Strain recovery of post-yield compressed semicrystalline poly(butylene terephthalate) Alessandro Pegoretti a, * , Stefano Pandini b , Theonis Ricco b a Department of Materials Engineering and Industrial Technologies and INSTM Research Unit, University of Trento, via Mesiano 77, 38050 Trento, Italy b Department of Chemistry and Physics for Engineering and Materials and INSTM Research Unit, University of Brescia, via Valotti 9, 25123 Brescia, Italy Received 29 November 2005; received in revised form 17 May 2006; accepted 6 June 2006 Available online 7 July 2006 Abstract Cubic specimens of a semicrystalline poly(butylene terephthalate) (PBT) have been compressed up to post-yield deformation levels with a fast (3.0  10 2 s 1 ) and a slow (1.5  10 4 s 1 ) strain rate at three different temperatures (25  C, 45  C, and 100  C, i.e. below, close and above the glass transition temperature of the material, T g , respectively). Differently from literature results reported for amorphous polymers, semicrystalline PBT shows that, after a post-yield deformation, recovery occurs also at temperatures higher than T g , and that an irreversible deformation, 3 irr , is set in the material. The irreversible strain component has been evaluated as the residual deformation after a thermal treatment of 1 h at 180  C. After unloading, isothermal strain recovery has been monitored for time periods of 1 h at various temperatures. From the obtained data, strain recovery master curves have been constructed by a timeetemperature superposition scheme. The features of the recovery process for the various deformation conditions have been analysed. In particular, it appears that specimens deformed below T g show a lower irreversible component, whereas, when deformed above T g , they display a higher irreversible deformation and a slower recovery process. Moreover, the effect of deformation rate appears particularly marked for samples deformed above T g . Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Strain recovery; Deformation; Poly(butylene terephthalate) 1. Introduction Post-yield deformation behaviour of polymers has been ex- tensively studied by several different methods, including calo- rimetric techniques [1e21], and measurements of dimensional variations after unloading (strain recovery) [3,4,7,11,15,16,19, 20,22e41]. In recent years, many efforts have been also focused on understanding the mechanical behaviour of highly deformed semicrystalline polymers by X-ray diffraction studies regarding the evolution of crystallographic texture [42e50]. Generally speaking, the deformation of polymers at high strain levels is always accompanied by changes in their microscopic structure that result in an alteration of the material state; after the applied load is released, specimens attempt to revert to their un-deformed state, eventually recovering the original shape. As pointed out by Moore and Turner [51], recovery tests were usually less intensively adopted to inves- tigate the viscoelastic or viscoplastic behaviour of polymeric materials. In fact, more common tests like creep and stress relaxation are preferred. Nevertheless, the strain recovery test has revealed itself as a powerful method for indicating the presence of both reversible and irreversible features in the deformation of polymeric materials [3,4,7,11,15,16,19, 20,22e41]. Amorphous polymers deformed in the glassy state have been proven to undergo a complete dimensional recovery in short times when heated to or above their glass transition tem- perature (T g ), even when deformed at high strain levels [4,22, 23,27,29,36,39]. To our knowledge, the only evidence of a permanent (even if very small) deformation in amorphous * Corresponding author. Tel.: þ36 0461 882452; fax: þ39 0461 881977. E-mail address: alessandro.pegoretti@ing.unitn.it (A. Pegoretti). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.06.020 Polymer 47 (2006) 5862e5870 www.elsevier.com/locate/polymer