Hierarchical self-assembly, relaxations and ergodic–non-ergodic transition in laponite ionogels

We report on the formation of entropic ionogels (gels in IL solutions) of laponite platelets in ionic liquid (IL), 1-ethyl-3-methyl imidazolium chloride, solutions at room temperature (0 < [IL] < 0.05% (w/v)). Self-assembled colloidal networks were formed following a growth mechanism given by, η r ∼ ( 1 − t / t g vis ) − p for t < t g vis ; and G 0 ∼ ( t / t g rheo − 1 ) β for t > t g rheo where ηr, G0 and t g vis , rheo represent relative viscosity, low frequency storage modulus and gelation time (determined from viscosity and rheology) respectively. The exponent p was found to increase from 0.23 to 0.85 with increase in IL concentration implying slower network growth kinetics, which helped in the formation of stronger ionogels (β decreased from 2.40 to 0.83). It was found that t g vis > t g rheo which indicated that the network, after formation, required considerable amount of equilibration time (longer for low IL concentration) to reorganize and attain a solid-like structure capable of sustaining shear deformation. Reorganization of clay platelets caused reduction in the inter platelet repulsion due the IL-double layer screening and facilitated formation of stronger ionogels for samples with [IL] ≤ 0.03% (w/v). Dynamic structure factor data revealed two primary relaxation modes: (i) the anomalous slow mode that was frozen at all IL concentrations, and (ii) the diffusive fast mode that contained two diffusion components both strongly dependent on IL concentration. Neutron scattering data could be split into two distinct scattering regions: (i) 0.001 < q < q# Å−1, I(q) ∼ q−α with α = 0.97–1.67 (Power–law region) and (iii) q# < q < 0.35 Å−1, I ( q ) ∼ e − ( q 2 R c 2 / 2 ) (Guinier–Porod region) with Rc = 4.6–5.0 Å−1. This implied existence of rod-like structures in homogeneous and rarefied clusters in fragile gels. In summary, it is shown that laponite gels could be systematically customized in IL solutions to generate a range of soft materials with properties solely dependent on IL concentration which may not exceed 0.05% (w/v).