Tuning the phase separation of cellulose nanocrystals with hydrolysis times: influence of effective dimensions

Abstract

This study attempts to quantify a relatively unexplored and very important subject: cellulose nanocrystal (CNC) bundles and their effective dimensions on phase separation and subsequent chiral resolution in CNC suspensions. Currently, there is little data discussing how effective bundle dimensions affect the onset of chiral nematic phase formation despite the fact that theory and experimental data indicate they are important factors. The effect of the extent of hydrolysis on the phase behavior of CNC suspensions was analyzed by correlating it with the critical weight concentration (w₀), which is the CNC weight corresponding to the onset of the chiral nematic phase. From Onsager theory and its extension, w₀ is primarily a function of CNC size while surface charge exerts a non-negligible effect. CNCs were produced from never-dried bleached softwood pulp under varying acid hydrolysis times to systematically alter sizes and surface charges. Concentration-dependent phase diagrams were mapped to ascertain the w₀ of the produced suspensions. The data revealed a clear decrease in w₀ when the hydrolysis time increased from 25 to 90 minutes, despite similar individual CNC size and increasing surface charges. This latter discovery following shape and size distribution indicated an increased area-equivalent (AE) diameter from extended hydrolysis, suggesting particle aggregation/bundling. This result was corroborated by elevated particle surface charges from enhanced lateral adherence between CNCs. In contrast to our findings that higher surface charge reduces the effective diameter, the observed decrease in w₀ suggests that an earlier onset of the anisotropic phase is driven by CNC bundles, which were more prevalent in samples with elevated surface charge. These observations indicate that CNC bundles play a significant role in promoting the anisotropic phase, counteracting the effect of surface charge on w₀. This work therefore provides invaluable insights into the complex interplay of CNC surface charge, shape, and size by shedding light on the importance of hydrolysis time on particle aggregation and phase behavior in CNC suspensions.