Tensile mechanical behavior and fracture toughness of MWCNT and DWCNT modified vinyl-ester/polyester hybrid nanocomposites produced by 3-roll milling

This study aims to investigate the tensile mechanical behavior and fracture toughness of vinyl-ester/polyester hybrid nanocomposites containing various types of nanofillers, including multi- and double-walled carbon nanotubes with and without amine functional groups (MWCNTs, DWCNTs, MWCNT-NH2 and DWCNT-NH2). To prepare the resin suspensions, very low contents (0.05, 0.1 and 0.3 wt.%) of carbon nanotubes (CNTs) were dispersed within a specially synthesized styrene-free polyester resin, conducting 3-roll milling technique. The collected resin stuff was subsequently blended with vinyl-ester via mechanical stirring to achieve final suspensions prior to polymerization. Nanocomposites containing MWCNTs and MWCNT-NH2 were found to exhibit higher tensile strength and modulus as well as larger fracture toughness and fracture energy compared to neat hybrid polymer. However, incorporation of similar contents of DWCNTs and DWCNT-NH2 into the hybrid resin did not reflect the same improvement in the corresponding mechanical properties. Furthermore, experimentally measured elastic moduli of the nanocomposites containing DWCNTs, DWCNT-NH2, MWCNTs and MWCNT-NH2 were fitted to Halphin–Tsai model. Regardless of amine functional groups or content of carbon nanotubes, MWCNT modified nanocomposites exhibited better agreement between the predicted and the measured elastic moduli values compared to nanocomposites with DWCNTs. Furthermore, Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were used to reveal dispersion state of the carbon nanotubes within the hybrid polymer and to examine the CNT induced failure modes that occurred under mechanical loading, respectively. Based on the experimental findings obtained, it was emphasized that the types of CNTs and presence of amine functional groups on the surface of CNTs affects substantially the chemical interactions at the interface, thus tuning the ultimate mechanical performance of the resulting nanocomposites.