New Porous Materials from Natural Polymers Processed by Supercritical CO_2

摘要

Supercritical carbon dioxide (CO_2) has been widely used for processing polymeric materials in a variety of applications. Polymer foaming is the production of porous materials through the interaction (e.g. sorption, plasticization, desorption) of CO_2 with a polymer matrix. Natural polymers such as cellulose, chitin, gelatin, and chitosan are crystalline and do not absorb CO_2 in the solid state. Also, they do not exhibit any temperature transitions (glass transition or melting point) before their decomposition temperature and, thus, the polymer foaming technique cannot be applied to them. We report here the CO_2 foaming of such polymers through an intermediate hydrogel state (hydrogel foaming). In an alternative route, the water in these gels can be replaced by an alcohol. The alcohol-gels can also be processed by CO_2 (critical point drying) resulting in interesting nanoporous materials (aerogels). The above mentioned two techniques were applied to various polymers. The influence of different processing parameters, such as pore size or density, on the properties of the obtained materials is investigated. Also, a mechanism for the hydrogel foaming is proposed and compared with the plain polymer foaming mechanism.