more than 100 years ago.¹¹ Chiral mesogens generally organize into an asymmetric, chiral liquid-crystalline superstructure with a superimposed macroscopic twist of the molecules like helical structure. Chiral liquid crystals and chiral LCPs have presented large potential for various applications because they can display marvelous mesophases such as cholesteric phase, chiral smectic phase and blue phases.^12–17 Recently, chiral LCIs are an emerging area in the fields of soft materials because they are promisingly used as novel functional materials which combine characteristic features of both chiral LCPs and LCIs in the same material. Now we focus on chiral LCIs using quaternary ammonium salts as ionic groups.

In the present effort, some novel pyridinium-based chiral LCIs were synthesized by introducing chiral mesogenic functional groups to the chemical structure. The mesomorphism of pyridinium salts has been known for a long time, and a lot of ionic liquid crystals containing pyridinium cations have been reported previously.^18–23 However, few reports are presented about chiral LCIs bearing pyridinium cations. It is interesting to investigate effect of pyridinium ionic groups on the order and property of the LC phases formed by chiral mesogens. These chiral LCIs were synthesized in three steps (see Fig. 1): (1) a series of alkyl bromine-containing LC polysiloxanes were synthesized; (2) LC pyridinium bromide salts were obtained by a quaternization using the bromine-containing LC polysiloxanes; (3) other LCIs bearing different anions were synthesized by metathesis of the bromide salts.

Fig. 1 Synthetic routes to the liquid crystalline ionomers.

Experimental section

Materials

Poly(methylhydrogeno)siloxane (PMHS, M_n ≈ 2300), 4-bromo-1-butene (4-BBE), cholesterol, sodium tetrafluoroborate, potassium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide and isonicotinic acid were obtained from ALDRICH without any further purification. Undec-10-enoic acid, hexachloroplatinic acid hydrate and reagent-grade solvents were purchased from Beijing HWRK Chemical Company. Tetrahydrofuran (THF) and pyridine were purified respectively by distillation over sodium metal and NaH before using. Other reagent-grade solvents were used as received.

Measurements

FTIR spectra of the samples were measured on a Perkin-Elmer instrument Spectrum One Spectrometer (Perkin-Elmer, Foster City, CA) by the KBr method. ¹H-NMR (300 MHz) spectra were carried out using a Varian WH-90PFT NMR Spectrometer (Varian Associates, Palo Alto, CA). Element analyses (EA) were performed by use of an Elementar Vario EL III instrument (Elementar, Germany). Optical rotation (α) was measured by a Perkin-Elmer instrument Model 341 Polarimeter using sodium light source (λ = 589 nm) at room temperature. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were carried out by use of a NETZSCH TGA 209C thermogravimetric analyzer and a NETZSCH instruments DSC 204 (Netzsch, Wittelsbacherstr, Germany) at heating and cooling rates of 10 °C min⁻¹ under a flow of dry nitrogen. The visual observation of optical textures was performed using a Leica DMRX polarized optical microscope (POM, Leica, Wetzlar, Germany) equipped with a Linkam THMSE-600 hot stage (Linkam, Surrey, England). X-ray diffraction (XRD) measurements of the samples were carried out using Cu Kα (λ = 1.542 Å) radiation monochromatized with a Rigaku DMAX-3A X-ray diffractometer (Rigaku, Japan). The spontaneous polarization of the samples was performed by a TD-88A ferroelectric material parameter test instrument (NDWH electron company, Nanjing, China).

Synthesis

LC monomers. The LC monomer cholest-5-en-3-ol(3β)-10-undecenoate (CUA) used in this work was synthesized according to previous reported procedure.¹⁸ The LC monomer cholesteryl isonicotinate (CIN) was synthesized using cholesterol and isonicotinic acid. Isonicotinic acid (12.3 g, 0.1 mol) and thionyl chloride (25.0 g, 0.21 mol) were added into a round flask and stirred at room temperature for 2 h, then heated to 60 °C for 3 h in a water bath to ensure that the reaction finished. The excess thionyl chloride was distilled under reduced pressure. Then 100 mL cold THF was added to the residue at 20 °C to obtain THF solution of isonicotinoyl chloride. To a solution of cholesterol (38.6 g, 0.1 mol), pyridine (31.6 g, 0.40 mol) in 200 mL of dry THF was added the isonicotinoyl chloride solution and reacted at reflux temperature for 23 h. Some THF was distilled out under reduced pressure. After cooling to room temperature, the residue was poured into 1000 mL of cold water. The precipitates were isolated by filtration, washed successively using sodium bicarbonate dilute solution and water. The solid was recrystallized in alcohol, isolated by filtration, and dried in a vaccum oven to obtain white crystals of LC monomer CIN. Yield: 82%. IR (KBr, cm⁻¹): 3116, 3059 (C–H stretching vibration), 2940, 2867 (–CH₂– stretching), 1720 (C=O stretching), 1594, 1559 (C=C stretching), 1466, 1441 (C=N stretching), 1410, 1382, 1367, 1323 (C–H bending vibration), 1280, 1257 (C–O bending), 1216 (C–N bending). Anal. calcd for C₃₃H₄₉NO₂: C, 80.60%; H, 10.04%; N, 2.85%. Found: C, 80.42%; H, 10.16%; N, 2.79%. ¹H-NMR (CDCl₃, δ, ppm): 0.67–2.01 (41H, alkyl-H); 2.84–2.91 (m, 2H, –CH₂–C=C– in cholesteryl); 4.29–4.34 (m, 1H, –O–CH– in cholesteryl); 5.06–5.10 (m, 1H, =CH– in cholesteryl); 7.58 (d, J = 9.0 Hz, 2H, pyridyl-H); 8.54 (d, J = 9.0 Hz, 2H, pyridyl-H). ¹³C-NMR (CDCl₃, δ, ppm): 14.0, 18.6, 19.0, 22.2, 22.5, 24.0, 24.4, 27.7, 28.3, 31.3, 32.2, 35.4, 36.4, 36.6, 38.1, 38.8, 39.1, 43.0, 49.2, 56.7, 57.2, 74.4 (–OC– in cholesteryl), 118.1 (C– in pyridyl), 123.5, 139.6 (–C– in cholesteryl), 142.8, 149.5 (–C– in pyridyl), 166.9 (–C=O).

