Microscopical Evaluation of Cellulose Esters with Low Degrees of Substitution

Abstract

The effects on cotton of esterification to low degrees of substitution have been investi gated by light and electron microscopy. Observations were made on partially esterified celluloses which included esters of acetic, palmitic, stearic, 12-hydroxystearic, linoleic, and ricinoleic acids and of the aromatic benzoic, cinnamic, naphthoic, and pkenylundecanoic acids. The uniformity of the esterification of cellulose was followed by dyeing and swelling techniques. Refractive index measurements were used to follow changes in optical anisotropy which accompanied the chemical modification. The average refractive indices of cellulose esters containing aromatic groups were greater than those of the esters with aliphatic substituents, the greatest difference being between indices measured perpendicu lar to the fiber axis. The normal fibrillate texture of the scoured surface appeared to become smooth upon esterification. Fragmentation of the esters in water in a laboratory blendor produced long strands of fibrillate material intermingled with clumps of spongy or amorphous material. On de-esterification of the fragments with an alcoholic base, the structure of the material reverted to that of unesterified cotton—fibrils became distinct and, in some cases, saponification proceeded to a stage where hydrocellulose-like particles were formed. , Many of the esterified fibers of low degrees of substitution swelled in the conventional methacrylate embedding technique. The swelling caused the cell wall to separate into layers, and the internal structure could then be studied by examination of thin sections of these fibers with the electron microscope. By use of an alternative embedding medium, aqueous polyvinyl alcohol, in the preparation of thin sections, compact unlayered struc tures of the esterified cottons were obtained. This permitted observations of the undis turbed structure of the modified cottons. Both types of observations were used in the microscopical evaluation of cotton fibers esterified to low degree of substitution to demonstrate structural changes brought about by the esterification reaction.