Novel cholesteric liquid crystalline elastomers containing dimer type nematic and chiral liquid crystalline side-chains

Abstract

A new set of cholesteric side chain liquid crystalline elastomers (ChLCEs) E₁–E₇ were graft copolymerized by hydrosilylation reaction with poly(methylhydrogeno)siloxane, nematic monomer (M₁), chiral monomer (M₂), and crosslinking agent (CL). The two monomers were both dimers, and the chiral 2-octyl terminal group was firstly used in the ChLCE systems. The chemical structures of M₁, M₂ and CL were carefully examined by Fourier transform infrared (FT-IR), elemental analysis (EA), proton nuclear magnetic resonance spectroscopy (¹H NMR); and their mesomorphic phases were determined by observation using polarizing optical microscopy (POM), and double confirmed by calculations from the results of X-ray diffraction (XRD). The helical structure of combining the chiral and achiral liquid crystalline side chains endowed the obtained ChLCEs with cholesteric liquid crystalline properties. By tailoring the dosage of the crosslinking agent, the mesomorphic properties of ChLCEs could be adjusted. The mesophase-isotropic phase ranges of the ChLCEs were gradually narrowed with the increasing addition of CL according to differential scanning calorimetry (DSC). Thermal analysis (TG) results showed the temperatures at which 5% weight loss occurred were greater than 300 °C for all the ChLCEs. The effective crosslink density (M̄_c) of the ChLCEs was characterized by swelling experiments showing that the molecular weight between the crosslinking points decreases with the increase of the CL. Fourier transform infrared imaging (FT-IR imaging) and FT-IR indicate the functional groups of all the ChLCEs are finely distributed.

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1. Introduction

Today, the fast growing development of liquid crystalline materials has received a lot of attention due to their multifunctional and oriented structures.^1–3 One typical example of such a supermolecular system is liquid crystalline elastomers (LCEs). Since the first report of the synthesis of LCEs in 1981 by Finkelmann,⁴ this group of materials has developed very fast because of their remarkable entropic elasticity and reversible liquid crystalline (LC) phase transition properties. The combination of orientational ordering of the rigid-rods with the entropic elasticity of a flexible backbone associates LCEs with many unusual phenomena, such as spontaneous shape changes, color changes or length changes at LC phase transitions under different external conditions, etc.^5–10 Presently, the study of LCEs has become a major subject of the liquid crystal field, and considerable achievements have been attained.^11–14

Comparing with conventional LCEs, cholesteric liquid crystalline elastomers (ChLCEs) introducing the basic features of chirality into the polymer elastomer network to form a unique helical structure with a spontaneous and uniform orientation, are less focused but possess much more development space. Several research groups have been doing the related studies for a long time and received interesting results in different respects. In 1989, Brand gave a theoretical study about the macroscopic characterization of the electromechanical properties of cholesteric and chiral smectic liquid crystalline elastomers.¹⁵ In 2001, Finkelmann group synthesized the first mono-domain ChLCEs in the application of tunable mirrorless lasing,¹⁶ aroused wide attention from the scientific world. One year later, Terentjev group published a paper and extensively explored the influence of uniaxial strain on the ChLCEs.¹⁷ While Warner group calculated the photonic band structures along and oblique to the helix axis of ChLCEs and found they are highly deformable and self-assembling systems.¹⁸ The research and investigation on ChLCEs were rumbling into season from then on, many new structures were synthesized and several applications have been developed. Usually ChLCEs possess the common electro-optical and mechanical properties like the common nematic liquid crystalline elastomers; moreover they have the special properties of piezoelectricity, tunable mirrorless lasing and photonic, etc.^19–21 Presently the main focus of the study is still on the utilization of their properties and exploration of new structures and applications.

