Carbon-13 Cross-Polarization Magic-Angle-Spinning Nuclear Magnetic Resonance Investigation of the Interactions between Maleic Anhydride Grafted Polypropylene and Wood Polymers

Abstract

The chemical interactions between maleic anhydride grafted polypropylene (MAPP) and wood were studied with solid-state carbon-13 cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CP-MAS NMR) spectroscopy. MAPP was synthesized with 100% ¹³C enrichment at the C₁ and C₄ carbons to allow detection of the [1,4⁻¹³C₂]MAPP functional groups and was melt blended with cellulose, lignin, and maple wood. In the cellulose/MAPP blend, changes in ¹³C CP-MAS NMR corrected signal intensities for the anhydride and dicarboxylic maleic acid functionalities suggested that esterification may have occurred predominantly from the more numerous diacid carbons. A single proton longitudinal relaxation in the rotating frame, ^HT_1ρ, for the MAPP and the cellulose carbons in the blend suggested that they were spin coupled, i.e., homogeneous on a 10–200 Angstrom scale. Esterification was also suggested in the lignin/MAPP blend. Furthermore, the more significant changes in the intensities of the carbonyl signals and ^HT_1ρ values suggested that lignin may be more reactive to MAPP than cellulose. Finally, when maple was melt blended with MAPP, the same trends in the ¹³C CP-MAS NMR spectra and ^HT_1ρ behavior were observed as when MAPP was blended with cellulose or lignin. This study therefore clarifies that during melt compounding of wood with MAPP, esterification occurs with wood polymers, preferentially with lignin. Understanding the interactions of MAPP with wood is of significance for the development of natural-fiber-reinforced thermoplastic composites.