Synthesis of amide-type smectic a side-chain polysiloxane liquid crystal

Article abstract of DOI:10.1080/15421406.2012.708309

In this paper, monomer N-(4-hydroxyphenyl-1) 4-alkyloxybenzamide was obtained by the Williamson reaction, acylation, and aminolysis. Side-chain polysiloxane liquid crystal was then successfully synthesized by a grafting reaction of the monomer. The monomer and liquid crystal were characterized by Fourier-transform infrared spectroscopy, ¹H nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and polarization microscopy. The results showed that both monomer and polymer exhibited thermotropic smectic A behavior, and they were very stable within the studied temperature range. They may be very sensitive to both electric and magnetic fields because of their liquid crystalline properties.

Full text of DOI:10.1080/15421406.2012.708309

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Synthesis of Amide-Type Smectic A Side-
Chain Polysiloxane Liquid Crystal
Jianfeng Gao a , Zhongzhan Sun a , Tao Peng a & Lina Yang a
a Department of Chemistry, School of Science , North University of
China , Taiyuan , Shanxi , China
Published online: 22 Jan 2013.
To cite this article: Jianfeng Gao , Zhongzhan Sun , Tao Peng & Lina Yang (2013) Synthesis of Amide-
Type Smectic A Side-Chain Polysiloxane Liquid Crystal, Molecular Crystals and Liquid Crystals, 570:1,
62-66, DOI: 10.1080/15421406.2012.708309
To link to this article: http://dx.doi.org/10.1080/15421406.2012.708309
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Mol. Cryst. Liq. Cryst., Vol. 570: pp. 62–66, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.708309
Synthesis of Amide-Type Smectic A Side-Chain
Polysiloxane Liquid Crystal
JIANFENG GAO,∗ ZHONGZHAN SUN, TAO PENG,
AND LINA YANG
Department of Chemistry, School of Science, North University of China,
Taiyuan, Shanxi, China
In this paper, monomer N-(4-hydroxyphenyl-1) 4-alkyloxybenzamide was obtained by
the Williamson reaction, acylation, and aminolysis. Side-chain polysiloxane liquid crys-
tal was then successfully synthesized by a grafting reaction of the monomer. The
monomer and liquid crystal were characterized by Fourier-transform infrared spec-
troscopy, 1H nuclear magnetic resonance spectroscopy, differential scanning calorime-
try, and polarization microscopy. The results showed that both monomer and polymer
exhibited thermotropic smectic A behavior, and they were very stable within the studied
temperature range. They may be very sensitive to both electric and magnetic fields
because of their liquid crystalline properties.
Keywords Amide linkage; liquid crystal; polymers; smectic A
Introduction
Side-chain liquid crystalline polymers have been extensively studied over the last decades
[1–5] because of their novel properties, such as light absorption, electric, magnetic, and
separation characteristics. Side-chain liquid crystals have potential uses in many applica-
tions, such as optoelectronic display, recording, storage, modulation, membrane separation,
biological materials, etc.
In recent years, significant progress has been made in the synthesis and application of
liquid crystal molecules containing amide linkages [6–10]. Aharoni [11] has synthesized
a polymer liquid crystal with an amide linkage and an isolated benzene ring. Kumar et al.
[12] have synthesized triphenylene discotic liquid crystalline compounds containing an
amide linkage. However, the synthesis of polysiloxane liquid crystals with amide linkages
is rarely reported. In this paper, the thermotropic liquid crystalline polysiloxane with an
amide linkage was synthesized successfully. The obtained monomer and liquid crystal
polymer had good stabilities within the studied temperature range.
∗Address correspondence to Jianfeng Gao, Department of Chemistry, School of Science, North
University of China, Taiyuan Shanxi 030051, China. Tel.: +86-351-3628189; Fax: +86-351-
3628189. E-mail: jianfenggao@163.com
62

Synthesis of Side-Chain Polysiloxane Liquid Crystal
63
Experimental
Synthesis of 4-Allyloxybenzoyl Acid (Compound 1) and 4-Allyloxybenzoyl
Chloride (Compound 2)
4-Allyloxybenzoyl acid was synthesized according to a previous report [13]. The obtained
compound 1 (14 g, 80 mmol) was reacted with thionyl chloride (12 mL, 165 mmol)
containing a few drops of N,N-dimethylformamide (DMF) at 60◦C for 5 h. Then, 4-
allyloxybenzoyl chloride (compound 2) was purified by reduced pressure distillation.
Synthesis of Monomer (Compound 3)
The obtained compound 2 (3.4 g, 20 mmol) was dissolved in 20 ml of acetone, and a mixture
of 2.18 g of aminophenol, 12 mL of acetone, and 2 mL of DMF was slowly added to the
reaction vessel. The reaction was carried out at 0◦C for 30 min. Thereafter, the product
was cooled and placed in a refrigerator overnight. After the filtrate was concentrated, the
product was precipitated by adding water to the residue. Compound 3 was obtained by
filtration and recrystallization from dichloromethane (yield: 76.8%; mp: 199–200◦C).
Synthesis of Side-Chain Polysiloxane Liquid Crystal (Compound 4)
About 2.2 g (25 mmol) of compound 3, 1.6 g of polysiloxane, 100 μg of potassium
chloroplatinate, and 17 ml of toluene were poured into a 100-mL three-neck round-bottom
flask equipped with a reflux condenser and magnetic stirrer. The mixture was refluxed
under nitrogen atmosphere at 110◦C for 24 h. After two times of recrystallization in
dichloromethane, compound 4 was dried in a vacuum oven at 110◦C for 24 h (yield: 51.1%).
The preparation is shown in Scheme 1.
Scheme 1. Preparation of target molecules.
Results and Discussion
Figure 1 shows the Fourier transform infrared (FT-IR) spectra of (a) polysiloxane,
(b) monomer (compound 3), and (c) side-chain polysiloxane liquid crystal (compound 4).
Clearly, (a) the Si-H stretching bond at 2162 cm−1 and (b) the C=C bond at 1653 cm−1 were

