STRUCTURE DEVELOPMENT AND DYNAMICS OF VIBRATION WELDING OF POLY(ETHYLENE NAPHTHALATE) FROM AMORPHOUS AND SEMICRYSTALLINE PRECURSORS

The influence of process conditions on the structural hierarchy develo ped in the heat-affected zone (HAZ) of polyethylene 2,6-naphthalene di carboxylate (PEN) welded both from amorphous, as well as crystalline, precursors was studied using optical microscopy and a matrixing microb eam X-ray diffraction camera developed in our laboratories. The struct ural gradients developed in the HAZ were found to be highly dependent on welding conditions as well as on the state of crystallinity in the precursor samples. In the amorphous sample, the HAZ is composed of 2 d istinct layers. The inner layer (II) (HAZ-II) is formed at the immedia te vicinity of the original weld interface, and, within this region, t he sample is highly oriented and crystallized. In the outer layer (I) (HAZ I), between this layer and the unaffected region, the polymer exh ibit deformed zones, but little or no crystallinity is noticed. In HAZ -I, the polymer chains melt and deform significantly and crystallize u pon cooling; and in the HAZ I, the polymer chains located in this regi on increase their temperature to reach temperatures T > T-g, where the y are deformed under the dynamic forces generated at the weld interfac e as well as under the normal forces. Due to the low crystallizability of PEN chains, the material mostly remain amorphous at this location. In the HAZ, a wave-like interface was observed at the bothe HAZ-II an d HAZ I boundaries. This was attributed to the presence of 2 regions o f deformable bodies of large difference in viscosities which, in the p resence of oscillatory stress field, results in wavy interface. The st ructural analysis revealed that as a result of significant shearing, t he naphthalene planes are oriented parallel to the weld interface rese mbling graphitic structure. This had a detrimental effect on the mecha nical performance of the parts, particularly in those samples welded f rom amorphous precursors. The samples welded from crystalline precurso rs exhibited more traditional HAZ structure gradient, and the boundary between the heat-affected and unaffected region is rather diffuse ref lecting the rather broad nature of the melting temperature range in th is polymer due to dynamic temperature gradient developed during and af ter the vibration stage. (C) 1998 John Wiley & Sons, Inc.