Medium-induced selective reconstruction of cellulose crystal planes: insights into the CNC formation pathway

Objective The formation of cellulose nanocrystals (CNC) is generally attributed to the preferential degradation and removal of amorphous regions during acid hydrolysis. However, this mechanism cannot adequately explain the spontaneous recrystallization of amorphous cellulose under aqueous or hydrothermal conditions. This study addresses how amorphous structures reconstruct crystal planes in different media, focusing on the evolution of crystal plane proportions during recrystallization and the effects of media on polymorphic transformation and the sequence of crystal plane recovery.

Method This study first compared the variation characteristics of different crystal planes at different ball-milling times to analyze the selective disruptive effect of ball milling on the crystalline structure of cellulose. On this basis, amorphous cellulose was used as the experimental material to investigate its recrystallization behavior in hydrothermal and oxalic acid systems. The evolution of the (200), (1-10), (110), and (004) crystal planes was analyzed by combining Gaussian peak deconvolution of X-ray diffraction (XRD) patterns with Fourier transform infrared spectroscopy (FTIR).

Result Ball milling caused the characteristic peaks of cellulose I to broaden significantly and decrease in intensity, and the relative crystallinity decreased from 80.1% to 28.9%. Crystal plane analysis showed that the decay rate of the (1-10) plane was faster than that of the (110) plane, while the (004) plane broadened markedly, exhibiting a selective deconstruction feature characterized by the preferential disruption of the lateral hydrogen-bonding network. After hydrothermal and oxalic acid treatments at 100 ℃ for 2 h, the relative crystallinity of the samples increased to 65.5% and 68.2%, respectively, and characteristic peaks of cellulose II appeared in both cases. Analysis of the crystal plane composition indicated that recrystallization was not a recovery of the original structure, but rather the formation of a new proportional equilibrium under the coexistence of cellulose I and cellulose II. The hydrothermal system preferentially restored the (200) plane, whereas the oxalic acid system promoted the recovery of the (004) plane at the lower-temperature stage; at higher temperatures, both systems tended toward a typical cellulose II distribution.

Conclusion Ball-milling-induced amorphization alters the proportional distribution of crystal planes. The recrystallization process can be described as a reconstruction pathway involving “selective disruption–proportional redistribution–new equilibrium formation.” Different media regulate the sequence of crystal plane recovery by modulating chain swelling and hydrogen-bond reorganization, without altering the direction of polymorphic transformation. These findings suggest that, in addition to the conventional “amorphous region removal” pathway, CNC formation may also originate from medium-induced rearrangement of disordered chains, providing a theoretical basis for the green and low-acid preparation of CNC.