Akwasi Asamoah

What makes cellulose auxetic?

Blog Post created by Akwasi Asamoah on Sep 24, 2015

Unlike conventional (positive Poisson’s ratio) materials, when auxetic (negative Poisson’s ratio) materials are stretched, they expand in at least one direction at 90° to the direction of stretch. Cellulose is believed to be auxetic. To fully appreciate the auxetics of non-native (processed) cellulose, a 1D bundle of cellulose microfibrils and 2D networks of cellulose mircofibrils were stretched in a Deben micro-tester, and their molecular straining followed in- plane with Raman spectroscopy. It was found that cellulose exhibits three distinct yielding points. Also, it was found that amorphous cellulose is more auxetic than crystalline cellulose while hydrogen bonds have not yielded. The 2D network of cellulose mircofibrils rather limits auxetics of single 1D cellulose microfibrils in a network. Differences in auxetics between crystals and amorphous must arise from the extra intermolecular hydrogen bonding in crystalline cellulose and associated differences in intermolecular bonding geometry. Similarity of trends of in-plane auxetics of cellulose to the off-axis auxetics of zeolites indicates similarity of structure at the nano-scale and the possibility of combining both of these semi-crystalline materials to produce ones with photo-electro-mechanical properties.