64
J. Gao et al.
2162
1685
1653
1610
3340
3274
1678
1613
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm−1
)
Figure 1. FT-IR spectra of (a) poly(methylsiloxane), (b) compound 3, and (c) side-chain polysiloxane
liquid crystal compound 4.
not present in the (c) resultant compound 4. The characteristic Si–O–Si stretching bond ap-
peared at 1000–1300 cm−1, whereas the absorption bands of amide C=O (at 1678 cm−1) and
aromatic band (at 1613 cm−1) still existed in compound 4. Thus, the appearance of the char-
acteristic absorption bands of corresponding compound 3 indicated the successful incorpo-
ration of monomers into the polysiloxane chains for all liquid crystal polysiloxanes [14].
1
Figure 2 shows the H NMR (400 MHz, DMSO, ppm) spectra of compound 4: δ =
0.01033 (a), 1.30881 (b), 3.15539 (c), 3.40765 (d), 6.86120 (e), 7.1894 (f), 7.71279 (g),
8.11688 (h), 9.65872 (i), and 9.95479 (j). In addition, δ = 4.74 (Si-H) was not detected
1
by H-NMR, suggesting the successful incorporation of monomers into the polysiloxane
chains. The result of the analysis was the same as that of FT-IR spectroscopy. δ = 2.90476
was the result of the α or β grafting reaction of compound 3.
Figure 2. 1H-NMR spectra of the obtained side-chain polysiloxane liquid crystal compound 4.

Synthesis of Side-Chain Polysiloxane Liquid Crystal
65
Figure 3. (a) POM photos of compound 3 (200×) at 189◦C; (b) POM photos of compound 4 (200×)
at 305◦C.
The optical textures of the samples were studied by polarized optical microscopy
(POM) with a hot stage. The representative optical textures of monomeric compounds 3
and 4 are shown in Figs. 3(a) and (b).
For compound 3, the images became bright and liquid crystal textures appeared when
the temperature was higher than 104◦C. The textures became clearly visible as the sample
was heated continuously. Ultimately, compound 3 exhibited textures with some small
rods, as shown in Fig. 3(a). The bar-like texture of compound 3 disappeared when the
temperature was higher than 205◦C, and the sample became isotropic. Compared with
nematic spherules, the smectic phase formed a small rod called a “batonnet.” Therefore, the
appearance of the batonnet showed that compound 3 had new high-ordered smectic phase
[15]. The POM results revealed that the liquid crystal temperature range of compound 3
was from 104 to 205◦C.
The POM also showed that compound 4 had a smectic A texture. The images became
bright and liquid crystal textures appeared when the temperature was higher than 247◦C.
The bar-like texture existed until the temperature of isotropic transition was 310◦C. This
phenomenon indicated the appearance of smectic textures upon heating compound 4.
When compound 4 was cooled from the isotropic state, similar bar-like textures appeared
gradually, and a broken fan-shaped texture appeared, as shown in Fig. 2(b). The POM results
revealed that the liquid crystal temperature range of compound 4 was from 247 to 310◦C.
The transition temperature can be obtained by differential scanning calorimetry (DSC).
Compounds 3 and 4 were further characterized by DSC measurements using a DSC 822e
instrument (METTLER TOLEDO, Switzerland) under nitrogen flow at a heating rate of
10◦C/min. The results are shown in Figs. 4(a) and (b).
The melting point (Tm) of compound 3 was observed at 106.13◦C, and the smectic-
isotropic transition temperature (TS-I) was 200.22◦C. The temperature range and the charac-
terization were consistent with the POM results. However, there was only one endothermic
peak in the DSC thermogram of compound 4, i.e., a melting point at 255.27◦C. This finding
can be attributed to the higher glass transition temperature when the isotropic liquid crystal
phase change cannot proceed until the sample decomposed [15]. However, the performance
of the liquid crystal was not affected.

66
J. Gao et al.
Figure 4. DSC thermograms of (a) monomer compound 3, and (b) side-chain polysiloxane liquid
crystal compound 4.
Conclusions
A new amide-type smectic A side-chain polysiloxane liquid crystal was synthesized, and its
phase behavior was melting point (Cr) = 255.27◦C. Its liquid crystal domain was narrower
than that of the monomer. Both side-chain polysiloxane liquid crystal and monomer had
good stabilities in wide temperature ranges. They can be applied in optoelectronic displays
as well as recording and storage materials.
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