Generally speaking, the preparation of ChLCEs could be achieved by two methods, (a) hydrosilylation reaction of at least one chiral mesogenic monomer and crosslinking agent to a poly(methylhydrogeno)siloxane polymer backbone; or (b) by reacting the chiral LC prepolymer that containing extra functional groups with crosslinking agent to form a network. Crosslinking formation could be realized by chemistry, or by UV or gamma radiation processes. Many new ChLCEs have been developed yet, but it is still necessary to synthesize various kinds of ChLCEs to explore their potential applications. Our team has done many relevant studies in this field recently.^22–27

In this study, we reported the synthesis of a series of side-chain cholesteric liquid crystalline elastomers derived from two new monomers: an achiral monomer with a terminal cyano-group (M₁) showing nematic phase, a chiral monomer with a terminal chiral 2-octyl group (M₂) showing chiral smectic C (S^*_C) and chiral nematic (N*) phases, and one crosslinking agent (CL) showing smectic and nematic phases. The side chain monomers are all dimer-type in relatively long and sophisticated structures with very good liquid crystalline behaviors, and until now have never been used in the LCEs systems. Especially, chiral 2-octanol, to our best knowledge, has never been used in any research based on liquid crystalline monomer or elastomer systems. We adopted (S)-2-octanol to form the chiral terminal group of M₂ and found the achieved chirality of M₂ was powerful enough to make all the system twisted to form the chiral LC phase, no matter in M₂ itself or in the final cholesteric liquid crystalline elastomers. The flexible spacers between the mesogenic cores also helped to reduce the steric effect inside the polymer network and realize good mesomphases. The mesomorphism and thermal stabilities of all obtained products were carefully characterized by reliable analytical methods. The influence of the cross-linked density on the mesomorphic properties and phase behaviors of the elastomers was also discussed.

2. Experimental

2.1. Materials

Poly(methylhydrogeno)siloxane (PMHS, M_n = 582) was obtained from Jilin Chemical Industry Co. (China); potassium iodide (KI), 1-ethyl-3-(3-dimethyllaminopropyl)carbodiimide hydrochloride (EDCI), 4-(N,N-dimethylamino)pyridine (DMAP), toluene, para-xylene, p-toluenesulfonic acid, dichloromethane (DCM), ethyl alcohol (EtOH), triethylamine (TEA), lithium hydrate (LiOH), potassium carbonate (K₂CO₃), ammonia water, petroleum ether, ethyl acetate, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), triethylamine (TEA) and chloroplatinic acid (Pt catalyst) were purchased from Sinopharm Chemical Reagent Co., Ltd (China); methyl chloroformate was purchased from Xiya Reagent Co., Ltd (China); diisopropyl azodicarboxylate (DIAD) and triphenylphosphine (TPP) were purchased from Shanghai Demo Medical Tech Co., Ltd; (S)-2-octanol was purchased from Liaoning Huifu Chemical Co., Ltd (China); 6-bromohexanonate were purchased from Jintan Hengxin Chemical Co., Ltd (China); 4′-hydroxy-4-biphenylcarbonitrile and 4′-hydroxy-4-biphenylcarboxylic acid were purchased from Shijiazhuang Sdyano Fine Chemical Co., Ltd (China). Toluene and THF were purified by distillation over sodium sand before use. All other solvents and reagents were purified by standard methods. 4-Hydroxyphenyl 4-(allyloxy) benzoate, 6-(4-((4-(allyloxy)benzoyl)oxy)phenoxy)-6-oxohexanoic acid, and 10-undecylenoyl chloride were prepared in the lab.

