(188j) Molecular Dynamics Analysis of Chitosan/Carbon Nanotubes Composite

Bionanocomposites, a new group of materials are the result of the combination of biopolymers and inorganic solids at the nanometer scale. These hybrid organic?inorganic materials are obtained from combinations of biopolymers and different inorganic solid particles such as metal nanoparticles or carbon nanotubes. The importance of bionanocomposites is based on the fact that biopolymers are biocompatible and biodegradable compounds and, therefore, their composites are of great interest for advanced biomedical materials, as for instance tissue engineering, membranes, sensors, artificial bones or gene therapy, etc. In this context, an attractive combination is produced by chitosan (CS) and carbon nanotubes (CNTs), since carbon nanotubes can provide improved mechanical strength and better structural integrity especially under physiological condition. This combination is not new, however CS/CNTs biocomposites reports are mainly focused on their preparation, structural characterization, mechanical properties and application. However their relaxations processes are scarcely reported, there exist only few reports about the Dynamical Mechanical Analysis (DMA) relaxation processes and none about their analysis by dielectric measurements. The knowledge about relaxations processes in bionanocomposites is very important for their application in the different areas that has been tested until now, for the future ones and for optimizing its processing parameters as well. That is why the aim of this work is to investigate thermal relaxation properties of the CS/CNTs biocomposite, to get a better understanding about the nature of its molecular dynamics. The relaxation processes analysis in materials give rise to get information about the reorganization of the structure in composite materials and about the nature of their molecular dynamics, such is the case of the glass transition temperature (T_g), an important parameter during processing of materials. Regarding the T_g of chitosan, there exists a big controversy about the value of the glass transition temperature in neat chitosan. The reported values range from 30 to 222 °C, while other authors have not observed a glass transition. The glass transition temperature of chitosan can be significantly affected by the addition of carbon nanotubes changing its thermal relaxation behavior. This work addresses the molecular dynamics of chitosan/ carboxy-functionalized multiwall carbon nanotubes (CS/MWCNT's) thin films composite analyzed by means of dielectric spectroscopy and dynamic mechanical analysis (DMA). Chitosan films with MWCNT's (0.5, 1, 3, 5 and 10 wt. %) composites were prepared by the solvent cast method. Their mechanical and electrical properties have been investigated as a function of water content in the 25 to 330°C temperature range. Three relaxation processes have been observed at different temperature ranges. The low temperature relaxation process (between 25 and 70°C) can be related to the α-process associated with the glass-rubber transition, it can be described by the Vogel-Fulcher-Tammann dependence of the relaxation time versus reciprocal temperature. The glass transition temperature is strongly affected by water content and it vanishes in the dry material. The second relaxation process was observed in the 70-190°C temperature range and it is identified as the σ-relaxation process which is often associated with the hopping motion of ions in the disordered structure of the biomaterial. This relaxation exhibits a normal Arrhenius-type temperature dependence with activation energy of 81-86 kJ/mol and it is independent of water content. Finally, a relaxation process related to film degradation was observed in the 200-300°C temperature range.