LC bromo-polysiloxanes. For synthesis of LC bromo-polysiloxanes (IP, IIP and IIIP), the same method was adopted. The samples were synthesized by analogous one-step hydrosilication reaction using poly(methylhydrogeno)siloxane (PMHS) with an averaged degree of polymerization of 35, the LC monomer CUA, and the bromo-olefin 4-bromobut-1-ene (BBE). The polymerization experiments are summarized in Table 1, and the synthesis of polymers are shown in Fig. 1.

Table 1 Polymerization, structural analysis and some properties of the bromo-polymers

Sample	Feed			x^a	Mn^b (×10^4)	Specific rotation^c	TGA^d
	PMHS (g)	CUA (g)	BBE (g)				Td (°C)	Inf. (°C)	Res. (%)
a Mole fraction of BBE component in one mol polysiloxane molecule was calculated from ^1H NMR spectra.b Molecular weight was calculated according to polysiloxane composition in ^1H NMR spectra.c Specific rotation of polymers ([α]^20D) was performed by 0.1 g in 50 mL chloroform.d Td, temperature of the samples at 5% loss weight; Inf., inflection of the samples on heating to 400 °C; Res., residue weight of the samples on heating to 400 °C.
IP	1.38	9.29	0.57	0.19	1.89	−22.6	282	325	40.2
IIP	1.38	6.97	1.13	0.39	1.59	−19.7	287	333	40.5
IIIP	1.38	3.48	1.98	0.71	1.13	−19.4	285	328	41.3

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The synthesis of polymer IIP is described as an example. The olefin LC monomer CUA (6.97 g, 12.6 mmol), the bromo-olefin BBE (1.14 g, 8.4 mmol) was dissolved in 200 mL of dry toluene, and the reaction solution was stirred at room temperature. PMHS (1.38 g, 0.6 mmol) and 5 mL of a 0.5% hexachloroplatinic/THF solution were added to the stirred reaction solution. Then reaction mixture was kept refluxing for 55 h under nitrogen atmosphere. After this reaction time the FT-IR analysis showed that the hydrosilation reaction was complete. Some solvent (175 mL) was distilled out, and the residue was cooled and poured into 200 mL methanol. The precipitated crude polymer was separated, purified by several reprecipitations from THF into methanol, and dried at 80 °C under vacuum for 24 h to obtain 9.13 g of polymer. FTIR (KBr, cm⁻¹): 3053, 2931, 2837 (C–H), 1748 (C=O), 1586 (C=C in cholesteryl), 1272 (C–O), 1259 (Si–C), 1112 (Si–O). ¹H-NMR (CDCl₃, δ, ppm): 0.05–0.11 (5.31H, Si–CH₃), 0.38–0.54 (3.28H, Si–CH₂–), 0.69–2.24 (63.54H, alkyl-H), 3.21–3.39 (1.29H, –CH₂–Br), 4.80–4.94 (0.99H, –O–CH– in cholesteryl), 5.03–5.15 (0.98H, olefinic-H in cholesteryl). ¹³C-NMR (CDCl₃, δ, ppm): 2.8, 6.8, 7.0 (Si–CH₂– and Si–CH₃), 14.9, 18.8, 19.3, 22.6, 22.8, 24.2, 24.5, 28.8, 28.9, 32.9, 38.7, 39.4, 39.6, 48.6, 56.5, 56.9, 74.1 (–OC– in cholesteryl), 122.2 (–C=CH in cholesteryl), 140.7 (–C=CH in cholesteryl), 174.6 (–C=O).

