The thermal stability of nanocellulose and its acetates with different degree of polymerization

Geared towards reinforcing thermoplastics of high melting points with nanocellulose, this study evaluated the factors affecting the thermal properties of and the thermal stabilizing effect of acetylation on nanocellulose with different average degree of polymerization (DPv) from bacterial cellulose (BC). Cellulose nanocrystals with DPv values of 300 and 500 were prepared by hydrolyzing BC nanofibers with a DPv of 1100 using hydrochloric acid. The thermal stability decreased after acid hydrolysis and showed a decreasing trend with decreasing DPv. The decrease in thermal stability is attributed to the increase in the number of reducing ends (REs) with decreasing DPv. Heterogeneous acetylation to an average degree of substitution of 0.38 improved the thermal stability, and the degree of improvement increased with decreasing DPv. The dependence of the degree of improvement on the DPv is attributed to possible protection of the REs by more stable acetyl groups. The influence of protecting the REs on the degree of improvement in thermal stability was further confirmed by sodium borohydride (NaBH4) reduction. The findings suggest that the thermal stabilization caused by acetylation to nanocellulose with small DPv is a combined effect of protecting both the surface OH and the REs; while for nanocellulose with high DPv, the thermal stabilization caused by acetylation is mainly due to protection of the surface OH.