The role of microstructure, molar mass and morphology on local relaxations in isotactic polypropylene. The a relaxation Mario Hoyos, Pilar Tiemblo, Jose ´ Manuel Go ´mez-Elvira * Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain Received 19 July 2006; received in revised form 17 October 2006; accepted 16 November 2006 Available online 5 December 2006 Abstract This study aims to assess the relative influence of microstructure, molar mass and morphology on the local a relaxation of isotactic poly- propylene (iPP). For that purpose, three families of samples have been prepared. The ZieglereNatta type provides a wide molar mass range where both the isotactic average length (n 1 ) and the nature of the isotacticity interruptions change according to chain size. The metallocene type allows to analyse also the samples in which molar mass and n 1 are related but the nature of the defects stays the same. Finally, three propylene-like EP copolymers (containing only isolated ethylene units) have been synthesised via metallocene catalysis in order to study samples which combine both relatively high molar masses and short n 1 . Among the factors considered, microstructure and n 1 in particular have been found to drive mainly the quality of the a relaxation. Concerning the intensity, it has been observed that some microstructural features related to n 1 must be fulfilled for this dynamics to take place. Provided this requirements are met, the intensity of the a relaxation depends on the final crystalline distribution resulting from a given molar mass, micro- structure and processing conditions. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polypropylene; a Relaxation; Microstructure 1. Introduction It is well known that local dynamics of chains determine the mechanical performance of isotactic polypropylene (iPP) at temperatures below 0  C. As a matter of fact, the small scale g relaxation drives the damping response under T g , typically between 130  C and 0  C. Furthermore, local dynamics contribute also, together with the glass transition (b relaxa- tion), to the mechanical response of this thermo-plastic at tem- peratures well-above room temperature. In fact, it is between 50 and 100  C approximately when the a relaxation is active. The implications of local dynamics features in the iPP per- formance are not only restricted to the mechanical behaviour, but also involve the thermo-oxidative stability of this polymer. Previous work has evidenced a relationship between the thermal stability of iPP and both a and g relaxations [1e3]. In particular, it was observed that (i) initiation of oxidation is concomitant with a partial vanishing of the g relaxation [2,3] and (ii) the a relaxation is associated with an abrupt change of p 0 [1], a parameter which reflects the ratio between the contents of initiating and propagating units in the thermo- oxidation [4]. From these results, it turns out that the g relax- ation is related to units which promote the thermal initiation while a relaxation is associated with a stabilisation effect. The opposite influence of both relaxations on the thermal stability of iPP has been confirmed in a recent study [5], which evidences that long degradation induction times are exhibited by iPP specimens characterized by high a and low g relaxa- tions, and short induction times by those iPP with low a and high g relaxations, whatever the type (ZieglereNatta or metal- locenic), the molar mass and the microstructure may be. Initiation of oxidative degradation is admitted to start at the inter-phase region [6]. This fact, together with the implication of both relaxations in the thermal oxidation, suggests that these dynamics are to a high extent associated with this zone. Actually, the a relaxation has been described in * Corresponding author. Tel.: þ34 915622900; fax: þ34 915644853. E-mail address: elvira@ictp.csic.es (J.M. Go ´mez-Elvira). 0032-3861/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.11.034 Polymer 48 (2007) 183e194 www.elsevier.com/locate/polymer