Structural formation of amorphous poly(ethylene terephthalate) during uniaxial deformation above glass temperature Daisuke Kawakami a , Benjamin S. Hsiao a, * , Shaofeng Ran a , Christian Burger a , Bruce Fu a , Igors Sics a , Benjamin Chu a , Takeshi Kikutani b a Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400, USA b Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan Received 27 August 2003; received in revised form 11 November 2003; accepted 14 November 2003 Abstract An in situ study of structural formation of amorphous poly(ethylene terephthalate) (PET) during uniaxial deformation above its T g (at 90 8C) was carried out by wide-angle X-ray diffraction (WAXD) with synchrotron radiations. Results indicate that the relationships between structure and mechanical property can be divided into three zones: I, II and III. In Zone I, oriented mesophase is induced by strain, where the applied load remains about constant but the amount of mesophase increases with strain. In Zone II, crystallization is initiated from the mesophase through nucleation and growth, where the load starts to increase marking the beginning of the strain-hardening region. The initial crystallites are defective but they form an effective three-dimensional network to enhance the mechanical property. The perfection of the crystal structure and the orientation of the crystals all increase with strain in this zone. In Zone III, the ratio between load and strain is about constant, while the stable crystal growth process takes place until the breaking of the sample. The sample damage is probably dominated by the chain pull-out mechanism from the crystal amorphous interface. The increase in molecular weight was found to enhance the overall mechanical properties such as the load to induce the mesophase and the ultimate tensile strength before breakage. q 2003 Elsevier Ltd. All rights reserved. Keywords: PET; Strain-induced crystallization; Mesophase 1. Introduction Poly(ethylene terephthalate) (PET) is one of the most widely used thermoplastic polyesters. Its most significant applications lie in the form of fibers and films. The production of PET films usually involves multiple-staged processes. In one scenario, the deformed PET melt is rapidly quenched below the glass transition temperature ðT g Þ and solidified into the amorphous state. The subsequent drawing processes of amorphous PET films can be carried out either below or above T g , resulting in very different structure, morphology and properties. In our laboratory, we are particularly interested in understanding the effects of different processing conditions and material variables on the development of structure and morphology from quenched amorphous PET films. The processing conditions involve high temperature drawing above and below T g ; uniaxial and multi-axial deformation, as well as varying deformation rates. The material variables involve different molecular compositions in copolymer and polymer blends, molecular weights and distributions, as well as inclusion of nanofillers. The structural development through the pathway of mesophase formation by the deformation of amorphous PET films below T g has been studied quite extensively (most studies were carried out at room temperature). For example, Bonart was the first scientist who reported the structure changes during uniaxial stretching of amorphous PET, first forming a nematic phase and then a smectic phase [1]. Yeh and Geil pointed out that the strain-induced crystallization could be explained by rotation, alignment, and perfection of the internal order of the granule-like structure [2–6]. Asano et al. investigated the mesomorphic structural changes during the annealing of cold-drawn amorphous PET films and determined the mechanical properties by micro- indentation techniques [7]. They also described the transition from the nematic phase to the triclinic crystalline structure during the annealing process. Deformation studies of amorphous PET above T g were 0032-3861/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2003.11.027 Polymer 45 (2004) 905–918 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 1-631-632-7793; fax: þ1-631-632-6518. E-mail address: bhsiao@notes.cc.sunysb.edu (B.S. Hsiao).