LC pyridinium bromide salts. The LC polysiloxane-based pyridinium bromide salts (PBSs, i.e. [IP–IIIP-Py][Br]) were synthesized by a quaternization reaction using CIN and the bromo-polymers (IP, IIP and IIIP), as shown in Fig. 1. All these polymers were synthesized via the same synthetic procedure, and the synthesis of [IIP-Py][Br] is described as an example. To a solution of IIP (1.25 g) dissolved in dry toluene, excess equivalent amount of CIN (1.28 g, 2.6 mmol) dissolved in dry toluene was added and stirred vigorously under dry nitrogen atmosphere. The reaction mixture was kept refluxing for 38 h under nitrogen atmosphere. During this period of time the compound precipitated out of the solution. This precipitate was filtered, washed with methanol, crystallized from chloroform, and dried under high vacuum for 24 h to obtain [IIP-Py][Br] in yield of 75%. The product was put into a N₂-atmosphere glovebox for storage. FTIR (KBr, cm⁻¹): 3124, 3056, 2935, 2838 (C–H), 1721, 1746 (C=O), 1589 (C=C in cholesteryl), 1560, 1467, 1440 (pyridyl), 1271 (C–O), 1260 (Si–C), 1112 (Si–O). ¹H NMR (DMSO-d₆, δ, ppm): 0.05–0.12 (4.17H, Si–CH₃), 0.36–0.51 (2.56H, Si–CH₂–), 0.72–2.23 (89.94H, alkyl-H), 2.95–3.09 (1.02H, –CH₂–N⁺– in pyridinium), 4.69–4.85 (1.80H, –O–CH– in cholesteryl), 5.13–5.25 (1.78H, olefinic-H in cholesteryl), 8.18–8.31 (0.94H, pyridyl-H), 8.80–8.91 (0.99H, pyridyl-H). ¹³C-NMR (DMSO-d₆, δ, ppm): 2.7, 6.7, 6.9 (Si–CH₂– and Si–CH₃), 14.3, 18.8, 19.2, 22.4, 22.7, 24.3, 24.6, 28.2, 29.0, 36.6, 38.9, 39.2, 39.4, 48.5, 56.6, 57.0, 60.1, 74.0 (–OC– in cholesteryl), 122.2, 122.4 (–C=CH in cholesteryl), 124.1, 138.4, 139.0 (–C– in pyridyl), 140.7 (–C=CH in cholesteryl), 168.4, 172.5 (–C=O).

Metathesis of PBSs using sodium tetrafluoroborate, potassium hexafluorophosphate and lithium bis(trifluoromethylsulfonyl)imide produced tetrafluoroborate salt polysiloxanes (TSPs, i.e. [IP–IIIP-Py][BF₄]), hexafluorophosphate salt polysiloxanes (HSPs, [IP–IIIP-Py][PF₆]) and bis(trifluoromethylsulfonyl)imide salt polysiloxanes (SSPs), [IP–IIIP-Py][Tf₂N], respectively. All these polymers were synthesized by the same synthetic procedure as shown in Fig. 1, and the synthesis of [IIP-Py][BF₄] is described as an example. [IIP-Py][Br] (1.0 g) was added to a excess equivalent amount of NaBF₄ (3.3 g, 30.0 mmol) and 100 mL of THF–H₂O (80 : 20 v/v) solvent. The reaction mixture was kept vigorous stirring at room temperature for half an hour, then the solid were filtered. Br⁻ ions in the filtrate was monitored by silver nitrate indicator solution. The anion exchange reaction was performed three times until disappearance of Br⁻ in the filtrate, indicating that Br⁻ in [IIP-Py][Br] should be completely replaced by BF₄⁻. The precipitates was washed three times by use of ethanol, dried under high vacuum for 20 h to obtain [IIP-Py][BF₄]. The product was put into a N₂-atmosphere glovebox for storage. FTIR (KBr, cm⁻¹): 3128, 3049, 2943, 2841 (C–H), 1720, 1742 (C=O), 1586 (C=C in cholesteryl), 1564, 1469, 1448 (pyridyl), 1278 (C–O), 1264 (Si–C), 1118 (Si–O), 1062 (B–F). ¹H NMR (DMSO-d₆, δ, ppm): 0.06–0.14 (4.10H, Si–CH₃), 0.32–0.54 (2.53H, Si–CH₂–), 0.69–2.25 (88.86H, alkyl-H), 4.74–4.89 (1.72H, –O–CH– in cholesteryl), 5.09–5.21 (1.83H, olefinic-H in cholesteryl), 5.56–5.65 (1.08H, –CH₂–N⁺–), 8.13–8.38 (0.98H, pyridyl-H), 8.86–8.98 (1.00H, pyridyl-H). ¹³C-NMR (DMSO-d₆, δ, ppm): 2.8, 6.8, 6.9 (Si–CH₂– and Si–CH₃), 14.4, 18.7, 19.4, 22.2, 22.4, 22.8, 24.2, 24.8, 28.6, 29.2, 32.4, 36.4, 38.8, 39.3, 48.6, 56.8, 57.2, 60.2, 74.2 (–OC– in cholesteryl), 122.4, 122.8 (–C=CH in cholesteryl), 125.8, 137.6, 139.8 (–C– in pyridyl), 142.4 (–C=CH in cholesteryl), 170.6, 176.8 (–C=O).

