The effect of the carboxylation degree on cellulose nanofibers and waterborne polyurethane/cellulose nanofiber nanocomposites properties

International audienceThere has been an exponential rise in the interest for waterborne polyurethanes (WBPU), due to the easy customizability of their properties and their ecofriendly nature. Moreover, their aqueous state facilitates the incorporation of hydrophilic reinforcements. Cellulose nanofibers (CNFs) have shown great potential, thanks to their renewability, large natural availability, low cost and great specific properties. However, CNFs often require some modification to obtain optimal compatibility. In this work, standard bleached hardwood kraft pulp has been subjected to a carboxylation process followed by mechanical disintegration. Varying treatment times and passes, CNF samples with different carboxylation degrees have been obtained. WBPU/CNF nanocomposites with different CNF content have been prepared. The effect of the carboxylation degree on the CNFs and on the nanocomposites properties has been studied. Although carboxylation damaged the cellulose structure, decreasing the crystallinity degree of CNF and reducing the thermal stability of fibers, composites showed better thermal and thermomechanical stability and improved mechanical properties than the unreinforced matrix counterpart. A maximum increase of 1670% in modulus, 377% in stress at yield and 86% in stress at break has been achieved for composites reinforced with carboxylated fibers. Therefore, it was observed that carboxylation improved matrix/reinforcement interactions

Molecular Engineering of Elastic and Strong Supertough Polyurethanes

Spider silk is an icon of supertough energy absorbing polymeric material which its macromolecular multiblock composition has been attributed to be responsible for such remarkable properties. As in spider silk, polyurethanes can be synthesized with two distinct block which can differ in nature, combining properties like deformability and relatively high strengths. Here we synthesized and studied four different block polyurethanes with two different soft segments (SS) and two different hard segments (HS), with the aim of discovering the best molecular architecture to develop best mechanical performance after macromolecular alignment. The difference between soft segments is the crystalline nature, one in the rubbery state (<i>T</i>_g, SS < <i>T</i>_room) and the other in the semicrystalline state at room temperature (<i>T</i>_room < <i>T</i>_m, SS). In parallel one hard segment was amorphous in the glassy state (<i>T</i>_room<i> < T</i>_g, HS) and the other semicrystalline (<i>T</i>_g, HS < <i>T</i>_room < <i>T</i>_m, HS). Results indicate that polyurethane with crystalline soft segments produce stronger materials after drawing than polyurethanes with rubbery soft segments, but the most exciting finding is the influence that hard segment has on the mechanical performance of predrawn materials, having polyurethanes prepared with semicrystalline hard segments more capability to undergo macromolecular alignment than materials with glassy segments, developing stiffer, stronger, and tougher materials