2.2. Measurements

Fourier transform infrared (FTIR) spectra were measured by a Nicolet 510 FTIR spectrometer (PerkinElmer, USA). ¹H NMR (600 MHz) spectra were measured by Bruker AV 600 (600 MHz) NMR spectrometer (Bruker, Germany) with tetramethylsilane (TMS) as an internal standard. Element analyses were carried out by an Elementar Vario ELIII (Elementar, Germany). Specific optical rotations were measured by an Autopol IV automatic polarimeter (Rudolph Research Analytical, USA). Polarized optical microscopy (POM) study was performed with a Leica DMRX (Leica, Germany) equipped with a Linkam THMSE-600 (Linkam, UK) heating stage. X-ray diffraction (XRD) measurements of the samples were performed using Cu Kα (λ = 1.54 Å) radiation monochromatized with a Bruker D8 ADVANCE X-ray diffractometer (Bruker, Germany). FTIR imaging was carried out using a Spotlight 300 infrared imaging system (PerkinElmer, US). The thermal transition properties were characterized using a Q2000 differential scanning calorimetry (DCS, TA instrument, USA) at heating and cooling rate of 10 °C min⁻¹ under nitrogen atmosphere. The thermal stabilities of the elastomers were measured with a Netzsch TGA 209C thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere.

2.3. Synthesis of monomers and crosslinking agent

The synthetic routes of the nematic liquid crystalline monomer 4-((6-((4′-cyano-[1,1′-biphenyl]-4-yl)oxy)hexanoyl)oxy)phenyl 4-(allyloxy)benzoate (M₁), the chiral liquid crystalline monomer (R)-4-((4-(allyloxy)benzoyl)oxy)phenyl (4′-((octan-2-yloxy)carbonyl)-[1,1′-biphenyl]-4-yl)adipate (M₂) and the crosslinking agent 1,4-phenylene bis(4-(undec-10-enoyloxy)benzoate) (CL) are shown in Fig. 1. The chemical structures of monomers and crosslinking agent were characterized by ¹H NMR spectra, FT-IR and elemental analysis, respectively. The detailed synthetic procedures and analytical data were collected in ESI.†

Fig. 1 Synthesis routes of monomers M₁, M₂ and crosslinking agent CL.

2.4. Synthesis of cholesteric side-chain liquid crystalline elastomers

The synthetic routes of the elastomers are outlined in Fig. 2; the feed ration, yield, and the optical rotations of the elastomers are summarized in Table 1. The synthesis of ChLCEs E₁–E₇ were adopted same method. Here use the synthesis of E₂ as an example. The monomers M₁, M₂, crosslinking agent CL and PMHS were dissolved in anhydrous THF. The reaction mixture was stirred and heated to 60 °C under nitrogen atmosphere without water, and then a proper amount of hexachloroplatinate hydrate catalyst in tetrahydrofuran was slowly syringed. The reaction was continued for at least 24 hours and the white powder elastomers were obtained by multiple precipitation with methanol, and then dried over vacuum.

Fig. 2 Synthetic routes of E₁–E₇.

Table 1 Feed ratio and yield of elastomers

Samples	Feed (mmol)				CL (mol%)	Yield (%)	Specific rotations^a
	PMHS	M1	M2	CL
a Measured in dichloromethane at 0.025 g ml^−1.
E1	0.1428	0.5	0.5	—	—	74	−10.28
E2	0.1428	0.48	0.48	0.02	2%	69	−9.27
E3	0.1428	0.46	0.46	0.04	4%	78	−8.85
E4	0.1428	0.44	0.44	0.06	6%	75	−7.63
E5	0.1428	0.42	0.42	0.08	8%	66	−7.41
E6	0.1428	0.40	0.40	0.10	10%	69	−6.73
E7	0.1428	0.35	0.35	0.15	15%	72	−5.60

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2.5. Swelling experiments for liquid crystalline elastomers

The obtained liquid elastomer samples (each 0.3 g) and toluene (each 10 ml) were put into the test tubes with stoppers, respectively, closed the test tubes hermetically. Then subsequently treated by homogenization at ambient temperature by ultrasound. The test tubes were weighted after and before homogenization. The swelling experiment would last for several days at room temperature to reach to equilibrium. Then the swollen elastomers were recovered from toluene at regular intervals, put them on filter paper and dried in the vacuum oven, and finally weighed until reached to the constant weight for each sample under the same condition.

