Vapour phase modification of kevlar fibres
Azpitarte Iraculis, Itxasne
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Nature has always been a great source of inspiration for the human being in general and for material scientists in particular. Over millions of years of evolution, nature has developed different strategies to adapt the properties of biomaterials to different specific needs. Among these strategies, the intercalation of inorganic fractions into organic matrices plays a critical role in the performance of many biomaterials. These hybrid organic-inorganic materials have attracted researchers interest over the past years. Hybrid materials present unique properties, bridging the gap between the organic materials (such as polymers) and inorganics (metals and ceramics). Most of the existing approaches for the synthesis of hybrid materials are carried out in liquid phase, which has some drawbacks such as the need of post processing treatments for the elimination of the residual solvent. The vapour phase approaches have demonstrated to be an efficient and reliable alternative for the synthesis of hybrid materials. Vapour Phase Infiltration (VPI), a modification of Atomic Layer Deposition (ALD), has been successfully used for the infiltration of metal oxides into polymeric matrices, thus creating hybrid materials. The initial target of this approach was the improvement of the mechanical properties. However, it has later been demonstrated that the hybridization of polymers with metal oxides leads to the modification of many other properties. Among others, high strength polymers, such as Kevlar, are great candidates for the vapour phase hybridization with a metal oxide. These polymers have a highly ordered crystalline structure, which is responsible for their outstanding properties. This makes their post synthetic functionalization difficult. However, high strength polymers are extremely sensitive to UV and high temperatures, so they need to be protected. Coating the polymeric fibres with a resin or a metal oxide, which is the so far used strategy, shows several drawbacks such as induced brittleness of the polymer and possible detachment of the coating. In the first part of this thesis, the hybridization of Kevlar fibres with ZnO through VPI is studied and compared to ZnO ALD coated fibres. The VPI process is carried out with 200 deposition cycles at 150 ºC, using diethylzinc and water as precursors. As a consequence of the hybridization, a hybrid Kevlar-ZnO material is created in the subsurface area of the fibres. The structure of this hybrid material is theoretically and experimentally characterized, consisting of ZnO clusters grown among covalently cross-linked Kevlar chains. Thanks to the substitution of the original hydrogen bonds by N-Zn-O bonds, the infiltrated fibres show an increase of 8 ºC in their degradation temperature. Besides, the mechanical properties of the fibres are tested together with their stability against UV irradiation. The infiltrated fibres maintain 90% of the modulus of toughness of untreated Kevlar. When exposed to UV light, the untreated Kevlar fibres lose more than 50% of their toughness. The ZnO coated fibres, prepared for comparison, suffer an enhanced photodegradation when exposed to UV light as a consequence of the photocatalytic activity of the ZnO film deposited on the surface of the polymer. However, the ZnO infiltrated fibres do not show any degradation of their mechanical properties after being irradiated with UV light. Therefore, it is demonstrated that thanks to the hybridization of Kevlar fibres with ZnO through VPI the thermal and UV stability of the polymer is enhanced. In the second part of this thesis a new ALD/VPI combined process is used for the optimization of the results presented in the previous part. With this combined ALD/VPI process a polymeric substrate can be coated with a metal oxide and infiltrated with a different metal oxide within a single process. In this case, Kevlar fibres coated with Al2O3 and infiltrated with ZnO are prepared and studied. The Al2O3 coating resulted in the enhancement of the modulus of toughness by 10%, while the ZnO-Kevlar hybrid created in the subsurface area supressed the UV sensitivity of the fibres. Thus, Kevlar fibres with an enhanced modulus of toughness and UV radiation resistance are created. In the last part of this thesis, the effect of the ZnO infiltration on the electronic properties of Kevlar is analysed. Theoretical DFT calculations predict a conversion of the benzenoid rings of Kevlar to quinoid rings resulting in a shift of the energy band gap towards lower energies. The change in the ring structures is experimentally proven by nano-FTIR and the band gap shift by UV-Vis spectroscopy. The shift in the band gap results in enhanced electrical conductivity of the ZnO infiltrated fibres, which is maintained even after bending the fibres and removal of the contribution from the coating to the conductivity. The conductivity of the hybrid fibres can be tuned by changing the exposure time or the number of infiltration cycles. However, in both cases a threshold value has to be overcome to observe a significant improvement. Moreover, the generation of photocurrent when illuminating the hybrid fibres with visible light is also studied. Finally, the ZnO infiltrated Kevlar fibres show also photocatalytic activity when illuminated with visible light, further confirming the shift of the electronic band gap with the creation of the hybrid material. The photocatalytic activity, which is tuneable with the number of infiltration cycles, is studied by following the degradation of Rhodamine B.