Introducing Students to Energy-Efficient Mechanochemistry of Biopolymers

Abstract

A comprehensive laboratory work that allows students to gain hands-on skills in energy-efficient mechanochemistry is described. Standard university curricula are paying very little attention to mechanochemistry, although skills in this discipline are highly demanded among chemistry graduates. In this study, a comprehensive description of mechanical destruction of α-cellulose was chosen as a model task for students to perform. The structure of cellulose is best suitable for demonstrating the effects occurring upon mechanical treatment. While being sufficiently complex (as it is characterized by high molecular weight and the presence of crystalline and amorphous regions), cellulose is simultaneously rather simple (as it consists of glucose units only and contains no branching) and allows one to consider rupturing of a macromolecule chain as the only possible mechanochemical reaction. This study employs the view commonly held in mechanochemistry that the field of mechanical forces undergoes relaxation through the following pathways: heating of the system, grinding of material, amorphization of the crystalline structure, and occurrence of mechanochemical reactions. This multilevel investigation of the process according to the "bottom up" approach has allowed the students to identify the relaxation pathways through which energy is expended during treatment of amorphous/crystalline biopolymers. It is safe to say that this approach allows the new generation of chemists not only to routinely assess and enhance the energy efficiency of the existing technologies but also to develop novel technologies employing hardly accessible and nonreactive substances.

Mechanical Amorphization of Chitosan with Different Molecular Weights

“…The maximum amorphization degree ( AD of ~50%) is attained within ~600 s; the specific power consumption is ~28 kJ/g. It is five- to sevenfold higher than power consumption for amorphization of polymorphic modifications of starch (4–6 kJ/g) [ 28 ] and is comparable to power consumption for amorphization of cellulose [ 29 ] whose supramolecular structure is very similar to that of chitosan.…”

“…Mechanical amorphization is considered to be a highly energy-intensive method for biopolymer pretreatment; energy is inevitably spent on heating the system, grinding the material, amorphization of the crystal structure, and the course of mechanochemical reactions. Studies on energy consumption allow not only to evaluate [ 27 , 28 ] and improve the energy efficiency of existing technologies, but also to develop new technologies for processing amorphous-crystalline polymers [ 29 ].…”