3. Results and discussion

3.1. Thermal and mesophase behaviors of M₁, M₂ and CL

The mesophase behaviors of the synthesized monomers M₁, M₂ and crosslinking agent CL were confirmed by DSC, POM, XRD and polarimeter. Fig. 3 displays the DSC thermograms of M₁, M₂ and CL. Table 2 summarized the phase transition temperatures, corresponding enthalpy changes and optical rotation properties of M₁, M₂ and CL obtained. It should be noted M₁ has no obvious peak for crystallization on the cooling cycle which has been confirmed by several retests by DSC possibly due to the enthalpy change of crystallization is too small. But the crystallization can be observed by polarized optical microscopy at 72.3 °C. The large nematic (N) phase temperature range probably owing to the cyano group connected to the biphenyl via an ether bond, which according to the theory of George Gray, will promote nematic stability in terms of increasing clearing temperatures.²⁸ For M₂, a melting transition peak, subsequent chiral smectic C (S^*_C) and chiral nematic (N^*) phases were observed; while CL was basically smectic C (S_C) and N phases.

Fig. 3 DSC thermograms of monomers and crosslinking agent: (a) LC monomer M₁; (b) LC monomer M₂; (c) crosslinking agent CL.

Table 2 Phase-transition temperatures and specific rotations of the monomers M₁, M₂ and crosslinking agent CL

Samples	Transition temperature^a^,^b in °C (corresponding enthalpy changes in J g^−1)	ΔT^c	Specific rotations^d
a K = solid, SA* = smectic A^*, SC = smectic C, S^*C = smectic C^*, N = nematic, N^* = chiral nematic, I = isotropic.b Peak temperatures were taken as the phase transition temperature.c Mesophase temperature ranges on heating cycle.d Measured in dichloromethane at 0.025 g ml^−1.
M1		88.8	—
M2		52.9	−24.9
C		67.3	−2.51

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Fig. 4 gives some representative optical textures of M₁, M₂ and CL from the observation of POM. M₁ shows typical marble texture for N phase during the temperature cycles without doubt. When M₂ was heated to 107.2 °C, the appeared blurred broken fan-shaped texture of S^*_C phase meant the chiral 2-octyl terminal group deformed the long molecule structure and made its terminal part tilted with the layer normal and precessed simultaneously about the layer normal; by further heating to 111.79 °C, the molecule structure was stretched and tilted to a higher degree and completely turned into a helical structure paralleled in multiple layers, presented a much vivid fan-shaped focal conic texture of N* phase with occasionally observed oily streak texture. At the cooling sequence same textures could also be observed. According to the results of optical rotation test, M₂ has a specific rotation of −24.9° in dichloromethane, which means it may possess enough spiral force to organize a stable helical state. It should also be noted though CL showed a typical combination of grainy schlieren texture of S_C phase and planar thread-like texture of N phase during the heating and cooling cycles, it has a weak specific rotation of −2.51° in dichloromethane. It is perhaps due to the soft ester groups between benzene rings and long carbon chains slightly twisted during the heating and cooling periods, anyway no obvious chirality happened.

Fig. 4 Optical textures of M₁, M₂ and CL: (a) marble texture of N phase on heating to 150 °C for M₁ (500×); (b) broken fan-shaped texture of S^*_C phase on cooling to 90 °C for M₂ (200×); (c) fan-shaped focal conic texture of N^* phase on cooling to 150 °C for M₂ (200×); (d) oily streak texture of N^* phase on cooling to 140 °C for M₂ (200×); (e) grainy schlieren texture of S_C phase on cooling to 150 °C for CL (500×); (f) planar thread-like texture of N phase on heating to 180 °C for CL (500×).

