Versatile gas-phase reactions for surface to bulk esterification of cellulose microfibrils aerogels.

Aqueous suspensions of microfibrillated cellulose obtained by a high pressure homogenization process were freeze-dried after solvent exchange into tert-butanol. The resulting aerogels, which displayed a remarkable open morphology with a surface area reaching 100 m(2)/g, were subjected to a gas-phase esterification with palmitoyl chloride. Under these conditions, variations of the reaction temperature from 100 to 200 °C, of the reaction time from 0.5 to 2 h, and of the initial quantity of reagent, led to the preparation of a library of cellulose palmitates with DS varying from zero to 2.36. These products were characterized by gravimetry, FTIR, and (13)C solid-state NMR spectroscopy. Of special interest were the cellulose palmitate samples of low DS in the range of 0.1-0.4, which corresponded to hydrophobic cellulose microfibrils exclusively esterified at their surface while keeping intact their inner structure.

Synthesis and characterization of solvent-invertible amphiphilic hollow particles.

Previous researches on solvent-dependent polymer systems mainly focus on amphiphilic invertible polymers (AIPs), which are capable of forming solvent-dependent micellar or inverse micellar assemblies. However, polymer particles that are invertible in response to solvent polarity are almost unexplored. In this paper, we report a new type of invertible hollow polymer (IHP) particle that is comprised of polyethylenimine-g-poly(methyl methacrylate) (PEI-g-PMMA) copolymer. The amphiphilic PEI-g-PMMA hollow particles were first prepared through synthesis of well-defined PEI/PMMA core-shell particles, followed by removal of PMMA homopolymer from the core. The resulting IHP particles can be stably dispersed in both nonpolar solvent and water. We have investigated the morphology and surface property of the particles in both dichloromethane (DCM) and water using transmission electron microscopy, water contact angle measurement, and X-ray photoelectron spectroscopy analysis to gain insight into this unique particle dispersibility. Sustainability of the solvent-invertible property was carefully studied through repeated treatment of the IHP particles in DCM or water for up to six cycles. Solvent-dependent property of the dry films formed by IHP particles was also investigated through water contact angle measurement. Increasing water content on the DCM-treated IHP particle film could reduce the water contact angle from 94° to 51°. Our results demonstrate that the amphiphilic hollow particles are a new type of polymer design for smart materials that are invertible in response to nonpolar and aqueous media in both dispersed and solid states.