A revised solid-state NMR method to assess the crystallinity of cellulose

Abstract

The crystallinity of cellulose has a strong impact on various material properties. Over the years, many methods have become available to estimate the crystallinity. The purpose of this work was to revise existing NMR-based methods and to introduce a complementary NMR method related to the 13C T1 relaxation time. The 13C T1 differs by an order of magnitude for amorphous and crystalline polymers among them cellulose. We have utilized the signal boost of 1H–13C cross polarization and the difference in 13C T1 as a filter to calculate the degree of crystallinity. The evaluation of the method is based on the difference in peak integrals, which is fed into a simple equation. The method was applied to five cellulosic samples of different nature and compared the obtained degree of crystallinity with the degree estimated from deconvoluted X-ray scattering patterns. Furthermore, an attempt has been made to give a basic understanding on the origin of CP enhancement in order to validate various proposed NMR methods. With the recent progress of NMR equipment, the presented method can be automatized and applied to a series of samples using a sample changer.