Polymerization Topochemistry of Cellulose Nanocrystals: A Function of Surface Dehydration Control

The activation (dehydration) of cellulose nanocrystals (CNCs) toward surface brush polymerization is accomplished either by freeze drying or solvent exchange. However, the question of which one of these protocols to choose over the other is generally open-ended. The current study attempts to shed light on this question by installing a standard polymer, polycaprolactone (PCL), onto the surface of both freeze-dried and solvent-exchanged CNCs by ring-opening polymerization (ROP) and examining the differences in polymerization and final product properties. The work is the first to demonstrate that the efficiency of surface polymerization and final product properties are in fact influenced by the protocols. The differences between the two sample PCL-grafted CNCs were investigated by X-ray photoelectron spectroscopy (XPS), elemental analysis, gel permeation chromatography (GPC), and contact-angle measurements. The freeze-dried samples had a significantly reduced PCL surface density. The crystallinity of the solvent-exchanged PCL-grafted CNCs (SECNC-g-PCL), however, was lower than that of either pure CNCs or freeze-dried PCL-grafted CNCs (FDCNC-g-PCL). It was determined that solvent exchange sufficiently modified the CNC surface to provide enhanced reactivity, an effect that was not as apparent for FDCNC-g-PCL. The solvent-exchanged CNCs tended to have more porous, nanotextured surfaces that were tended to be more responsive toward brush polymerization. In addition to the physical dissimilarities in surface morphology and surface accessibility contributing to topochemical differences between the two species, it was also found that the dispersibility, aggregation, and thermal stability were different.