Toughness improvement of epoxy thermosets with cellulose nanocrystals

Abstract

We report the toughness improvement of epoxy thermosets by the use of cellulose nanocrystals (CNCs). Pristine CNCs were functionalized via surface‐initiated reversible addition–fragmentation chain transfer/macromolecular design via interchange of xanthates (RAFT/MADIX) polymerization. The pristine CNCs were first surface‐treated with 4‐cyano‐4‐(isopropoxycarbonothioylthio)pentanoic chloride and then poly(N‐vinylpyrrolidone)‐grafted CNCs (CNC‐PVPy) were synthesized with the xanthate‐functionalized CNCs as chain transfer agent. Notably, the as‐obtained CNC‐PVPy can be dispersed into epoxy thermosets in the form of single fibrous nano‐objects, i.e. the nanocomposites of epoxy with CNCs were successfully obtained. The uniform dispersion of CNCs in epoxy matrix is attributable to the compatibilizing effect of PVPy chains. The introduction of CNCs markedly improved the toughness of epoxy thermosets. When the mass fraction of CNC‐PVPy was 30 wt%, the nanocomposites displayed the maximum critical stress intensity factor (K1C) and critical fracture energy (G1C) of 1.76 MN m−3/2 and 0.66 kJ m−2, respectively. The toughening of epoxy thermosets is mainly attributed to the plastic deformation of epoxy matrix, which is initiated from CNCs. © 2021 Society of Industrial Chemistry.

Role of cellulose nanocrystals in epoxy-based nanocomposites: mechanical properties, morphology and thermal behavior

“…The T 5% for the pure epoxy was 338 °C, and this was reduced when CNCs were added for the range of 336 to 319 °C. This behavior has already been described in the literature, in which T onset is around 320 °C and it decreased when CNCs were added to the epoxy matrix, and is due to the lower T onset temperature of the CNCs, which is around 230 °C [17,35] . Moreover, for T max , there was no relevant difference between the pure epoxy and the nanocomposites.…”

“…Table 3 also shows that the nanocomposites' tensile modulus and flexural modulus values were higher than that of the neat epoxy. This behavior indicates that the high surface area of the CNCs and the presence of O-H groups in their surface caused, at a certain level, good interaction of the CNCs with the epoxy resin molecules [17] , and includes possible chemical bonding between the CNCs and the epoxy matrix [40] . The values of elongation and tensile and flexural strengths have a tendency to reduction with the increase in the CNC concentration.…”

“…These bands refer to the hydroxyl groups adsorbed on the surface of the CNCs. This behavior indicates that there may have been a low interfacial interaction between the CNCs and the epoxy in the production of the nanocomposites [17] , although the CNCs were expected to be trapped in the cross-linked epoxy resin and completely covered by the polymer chains via hydrogen bonds [18] . The TEM images confirm low interaction and, as a consequence, the formation of clusters.…”

“…Data on the mechanical properties were obtained by tensile and flexion tests in which the nanocomposites showed an increase in the flexural modulus of approximately 50%, and of 20% in the tensile strength in relation to the polymer without CFN. Qi et al [17] produced CNC/epoxy nanocomposites using superficially modified CNCs by grafting with poly (n-vinylpyrrolidone), which significantly increased the toughness of epoxy resin and is explained by the excellent dispersion of the CNCs and its effect in promoting plastic deformation in the system. Some other works mention the modification of epoxy resin with CNCs identifying, in addition to changes in mechanical properties, changes in thermal characteristics [18] .…”

One-Pot Synthesis of UPy-Functionalized Nanocellulose under Mechanochemical Synergy for High-Performance Epoxy Nanocomposites

“…Although previous studies documented the enhancing effects of modified NCC on epoxy, the effects were unsatisfactory when the modified NCC content was 0.5 wt.% (45.22 MPa) [28]. Other studies had to work at very high modified-NCC concentrations (about 50 wt.%) to obtain similar enhancements to those of NCC-UPy at this low concentration [32,33]. This was mainly attributed to the good dispersibility of NCC-UPy in the epoxy matrix as well as the strong interfacial bonding force between NCC-UPy and the epoxy induced by their multiple hydrogen bonds, meaning that part of the load applied to the epoxy was transferred to the high-strength NCC-UPy molecular chains.…”