Recovery of post-yielding deformations in semicrystalline poly(ethylene-terephthalate) A. Pegoretti * , A. Guardini, C. Migliaresi, T. Ricco University of Trento, Department of Materials Engineering, via Mesiano 77-38050, Trento, Italy Received 12 February 1999; received in revised form 12 April 1999; accepted 5 May 1999 Abstract In this paper, aspects of the non-elastic deformation of semicrystalline poly(ethylene-terephthalate) (PET) films were studied from strain recovery and differential scanning calorimetric tests. The results show the existence of two components of non-elastic deformation, i.e. a fast- relaxing component (called anelastic) and a slow-relaxing component (usually called plastic). These strain components are both reversible and distinguished only on the basis of their different recovery times at temperatures far below the glass transition. A strain recovery master curve was built from the results of recovery tests at increasing recovery temperature. The shift-factor for the strain recovery master curve was then compared with the shift-factor for the construction of the dynamic storage modulus master curve obtained in linear regime (small strain). The aim of this comparison was to investigate the viscoelastic nature of yielding and post-yielding behavior in a semicrystalline polymer.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Non-elastic deformation; Semicrystalline polymers; Viscoelasticity 1. Introduction The viscoelastic nature of polymers determines a mechanical behavior strongly dependent on parameters like time, temperature and strain rate, on the basis of rela- tionships which, in linear regime (small strain), are described by viscoelastic models and relative constitutive equations [1]. However, the viscoelastic processes involved during deformation make the determination of the yielding onset difficult. This is conventionally fixed, by following the Conside ´re’s criterion [2], as the point of relative maximum of the nominal stress–strain curve. Nevertheless, experi- mental evidences for amorphous glassy polymers suggest that a more attentive analysis is required [3–10]. In fact, for amorphous glassy polymers like poly(methyl-methacry- late) (PMMA), polystyrene (PS), and polycarbonate (PC), it has been proven how non-elastic deformation consists of two distinct contributes, called anelastic and plastic, which are both reversible upon heating, for strains up to 50% and more [3,4]. The distinction between the two components is made on the basis of different characteristic recovery times at temperatures below the glass transition, i.e. at T  T g - 20°C [4,6]. A molecular model for glassy polymers, first proposed by Oleynik [3], was based on a “crystal like” mechanism of plasticity. According to this model, further developed by Perez et al. [11–14], and other authors [7–9,15], at tempera- tures far below the glass transition, plastic strain nucleates as thermo-mechanically activated localized shear induced defects, called shear micro domains (SMD), in the presence of pre-existing quasi point defects (QPD). These are points of fluctuation in the value of local free volume of the amor- phous matrix, predisposed for the easy nucleation of defects in a way that recalls the nucleation of dislocations in the crystal habit of a metal [7,9]. The borderline elastic constraint between SMDs and the undeformed matrix is the driving force for the partial strain recovery after unloading. Due to the lack of literature information regarding the non-elastic deformations in semicrystalline polymers, the aim of the present study is to provide a contribution to the understanding of yield and post-yield behavior of a semicrystalline poly(ethylene-terephthalate) (PET) film. 2. Experimental 2.1. Materials A commercial PET film, Mylar (Du Pont), 52 mm in thickness, was used. Due to the biaxial orientation of the Polymer 41 (2000) 1857–1864 0032-3861/00/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(99)00333-X * Corresponding author. Tel.: +39-0461-882413; fax: +39-0461-881977. E-mail address: alessandro.pegoretti@ing.unitn.it (A. Pegoretti)