| dc.description.abstract | The objective of this research was to utilize cellulose nanocrystals (CNC), a biobased nanomaterial, to understand their fundamental dispersion behavior and to develop CNC based films with high transparency, flexibility, and good optical qualities, which can be used in film packaging, coatings in the electronics industry, or as piezoelectric and optical sensors.
First, the research aimed at exploring the impact of electrolytes on the orientation of cellulose nanocrystals (CNC) in a mechanically shear-cast thin solid films prepared from CNC aqueous gels. Alignment in CNC films are the result of both the ease of achieving shear-induced alignment in the dispersion and the ability to retain that alignment during drying. Changes in the aqueous CNC gels’ rheological properties with electrolyte addition were correlated to the orientation and optical properties of dried CNC films. Film alignment was qualitatively assessed using cross-polarized optical microscopy and quantified by order parameters computed by UV-Vis transmission spectroscopy. Electrolyte addition resulted in an increased alignment in dried CNC films. For pure CNC, the film order parameters remained constant at approximately 0.3 for shear rates from 20 s-1 to 100 s-1. However, higher order parameters were achieved in the presence of electrolytes. Notably, an order parameter of 0.88 was achieved at a shear rate of only 20 s-1. In addition, films produced from dispersions containing electrolytes exhibited improved clarity and haze as well. The results of this work highlight that electrolyte addition can enable higher order parameters at lower shear rates and facilitate the development of aligned CNC films for applications such as polarizers, clear coatings, and piezoelectric materials.
CNCs produced from sulfuric acid hydrolysis contain an anionic sulfate ester group, which play a crucial role in the structural orientation. Thus, its effect on self-assembly was investigated in the CNC self-assembled solid film. NaOH treatment was performed on commercially purchased sulfated CNC to partially desulfate the CNC, and the self-assembled film was produced by pouring it into a Petri dish and allowing to evaporate in ambient conditions. Comparison between pure CNC and NaOH-treated CNC revealed differences in structural orientation of self-assembled solid films. Complete absence of chiral nematic organization was observed after reaction and the formation of the nematic structure was identified through SEM, cross-polarized microscopy, and UV-Vis spectroscopy. The optical properties of the films were thoroughly investigated and exhibited lowered haze and high clarity and sharpness. Lastly, polyvinyl alcohol (PVA) was added to the CNC to observe the effect of PVA on the optical quality of the film. Overall, a transparent and flexible composite film was developed, which could be utilized as a promising candidate in the film packaging application, especially in the electronic industry.
Moreover, a facile method, the freeze-thaw technique, was introduced where, without any external force or chemical reaction, a completely transparent CNC film can be produced by locking the CNC in an oriented manner. To date, self-assembled transparent CNC solid film can be obtained through chemical doping. It was found that the freeze-thaw method completely eliminated the chiral nematic structure, resulting in transparent films without structural color. Detailed internal structure characterization using SEM, XRD, and UV-Vis spectroscopy coupled with optical property analysis revealed a remarkable improvement in the clarity and sharpness of freeze-thaw based CNC films without compromising transmittance and haze, showing the promise of freeze-thaw based CNC films in packaging and coating applications.
Lastly, the study focused on utilizing cellulose nanocrystal (CNC) – polyvinyl alcohol (PVA) composites as optical sensors to detect high humidity conditions and determine water concentration in ethanol. The chiral nematic structure of CNC was used to prepare a colorimetric sensor. Upon the moisture absorption, the composites demonstrate a visual color change. A CNC-PVA sensor was developed, which can detect high humidity with 2 hours of exposure time. 2,2,6,6-tetramethylpiperidin-1-piperidinyloxy oxidized CNC (TEMPO-CNC) having carboxylic functionality was also used to prepare CNC-PVA composite films in order to compare the effect of functional groups on moisture sensitivity. Finally, we demonstrated a facile method for the utilization of the composite as an optical sensor to detect water concentration in ethanol efficiently, which can have applications in polar organic solvent dehydration. | en_US |
