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A novel thermotropic liquid crystalline – Benzoylated bacterial cellulose
Yan Wang, Qingping Luo, Bihui Peng, Chonghua Pei *
School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang City, Sichuan Province 621010, PR China
a r t i c l e i n f o
Article history: Received 26 March 2008 Received in revised form 28 April 2008 Accepted 7 May 2008 Available online 15 May 2008
Keywords: Bacterial cellulose Biofibers Degree of substitution Biopolymers Thermal properties
a b s t r a c t
Using the esterification of bacterial cellulose (BC), we have synthesized Benzoylated bacterial cellulose (BBC). The molecular structure of the BBC was characterized by means of Fourier transform infrared (FT-IR) spectroscopy, 1 H and 13 C nuclear magnetic resonance (NMR). The BBC is found to display thermo- tropic liquid crystalline feature determined with differential scanning calorimetry (DSC), polarized opti- cal microscope (POM) and wide-angle X-ray diffraction (WAXD). Here, we demonstrate that it is possible to obtain the BBC with degree of substitution (DS) from 0.88 to 2.46 by applying the different molar ratio of benzoyl chloride to the anhydrous glucose unit (AGU). The glass transition temperatures (Tg) of the liquid crystalline phases lie between 281.2 and 281.8 °C and the isotropic melt transition temperatures (Ti ) vary from 341.6 to 362.8 °C, depending on the DS. Ó 2008 Elsevier Ltd. All rights reserved.
- Introduction
Cellulose is one of the oldest and the most abundant natural poly- mers, which is renewable and biocompatible (Klemm, Heublein, Fink, & Bohn, 2005). Cellulose with a rigid or semirigid backbone fac- ilely forms liquid crystalline phases. Werbowyj and Gray (1976) firstly reported the cholesteric liquid crystalline phase of hydroxy- propyl cellulose in aqueous solutions in 1976. Thereafter, investiga- tions have been focused on the liquid crystalline of cellulose and its derivatives. Recently, the study on liquid crystal phases of functional cellulose ester or ether derivative gels or composites becomes attractive owing to their potential advantages in high intensity, toughness, and excellent processability (Huang & Li, 1995; Li, Huang, Hu, Lin, & Yang, 1999; Li, Huang, & Lin, 1996). Meanwhile, the study making use of liquid crystalline properties of cellulose for electro-optical applications opens new horizons for these tradi- tional materials (Godinho, Martins, & Figueirinhas, 1996; Westlund, Carlmark, Hult, Malmstrom, & Saez, 2007). Furthermore, the chole- steric liquid crystalline of cellulose derivative is of great scientific and technological interest in the consequence of its unique selective reflection of light (Huang, Ge, Li, & Hou, 2007; Ifuku, Kamitakahara, Takano, Tanaka, & Nakatsubo, 2004; Wang & Huang, 2004). While a lot of cellulose derivatives have been found to display the liquid crystalline character, all of them derive from plants. Besides plants, some bacteria also can produce cellulose (Brown, 2004; Hestrin & Schramm, 1954). The cellulose produced by bacteria is called ‘‘microbial cellulose” or ‘‘bacterial cellulose (BC)”, which is an unbranched polymer by b-1,4-linked glucopyranose unit (Jonas &
Farah, 1998). Although chemically identical to plant cellulose, BC is distinctly different from the cellulose derived from plants. BC is devoid of lignin, hemicellulose, and other complex carbohydrates. Moreover, BC has unique properties, such as ultrafine network structure, high good mechanical strength and a high degree of poly- merization. Owing to these, BC and its derivatives can be used in many fields, such as paper, electronic industries and medical mate- rials (Czaja, Young, Kawecki, & Brown, 2007; Ifuku et al., 2007; Nishi et al., 1990; Shah & Brown, 2005; Yano et al., 2005). In this paper, we report the preparation of benzoylated bacterial cellulose (BBC) through the esterification of BC with benzoyl chlo- ride. Benzoyl chloride reacts with the hydroxyl groups on the glu- copyranose unit of BC, and eventually benzoyl substituents are attached to the BC backbone to form the BBC. With gradual substi- tution, the molecular chain of the BBC becomes semirigid and dis- play an interesting thermotropic liquid crystalline phase. The BBC is one of potential candidates for sensors, high-level piezoelectric and optical materials.
- Experimental part
2.1. Materials
Acetobacter xylinum (Hainan-1) was obtained from Hainan Uni- versity and used to produce the BC pellicles. The A. xylinum (Hai- nan-1) was grown in a sterile liquid medium consisting of 0.4% ammonia sulfate ((NH 4 ) 2 SO 4 ), 0.05% magnesium sulfate (MgSO 4 ), 2% glucose, and 0.1% potassium dihydrogenphosphate (KH 2 PO 4 ) at pH of 4.5, 30 °C for 36 h. The strains were pre-cultured in a tube and then the strain (20 mL) inoculated into the 1 L flask containing 400 mL of the medium described above. The flasks were incubated
0144-8617/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2008.05.