To further study the multiple phase structures of M₂ and CL, XRD was performed to provide more detailed information, and the results are shown in Fig. 5. From Fig. 5a, all of the diffraction patterns of M₂ selected from different temperature points exhibited two sharp reflections (the weak one is the second order of the layer reflex²⁹) in the small angle region. A broad reflection at wide angles (associated with the lateral packings) and a sharp reflection at small angle (associated with the S^*_C layers or N^* layers) were respectively shown by the curves of cooling to 90 °C, 125 °C and heating to 109 °C, corresponding to the layer spacing d = 43.9, 35.3 and 41.5 Å, calculated by Bragg equation λ = 2d sin θ, respectively. While the theoretical molecule length (l) including alkyl chains in the trans conformation calculated using molecular modelling software Material Studios 6.0 is 43.39 Å (see Fig. S14 in ESI file†). The ratio d/l ≈ 1 indicates the molecules of M₂ are arranged in an almost interdigitated structure in S^*_C phase. This phenomenon is commonly observed in non-symmetric liquid crystal dimers in which the overlapping of alkyl chains and aromatic cores may happen.³⁰ The ratio d/l < 1 indicates the lay spacing is shorter than the length of the molecule, along with the diffuse peak around 2θ = 18.5°, give the possibility of the formation of N^* phase.³¹ The curve of cooling to 70 °C showed a typical crystallization phase meaning the existence of a solid state phase transformation. From Fig. 5b, the curve of cooling to 150 °C had small sharp peak in the small angle region with a diffuse peak at 2θ = 19.3° giving a layer spacing of d = 37.8; while the calculated theoretical molecular length is l = 47.15 Å (see Fig. S14 in ESI file†); the ratio d/l ≈ 0.8. Combining with the result from POM observation it could be determined a typical S_C phase had been formed at that point. The curve of cooling to 180 °C exhibited no peak around the small angle region, but gave a broad hump at 2θ = 18–22°; which usually meaning a nematic phase exists at that point. The curve of heating to 130 °C exhibited a crystallization phase also explaining a solid state phase transformation happened during that period.

Fig. 5 XRD patterns for (a) M₂ and (b) CL.

3.2. Structure and thermo behavior of the liquid crystalline elastomers E₁–E₇

Generally speaking, the thermo behaviors of the liquid crystalline elastomers usually rest with the structure of the elastomer backbone, the lengths of the flexible spacers, the rigidity of the mesogenic units and crosslinking agents, as well as the chirality of the mesogens. The backbones of the LCEs are basically categorized in three kinds: polysiloxanes, polyacrylates and polymethacrylates. Among which, the polysiloxanes exhibit lower glass transition temperature and higher thermal stability, hence become the most widely employed backbones for the synthesis of liquid crystalline elastomers. Sometimes there exists the competitions between the polymer backbones and the mesogenical units, which make the structures of the elastomers are usually much more complicated than those of the pure monomers. When the monomers become parts of the elastomers, the polymer backbones are inclined to form a random coil-like configuration; at the same time, the mesogenic units try to stabilize the long-range orientation order. Due to the monomers are generally connected to the polymer backbones via the flexible spacers, which may unfix the mesogenic groups and permit them to order in certain form.^32,33 For this reason, the polysiloxanes were preferably taken as the polymer backbones concerted with the flexible spacers to achieve better motility of the mesophases and liquid crystalline properties at appropriate temperature ranges.

In this article, the two monomers M₁ and M₂ are both dimer-like molecules with relatively long spacers between two hard mesogens, the liquid crystalline phases are nematic and chiral type respectively. The only crosslinking agent CL has longer molecule with two long flexible chains and three benzene rings in the middle as the hard mesogen, the liquid crystalline phase is from smectic to nematic. Ideally speaking, the co-elastomers composed of chiral and achiral side chains present better optical properties by the easiness of tuning the phase behaviors and helical pitches. By the change of the additions of these three elements, the structures of the elastomers can be tailored and modified. Due to the obtained ChLCEs are mainly made up of bendable moieties (PMHS backbone and flexible spacers of the dimers) and rigid moieties (crosslinking mesogens, chiral and achiral mesogenic units of the dimers), the PMHS backbone, the length of the flexible spacers and the rigidity of the mesogenic units and chirality of the mesogens may significantly affect the mesophase behaviors of the elastomers. Here we only changed the relative additions of the crosslinking agent to see the impact. The desirable perspective schematic structural illustration of the obtained elastomers is shown in Fig. 6. As we can observe, the ChLCEs are formed in different layers at a certain tilt angle like a twisted nematic phase, which conforms to the demand for the formation of chiral nematic phase. The chiral mesogens of M₂ make the local director helically arranged perpendicular to the vertical axis, turning into the main driving factors of this phenomenon.

