Biocomposites from linters cellulose: cellulose acetate/cellulose and chitosan/cellulose films

Author(s)

Daniella Lury Morgado

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2009-12-11

Examining board members

Elisabete Frollini; Luiz Alberto Colnago; Edvani Curti Muniz

• Advisor: Elisabete Frollini

Abstract

This work was aimed at studying the chemical modification of linters cellulose extracted from a source of rapid growth and considered the most pure cellulose from vegetable sources. Derivatization was carried out in a homogeneous medium to obtain materials with well-defined properties via a reproducible method. Here cellulose acetate was obtained with various degrees of substitution (DS) using the lithium chloride/dimethylacetamide system (LiCl/DMAc), being characterized with 1H NMR, infrared spectroscopy, viscometry measurements and thermal analysis (DSC and TG). The thermogravimetric curves were analyzed quantitatively, which allowed the determination of kinetics parameters for the thermal decomposition, including the activation energy (Ea). Ea and the substitution at C2 and C3 increased with increasing DS. Cellulose acetates with distinct DS were obtained in the form of films using the solvents mentioned above. Furthermore, biocomposite films were prepared with different contents of cellulose, in which the acetates were considered as matrices and the cellulose was the loading material. It is assumed that the cellulose chains form aggregates in solution, which will be preserved in the films, thus acting as reinforcement. This hypothesis was based on previous work and confirmed here with rheological data. We show that the cellulose chains are aggregated even at low concentrations. These films were characterized using X-ray diffraction, thermal analysis (DSC, TG and DMTA), size exclusion chromatography (SEC), atomic force microscopy (AFM) and scanning electron microscopy. No residual solvent was present after film preparation. The SEM images indicated that the cellulose fibers in the biocomposite films are not visible at the microscopy scale, thus suggesting the presence of cellulose nanofibers. This is promising due to the possible enhancement in the mechanical properties, which was actually observed with a threshold percentage of only 5% of cellulose with DS 0.8. The cellulose chains apparently interacted among each other, generating supramolecular structures with aggregated chains in the LiCl/DMAc solvent. The film roughness investigated with AFM was altered by the presence of cellulose in the composite film. For the film obtained with cellulose acetate with GS 1.5, the effects from cellulose as reinforcement were only observed with higher content of cellulose (15%). According to the stress-strain tests, the films may be employed in applications requiring rigid, mechanically resistant materials. Cellulose/chitosan films were also prepared using NaOHaq./thiourea as solvent, in which chitosan served as the matrix. As in the biocomposite with cellulose acetate, the cellulose chains formed domains. The films were characterized using X-ray diffraction, thermal analysis (DSC, TG and DMTA), biodegradation tests, humidity sorption isotherms and AFM. The solvent did not affect the crystallinity of the sample, according to the XRD data. Through thermal analysis, it was inferred that the thermal stability was affected by the presence of chitosan in the biocomposite films. The study of biocomposite film degradation in a simulated soil showed that the rate of biodegradation is associated with the crystalline regions of the sample, which are more accessible to the water and the microorganisms. In other words, the higher the crystallinity the lower the biodegradation rate is. It is worth mentioning that the biodegradability also depends on the film morphology. The analysis of AFM images indicated that the film roughness increased with the content of chitosan. The results obtained with the films made with chitosan, cellulose and biocomposites (chitosan/cellulose), as well as for the films from cellulose acetate and cellulose acetate/cellulose, are promising. (AU)