Thermoresponsive properties of acenaphthylene-labelled chitosan-graft-poly(N-isopropylacrylamide) copolymers

Abstract

Acenaphthylene-labelled chitosan-graft-poly(N-isopropylacrylamide) (CS-g-PNIPAAm-ACE) copolymers of different composition were synthesized by free-radical polymerization of chitosan (CS), N-isopropylacrylamide (NIPAAm) and acenaphthylene (ACE) in aqueous solution using potassium persulfate (PPS) as an initiator. By changing the molar ratio of CS:NIPAAm:ACE from 1:0.25:0.0125 to 1:10:0.11 the ACE-labelled copolymers with different content of poly(N-isopropylacrylamide) (PNIPAAm) grafts were prepared. The chemical structure of the obtained CS-g-PNIPAAm-ACE copolymers was confirmed by FT-IR and ¹H NMR spectroscopy. In addition, ¹H NMR spectra were also used to calculate the content of CS and NIPAAm in the synthesized copolymers. The introduction of ACE label into PNIPAAm grafts was also achieved as confirmed by UV spectroscopy. Moreover, the lower critical solution temperature (LCST) behaviour of synthesized copolymers was assessed by cloud point, differential scanning calorimetry (DSC) analysis, particle size, zeta potential and steady-state fluorescence measurements. The aqueous solutions of copolymers containing ≥ 26.5 molar percent of PNIPAAm side chains demonstrated LCST behaviour with the phase separation at around 29.3–31.3 °C. The intensity of thermoresponsiveness depended on the composition of copolymers and was higher for the copolymers with higher content of poly(N-isopropylacrylamide) moieties. Moreover, ACE chromophore attached to PNIPAAm grafts served as a fluorescent marker in the steady-state fluorescence investigation of thermoresponsive behaviour of synthesized copolymers.