Multi-scale investigation of the effect of photocurable polyethylene glycol diacrylate (PEGDA) on the self-assembly of cellulose nanocrystals (CNCs)

Cellulose nanocrystals (CNCs) are appealing building blocks to replace plastic for the fabrication of more sustainable photonic and barrier materials. Aqueous suspensions of CNCs are typically mixed with polymers to improve the mechanical flexibility of the dry nanocomposite, adding further functionalities. However, the role of the additives in the suspension state, and how they affect the processing and final properties of the material still requires further studies. In this work, the interactions between CNCs and polyethylene glycol diacrylate (PEGDA), a water-soluble, photocurable polymer, were investigated through a multi-scale approach involving small-angle X-ray and light scattering (SAXS and SALS). PEGDA was found to adsorb on the surface of CNCs, exhibiting two behaviour regimes depending on the mass ratio between PEGDA and CNCs (R P C ). Below a threshold ratio (R P C =0.5), PEGDA weakened the chiral interactions between CNCs, reducing the anisotropic volume fraction and increasing the cholesteric pitch. Above this threshold (R P C >0.5), the adsorbed polymer enhanced the helical twisting power, resulting in higher amounts of anisotropic phase and shorter pitch values. This non-monotonic trend was attributed to the structure of the adsorbed PEGDA layer on the surface of the CNCs, first masking the chiral features of the twisted nanoparticles, then enhancing the transmission of chirality due to changes in particle morphology and effective particle volume fraction. A simultaneous decrease in the suspension dielectric constant amplified this effect, strengthening interparticle interactions. Finally, rheo-optical experiments provided further insights into the formation of the cholesteric structure, showing that larger amounts of polymer delayed the isotropic-cholesteric phase transition. • The self-assembly behaviour of CNCs with PEGDA 700 g⋅mol −1 has been investigated with a multi-scale approach. • A balance between particle morphology and dielectric constant cause a non-monotonic helical twisting power variation. • The pitch and viscosity of the CNC/PEGDA suspensions can be modulated for further processing.