PALS and nanoindentation as complementary methods for investigating cellulose derivatives (esters and carbamates)

Abstract

New cellulose derivatives were synthesized differing in the nature of the substituent bonded with the main chain. The substituent contains a halogen atom and a methyl group at different positions. The new cellulose derivatives were studied by 1H NMR, Positron Annihilation Lifetime Spectroscopy (PALS), and Depth-Sensing Indentation (DSI) (nanoindentation). PALS is a powerful material characterization technique that can give information about the dimensions of the free volume holes (fvh) and their relative number density in polymer materials. The fvh are believed to be related to the molecular and segmental mobility of the molecular chains and, therefore, to the mechanical properties of a polymer. On the other hand, the nanoindentation characteristics such as universal, Martens, and indentation hardness, indentation modulus, creep, and a number of others give information about the resistance of a material against elastic and plastic deformations. This indirectly leads the interpretation of the nanoindentation results to the corresponding structural elements and their changes due to the indentation deformation. It was established that in the cases of cellulose derivatives the nanoindentation characteristics were influenced non-uniformly by the dimension and architecture of the substituent and the degree of substitution. Usually, the indentation modulus and all hardness characteristics are more sensitive to structural parameters than the creep and the elasticity. The obtained NMR, PALS, and DSI results were discussed in order to shed light on to what extent the free volume holes affect the mechanical properties of the studied groups of cellulose derivatives.