Polymer Nanocomposites based on UV-Curable Thiol-Ene System and Surface-Modified Cellulose Nanofibrils for Bio-Inspired Super-Hydrophobic Coatings

Thiol-ene ¿click¿ chemistry is very attractive as it enables rapid photo-polymerization reactions insensitive to oxygen presence and without side products generation, by irradiation with UV radiation. For this reason, a thiol-ene system was used in this thesis project as a polymer matrix for the production of composites. Despite this, thiol-ene polymers are characterized by very low mechanical properties. As a consequence, the aim of the thesis is the preparation of nancomposites containing cellulose nanofibrils (CNFs) as fillers in order to obtain higher mechanical performances. TEMPO-oxidized CNFs (TOCNFs) were used, due to their optimal dispersion in water, which allows them to maintain their nanometric dimensions without generating agglomerates, which is essential to maintain the transparency, typical of the chosen thiol-ene system. Following numerous experiments to optimize the processing of the raw materials, a method was chosen to achieve optimal dispersion of the TOCNFs in the thiol-ene resin, involving specific process steps such as solvent exchange, centrifugation and ultrasonification. Furthermore, due to the hydrophilic nature of cellulose, it was also necessary to apply surface modifications to the nanofibrils in order to improve compatibility with the polymer resin. The result was a fluid, homogeneous and processable resin for the production of high-quality transparent films. In addition, a complete characterization of the pure and nanocomposite resins was performed to assess flow behaviour and UV curing kinetics, as well as on the TOCNFs to evaluate their thermal stability and the effectivity of the applied surface modifications. The composite films produced were also analyzed in order to evaluate their transparency, mechanical and surface properties as a function of fillers concentration. The results show how the addition of modified TOCNFs within the thiol-ene matrix not only improves its mechanical properties such as surface hardness, but also maintains a relatively high thermal stability (Tonset = 340 ¿C) and transparency (85-90% UV-Vis transmittance), without hindering or even speeding up the photo-crosslinking reaction. Focusing on the wettability properties, it was observed that the slight hydrophobicity of the surface (78¿ water contact angle ¿WCA¿) was increased by the incorporation of the functionalized TOCNFs due to both the effect of surface roughness increase and the use of non-polar surface modifiers on cellulose nanofibrils. Finally, by considering the observed surface properties, a conceptual application, that involves the UV nanoimprint lithography (UV-NIL) technique to mimic the hierarchical surface of lotus leaves with the nanocomposite resin, was proposed. As can be seen from the SEM images, the replication of the micro- and nano-structure is satisfactory and allows to achieve impressive super-hydrophobicity (155¿ WCA) and self-cleaning properties, opening up potential use in application fields related to the use of super-hydrophobic protective and transparent coatings.