- Corresponding author. Tel.: +86 816 241 9280; fax: +86 816 241 9206. E-mail address: peichonghua@swust.edu.cn (C. Pei).
Carbohydrate Polymers 74 (2008) 875–
Contents lists available at ScienceDirect
Carbohydrate Polymers
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a r b p o l
statically at 30 °C for 7 days. The gel-like pellicles of BC were washed with water, and then boiled in 0.1 M aqueous solution of NaOH for 2 h to remove the impurities. The BC pellicles were again rinsed with water to remove the superabundance of NaOH, and fi- nally dried at 70 °C and pulverized to 40 mesh. Benzoyl chloride, nitrobenzene and pyridine with analytical reagent were used with- out further purification.
2.2. Measurement
We performed Fourier transform Infrared (FT-IR) spectroscopy at the room temperature by using a G988 FT-IR spectrophotometer with the KBr pellet method. The thermal properties were studied by thermogravimetric analyzer–differential scanning calorimetry (TG-DSC) and polarized optical microscope (POM). TG-DSC curves were recorded with a STA499C DSC in a N 2 atmosphere. The sam- ple with 3 mg mass was heated at a constant rate of 20 °C/min. The birefringence observation was made on an Olympusb 51 POM. The BBC powder heated on a CSS450 hot stage was placed between a microscope slide and cover glass. Wide-angle X-ray diffraction (WAXD) pattern was performed with a D/max-RB nickel-filtered
Cu Ka (k = 0.154 nm) the X-ray generator. The voltage and the cur-
rent were 35 kV and 60 mA, respectively. 1 H and 13 C nuclear mag- netic resonance (NMR) spectra were recorded on a Bruker spectrometer of 600 and 74.5 MHz, with deuterated tetrahydrofu- ran as solvent.
2.3. Typical preparation of benzoylated bacterial cellulose
BC powder and nitrobenzene (14 mL/g) were mixed. After standing for half an hour at room temperature, the above mixture was added weighted benzoyl chloride (12 mL/g) and pyridine (10 mL/g). The mixture was heated at 130 °C for 20 h, a homoge- nous solution was formed (Choi et al., 2004; Kim, Nishiyama, & Kuga, 2002; Zhou, Zhang, Okamura, Minoda, & Miyamoto, 2001). The solution was subsequently poured into bulky ethanol. The BBC was precipitated as a solid from the solution. The obtained BBC was separated by filtration, alternatively washed with ethanol and acetone, and finally dried at 70 °C.
2.4. Determination of degrees of substitution (DS) for the BBC
The BBC (100 mg) was added into 40 mL of 75% ethanol in a glass bottle, and heated to 60 °C for 30 min. Then 40 mL of 0.1 M NaOH solution was added to the BBC solution. After reheated to 60 °C for 15 min, the solution was cooled to room temperature, and maintained for 48 h. The excessive alkali was titrated with 0.1 M HCl with phenolphthalein as an indicator.
- Results and discussion
3.1. DS of the BBC vs the molar ratio of benzoyl chloride to the anhydrous glucose unit (AGU)
Fig. 1 shows the DS of the molar ratio of benzoyl chloride to AGU. It reveals that we can obtain BBC with DS from 0.88 to 2. by changing the molar ratio of benzoyl chloride to AGU. The DS in- creases nonlinearly with increasing of the molar ratio of benzoyl chloride to AGU. The probable reason for the nonlinearity is the trace water in the BC (Chen & Wu, 1980).
3.2. The structure of the BBC
As shown in Fig. 2a, the broad peak of the BC at 3435 cm^1
assigned to the O–H stretching vibration (mO–H) model obviously
decreases in intensity with benzoyl substitution. The peak of the BC and BBC at 1058 cm^1 assigned to the C–O–C stretching vibra-
tion (mC–O–C) model. Meanwhile, three new peaks at 1731, 1606,
711 cm^1 assigned to C@O stretching vibration (mC@O) model,
C@C stretching vibration (mC@C) model and C–H stretching vibra-
0 5 10 15 54 56
DS
The molar ratio of benzoyl chloride to anhydrous glucose unit
Fig. 1. Plot of DS of the BBC vs the amount of benzoyl chloride.
4000 3500 3000 2500 2000 1500 1000 500
BC
BBC with DS 2.
Wavenumber (cm -1)
1.0 1.2 1.4 1.6 1.8 2.4 2.
A(1606cm-1)/A(1058cm -1 (^) ) A(1731cm-1)/A(1058cm -1 (^) ) A(3435cm-1)/A(1058cm -1 (^) )
Absrobance/Absorbance
DS
Fig. 2. FT-IR spectra for (a) BC and the BBC with DS 2.46 and (b) the BBC with different DS.
Acknowledgements
The authors acknowledge Prof. Chaoping Xiao for the BBC mea- surement. We are also grateful to Dr. Yong Zhou, Dr. Yongjun Ma and Dr. Bo Dai for valuable advice. This investigation was sup- ported by Sichuan Province Science Foundation for Youths (08ZK026-062).
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