Internal Structure of Mature and Immature Cotton Fibers Revealed by Scanning Probe Microscopy

Abstract

A scanning probe microscope (SPM) is used to investigate morphological, nano-me chanical, and moisture absorption properties of transverse sections of mature and imma ture cotton fibers. Using TappingModeTM, images of whole sections reveal growth bands and the lumen. Increasing the imaging force produces contrast in the phase images, revealing discrete fibrils and variations in fibril density across the fiber cross sections. The effect of relative humidity (RH) on the fiber structure and hardness is also examined with the SPM. As RH increases, immature fibers swell above the face of the resin block in which they are mounted and become "softer," as indicated by a series of force-distance mea surements. What is thought to be fibril aggregation is evident when the immature fibers dry. Mature fibers in the resin block also "soften" to some degree but fail to swell, and fibril aggregation is not evident. Transverse sections of both mature and immature fibers. which have been cut from the resin block and floated in water onto glass microscope slides, show fibril aggregation. Both the mature and immature fibers are believed to pass through a glass transition at about 78% RH at 22°C. To our knowledge, this is the first study to reveal the fine structure of cotton fiber cross sections that have not received chemical or physical pretreatment. Proposed future work will attempt to relate both fibril density and hardness to the degree of crystallinity and, in turn, fiber maturity. In addition, the ability to obtain images of the growth bands that have been unaltered since the initial drying of the fiber may be useful in studying growing conditions.