Synthesis and Physicochemical Characterization of Chitin Derivatives

Firstly, chitin derivatives were synthesized. For this purpose, chitin was modi�ed via ring-opening reaction with cyclic anhydrides in lithium chloride/N,N-dimethylacetamide.en, chitin derivatives synthesized were characterized by FTIR, HNMR, CNMR, and U-Vis spectroscopies and scanning electronmicroscopy.ermogravimetric analysis was performed to investigate the thermal stability of chitin derivatives.ermogravimetric analysis results showed that chitinmodi�edwith trimellitic anhydride is thermally more stable than chitin modi�ed with phthalic anhydride. In addition, the electrical conductivity of chitin modi�ed with phthalic anhydride and trimellitic anhydride was also measured. Electrical conductivity measurement results showed that the electrical conductivity of chitin modi�ed with trimellitic anhydride (1.2 × 10 S cm) is higher than that of chitin modi�ed with phthalic anhydride (9.2 × 10 S cm).


Introduction
Polymers have been considered as good electrical insulators for years, and they have widely been used as inactive packaging and insulating material.But this narrow perspective is rapidly changing as a new class of polymer known as intrinsically conducting polymers (CPs) or electroactive polymers (EAPs) is being discovered.Conjugated polymers are plastic semiconductors.ey have band gaps that can be tuned by alternating the chemical nature of either polymer backbone or side groups present in chain.Conducting polymers contain conjugated -electron system, responsible for their unusual electronic properties such as electrical conductivity, lowenergy optical transitions, low ionization potential, and high electron affinity [1,2].CPs, such as polyacetylene [3,4], polyaniline [5][6][7][8][9][10][11][12][13], polypyrrole [10,14], polythiophene [10,15], poly(p-phenylene) [16], and poly(p-phenylene vinylene) [17], have received a great deal of attention and constitute a class of materials that possess the properties of both organic polymers and inorganic conductors or semiconductors.e preparation, characterization, and application of electrochemically active and electronically conducting polymeric systems are still in the foreground of research activity in electrochemistry.ere are at least two major reasons for this intense interest.e �rst is the intellectual curiosity of scientists, which focuses on understanding the behavior of these systems, in particular on the mechanism of charge transfer and on charge transport processes that occur during the redox reactions of conducting polymers.e second is the wide range of promising applications of these compounds in the �elds of energy storage, electrocatalysis, organic electrochemistry, bioelectrochemistry, photoelectrochemistry, electroanalysis, sensors, electrochromic displays, microsystem technologies, electronic devices, microwave screening, corrosion protection, and so forth.Many excellent monographs reviewing the knowledge accumulated regarding the development of conducting polymers, polymer �lm electrodes, and their applications have been published [18].Studies for preparing alternative CPs or semiconductors from natural and/or low-cost materials have been rapidly continuing.Chitin is a linear polymer composed of 2-acetamido-2deoxy-D-glucopyranose (N-acetyl-D-glucosamine, GlcNAc) units linked by -(1 → 4) linkage.It is distributed widely in nature as the skeletal material of crustaceans and insects and as a component of cell walls of bacteria and fungi.Chitin occurs naturally in three polymeric forms known as -, -, and -chitin [20].-chitin is arranged in an antiparallel con�guration while -chitin is organized in a parallel con�guration.However, -chitin has a parallel and antiparallel structure, which is a combination of -chitin and -chitin [21].Molecular structure and hydrogen bonding in -chitin and -chitin are shown in a paper [22].-Chitin is the most abundant form found in nature.Both -chitin and -chitin are crystalline [23].Chitin has strong inter-and intramolecular hydrogen bonds between the polymer chains and is water insoluble due to its rigid crystalline structure [24].It is the second most abundant organic compound, with 10 11 tons produced annually, aer cellulose on earth [25].In recent years important studies with respect to the electrical conductivity of some chitin derivatives are done [26][27][28][29].
Phthalic anhydride is an important industrial chemical, especially for the large-scale production of plasticizers for plastics.It is presently obtained by the catalytic oxidation of ortho-xylene and naphthalene.Trimellitic anhydride is mainly used for producing polyester resin, polyimide resin, water-soluble alkyd resin, water-soluble polyurethane resin, plasticizer, water-soluble amino-alkyd resin, curing agent for epoxy resin, aeroengine oil, electric capacitor maceration oil, and so forth.e aim of this study was to synthesize chitin derivatives, to characterize physicochemically the derivatives to be synthesized and to compare them from the point of view of stability and conductivity.

