Nanoparticle rearrangement under stress in networks of cellulose nanofibrils using in situ SAXS during tensile testing.

This study aims to describe and evaluate the mechanism for increased strain-at-break of composites made of cellulose nanofibrils (CNFs) reinforced with nanoscopic latex particles (<200 nm) stabilized by a cationic polyelectrolyte as corona. The applied latex nanoparticles (NPs), synthesized by polymerization-induced self-assembly (PISA), are composed of a neutral core polymer, either poly(butyl methacrylate) (PBMA) or poly(methyl methacrylate) (PMMA). At room temperature, PBMA is close to its glass transition (Tg), while PMMA is below its Tg. Nanocomposites with 75 wt% CNFs and 25 wt% NPs were analyzed using in situ small angle X-ray scattering during tensile testing, monitoring the structural evolution of the NPs under strain. The scattering of the spherical PMMA NPs, which do not coalesce like the PBMA NPs, shows changes to the organization of the NPs in the CNF-network. The observations are corroborated by cross-sectional transmission and scanning electron microscopy. No distinct change from spherical to ellipsoidal shape is evidenced for the PMMA NP cores during tensile strain. Changes in anisotropic scattering produced by the three-dimensional NP structure appear to be very different between nanocomposites loaded with PMMA or PBMA NPs, contrasting commonly described two-dimensional CNF networks. The discrete PMMA NPs can reorganize within the CNF-NP double network under strain, resulting in maintained strength and increased strain-at-break. Increasing the humidity (20, 50 and 80% RH) during in situ measurements further emphasizes this effect in the PMMA composite, relative to the PBMA composite and CNF reference films. The onset of deformation occurs at strain values beyond the fracture of the more brittle films, indicating the effect of secondary nanoscale interaction available only for the PMMA composite, extending the plastic deformation and increasing the ductility. These results provide key insights into the deformation mechanism occurring during tensile testing in the CNF composites loaded with PMMA NPs.