Chemical modification of cellulose for electrospinning applications

Abstract

The aim of the thesis is to develop technology for producing cellulose fatty acid esters that later will be used to produce fibrous materials by means of electrospinning. Main material of the study is cellulose-stearate which is a polymer synthesised by reaction between stearoyl chloride and cellulose. The experimental part consists of synthesis of it by chemical modification of cellulose using ionic liquid as a reaction media. In addition, ionic liquid is also synthesised from the beginning. Various syntheses are carried out in order to obtain polymers with different degree of substitution because it is known that it affects solubility a lot. Three ratios cellulose: stearoyl chloride are tested as well as the use of catalyst or not. Those tested are 1:1, 1:3 with and without catalyst and 1:5 with and without catalyst too. Moreover, high purity reagents for producing IL and CS are purchased (methylimidazole and stearoyl chloride) in order to compare its use in CS obtention. The synthesis chosen for repeating is 1:5 without catalyst. All samples are characterized by means of differential scanning calorimetry (DSC), infra-red spectroscopy (FTIR) with ATR accessory and saponification method for determining the degree of substitution. As expected, as bigger the ratio is, bigger is the DS obtained but one important point is that the results found are all under 1 what means that the DS is really low. Using catalyst or not does not affect to DS and it does not modify the properties so it is better to use it because reaction time becomes half (1,5h). The method for determining that is a manual test based in a saponification reaction and a back titration. From the calorimetry test it is observed that the main impurity remaining in the samples after washing them with methanol is stearic acid. Separate it from CS is difficult because this compound may not be enough soluble in this organic solvent. The FTIR spectra are a useful technique to control if the reaction is being done or not. In the cellulose stearate graphs obtained, the typical peaks from ester (1740 cm-1 for C=O and 1240 cm-1 for C-O-C) can be found but they cannot be found in cellulose spectra, as expected. Comparing cellulose spectrum and a derivative one it can be seen that the peak corresponding to bond C-O from the hydroxyl groups (1000-1100 cm-1) ecreases while the other ones from ester mentioned before increases. With these observations it can be said that the reaction has taken place, thus the polymer has been obtained. Another important point is to find appropriate solvent or solvent mixture and spinning conditions for obtaining the fibrous materials with controllable parameters. The technique used for producing fibres, called electrospinning, is nowadays an important field of research and it is being developed for future applications related with nanoscale fibers diameter. It is based in the obtention of fibers by applying a difference of voltage between mixture supplier (syringe) and the collector. The attempts with this method have not been successful at all so no fibers have been obtained (some more experiments are required for finding the optimum electrospinning conditions). Anyway, while doing these tests two suitable solvents for the polymer have been found, toluene and chloroform. The fact that they are highly volatile makes electrospinning more difficult. The range of polymer concentration in the mixtures has to be between 11-13%, more than that makes the mixture become jelly and less makes it not enough viscous. Definitely, using high purity reagents (especially pure stearoyl chloride) makes DS increase a lot. Testing the solubility of the polymers obtained with these conditions showed that are highly soluble in a lot of organic solvents. This result leads to think that DS is much more bigger in polymer obtained with high purity reagents than in technical ones. It is more important that stearoyl chloride is high purity one than the IL reagents. All in all, this polymer, as well as electrospinning technique, should be further studied in order to obtain the fibers expected.