Synthesis of Chitin
Derivatives.e 1.0% (w/v) chitin solution was prepared as follows: chitin (3.0 g) was added to 300 mL of 5% (w/v) LiCl/DMAc solution, and the mixture was stirred at room temperature for 3 h to give a clear solution.Cyclic anhydride (100 mmol) was added to 200 mL of the chitin solution (Scheme 1).Aer stirring for 24 h, reaction mixture (containing a gel) was poured into MeOH (100 mL).e precipitate was �ltered, and then dispersed in 200 mL of water.e product was precipitated from the solution or suspension by adjusting the pH to 1-2 with 3 M HCl.e precipitate was �ltered, washed with MeOH, and dried in vacua [30].

Characterization.
FTIR spectra were recorded between 450 and 4400 cm −1 with a resolution of 4 cm −1 from KBr pellets on a PerkinElmer RXI FT-IR spectrometer (PerkinElmer Inc., USA). 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded in DMSO on a BRUKER DPX-400 high-performance digital FT-NMR spectrometer (Bruker Corporation, Germany).Tetramethylsilane was used as an internal standard.UV-Vis absorption spectra were obtained with a PerkinElmer Lambda 25 UV/Vis spectrometer (PerkinElmer Inc., USA) working in the wavelength range of 190-1100 nm by using a quartz cell of 1 cm path length.SEM micrographs were taken with a VEGA-II LSU scanning electron microscope (Tescan Inc., USA).TGA thermograms were recorded by using a Pyris 1 thermogravimetric analyzer (PerkinElmer Inc., USA).Chitin modi�ed with phthalic anhydride (CPA) and chitin modi�ed with trimellitic anhydride (CTA) were heated from 25 to 900 ∘ C and from 35 to 900 ∘ C, respectively, at a heating rate of 10 ∘ C min −1 under N 2 atmosphere.e direct current electrical conductivity of chitin derivatives was measured by the standard four-point probe method by using PCI-DAS6014 for a current source, voltmeter, and temperature controller.For this purpose, dry and powdered samples were transformed into pellets by using a steel die of 13 mm diameter under a pressure of 700 MPa.

FTIR Spectra.
Figure 2 shows the FTIR spectra of CPA and CTA.Characteristic absorption bands of CPA and CTA are given in Table 3. Carbonyl stretching vibrations (1739 cm −1 for CPA and 1712 cm −1 for CTA) related to ester and/or carboxylic acid and aromatic C=C stretching vibrations (1493-1451 cm −1 for CPA and 1503-1476 cm −1 for CTA) related to benzene ring in the FTIR spectra of CPA (Figure 2(a)) and of CTA (Figure 2(b)) con�rmed that chitin was successfully modi�ed with phthalic anhydride and trimellitic anhydride. 13C NMR Spectra.Figures 3 and 4 display the 1 H and 13 C NMR spectra of CPA and CTA, respectively.Broad singlet at 3.55 ppm with 1H intensity related to the secondary hydroxyl groups of CPA and broad singlet at 3.53 ppm with 1H intensity related to the secondary hydroxyl groups of CTA in their 1 H NMR spectra show that esteri�cation reaction occurred with only or usually primary hydroxyl groups.e primary hydroxyl groups according to the secondary hydroxyl groups in chitin react easier to form an ester because the nucleophilic attack of the secondary hydroxyl groups gets difficult due to the steric hindrance of chitin ring.Scheme 1 in [30� is con�rming this idea.

