REPOSITORY > RESULTS

Abstract

Synthesized in green plants, cellulose is the most abundant natural biopolymer on the planet and is thus widely available, cheap and renewable. Furthermore, its hierarchical structure allows extraction of nanosized particles in the form of cellulose nanocrystals (CNCs), obtained with hydrolysis or cellulose nanofibrils (CNFs), isolated through chemical, biological pretreatment and mechanical fibrillation. The latter can also be produced by some bacteria, yielding bacterial nanocellulose (BNC). All cellulose nanomaterials exhibit high specific surface and mechanical strength, that are highly desirable in numerous applications.
In the dissertation, firstly, all three types of cellulose nanomaterials were used as primary biopolymer for film fabrication or as a reinforcing agent in chitosan and alginate-based composite films. To indicate their potential as sustainable packaging material, technical specifications of the films were evaluated: tensile strength (TS) and elongation-at-break (ε), together with oxygen permeability (OTR), water vapor transmission (WVT) and water contact angle (WCA). To enable broader applications, improvement towards the water resistance through functionalization of either cellulose nanomaterials prior to their incorporation in natural biopolymer matrix or by surface treatment of the already fabricated biopolymer film is needed.
Literature on hydrophobization of cellulose nanomaterials was thoroughly reviewed to determine possible approaches to modifications as well as to identify gaps in knowledge. While routes of chemical functionalizations (esterification, silylation, carbamation, etherification and click chemistry) are well known and researched, there is a lack of thorough studies on mechanisms and kinetics of these reactions, the understanding of which is crucial in transfer to industrial scale. Furthermore, the comprehensive review revealed that although the use of plasma treatment is fast and effective for hydrophobization of cellulose (nano)materials, only very limited available scientific contributions have been presented.
With this in mind, the films based on CNFs were processed with fluorocarbon plasma, resulting in a drastic increase of WCA from initial 46° to 130° in already 30 s of treatment due to newly formed C-F3, C-F2, and C-F bonds that were identified with high resolution C 1s XPS, Raman and ATR-FTIR spectroscopy. The treatment was extended to chitosan-based films with incorporated CNCs, studying stability of properties relevant for packaging applications as well (TS, ε, WVT and WCA) over the course of 30 days after hydrophobization. Additionally, no leaching of fluorine components into liquid environments was found through LC-MS analysis. While the treatment with RF-generated plasma in fluorocarbon was found to be ultrafast and providing stable hydrophobic coating, fluorinated compounds might cause a disturbance in metabolism at higher concentrations, therefore in this work we aimed to find a substitute gas. It was discovered that plasma generated in N2 provides the same result of hydrophobic surface while avoiding fluorine-related compounds.
To fulfil the other objective, contribution to the knowledge on mechanisms and kinetics of functionalization reactions, an esterification reaction of cellulose nanomaterials with acetic anhydride in the presence of pyridine was revisited. A combined computational and experimental study of this reaction was carried out on both CNCs and CNFs, indicated two competitive reaction mechanisms and yielded kinetic parameters through microkinetic modelling of both materials. To demonstrate the practical use of surface acetylation of cellulose nanomaterials, modified CNCs of various degrees of substitution were incorporated into alginate and chitosan films that were subjected to various environmental humidity to evaluate its effect on TS, ε, moisture content and WVT. Finally, to gain an insight into the end-of-life of such films, biodegradation of alginate and chitosan-based with pristine and acetylated CNCs in activated sludge was followed through the course of 5 days through respirometry in OxiTop system.