Improvement of dielectric properties of cyanoethyl cellulose via esterification and film stretching

This study offers a concept of material functionalization for cellulose derivatives via chemical modification followed by orientation control. Namely, we improved the dielectric properties of cyanoethyl cellulose (CyEC) by a combination of esterification and uniaxial heat-stretching. To this end, we prepared CyEC ester derivatives (Cm-CyEC: m = 3, 4, 5, 8, 12 and 18; m is the carbon number of the side chain) with almost no residual hydroxyl groups from CyEC (degree of cyanoethyl substitution = 2.54) in a homogeneous system. The introduction of higher fatty acids (m ≥ 8) improved the thermal processability of the derivatives, because of diminishing crystallinity. Cast films were stretched at temperatures close to the glass transition temperature (T _g) (152–166 °C), and different manners of orientation between the original CyEC and C18-CyEC were confirmed by IR dichroism, wide-angle X-ray diffraction and birefringence measurements. The dielectric constant for as-cast films monotonically decreased with an increase in m. The constants of the drawn CyEC films also decreased with increasing percent elongation (γ), but those of the C18-CyEC films increased with increasing γ. Such contradictory behaviour may be attributed to a local segmental orientation, which greatly increased the sum of the dipole moments of carbonyl and cyano moieties for C18-CyEC. Consequently, the dielectric constant of the stretched film of C18-CyEC increased up to ~23 (at 10² Hz), a 2.5-fold increase beyond the maximum dielectric constant of the unstretched C18-CyEC film, while the dielectric loss remained ≤0.05 at 10²–10⁵ Hz and 25 °C.