Mesomorphic form of isotactic polypropylene in stereodefective polypropylene: Solid mesophase or liquid-crystal like structure Claudio De Rosa * , Finizia Auriemma, Nella Galotto Galotto, Rocco Di Girolamo Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy article info Article history: Received 8 December 2011 Received in revised form 24 March 2012 Accepted 27 March 2012 Available online 2 April 2012 Keywords: Mesophase Stereodefective polypropylene Disordered crystals abstract A study of the crystallization of the mesomorphic form of isotactic polypropylene (iPP) in samples of different stereoregularity prepared with metallocene catalysts is presented. Contrary to what claimed in the literature, we have found that the mesomorphic form can be obtained by quenching of the melt at 0  C also in the case of low stereoregular samples, provided that the samples are kept at 0  C for long time. The key is the formation of mesomorphic bundles with chains in ordered 3/1 helical conformation of size large enough to be stable and inhibit the crystallization of the a form at room temperature. For stereoirregular samples the concentration of long ordered helical stretches is low and this requires long residence time at 0  C for the formation of mesomorphic aggregates of size larger than the critical size. This result provides evidence that the mesomorphic aggregates are not formed as a result of a cooper- ative process driven by entropy, as in the case of liquid crystals, but rather they form via a nucleation and growth mechanism, as in the normal crystallization processes, according to the idea that the mesophase of iPP is a highly defective crystalline form. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The mesomorphic form of isotactic polypropylene (iPP) is generally obtained by rapid quenching the melt in cold water [1e 10]. By 1984 the mesophase was identified as metastable and able to form only at temperatures below its glass transition temperature [11a]. A prerequisite for the formation of the meso- phase is prior suppression of development of the stable monoclinic a form crystals at low supercooling, by rapid cooling of the liquid faster than about 100  C/s [10]. Cooling to a temperature lower than the glass transition faster than 1000  C/s even inhibits mesophase formation, with formation of amorphous glass. The mesophase, however, then forms by cold crystallization on subsequent heating of the amorphous glass to room temperature [8,9]. The origin of formation and the structural organization of the mesophase of iPP are considered still open questions, and have deserved in the past a long discussion in the literature [1e6,8,11e 15]. In particular, the mesomorphic form of iPP has been described as a highly defective crystalline phase (a solid meso- phase) [13] or a liquid-crystal like structure [15]. The most important structural features of the mesomorphic form of iPP have been revealed mainly from analysis of the X-ray powder diffraction profile and of the fiber diffraction pattern [13,14]. The powder diffraction profile shows only two broad reflections at 2q ¼ 15  and 21.3  , which are in the fiber diffraction pattern confined on the equator and first layer line, respectively. The layering of the diffracted intensity in the fiber pattern of the mesophase and the corresponding measured value of the chain axis c of 6.5 Å [2,13,14], along with infrared spectra [2], have indicated that the mesophase of iPP is composed of bundles of parallel chains in the ordered three- fold helical conformation, as in the a form [1]. It was also recog- nized that in the small mesomorphic aggregates a long-range order is maintained only along the chain axes because of the ordered chain conformation, whereas large amounts of disorder in the lateral packing of the helical stretches would be present [1,2,13,14]. Owing to these structural features, Natta, Peraldo and Corradini [2] gave the name “smectic” to this mesophase, to indicate a degree of order higher than that of an ideal nematic liquid crystal, in which the only degree of order is the parallelism of the rod-like molecules. The presence of some correlation between the positions of adja- cent helices, their z coordinates, and their rotational coordinates was also suggested by Wykoff [3]. An even higher degree of order was proposed by Gailey and Ralston [5], who suggested that the quenched material is composed of very small (50e100 Å) hexagonal crystals, whereas Bodor et al. [4] assumed that the quenched form is composed of microcrystals of monoclinic polypropylene and concluded that crystal size broadening is responsible for the typical X-ray diffraction pattern. On the other hand, the rather diffuse X-ray diffraction pattern * Corresponding author. Tel.: þ39 081 674346; fax: þ39 081 674090. E-mail address: claudio.derosa@unina.it (C. De Rosa). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.03.045 Polymer 53 (2012) 2422e2428