Grafting of Chitosan and Chitosantrimethoxylsilylpropyl Methacrylate on Single Walled Carbon Nanotubes-Synthesis and Characterization

Abstract

Acid functionalized single walled carbon nanotubes (CNTs) were grafted to chitosan by first reacting the oxidized CNTs with thionyl chloride to form acyl-chlorinated CNTs. This product was subsequently dispersed in chitosan and covalently grafted to form CNT-chitosan. CNT-chitosan was further grafted onto 3-trimethoxysilylpropyl methacrylate by free radical polymerization conditions, to yield CNT-g-chitosan-g-3-trimethoxysilylpropyl methacrylate (TMSPM), hereafter referred to as CNT-chitosan-3-TMSPM. These composites were characterized by Fourier Transform Infrared Resonance Spectroscopy (FTIR), carbon-13 nuclear magnetic resonance (¹³C NMR), Thermogravimetric Analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composite showed improved thermal stability and could be of great potential use in bone tissue engineering.

1. Introduction

CNTs have attracted recent attention as suitable material for use as reinforcing materials for polymer matrices since their discovery in 1991. Their nanometer size, high aspect ratios, and more importantly their excellent mechanical, electrical and conducting properties make them highly suitable in biomaterial fabrication for tissue engineering [1]. CNTs are generally insoluble and severely aggregate due to Van der Waals attraction among the nanotubes, creating a problem when homogenous dispersion is desirable. To overcome this barrier, various physical and chemical approaches have been explored including direct suspension of CNTs in polymer solution via sonication, in situ polymerization in the presence of CNTs [2–5], and the chemical modification of CNTs to enhance solubilization. Overall, the grafting of CNTs to polymer through covalent functionalization appears to be one of the most attractive and effective ways to achieve homogeneous dispersion of CNTs in polymer matrices in order to make high quality nanocomposites. This technique has been used to make nanocomposites of CNT-polymers, mostly utilizing synthetic based polymers [6–8]. The use of natural biopolymer grafted to carbon nanotube is rare. Chitosan, a N-deacetylated derivative of chitin, consist of 2-amino-2-deoxy (1–4)-β-D-glucopyranose residue (or D-glucosamine units) and is derived from the partial alkaline deacetylation of chitin. Chitin, the precursor of chitosan, is a cellulosic type biopolymer which is widely distributed in nature, especially in the shell of crustaceans, the cell wall of fungi and exoskeleton of insects. Chitosan has been developed as new bioactive materials since these possess various biomedical activities [9, 10]. Chitosan’s solubility and poor mechanical properties limit its widespread applications. Chitosan is insoluble in water but dissolves in aqueous solutions of organic acids like acetic, formic and citric acids. It is attractive due to its biocompatibility, biodegradability, nontoxicity and exhibits excellent film-forming ability. It can be used as a modifier due to the abundance of -NH₂ and -OH functional groups which renders it ideal for a variety of chemical modifications.

Although some successful carbon nanotube chitosan based nanocomposites have been reported, to our knowledge, most of them were obtained by non covalent interactions such as blending [11], layer-by-layer self-assembly [12], surface deposition and crosslinking [13] and electrochemical deposition on the surface of CNTs [14]. Covalent grafting of CNT to chitosan will produce more stable composite, prevent leaching of materials during applications, improve hydrophilicity depending on the functionality introduced and ensure long term stability of CNT in the media.

In this study, we have taken advantage of the existence of some of the free amino groups on chitosan, to graft functionalized carbon nanotubes, which were further grafted to 3-trimethoxysilylpropyl methacrylate using free radical polymerization conditions, to yield CNT-chitosan-3-TMSPM. The chemical modifications were confirmed using FTIR, ¹³C-NMR and TGA.

2. Experiment

2.1 Materials

Low molecular weight chitosan (91.3% deacetylated), potassium persulfate (K₂S₂O₈), sulfuric acid (H₂SO₄), nitric acid (HNO₃), thionyl chloride and 3-trimethoxysilylpropyl methacrylate were obtained from Sigma Aldrich. High purity single walled CNTs were purchased from Helix Materials Solutions.

