Deformation-Induced Phase Transition and Superstructure Formation in Poly(ethylene terephthalate) Daisuke Kawakami, † Benjamin S. Hsiao,* ,† Christian Burger, † Shaofeng Ran, † Carlos Avila-Orta, † Igors Sics, † Takeshi Kikutani, ‡ Karl I. Jacob, § and Benjamin Chu † Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan, and School of Polymer, Textile & Fiber Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta, Georgia 30332-0295 Received April 5, 2004; Revised Manuscript Received October 24, 2004 ABSTRACT: Deformation-induced phase transitions and superstructure formation in poly(ethylene terephthalate) (PET) were studied by means of in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) as well as Raman spectroscopy. The deformation conditions involved uniaxial stretching of quenched PET films at a temperature just below its glass transition temperature (T g), where a notable “plastic deformation” stage was observed. WAXD results indicated that the initial sample contained a “slush” structure (amorphous + nematic), whereby deformation induced oriented amorphous, nematic, smectic (C and quasi-A), and stable triclinic crystalline phases. SAXS results indicated that the fibrillar superstructure was formed upon the formation of oriented slush. In-situ Raman spectroscopic data revealed the orientation information on ethylene glycol and benzene ring as well as the gauche-trans transition in deformation of PET chains, which are in good agreement with X-ray results. A mechanism for deformation-induced phase transitions and for hierarchical structure formation has been proposed to correlate the structural information with the mechanical properties. 1. Introduction The recent simulation work by Frenkel et al. 1 indi- cated that the isotropic-nematic transition occurs when the aspect ratio of the mesogenic molecule (L/D, where L represents the length and D represents the diameter) is in the range of 3-4. In view of poly(ethylene tereph- thalate) (PET), its persistence length is about 1.33 nm, and the average molecular diameter is about 0.66 nm based on the experimental work by Imai et al. 2 This suggests that the aspect ratio of the PET mesogenic unit is around 2, which falls short of being a liquid crystalline polymer. However, the existence of mesomorphic phases has been well documented in oriented PET samples. Bonart was the first to report the formation of nematic and smectic phases during the tensile stretching of PET using the conventional X-ray diffraction method. 3,4 Recently, with the use of high-intensity synchrotron X-rays, more detailed features of mesomorphic phases in oriented PET have been revealed by different re- search groups. For example, Windle et al. reported the formation of a transient smectic phase in oriented fibers made of random PET and PEN (polyethylene naphtha- lene-2,6-dicarboxylate) copolymers. 5,6 Asano et al. re- ported the appearance of smectic order at 60 °C having a spacing of 1.07 nm during the annealing of cold-drawn amorphous PET films. 7 Blundell and co-workers ob- served the smectic A structure during the fast extension of PET. They proposed that the smectic structure is a precursor of the crystalline structure based on the simultaneous appearance of the triclinic crystalline peak and disappearance of the smectic peak. 6,8-12 Hsiao and co-workers reported the smectic C phase during the deformation of amorphous PET film below the glass transition temperature (T g ) at 50 °C. 13 They observed that the mesophase developed immediately upon the neck formation. As the mesophase contained a sharp meridional peak 001′ (d ) 1.032 nm), which was smaller than the monomer length in the typical triclinic unit cell (c ) 1.075 nm), they concluded that the chains in the mesophase formed an inclined smectic C structure. Thus, PET may be considered as a “marginal” liquid crystalline polymer. Its mesomorphic transitions are not obvious in the unoriented (isotropic) state but are distinct in the oriented state. In a way, the kinetic pathways of the phase transition in PET may be too long to be observed under the isotropic conditions, while the deformation process can overcome the kinetics barrier and reveal all possible transitions. It is conceivable that the varying mesomorphic phases reported in the earlier studies are simply due to the different experimental conditions involved, where all mesomorphic structures (e.g., oriented amorphous, nematic, smectic C, and quasi-smectic A) can be induced by stretching. In the present study, our first objective was to construct a deformation-induced phase transition mechanism for PET, using an amorphous sample as the starting phase at a suitable temperature where all the reported me- somorphic phases can coexist. The second objective of this study was to correlate the phase transition with the superstructure formation in PET under uniaxial deformation, which was prompted by two recent studies. Asano et al. observed that the smectic A structure (observed by WAXD, with the characteristic meridional d spacing ) 1.07 nm) was related to the occurrence of lamellar layer structure having a 10 nm repeat spacing (by SAXS) during the † State University of New York at Stony Brook. ‡ Tokyo Institute of Technology. § Georgia Institute of Technology. * To whom correspondence should be addressed: Tel 631-632- 7793; Fax 631-632-6518; e-mail bhsiao@notes.cc.sunysb.edu. 91 Macromolecules 2005, 38, 91-103 10.1021/ma049333x CCC: $30.25 © 2005 American Chemical Society Published on Web 12/08/2004