Fig. 6 Schematic illustration of elastomers. The yellow rods represent the nematic mesogens, the cyan curved rods represent chiral mesogens, the dark red lines represent the backbones of liquid crystalline elastomers, and the black cylinder represent the crosslinking agents which form the elastomers by connecting different backbones through long alkyl chains.

The thermograms of the obtained ChLCEs were shown in Fig. 7, while Table 3 summarized the phase transition temperatures, corresponding enthalpy changes and thermal decomposition temperatures of the elastomers E₁–E₇ obtained with DSC and TGA. It could be suggested from the Fig. 7a that the T_g of the elastomers increased slightly with the augmentation of the crosslinking agents in the polymer system, while the corresponding T_i decreased to certain degree as the counterpart; which meant the mesophase temperature range ΔT was gradually narrowed due to these changes. T_lc of the elastomers was also in this trend, the feed ratio of the CL endowed the elastomers with great influence by dwindling the temperature range of their mesophases. All of the mesophase temperature ranges of the elastomers were relatively wide from almost 65 to 27 °C in the order from E₁ to E₇. The enthalpy change of the mesophase-isotropic phase transition ΔH_i was always a little bigger than that of the anisotropic-liquid crystalline phase transition ΔH_lc, meaning it needed more energy to transform from mesogenic phase to isotropic phase in order to achieve next level thermo stability. The thermal stability of elastomers could be observed in Fig. 7b, where the TGA curves of selected samples presented the temperatures at which 5% weight loss occurred were greater than 300 °C with the regular pattern of E₇ > E₅ > E₃ > E₁, meaning all the obtained elastomers possessed good thermal stabilities, and along with the increasing dosage of CL the thermal stabilities of the elastomers were better. Fig. 7c gave a better view of the thermal behaviors of the ChLCEs.

Fig. 7 (a) DSC thermograms of selected elastomers in the second heating cycles; (b) TGA curves of selected elastomers; (c) effects of CL content on phase transition and decomposition temperatures of elastomers.

Table 3 DSC and TGA results for elastomers

Sample	DSC					ΔT^d	TGA	LC phase
	Tg (°C)	Tlc^a (°C)	ΔHlc^b (J g^−1)	Ti (°C)	ΔHi^c (J g^−1)		T5%^e (°C)
a Temperature at which mesophase appeared.b Enthalpy change at which mesophase appeared.c Enthalpy change at which isotropy appeared.d Mesophase temperature ranges (Ti − Tg).e Temperature at which 5% weight loss occurred.
E1	88.6	105.6	0.38	170.2	0.58	81.6	309	N*
E2	91.8	111.7	0.41	169.7	0.48	77.9	311	N*
E3	93.3	120.5	0.35	167.4	0.51	74.1	314	N*
E4	97.5	116.5	0.42	171.2	0.49	73.7	316	N*
E5	100.2	124.9	0.37	172.4	0.55	72.2	323	N*
E6	101.5	128.2	0.52	168.9	0.67	67.4	324	N*
E7	102.7	135.1	0.35	162.5	0.43	59.8	325	N*

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3.3. Optical textures of the liquid crystalline elastomers E₁–E₇

