Surface chemistry driven dynamic rheology, microstructure of fumed and colloidal silica particles in nematic liquid crystals

Abstract

We investigate the influence of surface chemistry of silica particles on the dynamic rheology and underlying microstructure of silica in nematic liquid crystal (NLC), 4-cyano-4-pentylbiphenyl (5CB) suspensions. The hydrophilic fumed silica (Aerosil 200), synthesized (uncoated) and N-methyl-3-aminopropyltrimethoxysilane (MAP) coated colloidal silica particles in 5CB suspensions form gels at a critical particle concentration of 1.5 wt% (φ = 0.007), 15 wt% (φ = 0.075) and 20 wt% (φ = 0.103) respectively. On the other hand hydrophobic fumed silica (Aerosil RX300) and Dimethyl octadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP) coated colloidal silica particles in 5CB suspensions form gels at relatively higher particle loading of 3.5 wt% (φ = 0.016) and 50 wt% (φ = 0.314) respectively. The storage modulus (G′) of the Aerosil 200/5CB, uncoated silica/5CB and MAP coated silica/5CB suspensions follow power-law scaling G′ ~ φm, where m is 4.74 ± 0.5 and 5.89 ± 0.3 and 6.28 ± 0.5 respectively. In contrast, Aerosil RX300/5CB suspensions have a higher value of m (6.53 ± 0.4). Dynamic rheology data of solid gel-like Aerosil 200/5CB, uncoated silica/5CB and MAP coated silica/5CB suspensions exhibit signatures of soft glassy materials, exemplified by (a) an almost frequency (ω) independent G′ and a minima in loss modulus (G″) and (b) strain overshoot in G″, whereas Aerosil RX300/5CB and DMOAP coated silica/5CB suspensions show signatures of flocculated gels with strong dependence of G′ on ω, an absence of minima in G″ and no strain overshoot in G″. Furthermore, G′ of gel-like Aerosil 200/5CB suspension is insensitive to the nematic-isotropic phase transition (TNI) of 5CB, while the Aerosil RX300/5CB suspension shows a liquid-like behavior (G″ > G′) at temperature (T) > TNI of 5CB. We believe that the observed differences in the dynamic rheology are dictated by the surface anchoring or interactions of 5CB molecules with the –OH groups of hydrophilic silica. The microscopy results show that at T < TNI, the 5CB in Aerosil 200/5CB suspensions gets restricted in the network of Aerosil 200 nanoparticles, whereas the flocs of Aerosil RX300 nanoparticles are expelled by the nucleating 5CB domains to form a particulate network in Aerosil RX300/5CB suspensions. Moreover, the uncoated, MAP and DMOAP coated colloidal silica/5CB suspensions do not exhibit the prominent effect of T on rheology except the discontinuity in G′ and G″ at TNI of 5CB.