Results and discussion

Synthesis

The bromo-polysiloxanes (IP, IIP and IIIP) containing different bromine moiety were synthesized by olefinic LC monomer CUA, the bromo-olefin BBE, and Si–H bonds-containing PMHS using hexachloroplatinic acid as catalyst in a hydrosilylation reaction which was monitored by FTIR spectra. The characteristic band of the Si–H stretching vibration at 2165 cm⁻¹ weakens during the reaction. The reaction process finishes after the Si–H bond completely disappears. The bromo-polysiloxanes were purified by precipitating from THF/methanol solution in order to eliminate small molecular materials. The chemical structures of the bromo-polysiloxanes were characterized by FTIR, ¹H NMR and ¹³C NMR spectra.

The molecular weight and polymer composition of IP, IIP and IIIP (see Table 1) were calculated according to analogous method reported in our previous work.¹⁹ In this work, the integrations of –CH₂–Br at δ 3.3 ppm and –O–CH– at δ 4.8 ppm in the ¹H-NMR spectra are used as basic arithmetical units because these protons are respectively originated from BBE and cholesteryl in CUA.

The PBSs ([IP-Py][Br], [IIP-Py][Br] and [IIIP-Py][Br]) were synthesized in an amination reaction using CIN and the bromo-polysiloxanes (IP, IIP and IIIP), respectively. The chemical structures were characterized by use of FTIR and NMR spectra. Compared with the corresponding precursors, the PBSs show new absorption peaks around 1560, 1467, 1440 cm⁻¹ which are assigned to C=N stretching vibration in pyridyl units in the FTIR spectra. In ¹H NMR spectra, the bromo-polysiloxanes display protons of –CH₂–Br, but the PBSs show protons of N⁺–CH₂– instead of –CH₂–Br, indicating that the quaternary ammonium salts are synthesized successfully. For example, IIP shows chemical shifts of –CH₂–Br at δ 3.3 ppm, while [IIP-Py][Br] displays chemical shifts of N⁺–CH₂– at δ 5.6 ppm, as shown in Fig. 2. Furthermore, [IIP-Py][Br] shows chemical shifts of pyridyl H at δ 8.2 and 8.8 ppm, but IIP do not show these chemical shifts. The mass percent of cholesteryl pyridinium ion pairs (Py⁺X⁻) in the polymer matrix can also be calculated according to ¹H NMR spectra analysis, as listed in Table 2. In ¹³C NMR spectra, [IIP-Py][Br] displays new chemical shifts at 124.1, 138.4, 139.0 ppm which should be assigned to pyridyl groups.

Fig. 2 ¹H-NMR spectra of IIP (I) and [IIP-Py][Br] (II).

Table 2 Structural analysis and some properties of the pyridinium-based liquid crystalline ionomers

Sample	Py^+X^−^a (%)	Mn^b (×10^4)	Ps^c (nC cm^−2)	Specific rotation^d	TGA^e
					Td (°C)	Inf. (°C)	Res. (%)
a The mass percent of cholesteric pyridinium ion pairs (Py^+X^−) in the polymer matrix calculated according to ^1H NMR spectra analysis.b Based on polymer composition calculated from ^1H NMR spectra.c Measured by a TD-88A ferroelectric material parameter test instrument.d Specific rotation of polymers ([α]^20D), 0.1 g in 50 mL DMSO.e Td, temperature of the samples at 5% loss weight; Inf., inflection of the samples on heating to 400 °C; Res., residue weight of the samples on heating to 400 °C.
[IP-Py][Br]	17.1	2.21	240	−24.4	303	346	42.8
[IIP-Py][Br]	34.4	2.26	257	−22.8	302	350	42.4
[IIIP-Py][Br]	60.4	2.34	268	−23.2	306	351	43.6
[IP-Py][BF4]	17.3	2.22	241	−21.2	302	348	52.4
[IIP-Py][BF4]	34.7	2.23	256	−22.1	303	359	51.6
[IIIP-Py][BF4]	60.7	2.37	279	−22.4	3.2	356	52.9
[IP-Py][PF6]	18.7	2.25	249	−22.9	305	347	56.3
[IIP-Py][PF6]	36.9	2.35	264	−23.0	303	350	55.4
[IIIP-Py][PF6]	63.0	2.51	287	−22.8	307	342	56.1
[IP-Py][Tf2N]	21.8	2.34	265	−24.4	311	351	58.6
[IIP-Py][Tf2N]	41.5	2.53	298	−22.3	310	349	58.8
[IIIP-Py][Tf2N]	67.3	2.84	310	−22.8	312	349	57.9

