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MORPHOLOGY AND FORMATION OF FIBRILLAR STRUCTURE IN PBO FIBER C.C. CHAU, J.H. BLACKSON, H.E. KLASSEN AND W.-F. HWANG The Dow Chemical Company, Midland, MI 48674 ABSTRACT Electron microscopy studies showed that the porous structure of PBO fiber may contain fractal geometries; i.e., the void spaces are self-similar with variations in magnification. At the fiber surface, a dense skin which consists of fibrils was observed. In the matrix, the fibril size is about 5 to 50 nm with the voids distributed randomly among the fibrils. The fractal dimension of voids in PBO fiber as determined by microscopy and image analysis was found to be 2.44. The fibrillated fiber showed a continuous fibril size distribution with no evidence of fibril size hierarchy. These observations suggest a nucleation and growth mechanism for the formation of the fibrillar structure in PBO fiber. INTRODUCTION The structure of high performance fibers is an interesting subject and has been studied for many years. Published data indicates that organic fibers are highly oriented in the fiber direction [1]. Studies on Kevlar aromatic polyamide fibers showed that rod shaped crystallites are oriented [2-4] along the fiber axis with layered stackings [5-7] held together by H-bonds, and with weak van der Waals' attractions in the lateral direction. Observations on PBT [8] have shown that these fibers could contain fibrils, with a fibril size 10 nm or smaller, oriented along the fiber axis. The fibrillar nature seems to allow the fiber to be fibrillated easily by both tensile and compressive deformation [3, 9] or by a simple peel test [10]. Interest is further revolving around the possibility of fibrillar hierarchy. Although still not clear, fibrillar morphology has been recognized as one of the most important features of organic fibers since mechanical properties, such as high tensile strength and modulus, and weak compressive strength [11], are believed to be closely related to the fibrillar nature of the fiber. In a continuing effort to understand the fibrillar morphology and the mechanism of fiber formation, microtomed thin sections of PBO fibers were examined by transmission electron microscopy. Details of the fibrillar morphology were observed and analyzed quantitatively. Some considerations regarding the fibrillar formation mechanism are given. EXPERIMENTAL (1) Electron Microscopy: PBO fibers were prepared from dopes of a copolymer of poly(phenylene benzobisoxazole) (PBO) in polyphosphoric acid (PPA). The spun filaments were collected in water, dried, and subsequently heat treated at elevated temperatures under tension. Fibers with a diameter of about 15 um were embedded in epoxy resin. Flat silicon embedding molds were used with the fibers oriented parallel to the long dimension of the mold. After appropriate trimming of the epoxy embedded sample, ultramicrotomy was performed at room temperature with a Reichert Ultracut E microtome to produce sections ranging in thickness from 40-70 nm. New areas of a diamond knife were used continuously to avoid damage