Micromechanical and positron annihilation lifetime study of new cellulose esters with different topological structures

Two new groups of cellulose esters with a substituent, either trifluoromethylbenzoate or methylbenzoate, were synthesized in homogeneous conditions. The actual presence of aromatic substituents was demonstrated by ¹H-NMR spectra. The mechanical and structural peculiarities were determined by Depth Sensing Indentation (DSI)and Positron Annihilation Lifetime Spectroscopy (PALS)techniques. The relative position of CH₃ and CF₃ groups on the aromatic substituents influences the molecular packing differently and affects the formation of free volumes. The hardness characteristics and modulus of the corresponding cellulose esters were the lowest when the CH₃ and CF₃ groups are in m-position on the aromatic substituents. Meanwhile, they exhibited the highest number density of the free volume holes, as revealed by the o-Ps intensity values. An inverse linear relationship between the hardness, respectively on the modulus, and the o-Ps intensity was found. A simplified scheme of the structural feature of the investigated cellulose esters was proposed.