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The TSPs ([IP-Py][BF₄], [IIP][BF₄] and [IIIP-Py][BF₄]), HSPs ([IP-Py][PF₆], [IIP][PF₆] and [IIIP-Py][PF₆]), SSPs ([IP-Py][Tf₂N], [IIP-Py][Tf₂N] and [IIIP-Py][Tf₂N]) were synthesized by metathesis of the bromide salts using corresponding chemical reagents. These new pyridinium salts were also characterized by FTIR spectroscopy and NMR spectra. In the FTIR spectra, the exchange of the counter ion is readily observed that characteristic strong frequencies for the new groups are found (e.g. 1121 cm⁻¹ for BF₄⁻, 830 cm⁻¹ for PF₆⁻, and 860 cm⁻¹ for Tf₂N⁻, respectively). In the ¹H NMR spectra, the two protons in the vicinity of the nitrogen atom located in pyridine ring show upfield shifts for SSPs, PBSs, TSPs, and HSPs in sequence, indicating that the anion has some influence on the chemical shift of these protons. For example, [IIP-Py][Tf₂N], [IIP][Br], [IIP][BF₄] and [IIP][PF₆] display chemical shift peaks of the lowest field at δ 9.1, 8.8, 8.7 and 8.5 ppm respectively. Such a trend follows the one that was observed for other different series of pyridinium salts.²⁰

In these work, because the polymers were synthesized using cholesterol which is a chiral molecule, a super molecular helical structure should be formed in the liquid crystalline state, resulting in some particular properties. It is necessary to study the degree of chirality (i.e. optical rotation, α) due to introduction of different functional group in the liquid crystalline polymers. The specific rotation should be determined by the molecular stereostructure for these chiral alkyl bromide-containing polysiloxanes and pyridinium-based LCIs. The measured value of α (C = 1, in CHCl₃) of the LC monomers CUA and CIN is −30.8 and −31.4 respectively. The LC bromo-polysiloxanes (IP, IIP and IIIP) show different specific rotation in comparison with CUA due to cleavage of the double bond and the binding of monomers to the polysiloxane main chains, as shown in Table 1. IP, IIP and IIIP display lower absolute value of specific rotation than that of CUA indicating that the polymer show lower chirality due to existence of polysiloxane main chains. For the pyridinium-based LCIs, the HSPs, PBSs, TSPs, and SSPs display similar measured value of α as shown in Table 2, indicating that the anion has not a significant influence on the chirality for these ionic polymers.

LC properties of the monomers

The LC property of the monomer CUA was reported in previous work.²¹ The LC property of the monomer CIN was characterized by use of DSC, POM and XRD. The crystal solid state (K) of CIN shows a crystalline melting endotherm at 164.3 °C (ΔH = 78.64 J g⁻¹) and a chiral nematic (N*)–isotropic phase (Iso) transition at 181.3 °C (0.15 J g⁻¹) on heating cycle, as well as an Iso–N* transition at 137.2 °C (−0.06 J g⁻¹) and a N*–K transition at 116.2 °C (−35.97 J g⁻¹) on cooling cycle in the DSC thermograms, as displayed in Fig. 3(a). The optical textures of liquid crystal state are observed by means of POM with hot stage, and some representative optical textures are shown in Fig. 3(b) and (c). When CIN was heated, eyesight became bright and LC textures appeared when it was heated above 164 °C. The textures appeared vividly, and fan-shaped textures of N* phase were displayed, as shown in Fig. 3b. Then oily streaks texture of N* phase appeared when the sample was continuously heated (see Fig. 3(c)). Oily streaks texture is one of the most commonly observed textures of cholesteric phase prepared between two untreated glass substrates. The actual oily streaks can be seen as a network of defect lines dispersed in uniformly helical regions. The N* mesophase is also confirmed by XRD studies, which can provide some more detailed information on the liquid crystalline structure and type. A broad diffuse peak at wide angles (2θ ≈ 17°) and a weak diffuse reflection at small angles (2θ ≈ 2.7°) are shown in the profile curves when the sample is heated to LC state, indicating that natural N* phase of CIN.

Fig. 3 LC properties of the LC monomer CIN: (a) DSC thermograms on the second heating and the first cooling cycles; (b) fan-shaped texture on heating to 168 °C; and (c) oily streaks texture on heating to 179 °C.

LC properties of bromo-polysiloxanes

Liquid crystalline behaviors of the LC bromo-polysiloxanes (IP, IIP and IIIP) were studied by DSC, POM and XRD. For all these LC bromo-polysiloxanes, smectic A (S_A) mesophases were detected. The phase transitions and thermodynamic data, and the layer distances of mesophase at 100 °C of these bromo-polysiloxanes are shown in Table 3. Representative DSC traces for bromo-polysiloxane IIP are presented in Fig. 4(a). A glass transition at 3.6 °C is displayed here, followed by a S_A mesophase to isotropic phase transition at 189.5 °C on the heating cycle. The cooling scan displays an isotropic–S_A phase transition without the S_A–glass transition due to insufficient cooling efficiency.

