Thermo-responsive and fluorescent cellulose nanocrystals grafted with polymer brushes

Abstract

Cellulose nanocrystals (CNCs) grafted with fluorescent and thermo-responsive poly(N-isopropylacrylamide) (PNIPAAM) brushes were prepared via surface-initiated activators generated by electron transfer for atom transfer radical polymerization (SI-AGET-ATRP) in the CH₃OH–H₂O mixing solvent with different volume ratios. The successful grafting was supported by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) measurements. Gravimetric analysis plus ¹H NMR and gel permeation chromatography (GPC) measurements showed that there was an increase in the monomer conversion and molecular weight of polymer brushes with increasing H₂O proportion of the solvent system. The variation trend of graft length was further evidenced by the gradual change of decomposition and glass transition temperatures of the surface-grafted CNCs. A large scale of chain transfer occurred on the surface of CNCs in view of the minute quantity of free polymers generated by a sacrificial initiator. Free polymers cannot be used as a substitute to characterize grafted polymers in terms of the big difference between their molecular weights. The obtained surface-grafted CNCs showed thermo-enhanced fluorescence owing to the thermal-driven chain dehydration of the grafted PNIPAAM brushes.