Ionic Conductivity and Structure of Chitosan Films Modified with Lactic Acid-Choline Chloride NADES

Abstract

The natural deep eutectic solvent (NADES) based on choline chloride (ChCl) and lactic acid (LA) was used for the preparation of chitosan (CS) films by the solution casting method. The content of NADES in films was from 0 to 82 wt%. The impact of NADES on the morphology and crystalline structure of films was investigated using scanning electron microscopy as well as wide-angle and small-angle X-ray scattering. The experimental results allow to propose CS chains swelling in NADES. FTIR spectroscopy confirms the interactions between CS and NADES components via the formation of hydrogen and ion bonds. The thermal properties of the composite films were studied by simultaneous thermogravimetric and differential thermal analysis. Thermomechanical analysis demonstrated appearance of two transitions at temperatures between -23 and -5 °C and 54-102 °C depending on NADES content. It was found that electrical conductivity of film with 82 wt% of NADES reaches 1.7 mS/cm. The influence of the composition and structure of films on the charge carriers concentration and their mobility is discussed.

Keywords: chitosan; natural deep eutectic solvent; plasticization; polymer electrolytes.

Figure 1

Schematic illustration of the preparation of chitosan-lactic acid-choline chloride (CS-LA-ChCl) films.

Figure 2

FTIR spectra of the pure CS, CS-LA film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%) in the regions cm⁻¹ 1800–700 (a) and 1680–1500 cm⁻¹ (b).

Figure 3

SEM micrographs of the reference CS film (CS/DES-0) (a,b) and films with different NADES contents: 50 wt% (c,d), 67 wt% (e,f), 75 wt% (g,h) and 82 wt% (i,j). The surface (a,c,e,g,i) and the cross-sectional morphology (b,d,f,h,j) of films.

Figure 4

WAXD patterns of CS-LA film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%).

Figure 5

SAXS patterns of CS-LA film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%).

Figure 6

The SAXS patterns for CS-lactic acid film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%) in the Guinier coordinates. Straight lines demonstrate the slope of curves at the low q² values.

Figure 7

TGA (a) and DTG (b) curves for CS, CS-LA film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%).

Figure 8

TMA curves for CS—LA film (CS/DES-0) and films with different NADES contents (50, 67, 75 and 82 wt%).

Figure 9

Water sorption isotherms for CS film prepared with LA and with different concentrations of LA-ChCl NADES (a); sorption on active sorption centers (b) and in clusters (c).

Figure 10

Typical view of stress-strain curves for the reference CS film (CS/DES-0) and composites with different NADES contents (a). Dependences of mechanical properties of the films on the NADES content (b).

Figure 11

(a) Dependences of logarithm of electrical conductivity on the reciprocal temperature for the film CS with LA and CS films prepared with different NADES content (a) and (b) dependence of the number density of charge carriers (n) on the number density of -COOH groups (n₀) in the films.