Table 3 Phase transitions and thermodynamic data, and XRD data for the polymers

Sample	Phase transitions,^a °C (corresponding enthalpy changes; J g^−1)		d-Spacing^b (Å)
	Heating	Cooling	I	II
a Transition temperatures (°C) and enthalpies (J g^−1, in parenthesis) were determined at a scanning rate of 2 °C min^−1. G, glassy; SA, smectic A; S^*C, chiral smectic C; I, isotropic phase.b Determined by XRD analysis on heating to 100 °C: sharp reflections at low angles (I) and broad reflections at wide angles (II).
IP	G 5.8 SA 189.5 (0.96) I	I 175.4 (−0.61) SA	36.1	30.6
IIP	G 3.6 SA 188.9 (0.11) I	I 170.8 (−0.06) SA	36.3	5.2
IIIP	G 2.2 SA 185.4 (1.64) I	I 173.1 (−0.84) SA	36.0	30.1
[IP-Py][Br]	G 2.8 S^*C 178.7 (0.52) I	I 154.3 (−0.16) SA 77.9 (−0.11) S^*C	28.4	5.1
[IIP-Py][Br]	G 1.6 S^*C 174.9 (0.28) I	I 150.6 (−0.07) SA 78.6 (−0.08) S^*C	28.5	5.1
[IIIP-Py][Br]	G 1.1 S^*C 169.2 (0.36) I	I 152.1 (−0.16) SA 74.3 (−0.09) S^*C	28.3	5.1
[IP-Py][BF4]	G −0.8 S^*C 178.2 (0.24) I	I 154.1 (−0.12) SA 76.9 (−0.10) S^*C	29.4	5.2
[IIP-Py][BF4]	G −2.6 S^*C 174.4 (0.71) I	I 151.4 (−0.11) SA 84.4 (−0.09) S^*C	29.5	5.2
[IIIP-Py][BF4]	G −2.1 S^*C 170.2 (0.16) I	I 152.4 (−0.11) SA 78.2 (−0.06) S^*C	29.6	5.2
[IP-Py][PF6]	G −0.9 S^*C 174.3 (0.21) I	I 157.1 (−0.10) SA 75.2 (−0.08) S^*C	30.9	5.3
[IIP-Py][PF6]	G −2.6 S^*C 175.4 (0.27) I	I 158.1 (−0.52) SA 83.1 (−0.06) S^*C	30.6	5.3
[IIIP-Py][PF6]	G −3.1 S^*C 166.1 (0.46) I	I 151.1 (−0.18) SA 76.6 (−0.12) S^*C	30.8	5.3
[IP-Py][Tf2N]	G −4.9 S^*C 160.4 (1.26) I	I 144.2 (−0.13) SA 67.2 (−0.09) S^*C	32.3	5.4
[IIP-Py][Tf2N]	G −10.0 S^*C 144.7 (1.93) I	I 142.1 (−0.06) SA 63.1 (−0.08) S^*C	32.4	5.4
[IIIP-Py][Tf2N]	G −11.1 S^*C 149.2 (1.32) I	I 141.0 (−0.26) SA 64.2 (−0.11) S^*C	32.2	5.4

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Fig. 4 DSC thermograms of polymers IIP (a) and [IIP-Py][Br] (b) on the second heating and the first cooling cycles (G, glassy; S_A, smectic A; S^*_C, chiral smectic C; Iso, isotropic phase).

The POM image of IIP shows that upon heating and cooling, the sample exhibits weeny focal-conic textures as shown in Fig. 5(a)–(b). These textures do not look like typical focal-conic textures in S_A phase of low molecular weight liquid crystals due to restriction of the polymer matrix.

Fig. 5 Optical texture of the samples: weeny focal-conic texture of IIP on heating to 100 °C (a), and on cooling to 77 °C (b); (c) weeny fan-shaped texture of [IIP-Py][Br] on heating to 100 °C (d), and on cooling to 77 °C.

In the mesophases, a sharp reflection at low angles (associated with the smectic layers) and a broad reflection at wide angles (associated with the 1ateral packings) are respectively shown by all curves of the LC bromo-polysiloxanes in XRD measurements. The XRD diagram obtained from powder samples of IIP at 100 °C is presented in Fig. 6(a). This curve presents a sharp first-order reflection at 36.3 Å and a second-order reflection at 18.4 Å, which correspond to smectic layers; and a diffuse reflection at about 5.2 Å, which corresponds to lateral spacing of two mesogenic side groups. Considering that the optical polarizing micrograph reveals focal-conic textures for polymer IIP at this temperature range, both results are consistent with a smectic A structure.

Fig. 6 Representative XRD curves at 100.0 °C and schematic drawing of polymers: (a) IIP, and (b) [IIP-Py][Br].

LC properties of LC ionomers

Phase behavior. All these synthesized pyridinium-containing LCIs (PBSs, TSPs, HSPs and SSPs) exhibited similar liquid crystalline properties, which were characterized by POM, DSC and XRD in both heating and cooling cycles. The phase transitions and corresponding enthalpy changes are summarized in Table 3.

On the whole, the glass transition temperature (T_g) and the mesophase–isotropic phase transition temperature (T_i) of the LC ionomers decrease slightly with increase of cholesteryl pyridinium cations in every homologous anion series, but the enthalpy changes of mesophase–isotropic phase transition show no regularity. Representative DSC traces for polymer [IIP-Py][Br] are presented in Fig. 4(b). When it is heated, [IIP-Py][Br] showed glass transition and S^*_C–isotropic phase transition; when it was cooled from isotropic state, it displayed an isotropic phase–S_A transition and a S_A–S^*_C mesophase transition.

