Temperature-induced sol-gel transition and microgel formation in α-actinin cross-linked actin networks: A rheological study

We have studied the sol-gel transition, the viscoelastic and the structural properties of networks constituted of semiflexible actin filaments cross-linked by {α}-actinin. Cross-linking was regulated in a reversible way by varying the temperature through the association-dissociation equilibrium of the actin--{α}-actinin system. Viscoelastic parameters [shear storage modulus G{'}({ω}), phase shift tan({Φ})({ω}), creep compliance J(t)] were measured as a function of temperature and actin-to-cross-linker ratio by a magnetically driven rotating disc rheometer. G{'}({ω}) and tan({Φ})({ω}) were studied at a frequency {ω} corresponding to the elastic plateau regime of the G{'}({ω}) versus {ω} spectrum of the purely entangled solution. The microstructure of the networks was viewed by negative staining electron microscopy (EM). The phase shift tan({Φ}) (or equivalently the viscosity {η}) diverges and reaches a maximum when approaching the apparent gel point from lower and higher temperatures, and the maximum defines the gel point (temperature Tg). The elastic plateau modulus GN^' diverges at temperatures beyond this gel point TTg but increases only very slightly at T>Tg. The cross-linking transition (corresponding to a sol-gel transition at zero frequency) is interpreted in terms of a percolation model and the divergence of GN^' at TTg is analyzed by a power law of the form GN^'{~}[p(T)-pc{]}^{{γ}}$ where p(T) is the temperature dependent fraction of crosslinks formed. A power of {γ}=1.5--1.8 is found. Negative staining EM shows (1) that the gel is essentially homogeneous above the cross-linking transition (T>Tg), (2) that microscopic segregation takes place at T{}Tg leading to local formation of clusters (a state termed microgel), and (3) that at low actin--{α}-actinin ratios (r_Aα{≤}10) and low temperatures (T{≤}10 ^∘{}C) macroscopic segregation into bundles of cross-linked actin filaments and a diluted solution of actin filaments is observed. The three regimes of network structure are represented by an equivalent phase diagram. {} 1996 The American Physical Society.