2.2 Instruments and Measurements

FTIR was run using IR-200 Thermo-Nicolet 2.2 in the range of 400–4000 cm⁻¹ to confirm the presence of functional groups in chitosan and CNTs derivatives. Freeze-dried samples and dried potassium bromide (KBr) were used to prepare pellets. The thermal stability was analyzed in a Thermogravimetric analyzer (TGA) Q500 using Universal V3.4C TA Instruments program to analyze the resulting scan. The triplicate runs were performed in the temperature range of 30 °C to 800 °C and consisted of a ramp at a steady rate of 20 °C/min. The nitrogen flow rate was maintained at 60 mL/min. SEM procedure was performed using FEI Quanta 600 FE-SEM at 10 kV or using JEOL JSM at 10kV. Specimens for high-resolution imaging were coated with 4 nm Pt/Pd layers using Cressigton 208HR. The FE-SEM acquisition at Texas A&M University was supported by the NSF grant DBI-0116835, the Vice President for Research Office, and the Texas Engineering Experiment Station. Solid-state NMR experiments were performed Dr. Vladimir Bakhmoutov at NMR Laboratory, Department of Chemistry, Texas A&M University, College Station, Texas. Experiments were performed with a Bruker Avance-400 spectrometer equipped with a standard 4 mm MAS probe head. The ¹³C NMR spectra were recorded at spinning rates of 11 kHz with a CP-pulse sequence. Chemical shifts were referenced to TMS (external).

2.3 Preparation of Degraded Chitosan and Grafted Chitosan Derivatives

2.3.1 Purification

Ten grams (10 g) of chitosan was dissolved in 200 mL of 1% acetic acid solution, then filtered, using glasswool to remove insoluble sample. Two hundred (200) mL of 2 M sodium hydroxide (NaOH) was added to the filtered sample with continuous stirring. After removal of excess NaOH solution, the filtrate washed with distilled water until a pH 7.0 was reached.

2.3.2 Preparation of Grafted Chitosan Derivatives

Chitosan (0.5 g) was reacted with K₂S₂O₈ (0.02 g) for 20 hours before adding 3-TMSPM (0.0547 mole) for an additional 2 hours. Each reaction was stopped with 10 mL tetrahydrofuran before freeze-drying. The dried product was washed continuously with methanol to remove unreacted residue. The presence of 3-TMSPM was confirmed using FTIR and solid state NMR.

2.4 Preparation of Carbon Nanotube Derivatives

2.4.1 Synthesis of Functionalized CNTs

CNTs were synthesized as described in recent literature [15, 16]. Typically, CNTs were reacted with H₂SO₄:HNO₃ (3:1), then tip sonicated for 30 minutes using an ultrasonic processor with amplitude at 30% and 7s pulse to yield carboxylic acid functionalized CNTs (CNT-COOH). The carboxylic acid group was converted to formyl chloride via reaction with thionyl chloride for 24 h at 75 °C while refluxing, hereafter product is referred to as CNT-COCl. After the reaction was stopped, reaction mixture was cooled before centrifuging and washing to remove excess reactants. Samples were dried overnight at 90 °C and 30 in Hg. The presence of functional groups was confirmed using FTIR.

2.4.2 Synthesis of CNT-chitosan Derivative

CNT-COCl (400 mg) was reacted with chitosan (2 g) in 100 mL 2% acetic acid at 75 °C for 24 hours while stirring. After the reaction was stopped, the product was washed three times with 2% acetic acid to remove unreacted chitosan.

2.4.3 Synthesis of CNT-chitosan Grafted Derivative

CNT-chitosan (0.1 g) was reacted with K₂S₂O₈ (0.02 g) and 3-trimethoxysilyl propyl methacrylate (6 mL) in 2% acetic acid solution at 75 °C for 2 hours. Product was centrifuged at 20,000 rpm and washed twice with water before drying at 90 °C. Hereafter, the product will be referred to as CNT-chitosan-3TMSPM.

3. Results and Discussion

3.1 FTIR

Peaks for chitosan and CNT derivatives were as indicated by the incorporation of 3-TMSPM onto chitosan and CNT-chitosan (Fig. 2). The broad and strong band between 3200–3600 cm⁻¹ was attributed to overlapping -OH and -NH stretching while the strong broad band between 3300–3500 cm⁻¹ was characteristic of -NH stretching vibration. Other peaks which were characteristic of chitosan included absorption bands at 1153 cm⁻¹ due to asymmetric stretching of the C-O-C bridge (β (1,4) glycosidic bonds), 1091 cm⁻¹ due to chitosan’s saccharide moiety [17] and 2950 cm⁻¹ due to -CH stretching and bending of -CH₃ [18]. Native chitosan showed the presence of -C=O of -NHCO- near 1650 cm⁻¹ [6, 19] but when 3-TMSPM is grafted onto chitosan, the peak at 1650 cm⁻¹ disappears and a noticeable peak near 1637 cm⁻¹ appears and indicates the presence of 3-TMSPM. This also occurred when comparing FTIR scans for CNT-chitosan and CNT-chitosan-3-TMSPM. FTIR for chitosan-3-TMSPM (Fig. 2b) and CNT-chitosan-3-TMSPM (Fig. 2f) indicated the attachment of 3-TMSPM to chitosan as the presence of a strong peaks at 1720 cm⁻¹. Further evidence for the incorporation of 3-TMSPM was the presence of an increased intensity of the absorption peak at 2950 cm⁻¹ when compared to scans in Fig. 2a versus 2b and Fig. 2e versus 2f, and the strong peaks between 900 cm⁻¹ and 1,100 cm⁻¹ which were associated with Si-O-Si vibrations of the siloxane groups.