1 H and
F 2: FTIR spectra of (a) CPA and (b) CTA.

UV-Vis Absorption Spectra.
Figure 5 shows the UV-Vis absorption spectra of CPA and CTA.As it is known, UV-Vis spectroscopy is the measurement of the wavelength and intensity of absorption of near-ultraviolet and visible light by a sample.Ultraviolet and visible light are energetic enough to promote outer electrons to higher energy levels [31].
As can be seen from Figures 5(a) and 5(b), CPA has one absorption band but CTA has two absorption bands.e absorption band at about 259 nm in Figure 5(a) may be due to    * transition (R band).As for the absorption bands at about 265 nm and about 293 nm in Figure 5(b) that may be due to    * and    * transitions (R band), respectively.
3.5.SEM Micrographs.Figure 6 displays the SEM micrographs of CPA and CTA.e SEM micrographs of CPA and CTA show that CPA has more ordered and bigger macropores according to CTA.As it is known, pores decrease the electrical conductivity of material.is effect becomes more while it is gone from micropore to macropore.3.6.TGA ermograms.Two decomposition stages could be observed in the thermogram of CPA and CTA (Figure 7).In the thermogram of CPA (Figure 7 In the thermogram of both CPA and CTA, the �rst decomposition stage could be attributed to water evaporation, and the second decomposition stage could be attributed to the degradation of the polysaccharide structure of the molecule, including the dehydration of polysaccharide rings and the polymerization and decomposition of the acetylated and deacetylated units of CPA and CTA [32][33][34][35][36].It can be said that CTA is thermally more stable than CPA.3.7.Electrical Conductivity.e electrical conductivity of CPA and CTA was measured to be 9.2 × 10 −5 S cm −1 and 1.2 × 10 −4 S cm −1 , respectively.ese conductivity values are in the conductivity range ( = 10 −7 -10 −1 S cm −1 ) of semiconductors [37] and show that CTA is more conductive than CPA.
In addition, the electrical conductivity of especially CTA is also higher than the electrical conductivities of previous synthesized chitin derivatives [26][27][28][29], and it can be said that chitin modi�ed with trimellitic anhydride is a good semiconductor.

Conclusions
e following �ndings con�rmed that chitin was successfully modi�ed with phthalic anhydride and trimellitic anhydride: (1) carbonyl stretching vibrations (at 1739 cm −1 for CPA and at 1712 cm −1 for CTA) related to ester and/or carboxylic acid in the FTIR spectra of CPA (Figure 2 ( CPA has one absorption band, but CTA has two absorption bands.e absorption band at about 259 nm in Figure 5(a) may be due to    * transition (R band).As for the absorption bands at about 265 nm and about 293 nm in Figure 5(b) that may be due to    * and    * transitions (R band), respectively.e SEM micrographs of CPA and CTA show that CPA has more ordered and bigger macropores according to CTA.Besides, CTA is thermally more stable and more conductive than CPA.As a conclusion, the modi�cation of chitin with phthalic anhydride and trimellitic anhydride is of low cost, and the electrical conductivity of especially CTA is higher than the electrical conductivities of previous synthesized chitin derivatives.It can be said that CTA can be used as an alternative semiconductor.
(a)), the �rst decomposition stage is in the range of approximately 30-95 ∘ C, and the second decomposition stage is in the range of approximately 190-235 ∘ C. In the thermogram of CTA (Figure 7(b)), the �rst decomposition stage is in the range of approximately 50-110 ∘ C, and the second decomposition stage is in the range of approximately 200-285 ∘ C.

F 4 :
13 C NMR spectra of (a) CPA and (b) CTA.
cm −1 for CPA and at 1503-1476 cm −1 for CTA) related to benzene ring in the FTIR spectra of CPA (Figure 2(a)) and CTA (Figure 2(b)), (3) the multiplet peaks at 8.02-8.15ppm related to the protons of aromatic ring and the broad singlet at 10.62 ppm related to the proton of carboxylic acid in 1 H NMR spectrum (Figure 3(a)) of CPA, (4) the peaks at 7.49-8.55ppm related to the protons of aromatic ring in 1 H NMR spectrum (Figure 3(b)) of CTA, (5) the peaks at 130.77 ppm, 132.97 ppm, and 135.45 ppm related to the carbons of aromatic ring in 13 C NMR spectrum (Figure 4(a)) of CPA, (6) the peaks at 167.71 ppm and 167.91 ppm related to carboxyl carbon of carboxylic acids, the peak at 170.12 ppm related to carbonyl carbon of ester, and the peaks at 131.30 ppm, 132.70 ppm, 133.39 ppm, 134.90 ppm, and 138.30ppm related to the carbons of aromatic ring in 13 C NMR spectrum (Figure 4(b)) of CTA.
T 1: Some important properties of chitin (Sigma C 9213).
3.1.Solubility.CPA and CTA such as chitin are soluble in 5% (w/v) LiCl/DMAc solution too.e solubility of chitin, CPA, and CTA in 5% (w/v) LiCl/DMAc solution has been changing in the order of CTA>CPA>chitin.Besides, chitin is being soluted as more gel according to CPA and CTA in 5% (w/v) LiCl/DMAc solution.ere are no other solvents in Table 1 in our laboratory.
T 3: Characteristic absorption bands of CPA and CTA.