Temperature-Controlled Star-Shaped Cellulose Nanocrystal Assemblies Resulting from Asymmetric Polymer Grafting

We present here the grafting of thermoresponsive polyetheramines at the reducing ends of cellulose nanocrystals (CNCs) using a two-step protocol involving an end carboxylation followed by a peptide coupling with the primary amine moiety of the polyetheramine. In aqueous suspensions these end-modified CNCs became associated by their derivatized tips when the temperature was raised past a lower critical solution temperature (LCST), above which these polyetheramines are known to collapse and become hydrophobic. The CNC association was reversible when the temperature was lowered and the phenomenon of association/disassociation was totally reproducible in repeated temperature cycles as followed by dynamic light scattering (DLS). Small-angle neutron scattering (SANS) data revealed the presence of grafted chains with an extended conformation and showed the assembly of modified CNCs into swollen aggregates in suspension at T > LCST. Transmission electron microscopy (TEM) images confirmed that the once dispersed derivatized CNCs at low temperature became associated through their reducing ends above the LCST. At such temperatures, these modified CNCs attached themselves in a remarkable fashion, forming the arms of regular four-, five-, or six-branched stars.