Preparation And Properties Of Funtional Cotton And Novel Aerogels Based On Nano-structured ZnO And SiO₂

With the development of society and the improvement of standard living, people's demands for multi-functional textiles have become increasingly stronger. The rapid development of nanotechnology provides advantageous conditions to the functionalization of textiles. ZnO is an important direct wide band gap semiconductor. Applying one dimensional nano-structured ZnO materials onto textiles can endow the textiles with UV-blocking, antimicrobial and self-cleaning properties. Furthermore, SiO2 aerogel is such a kind of solid materials which consists of SiO2 nanoparticles and has a continuous random network structure filling with gaseous dispersive medium. Due to its unique structure, SiO2 aerogel is currently the world's best heat insulator. The textiles incorporated with SiO2 aerogel show excellent thermal insulation and inflame-retarding effects. Therefore, investigating the preparation and properties of functional textiles and novel aerogels based on nano-structured ZnO and SiO2 is very important. A series of studies have been carried out, and the major studies include:(1) In situ growth of ZnO nanorod arrays on cotton fibers.By hydrothermal method, oriented ZnO nanorod arrays were vertically grown onto the surface of cotton fibers which were precoated with layers of ZnO nanocrystals. The effects of the precoated ZnO nanocrystals, the concentration of the hydrothermal solution, hydrothermal temperature, hydrothermal reaction time on the preparation and morphology structure of ZnO nanorod arrays were investigated. The results show that the ZnO nanocrystals onto the cotton fibers are the prerequisite to ZnO nanorod growth. Morphology control of the ZnO nanorod arrays can be achieved by adjusting the hydrothermal reaction conditions. The grown ZnO nanorod arrays on cotton fibers endow the cotton fabrics with remarkable UV-blocking property.(2) The fabrication of microscale rough structures on cotton fibers and the corresponding superhydrophobicities.Different microscale rough structures were fabricated onto the surface of the cotton fibers by SiO2 nanoparticles and ZnO nanorod arrays. Then superhydrophobic cotton fabrics were prepared by subsequent modification with low surface energy material. Static contact angle and roll-off angle measurements show that SiO2 modified cotton fabric displays a higher contact angle while the ZnO modified cotton fabric shows a much smaller roll-off angle. It is believed by us that the difference in roll-off angle is related to the solid/liquid/gas three-phase contact line. Distorted and non-continuous solid/liquid/gas three-phase contact line is more easily formed for the ZnO modified cotton fabric when the water droplet contacts with the ZnO nanorod arrays, and therefore the water droplet is more easily to roll off from the ZnO modified cotton fabrics.(3) Preparation of elastic and superhydrophobic SiO2 aerogel by ambient pressure drying.Methyltrimethoxysilane (MTMS) derived aerogel was prepared by controlling the reaction temperatures via ambient pressure drying. Tedious solvent exchange and hydrophobic modification which are commonly used during SiO2 aerogel preparation by ambient pressure drying were avoided. The studies show that the ageing temperature and gelation temperature have great influences on the preparation of SiO2 aerogel. Increasing the ageing temperature will further improve the SiO2 alcogel network and strengthen the gel network skeleton, enhancing the resistance of gel network against capillary forces generated during the ambient pressure dyring and resulting in the decreased volume shrinkage and bulk density. Increasing the gelation temperature leads to larger pore sizes within gel network. Low density and low volume shrinkage SiO2 aerogel can be obtained by adjusting the gelation temperature and controlling the pore structure within the gel network. In addition, MTMS derived SiO2 aerogel shows excellent elasticity and superhydrophobicity.(4) Preparation of ambient pressure dried hybrid aerogel with enhanced mechanical property.Methyltrimethoxysilane (MTMS) and vinyltrimethoxysilane (VTMS) derived aerogel was prepared by ambient pressure drying. The studies show that the pore structure is the key factor in determining the morphology and properties of SiO2 aerogel by ambient pressure drying. If the gel network is dense, the capillary pressure effects during ambient pressure drying are more significant, leading to cracks. If the gel network is loose, the spaces between SiO2 clusters increase, reducing the impact of capillary effect on the gel network during ambient pressure drying. This is beneficial for the preparation of crack-free and monolithic SiO2 aerogel. However, too large pore sizes will decrease the overall connectivity of the SiO2 clusters, weakening the physical and mechanical properties of SiO2 aerogels and the prepared SiO2 aerogel is very fragile. The precursor concentration and gelation temperature have great influences on the pore structure of gel network. Therefore, adjusting the precursor concentration and gelation temperature is crucial for preparing SiO2 aerogel. In addition, MTMS/VTMS based SiO2 aerogel shows excellent elasticity and no obvious shrinkage was observed after repeated compression.To further improve the mechanical property of SiO2 aerogel, polystyrene (PS)/ SiO2 hybrid aerogel was prepared by adding styrene into the MTMS/VTMS derived alcosol. The prepared PS/SiO2 hybrid aerogels keep low bulk density and high porosity. The as-obtained hybrid aerogels show an increased cluster feature size and a decreased specific surface area with the increased volume of styrene. FTIR and solid state NMR spectra show that polystyrene was deposited onto the surface of the SiO2 particles within the gel network. The compression testing shows that the PS/SiO2 hybrid aerogel is elastic and the strength of the hybrid aerogel is drastically increased with the increase of the added styrene. Furthermore, the hybrid aerogel shows superhydrophobicity.