If LC monomers are used as starting materials for LC elastomers, it is crucial to consider that monomer and polymer networks may differ in their phase behaviors.⁵ Fig. 8 gives some examples of the optical textures of the obtained ChLCEs. All of the samples showed various N^* phase textures in the second heating and second cooling cycles indicating the combination of chiral and non-chiral long side chain dimers with one long crosslinking agent would possibly to give N^* phase if the adjust of the composition is good enough. The diversity of the optical textures for elastomers is perhaps due to the nondeterminacy of the macromolecular composition, which caused different texture evolutions by the formation of orientational order from the movement of the helical structures of chiral mesogens, as well as the low-temperature and wide nematic phase property infused by the cyano-terminated side group of the nematic mesogens. Anyway, no blue phase textures were observed in this case, though the chiral 2-octyl terminal group and cyano-terminal group are well recognized combination for making blue phase liquid crystals,^34,35 perhaps due to the sophisticated macromolecular network of the elastomers.

Fig. 8 Optical textures of the elastomers: (a) oily streak texture of N^* phase on heating to 170 °C for E₁ (500×); (b) fingerprint texture of N* phase on cooling to 146 °C for E₃ (200×); (c) droplet texture of N^* phase on cooling to 169 °C for E₄ (500×); (d) coalescent droplet texture of N^* phase on cooling to 165 °C for same sample of E₄ (500×); (e) fan-like texture of N^* phase on cooling to 144 °C for E₅ (500×); (f) oily streak texture of N^* phase on heating to 159 °C for E₆ (200×).

X-ray patterns of selected ChLCEs heating to 150 °C confirmed the presence of the N^* phases (Fig. 9). A broad reflection at wide angle 2θ ≈ 20° (associated with the lateral packings) and a relatively weak reflection around 2θ ≈ 12° can be observed; that is a strong evidence of the existence of chiral nematic phase. Moreover, the broad reflections were more and more diffuse and peak intensity decreased with the increasing crosslink agent dosage, which implies the chiral nematic layers are less ordered due to the interference of the crosslinking agent. At the meantime the optical rotations of the elastomers changed from −10.28 to −5.6 (Table 1). According to the Bragg equation λ = 2d sin θ, the corresponding d spacings were 4.5 and 7.36 Å, respectively. Hence the chiral nematic phase of the obtained ChLCEs were firmly verified by optical textures and XRD results.

Fig. 9 XRD pattern of selected elastomer samples on heating to 150 °C.

3.4. Distribution of functional groups in elastomers

To study the evolution of mesogens in the elastomer system, it is important to evaluate distribution morphology of the functional groups. The FT-IR and FT-IR imaging system were adopted here to give a better understanding of this issue. Fig. 10 gives a full expression of the distribution of the functional groups in the elastomer system. In the upper part of Fig. 10 for the FT-IR spectra for elastomers E₁ to E₇, the total disappearance of the Si–H band meant the successful incorporation of M₁, M₂ and CL onto the polysiloxane chains. The FT-IR typical values (KBr, cm⁻¹) are: 3100–2810 (–CH₃, –CH₂), 2225 (–CN), 1760–1697 (C=O), 1604, 1510 (–Ar), 1020–1150 (Si–O–Si). They also showed a tendency that the exclusive absorption peaks of cyano group at 2225 cm⁻¹ was gradually lessened along with the increasing dosage of CL, meaning the content of M₁ relatively decreased with the increasing dosage of CL in the order from E₁ to E₇, which was also agreed with the experimental evidence. The FT-IR imagine system was used to characterize the distribution of the functional groups in this elastomer system. In the lower part of Fig. 10, the yellow and green areas represent stronger and weaker absorption for the cyano group at 2225 cm⁻¹. Due to 2225 cm⁻¹ is the unique absorption peak of cyano group in M₁, it can be assumed that M₁ are finely distributed in the elastomer system according to the images Fig. 10a–d. It is obviously that the Fig. 10a has contiguous and bigger yellow areas, indicating there are a lot of cyano groups in E₁ elastomer system, while there is a proneness that less and less yellow areas appeared in the images in the order of E₁ > E₂ > E₅ > E₇, suggesting there are more and more crosslinking agents were involved in the elastomer system in this order, which was also well consistent with the experimental design. Meanwhile, the uniform yellow areas in Fig. 10 proved the efficient distribution of the functional groups is generally realized.

