Modelling of photodegradation effect on elastic–viscoplastic behaviour of amorphous polylactic acid films S. Belbachir a,b , F. Zaı¨ri a,Ã , G. Ayoub a , U. Maschke c , M. Naı¨t-Abdelaziz a , J.M. Gloaguen d , M. Benguediab b , J.M. Lefebvre d a Universite´ Lille 1 Sciences et Technologies, Laboratoire de Me ´canique de Lille (UMR CNRS 8107), Avenue P. Langevin, 59655 Villeneuve d’Ascq Cedex, France b Universite ´ de Sidi Bel Abbes, De´partement de Me´canique, 2200 Sidi Bel Abbes, Alge´rie c Universite´ Lille 1 Sciences et Technologies, Laboratoire de Chimie Macromole´culaire (UMR CNRS 8009), Bˆ at. C6, 59655 Villeneuve d’Ascq Cedex, France d Universite´ Lille 1 Sciences et Technologies, Laboratoire de Structure et Proprie ´te´s de l’Etat Solide (UMR CNRS 8008), Bˆ at. C6, 59655 Villeneuve d’Ascq Cedex, France article info Article history: Received 6 August 2008 Received in revised form 8 October 2009 Accepted 9 October 2009 Keywords: Photodegradation Elastic–viscoplastic modelling Finite strain Polymeric material abstract Polylactic acid (PLA) films were subjected to accelerated ultra-violet (UV) ageing. The UV irradiation leads to the alteration of the chemical structure which influences directly the mechanical response of the polymer. The chemical modification of the polymer was followed by gel permeation chromatography. Uniaxial tension tests were conducted at 50 1C and for different strain rates in order to characterize the large deformation response of PLA. The influence of UV irradiation on the alteration of the large deformation response of PLA was examined. A physically based elastic–viscoplastic model was used to describe the mechanical response of virgin PLA. The photodegrada- tion effect was incorporated into the constitutive model to capture the stress–strain behaviour up to failure of aged PLA. To that end, the measured molecular weight was used as a direct input into the model. The model is shown to be in good agreement with experimental results over a wide range of UV irradiation doses. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, bio-based polymers have attracted special attention in view of finding substitutes for petroleum-based polymers. Biodegradable polymers, derived from renewable biopolymers, are used for various applications such as agricultural mulch films, packaging films, containers for liquid foods, etc. Among this class of polymers, polylactic acid (PLA) is a synthetic polymer of great interest (Drumright et al., 2000; Garlotta, 2002; Auras et al., 2004) particularly for its properties close to conventional polymers. Many publications concerning the degradation mechanisms of PLA via microbes or enzymes have been recently reported (Li et al., 2000; Tsuji and Miyauchi, 2001; Tsuji et al., 2006a). In the meantime, it is well recognized that ultra-violet (UV) irradiation induces important physico-chemical changes in polymers involving a reduction of molecular weights and thus a decrease in their mechanical performance (Rabek, 1995). 1.1. Background on photodegradation The degradation of polymers has a great influence on their behaviour. Two kinds of processes may be distinguished, physical and chemical, and both are strongly linked. The degradation may be for example induced by UV irradiation, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmps Journal of the Mechanics and Physics of Solids ARTICLE IN PRESS 0022-5096/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmps.2009.10.003 Ã Corresponding author. Tel.: þ33328767460; fax: þ33 328767301. E-mail address: fahmi.zairi@polytech-lille.fr (F. Zaı¨ri). Journal of the Mechanics and Physics of Solids 58 (2010) 241–255