Cellulose–TBPH interactions from diffusion NMR, rheology and SAXS studies
Résumé
Abstract Self-diffusion coefficients, critical concentrations, characteristic radii and interactions between solute and solvent are rarely explored at high polymer concentrations matching realistic processing conditions. At these concentrations, heterogeneous dissolution takes place and common rheological or diffusion models can fail. In this work, these parameters were measured for dilute and concentrated solutions of two microcrystalline celluloses with two different average polymerization degrees in aqueous solutions of tetrabutylphosphonium hydroxide (TBPH). In order to do so, a systematic study was conducted by rheology, small angle X-ray scattering (SAXS) and pulsed field gradient (PFG) stimulated echo NMR. The critical concentrations c* were estimated from NMR diffusion studies and rheology. SAXS and diffusion NMR yielded the gyration and hydrodynamic radii. Cellulose concentration impacted the diffusivity of the system components, revealing different interactions between solvent and solute. The increased presence of poorly dissolved cellulosic fraction above c* abruptly changes the diffusivity of the mobile moieties. It was also found that the amount of tetrabutylphosphonium cations bound to the dissolved cellulose molecules saturated at 50%, regardless of the other conditions. The solutions clearly exhibited a multi-component composition with coexisting objects as diverse as crystallites or near-ideal polymer coils; their relative amount strongly depended on the polymer concentration. This article demonstrates that one technique alone cannot suffice in describing the complexity of these systems over the whole concentration ranges, but combining information from PFG NMR and SAXS data is promising in explaining a wide range of solution states.
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