Evidence for retention of the crystalline‐amorphous ratio of cellulose during heterogeneous acid hydrolysis

Abstract

AbstractThe literature reports that hydrolysis of cellulose under heterogeneous conditions proceeds by an initially rapid attack on the amorphous regions and then a much slower hydrolysis of the ordered regions or “crystallites.” During such hydrolysis, the degree of polymerization (D.P.) of cellulose initially decreases quite sharply and then levels off and declines at a much slower rate. This leveling off of the D.P. has been interpreted as being due to the slow hydrolysis of the “crystallites.” Some results obtained in our laboratories suggested that heterogeneous hydrolysis does not generate “crystallites” as has been assumed. Hence, a study of the crystallinity of the hydrolysis products was undertaken. The x‐ray crystallinity of cellulose was evaluated by comparing the integrated intensities of the main diffraction peaks before and after hydrolysis. The effects of variations in sample preparation and orientation of cellulose samples as well as fluctuations in the operation of the diffractometer were assessed and were found to introduce no artifacts. The x‐ray crystallinity of slash pine sulfate pulp did not increase upon hydrolysis as would be expected if hydrolysis had liberated “crystallites.” Cotton linters and slash pine sulfite pump exhibited only small increases in crystallinity upon hydrolysis. A decrease in x‐ray crystallinity was observed for gum sulfate pulp which cannot be explained at this time. Thus it appears that hydrolysis does not convert cellulose to a wholly crystalline or ordered state. It might be well to review past work in which complete crystallinity of cellulose “crystallites” has been assumed.