Microfibers Formed Ab Initio in Extensional Melt Flows
Authors: Tucker Paul , George Waller 
Published: July 2017

Recently, published work on the ultrastructure of filaments formed under quenching, stretching fields Icads to the suggestion that such fine structure is changed in kind as one approaches the practical operating conditions of commercial melt-extrusion processes. An analysis of details such as jet stretch, melt temperature, and calrillary flow velocities permits one to compare the pubtished work with data from an experimental extrusion of a polyethylene monofilament which was purposely developed to more closely approximate "commercial practice." The fine structure of the etched filament is revealed with the aid of electron microscopy, which reveals the as-extruded filament to be composed of a peripheral stockade of micro-shish-kebab fibers surrounding a central field (central in the cross section) of a lamellar type of microstructure containing numerous axially aligned bridging elements. A companion monofilament extruded from linear polyethylene oxide under relatively arbitrary conditions approximating commercial extrusion practice yielded a similar microstructure, but one which was considerably richer in detail than that observed in the case of the poly ethylene monofilament. Both monomaments exhibited a decyee of secondary confounding microstructure which remains to be extensively analyzed.

A phase transformation experiment has been sought which models the major features of the formation of the micro fibers seen in the examination of the stress-crystallized extrudates without the confounding features of the secondary structure or at least with those features substantially minimized. The scheme devised is based upon an intimate blend at a low concentration of high molecular-weight polymer with a major component of low molecular-weight material which can be raised to a temperature above the melting point of the high molecular-weight component; after supercooling to some appropriate temperature, the mixed material is sheared concurrently with the transformation of the high molec ular-weight component. Upon further cooling, the resulting solid is then treated to selectively remove the low molecular- weight component leaving stress-crystallized regions rich in the high molecular-weight material dispersed and ready for microscopy. Model microfibers obtained by this technique closely resemble the row-nucleated, shish-kebab structures seen in the peripheral regions of the extruded monofilaments. Electron diffraclion patterns of the model fibers have been analyzed and show in the case of linear polyethylene that the crystal structure is typical of that of solid linear polyethylene often obtained by other techniques. There is no evidence for a high temperature or shear-related triclinic phase. Detailed comparisons are made between the results of microscopic studies of the model microfibers and the morphologies observed in the as-extruded, stress-crystallized monofilaments obtained from the practical thread-line. The model microfibers appear to be closely related to the structures termed by Keller as micro-shish-kebab. However, in contradis tinction to the results of Keller and others, the microfibers made by our technique are notabte in that they are often re covered free of veils and do not require the normal severe-recovery treatments such as hot-solvent washing and acid deg radation to reveal well-resolvable, detailed structural features, particularly in the region of the central shish. The paper ends with additional evidence of the microstructural features developed at the initiation of deformation (cold drawing) in the experimental monofilaments prepared for the study. It is suggested that, as the local neck forms at the beginning of cold drawing, the axially aligned bridging structures typically seen in the central regions of the (undrawn) filament tend to dominate the visible fine structure.