Spatial Distribution of Functional Groups in Cellulose Ethers by DNP-Enhanced Solid-State NMR Spectroscopy

Abstract

Cellulose ethers are an important class of biosourced polymers. They are typically obtained from by chemical modification of natural cellulosefibers in aheterogeneous process, resulting in heterogeneous distributions of the functional groups in the materials. A direct consequence is that batches with otherwise similarcharacteristics (molecular weights, degree of substitution, and particle size) can differin their material properties. Here, we show that dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy can be used to reveal thespatial distribution of functional groups in cellulose ethers. Specifically, we show that we can clearly distinguish two samples ofhydroxypropyl methylcellulose (HPMC) with otherwise almost identical chemical characteristics. Wefind that one of the samplesexhibits a more homogeneous distribution of the substituents throughout the bulk material, while the second sample shows a more partitioned distribution of the hydroxypropyl moieties, which are found to be more present in the core than the surface of the HPMC particles. The method yields new insights into why different cellulose ether samples with almost identical chemical properties show significant variations in their physical properties. The heterogeneity of substituents in bulk polymers is a general analytical problem, and the method presented here is applicable to other polymers