Fig. 10 (Upper part) FT-IR spectra for elastomers E₁ to E₇ and (lower part) FT-IR images of (a) E₁, (b) E₃, (c) E₅ and (d) E₇.

3.5. Swelling characterization of ChLCEs

For the sake of studying the structure of the ChLCEs, it is crucial to calculate the effective crosslink density (M̄_c), i.e. the average molecular weight between crosslink points. M̄_c can be characterized by swelling experiments and calculated according to the below Flory swelling theory.³⁶

M̄_c = −ρV₁(V₂^1/3 − V₂/2)/[ln(1 − V₂) + V₂ + χV₂²]

where ρ is the polymer density before swelling, which is listed in Table 4 for the synthesized ChLCEs; V₁ is molar volume of the swelling solvent (105.91 cm³ mol⁻¹) for toluene in this work; the Flory–Huggins parameter or the interaction parameter between the solvent and the elastomer (χ) can be calculated according to the expression:³⁷

χ = (δ₁ − δ₂)²V₁/RT

where R is the gas constant; T the absolute temperature; δ₁ and δ₂ are solubility parameters of the solvent and polymers, respectively. The Hildebrand solubility parameters of toluene and polyorganosiloxane are 18 and 14.9 MPa^1/2, respectively.^38,39 The parameter V₂ is the polymer network volume fraction at swelling equilibrium, and can be calculated by:

V₂ = 1/Q

where Q is the swelling ratio of networks by volume. It was determined by gravimetrically via the following equation:²³

Q = 1 + (W₂/W₁ − 1)ρ_p/ρ_s

where W₁ and W₂ are the weights of the ChLCEs samples before and after the equilibrium swelling; ρ_p and ρ_s are the densities of ChLCEs samples and solvent toluene, respectively.

Table 4 Swelling properties of the elastomers

Sample	P (g cm^−3)	V2	M̄c (g mol^−1)	Solubility^a
				Toluene	THF
a Key: (+) dissolve; (−) insolubility or swelling.
E1	1.073	0.22	6292	−	+
E2	1.081	0.24	5136	−	+
E3	1.085	0.27	3847	−	+
E4	1.090	0.30	2949	−	+
E5	1.094	0.32	2497	−	+
E6	1.096	0.34	2125	−	+
E7	1.115	0.37	1712	−	−

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The data of the swelling properties of the obtained ChLCEs were collected in Table 4. As seen from the data, with the increasing concentration of cross-linking units in the networks, M̄_c values reduced, that is to say, the molecular weight between the crosslinking points decreased with the increase of the CL.

4. Conclusions

In this paper, a new set of novel side-chain ChLCEs containing dimer-type nematic and chiral side-chains were synthesized and characterized. The chemical structures and liquid crystalline properties of the obtained monomers and crosslinking agent were prudently characterized. All of the ChLCEs exhibited diversified optical textures, relatively wide mesophase temperature ranges and excellent thermal stabilities. The combination of optical textures and XRD curves of the elastomers showed all of the ChLCEs are chiral nematic liquid crystals. The dosage of the crosslinking agents greatly influence the thermal and mesophase behaviors of the elastomers. The elastomers containing more crosslinking agents showed less reversible mesophase transition, better thermo stability, smaller crosslinking density and harder elastomer system. The functional groups in the elastomers is finely distributed. This new set of side-chain ChLCEs may have potential usage in the piezoelectric, tunable lasing, photonic, optical devices and related applications.

Acknowledgements

The authors are grateful to the National Key Technology Support Program of China (contract grant number 2008BAL55B03) and the Science and Technology Department of Liaoning Province for financial support of this work.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis procedures of M₁, M₂, and CL; FTIR spectra and ¹H NMR (600 MHz) spectra thereof; additional textures and theoretical molecular model. See DOI: 10.1039/c6ra19330g

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