Fig. 2.

FTIR scans for (a) Chitosan, (b) Chitosan-3-TMSPM, (c)CNT-COOH, (d)CNT-COCl, (e) CNT-Chitosan and (f) CNT-chitosan-3-TMSPM.

3.2 Solid State ¹³C-NMR

Solid state ¹³C-NMR was performed by Dr. Vladimir Bakhmoutov at NMR lab at Texas A&M University. ¹³C-NMR analysis for chitosan (Fig. 3a) indicated the presence of -C=O group absorption peak near 175 ppm as a result of 91% deacetylation and a small amount of chitin [20]. The peak near 20 ppm (Fig. 3a and 3b) was attributed to the -CH₃ group of the chitosan while the peak absorption near 30 ppm in Fig. 3b showed the presence of -CH3 of 3-TMSPM and confirmed the attachment of 3-TMSPM onto chitosan as observed in earlier work [21].

Fig. 3.

¹³C-NMR of a) pure chitosan (top) and b) chitosan-3TMSPM (bottom).

3.3 TGA

The thermal stability of chitosan and its derivatives was analyzed using thermogravimetric analysis (TGA). The attachment of 3-TMSPM to chitosan and to CNT-chitosan is quite evident as the weight losses are increasingly noticeable (Fig. 4). In the scan, virgin chitosan starts to degrade near 250 °C. This is attributed to the degradation of and deacetylation of chitosan [22, 17]. Water evaporation occurred in all scans of chitosan derivative near 80 °C and is in agreement with the literature. A rapid degradation is observed for CNT-chitosan-3-TMSPM derivative near 400 °C which was attributed to the presence of silyl groups of the hybrid derivative. The presence of one endotherm for all peaks indicated that all samples were homogeneous.

Fig. 4.

TGA curves for chitosan and its CNT derivatives.

3.4 SEM

SEM was performed to assess the morphology of the CNTs and chitosan derivatives. While the SEM for chitosan resembles previous results [21], it also indicated the attachment of chitosan to the functionalized CNT as indicated by the thin strings in scans Fig. 5d and further attachments between CNT-chitosan-3-TMSPM was seen in Fig. 5e.

Fig. 5.

SEM micrographs for a) chitosan, b) CNT-COOH, c) CNT-COCl, d) CNT-chitosan and e) CNT-CH-3TMSPM.

3.5 TEM

TEM was used to give an indication of the attachment of chitosan and eventually chitosan-3-TMSPM to the functionalized CNT. TEM (Figs. 6d–e) indicated that coatings were clearly visible on the surface of CNTs and that the functionalized CNTs had attached to the chitosan surface. Energy Dispersive Spectroscopy (EDS) scans showed the presence of atoms present in each sample and indicated the presence of impurities. For example, Fig. 7 (upper) indicated the existence of sulfur (S) which is presence in the catalyst to initiate degradation of chitosan and was also seen in EDS of CNT-chitosan. Sodium (Na) atoms were also present in scans and were attributed to the NaOH used in the purification of chitosan. A silicon peak was observed in scans of CNT-chitosan-3-TMSPM but its presence in others scans was attributed to contamination of the grid used for performing TEM.

Fig. 6.

TEM scans for (a) chitosan, (b) CNT-COOH, (c) CNT-COCl, (d) CNT- chitosan and (e) CNT-chitosan-3-TMSPM. (scale -50 nm).

Fig. 7.

EDS scans for chitosan and CNT-chitosan-3-TMSPM.

4. Conclusions

The covalent grafting of chitosan and chitosan-3-TMSPM to CNTs has been accomplished. This composite showed improved thermal stability compared to neat chitosan or chitosan-3TMSPM and suggested other potential uses.

Fig. 1.

The reaction scheme for synthesis of CNT-chitosan-3-TMSPM.