| Summary: | We report on a versatile method for chemically grafting multiwalled carbon nanotubes (MWCNTs) onto the surface of conventional glass fibers (GFs), as well as depositing further silica (SiO<sub>2</sub>) or superparamagnetic (SPM) magnetite (Fe<sub>3</sub>O<sub>4</sub>) nanoparticles (NPs) creating novel hierarchical reinforcements. The CNT-grafted GFs (GF-CNT) were utilized further as the support to decorate nano-sized SiO<sub>2</sub> or Fe<sub>3</sub>O<sub>4</sub> via electrostatic interactions, resulting finally into double hierarchy reinforcements. SiO<sub>2</sub> NPs were first used as model nano-particulate objects to investigate the interfacial adhesion properties of binary coated GFs (denoted as GF-CNT/SiO<sub>2</sub>) in epoxy matrix via single fiber pull-out (SFPO) tests. The results indicated that the apparent interfacial shear strength (IFSS or <i>τ<sub>app</sub></i>) was significantly increased compared to the GF-CNT. Fe<sub>3</sub>O<sub>4</sub> NPs were assembled also onto CNT-grafted GFs resulting into GF-CNT/Fe<sub>3</sub>O<sub>4</sub>. The fibers exhibited a magnetic response upon being exposed to an external magnet. Scanning electron microscopy (SEM) revealed the surface morphologies of the different hierarchical fibers fabricated in this work. The interphase microstructure of GF-CNT and GF-CNT/SiO<sub>2</sub> embedded in epoxy was investigated by transmission electron microscopy (TEM). The hybrid and hierarchical GFs are promising multifunctional reinforcements with appr. 85% increase of the IFSS as compared to typical amino-silane modified GFs. It could be envisaged that, among other purposes, GF-CNT/Fe<sub>3</sub>O<sub>4</sub> could be potentially recyclable reinforcements, especially when embedded in thermoplastic polymer matrices.
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