The POM image of [IIP-Py][Br] shows weeny fan-shaped textures as shown in Fig. 5(c and d), which are different to the mother polymer IIP, indicating that introduction of cholestery pyridinium ions change the LC behavior of the bromo-polysiloxane.

XRD is utilized to characterize the mesophase structures. Each of the X-ray patterns shows a sharp peak in the small angle region and a diffuse reflection in the wide angle area. The wide angle diffuse reflection (d spacing near 5 Å) corresponds to the inter-molecular separation within the smectic layer arising due to the liquid-like positional correlation within the layer.

The sharp first-order reflection in the small angle region corresponds to smectic layers. The d spacings of [IIP-Py][Br] exhibit a value of 28.5 Å in the small angle region and 5.1 Å in the wide angle area at 100 °C, as shown in Fig. 6(b). In the mesophase, a decrescence of the layer distance to 28.5 Å compared with 36.3 Å of IIP is observed for [IIP-Py][Br]. This result suggests a structure of the tilted nature for the S^*_C phase.

These side-chain LCIs are composed of flexible moieties (polymer backbone and flexible spacer), rigid moieties (cholesteryl mesogenic units) and ion pair groups, thus the polymer backbone, the length of the flexible spacer, the rigidity of mesogenic units and ionic groups would influence LC behaviors of the polymers. However, the mother bromo-polysiloxanes are composed of these functional groups except pyridinium ionic groups, therefore the smectic layers should be made up of rigid component and flexible component and S_A should be formed easily.

For the LCIs, rearrangement of these smectic layers should be occurred due to electrostatic attraction and ion aggregations derived from introduction of the cholesteryl pyridinium ionic mesogens into the polymer systems. These can lead to decrescence of the layer distance and the tilted nature for the S^*_C phase. Representative schematic drawings of polymer structures are illustrated in Fig. 6.

Phase behaviors of these LCIs bearing different anions are also compared. XRD studies can provide detailed information on the liquid crystalline structure of S^*_C mesophase of these LC ionomers. A broad diffuse reflection at wide angles and a strong sharp reflection at small angles are observed in the profile curves of all these LCIs. The d spacing of wide angle diffuse reflections corresponding to the inter-molecular separation within the smectic layer increase slightly from 5.1 Å to 5.4 Å in the sequence of anions Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹ (Table 3), which should be due to ionic size of the anions. Because the reflections at small angles corresponds to the interlayer distances of S^*_C phase, the larger intensity of these peaks suggests the stronger regularity between the layer spacing. The smectic layer spacing elongates slightly from 28 Å to 32 Å in the homologous cation series bearing sequence of anions Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹. The alignment of the molecular rod perpendicular to the layer plane tends to elongate the layer spacing, and at the same time thermal motion of the chains at the mesophase tends to shorten the layer spacing.²² Obviously, thermal motion of the polymer chains should be restrained by the electrostatic attraction or ion aggregations originated by ion pairs in these LC ionomer systems. Furthermore, cholesteryl mesogen rod tends to form helical structure due to chiral molecules. The LCIs contains cholesteryl pyridinium mesogen ion pairs in the molecular rod structures, thus the alignment of the molecular rod perpendicular to the layer plane should be influenced by the ion aggregations. For the homologous cholesteryl pyridinium cation series, the smectic layers of SSPs bearing Tf₂N⁻¹ can form longer layer spacing than those of PBSs bearing Br⁻¹ due to large size of pyridinium–Tf₂N⁻¹ ion pairs.

These results suggest that the smectic layer spacing elongate slightly in sequence of anions Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹ for these LCIs. The subtle difference of S^*_C phase can also be exhibited in LC optical textures investigated by POM. All the LC ionomers show fan-shaped textures of S^*_C phase, but some subtle difference can be observed. For example, polarized optical textures of [IIP-Py][BF₄] and [IIP-Py][ Tf₂N] are shown in Fig. 7. [IIP-Py][Tf₂N] shows some smaller fan-shaped textures than [IIP-Py][BF₄] because of large size of pyridinium–Tf₂N⁻¹ ion pairs.

Fig. 7 Optical texture of the samples: fan-shaped texture of [IIP-Py][BF₄] on heating to 80 °C (a), 97 °C (b); 150 °C (c) and 170 °C (d); weeny fan-shaped texture of [IIP-Py][Tf₂N] on heating to 80 °C (e), 97 °C (f); 120 °C (g) and 140 °C (h).

