Non-linear rheological behaviors of polyvinyl alcohol/silver nanowire/silica nanoparticle suspensions under large amplitude oscillatory shear flows

Abstract

This study investigated the non-linear rheological behaviors of silver nanowire (AgNW) suspensions containing silica nanoparticles (SiNPs) dispersed in aqueous polyvinyl alcohol (PVA) solutions under large amplitude oscillatory shear (LAOS) flows with various methods including LAOS moduli, Fourier-transform (FT) rheology, and the sequence of physical processes (SPP). The microstructures of the suspensions depended on the ratio of SiNP and AgNW concentrations (φSi/φAg). Lower φSi/φAg ratios yielded entangled AgNWs, whereas high φSi/φAg ratios induced AgNW–SiNP bundle formation due to strong attraction between SiNPs and AgNWs. Non-linear rheological behaviors were classified into three new LAOS types (A, B, and C) based on distinct microstructures including entangled networks of individual AgNWs, stiff AgNW–SiNP bundles, and a combination of both. Type A, with dominant entangled AgNWs, displayed two-step strain thinning attributed to flocculated network formation. Network disruption aggravated the non-linearities due to strain-induced structure formations, followed by the appearance of minima in FT intensities due to disentangled and aligned AgNWs. Type B, comprising AgNW entangled networks and AgNW–SiNP bundles owing to higher SiNP contents, exhibited broad one-step strain thinning. Type C, with dominant stiff bundles, presented the simplest network structures, resulting in one-step strain thinning. Furthermore, intracycle structural changes during LAOS flows were examined using two SPP parameters: instantaneously recoverable elasticity Gt,max′ and internal area of SPP moduli. The SPP analysis also exhibited different behaviors depending on the LAOS types. Thus, employing FT rheology and SPP methods, complex microstructures containing PVA, AgNWs, and SiNPs and their structural changes during LAOS flows were investigated.