International audienceThe article is a critical review of all aspects of the dissolution of cellulose in NaOH-based aqueous solutions: from the background properties of the solvent itself, to the mechanisms of cellulose fibre swelling and dissolution, solution structure and properties and influence of additives and, finally, to the properties of various materials (fibres, films, aerogels, composites and interpenetrated networks) prepared from these solutions. A historical evolution of the research on this topic is presented. The pros and cons of NaOH-based aqueous solvent for cellulose are summarised and some prospects are suggested

The rheology of rigid rod cellulose whisker suspensions has been investigated. The isotropic-at-rest suspension flow curves show two plateaus, one at low shear rates and another at high shear rates, that reflect the flow of isotropic (at low shear rates) or oriented (at high shear rates) suspensions. From the low shear rate viscosity plateau vs concentration, we see that the system is in the semidilute region above 0.02 wt %, in agreement with the theoretical predictions. From the same curve, we can estimate a maximum packing concentration of whiskers rods that corresponds to the experimentally measured isotropic-to-anisotropic transition. The high shear rate plateau viscosity data show that the suspension is still in the dilute state above 0.6 wt %. The critical concentration at the dilute−semidilute transition is thus strongly dependent on the state of order, which suggests that care has to be taken when measuring parameters extracted from flowing solutions or suspensions, as for example occurring with Ubbelhode viscometry. Above a certain critical concentration, the suspensions become anisotropic at rest. The viscosity vs concentration curve has a maximum that vanishes at high shear rates as for liquid crystalline polymer solutions. Rheological and rheo-optical observations show fast inception and relaxation of both the rheological functions and the texture, in complete contrast with liquid crystalline polymer solutions.

The shear rheology of a microcrystalline cellulose dissolved in a 9% NaOH aqueous solution was studied in the steady and oscillatory modes. The cellulose-(9% NaOH-H(2)O) mixtures show not to be true solutions. In the dilute regime, with cellulose concentration below 1%, the rheological behavior is typical of the one of suspensions. The formation of cellulose aggregates is favored when temperature is increased. In the semidilute regime, an irreversible aggregate-based gelation occurs, being faster with increasing temperature.

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The aim of the paper is to investigate the structure of solutions of microcrystalline cellulose in NaOH/water mixtures and to determine the limit of cellulose solubility. The binary NaOH/water and the ternary cellulose/NaOH/water phase diagrams in the area of cellulose dissolution (7-10% NaOH below 0 degrees C) are studied by DSC. The NaOH/water binary phase diagram has a simple eutectic behavior. Because of the existence of this eutectic structure, it is possible to measure the influence of the addition of pure low molar mass microcrystalline cellulose. This shows that a minimum of four NaOH molecules should be linked to one anhydroglucose unit to allow for the dissolution of microcrystalline cellulose. The proportions between bound Avicel, NaOH, and water molecules as a function of cellulose concentrations are calculated. A tentative explanation about the origin of the dissolving power of NaOH/water is given.

Five modes describing the behaviour of cellulose fibres dipped in a chemical have been identified:In the case of the behaviour of wood and cotton cellulose fibres in N-methylmorpholine-N-oxide (NMMO) and water mixtures, four domains of water content have been identified. Below 17% of water up to monohydrate (13%), the fibres are disintegrated into rod-like fragments and dissolve (mode 1). In NMMO -water mixtures containing 19-24% water, the cellulose fibres exhibit a heterogeneous swelling by forming balloons (composed of dissolved cellulose holds inside a membrane) separated with non-swollen sections. The whole fibre will completely dissolve (mode 2) in four successive steps (growth of the balloons, burst of the balloons, dissolution of the non-swollen sections and finally dissolution of the membrane). With still greater water contents (25-30%), only the ballooning phenomenon is observed, with a partial dissolution inside the balloon (mode 3). Above 35% of water, the fibres swell homogeneously and are not dissolving (mode 4).

The swelling and dissolution mechanisms of native cotton and wood cellulose fibres in NaOH-water are studied. The cellulose fibres exhibit a heterogeneous swelling by ballooning in the best dissolving conditions (À5 8C, 7.6% of NaOH). This corresponds to the mode 3 of the swelling-dissolution (see companion paper). In this region of the mixture phase diagram, cellulose is only dissolved inside the balloons. A lot of insoluble parts are present. Increasing the temperature and/or the content of NaOH decreases the quality of the solvent. In this case, the cellulose fibres do not show ballooning, but only a homogeneous swelling (mode 4). Three components are tested as additives: urea, zinc oxide and N-methylmorpholine-N-oxide (NMMO). The swelling and dissolution mechanisms in NaOH-water and NaOH-water-additives stay the same. Adding urea to NaOH-water (À5 8C, 7.6% of NaOH) gives the same ballooning mechanisms, but with a larger expansion of the balloons, indicating that the solvent is better. Zinc oxide does not increase the expansion of the balloons, but the kinetics of swelling is faster. NMMO acts as a dissolution inhibitor.

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