Applications of cellulose nanocrystals : thermal, rheological and mechanical properties of new materials

Abstract : The preparation of composites based on cellulose nanocrystals (CNC) is normally performed using techniques such as melt processing or casting/evaporation. In the last one, impressive mechanical properties can be reached due to the creation of a particle 3D network that is based on new hydrogen bonds between the cellulose nanorods. This process of new H-bond formation normally takes time and is dependent of the nanoparticle size and its volume fraction. Besides, the quality of filler dispersion into the polymeric matrix is also an important parameter to provide the highest surface area and provides an ideal structure for the rigid structure. In this work, we tried to propose different preparation methods and characterizations to obtain nanocomposites with a simple preparation either by casting/evaporation or melt processing.First, we improve the compatibility between an hydrophobic matrix and CNC by the chemical modification of the former. This approach tried to be an alternative to standard modification process, normally performed on nanoparticle surface by –OH groups substitution. As a good model for the study of composite mechanical properties, a natural rubber matrix was used with double bonds oxidized by the use of a strong oxidant aiming to introduce new hydroxyl groups on the isoprene chains. These new groups seem to interact with the cellulose surface, creating new supramolecular interactions between particle and matrix. It was observed that the increase in the degree of oxidation can first increase the compatibility between the polymer and the filler, increasing the mechanical properties. Later, over-oxidation starts to cause the plasticization of the system.In sequence, we had use melt processing for produce nanocomposites at high temperatures by using amorphous and semi-crystalline polymers. In a first approach, CNC were coated with the matrix polymer (polycarbonate) by a system of dilution/precipitation in suitable solvents. The coated nanocrystals are extruded with polycarbonate at 230ºC thanks to the coating approach, that allows an increase in the processing temperature of CNCs. Also, this technique improves its dispersion in the matrix due to entanglement of the polymer chains and the individual nanocrystals. The thermal analyzes shows that the CNC presence and coating masterbatch preparation reduces the polycarbonate Activation Energy (Ea) causing an acceleration in its thermal degradation and a molecular weight (Mv) reduction. As consequence, an increase in the crystallinity of the material occurs. Mechanical characterizations (DMA) show that nanocrystals presence and Mv reduction incresed the mechanical properties of the materials. The modulus (E') values, after the Tg, are higher than theoretical values calculated by Halpin−Kardos model for all studied compositions.The last part of this work is focused on investigate the influence of extrusion and thermal history of the material on its mechanical and rheological properties.In a first approach, we investigated the role of CNCs with different sizes on the material’s final properties (i.e. crystallinity and mechanical properties). Different sources were used to obtain CNCs with different percolation volume fractions, aiming to investigate its capacity on crystallization of a semi crystalline thermoplastic matrix. The results showed that crystallinity and mechanical properties are, indeed, strongly affected by the aspect ratio of the nanorod. In fact, longer CNC particles seem to be more capable to create crystalline domains and reinforce the polymer despite the lower total number of particles. In a second step, the rheological properties of the materials were investigated to characterize the effect of particle micro-structure. The composite internal organization seems to be dependent of the system viscosity and filler volume fraction, what can bring strong impact on the mechanical properties of the material.
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Marcos Mariano. Applications of cellulose nanocrystals : thermal, rheological and mechanical properties of new materials. Material chemistry. Université Grenoble Alpes, 2016. English. ⟨NNT : 2016GREAI046⟩. ⟨tel-01560014⟩

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