SUBSECRETARIA DE CONTABILIDAD GUBERNAMENTAL a) Misión

8.1. Conclusions

The preparation of nano-sized cellulose and chitin crystals was successfully carried out by means of acid hydrolysis. TEM and AFM images indicated that the cellulose nanowhiskers produced have dimensions in the range of 50-150 nm (length) and 5-10 nm (width) and chitin nanowhiskers have lengths in the range of 50-200 nm and widths of 5-15 nm. The increase in crystallinity for the obtained cellulose nanowhiskers proved using the absorbance spectrum from FTIR. The ratio that was used to calculate the crystallinity from the FTIR spectra increased from 1.07 for MCC to 1.23 for cnw. TEM, POL and fluorescence microscopy results indicated that freeze dried cellulose and chitin nanowhiskers do not redisperse as well in water as the nanowhiskers which were kept in the water suspension. This observation may be due to strong hydrogen bonding once the whiskers have dried or the loss of ester sulphate groups from the freeze drying process.

Up to 10 wt% cellulose and chitin nanowhiskers were successfully incorporated into an EVOH27 and EVOH44 matrix by way of solution casting and electrospinning. TEM and FTIR analysis results of the produced EVOH/cnw and EVOH/chnw nanocomposites indicated successful incorporation of cellulose and chitin nanowhiskers. TEM images revealed good dispersion of nanowhiskers but agglomeration were definitely present at higher nanowhisker loadings. The electrospun EVOH/cnw and EVOH/chnw fibers were more challenging to analyse with TEM but the nanowhisker dispersion was still easily observed especially for the samples that were sectioned in length. The nanowhiskers were observed to be clearly orientated in the direction of the fiber axis which probably resulted from the electrostatic pull of the EVOH/cnw solution during electrospinning. Embedding the nanocomposites samples in resin and staining the ultra microtomed samples for proper observation under TEM were found to be the most effective method in observing nanowhisker dispersion.

During thermal characterization an increase in thermal stability was observed as well as an increase and decrease in percentage crystallization of EVOH/cnw and EVOH/chnw nanocomposites. The dispersion of nanowhiskers may cause a hindrance for the EVOH molecules to crystallize or even changing the formation of crystallization. It seems that in most cases, the lower nanowhisker loads act as nucleating agents, thus increasing the percentage crystallization in the EVOH nanocomposites. The chitin nanowhiskers seem to behave more unpredictable than the cellulose nanowhiskers due to their more hydrophobic character as well as stronger hydrogen bonding between chitin molecules than in the case of cellulose.

Thermogravimetric analysis results of the nanocomposites showed that the increase in thermal stability at high temperatures could be partly attributed to the presence of acid, causing chemical

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modifications. These observations were seen for both solution cast and electrospun EVOH/cnw nanocomposites. Some thermal behaviour is observed that does not indicate any trend and can probably be ascribed to the extremely small dimensions of cnw and chnw as well as the strong hydrogen interactions between nanowhiskers.

Preliminary DMA analyses revealed an increase in the storage modulus for the EVOH44/cnw nanocomposites which indicates that the cellulose nanowhiskers are increasing the stiffness of the matrix by decreasing the mobility of EVOH polymer chains. The mechanical analysis results also showed greater improvement in the storage modulus for the EVOH44/cnw(sus) nanocomposites which may be explained in terms of good dispersion of the nanowhiskers within the matrix.

Agglomeration of the nanowhiskers was observed during fluorescent microscopy but seemed to be evenly distributed in the EVOH/cnw nanocomposites. The results obtained are not yet conclusive but can be used as a basis for further investigations.

The last stage of the project involved the incorporation of electrospun EVOH fibers with and without cellulose nanowhiskers into a LDPE matrix. The aim was to investigate the changes in tensile properties for the LDPE/EVOH and LDPE/EVOH/cnw composites. Tensile results showed a general increase in tensile strength and Young’s modulus, especially for LDPE/EVOH27, with a decrease in elongation. The SEM images of tensile fractured surfaces agreed well with the stress strain results. The compatibility between EVOH27 and LDPE was clearly seen by less pull outs if any, or loose hanging fiber strains.

The thickness of the fiber mats seemed to play a significant role in the enhancement of mechanical properties. The thickness of the fiber mats may depend on several parameters including the solution concentration and the type of polymer. The volume and the fiber diameters should therefore be controlled and taken into consideration when the electrospun EVOH and EVOH/cnw fiber mat is incorporated into a LDPE matrix.

8.2 Recommendations for future work

A fluorescent microscope with higher resolution will especially make a more valuable contribution towards the investigation of nanowhisker dispersion within a polymer matrix

The cellulose nanowhiskers were found to contain acid traces on the surface even after extensive washing and treatment with a strong base. Better neutralisation of the cellulose nanowhiskers can be investigated further. Another approach is however to investigate different methods of nanowhisker production in order to avoid degradation during processing at high temperatures.

EVOH is often used in the film packaging industry where it gets sandwiched between hydrophobic polyolefin layers. Studies on water absorption and gas permeability of EVOH/cnw nanocomposites are therefore recommended to investigate.

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Further and detailed investigation regarding the incorporation of EVOH/cnw into a LDPE matrix can be carried out incorporating cnw loadings of more than 10 wt% into the EVOH fibers.