Thermal behaviors of these LC polymers are compared. The LCIs show lower T_g and T_i than corresponding mother bromo-polysiloxanes, leading to narrower LC temperature range (ΔT) in the DSC curves. For the LCIs bearing homologous pyridinium cations and different anions, T_g, T_i and ΔT tend to reduce slightly in sequence of Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹ anions. Fig. 8 illustrates phase behavior of [IP-Py][Br], [IP-Py][BF₄], [IP-Py][PF₆] and [IP-Py][Tf₂N] bearing homologous pyridinium cations with different anions. Before the DSC experiments are performed, thermal stability of the polymers is studied, and the TGA values are listed in Tables 1 and 2. Some representative TGA thermograms are shown in Fig. 9. It suggests high thermal stability in consideration of high temperature more than 300 °C of 5% weight loss temperatures (T_d) and more than 42% residue weight of the samples for all these LCIs, while the bromo-polysiloxanes display below 300 °C of T_d and below 42% residue weight. On the whole, the LCIs bearing homologous pyridinium cations exhibit higher thermal stability in sequence of Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹ anions.

Fig. 8 Phase behavior of the LCIs bearing homologous pyridinium cations with different anions which are derived from IP.

Fig. 9 TGA thermograms of representative polymers: IIP, [IIP-Py][BF₄] and [IIP-Py][Tf₂N].

Ferroelectric behavior. Chirality can be originated from molecular asymmetry of carbon atoms being substituted by four different ligands, which is due to introduction of chiral cholesterol units in this work. It is an inherent property of many systems in nature, and the introduction of chirality into mesogenic systems has a tremendous impact on the respective liquid crystalline behavior. For the S_C phase, the director is tilted by an angle with respect to the smectic layer normal, and the molecules' centers of mass within the smectic layer are isotropically distributed. The helical super-structure of the S^*_C phase appears in the S_C phase due to chirality, therefore some physical properties of LC phase are modified by the loss of mirror symmetry. The S^*_C mesophase has been proved ferroelectric theoretically and experimentally for a long time. Some side-chain LC polysiloxanes with chiral centers exhibit ferroelectric properties due to the S^*_C phase showing high spontaneous polarization values and fast response times.^23–25

The S^*_C phase can exhibit a spontaneous polarization, and this spontaneous polarization is switchable between two stable states by an applied electric field. The spontaneous polarization (P_s) of the S^*_C phase for the LCIs is measured by ferroelectric hysteresis loop method using a ferroelectric material parameter test instruments, as listed in Table 2. These measurements are carried out with cell thicknesses of polymer film about 4 μm. The liquid crystal alignment layer is quenched from the S^*_C phase under a DC electric-field (50 V μm⁻¹), which is confined between two glass plates coated with indium tin oxide (ITO) layers. All these pyridinium-containing LCIs display approximate P_s as shown in Table 2. [IIIP-Py][Tf₂N] displays the highest spontaneous polarization value in all the samples, while [IP-Py][Br] shows the lowest one. On the one hand, the P_s value increases slightly with the increase of pyridinium ion pairs in polymers of homologous anions, indicating the polarization should be positively associated with the pyridinium ion pairs. On the other hand, the P_s value of ionomers containing the same pyridinium cations increases slightly with the sequence of different anions Br⁻¹, BF₄⁻¹, PF₆⁻¹, and Tf₂N⁻¹, which should be due to ionic size of the anions. The Tf₂N⁻¹ ion has the largest volume in these anions, therefore the negative charge spread more easily leading to strong polarization.

Conclusions

Bromo-polysiloxanes IP, IIP and IIIP exhibiting S_A phase were synthesized in good yields via controlled hydrosilylation reaction by use of olefinic LC monomer and bromo-olefin. The polysiloxane-based pyridinium bromides PBSs were prepared in an amination reaction using the bromo-polysiloxanes and pyridinium-containing liquid crystal CIN, and the pyridinium tetrafluoroborates TSPs, pyridinium hexafluorophosphates HSPs, pyridinium bis(trifluoromethylsulfonyl)imides SSPs were synthesized by metathesis of the pyridinium bromides. All these LC ionomers show S^*_C phase on heating, and display S_A phase and S^*_C phase on cooling. The LCIs show lower T_g and T_i, and narrower LC temperature range than the corresponding mother bromo-polysiloxanes. For the LCIs bearing homologous pyridinium cations and different anions, T_g, T_i and ΔT tend to reduce slightly in sequence of PBSs, TSPs, HSPs, and SSPs. Thermal stability of these LC ionomers are superior to the corresponding mother bromo-polysiloxanes. All these LCIs containing the same pyridinium cations displayed approximate spontaneous polarization, and the P_s value increases slightly with the sequence of PBSs, TSPs, HSPs, and SSPs. For the LCIs, rearrangement of smectic layers would be occurred due to electrostatic attraction and ion aggregations derived from cholesteryl pyridinium ionic mesogens in the polymer systems, leading to decrescence of the layer distance and the tilted nature for the S^*_C phase.

Acknowledgements

This work was supported by National Natural Science Foundation of China [Project 51273035] and the Fundamental Research Funds for the Central Universities [grant number N130405001 and N130205001].

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Footnote

† Electronic supplementary information (ESI) available: Fourier transform infrared (FTIR) spectra of the monomer, NMR (300 MHz) spectra of the monomer, DSC and TGA thermograms of some polymers. See DOI: 10.1039/c5ra27247e

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