Using a servo hydraulically controlled vibration-welding machine, the temporal and spatial development of hierarchical structure in the heat-affected zone of Poly(vinylidene fluoride) homopolymer and a copolymer of vinylidene fluoride and hexafluorpropylene are investigated. The dynamical changes early in the welding process are rather complex, and start at highly localized regions that then develop rapid adhesive character. In subsequent stages, these regions coalesce and a wave pattern establishes with wave normals oriented in the vibration direction. The crests of the waves near the side surfaces then begin to extrude fibrillar structure. These fibers are also collected and extruded in the vibration direction with their long axes normal to the vibration direction. The production of such features is partly attributed to the high melt elasticity of these polymers that results in instability resulting in their production. X-ray analysis indicates that the fibers are unoriented in their crystalline regions but electron microscopy studies show that a spiral orientation develops in the extruded fibers indicating that they underwent extension plus twisting deformation.
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