Commercial, High-Impact Polypropylenes: Composition and Chain Structure as Revealed by Temperature-Gradient Extraction Fractionation Hossein Mahdavi, Mojtaba Enayati Nook School of Chemistry, University College of Science, University of Tehran, P. O. Box 14155-6455, Tehran, Iran Received 4 April 2011; accepted 7 September 2011 DOI 10.1002/app.35637 Published online 16 January 2012 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Four different grades of commercial, high- impact polypropylene (hiPP) were fractionated by temper- ature-gradient extraction fractionation, and the chain structure and melting behavior of the fractions were stud- ied by Fourier transform infrared spectroscopy and differ- ential scanning calorimetry. Furthermore, the morphology of the disperse phase in the resins was characterized by scanning electron microscopy of the microtome-cut etched and original samples. The results show that there was a strong relation between the chain structure, content, and distribution of the dispersed phase and the mechanical properties of hiPP. These parameters of the elastomeric phase are really critical in reaching the best rigidity-impact balance in hiPP. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: 1606–1615, 2012 Key words: fractionation of polymers; morphology; polyolefins INTRODUCTION Isotactic polypropylene (iPP) is one of the most important commodity polymers; it has many appli- cations because of its good mechanical properties and favorable cost-to-performance ratio. Despite its good mechanical properties, the brittleness and low impact properties of polypropylene (PP) in low temperatures seriously limit its applications. 1 This drawback of iPP has been well known for a long time, and a variety of techniques are available to eliminate it; these mainly focus on improving iPP’s toughness. 2–6 Some of these methods include the blending of iPP with rubbers, 7 the addition of nucle- ating agents to reduce the average size of iPP spher- ulites, 8,9 and copolymerization with ethylene. 10 The latter approach, however, is the best method for toughening iPP with respect to both the polymer properties and production cost. 11,12 There are some industrial processes for the copolymerization of propylene with ethylene in a two-reactor system, such as the Catalloy and Spheri- pol techniques of Montell and Himont. 13,14 In these processes, the homopolymerization of propylene proceeds in the first reactor with a spherical super- active TiCl 4 /MgCl 2 -based catalyst; then, polymer particles are transferred into the second reactor, where ethylene is introduced to be copolymerized with propylene. Because active site types on the catalyst differ in their propagation/transfer ratios and comonomer reactivity ratios, in the second reactor, a variety of ethylene–propylene copolymers, from completely amorphous ethylene–propylene random copolymers (EPRs) to semicrystalline ethylene–propylene block copolymers, are formed and finely dispersed in the cavities of the preformed iPP particles. The high-impact polypropylene (hiPP) produced in this two-reactor system (called in situ or in reactor blend/alloy) is a mixture of multiple components with a complex structure and a high-tech commodity polymer with a yearly increase of 10%. 15 The microstructures of polyolefin alloys mainly determine their properties and applications. There- fore, the determination of the complete molecular structure and morphological analysis of such poly- mers is a desirable task, and a variety of techniques has been applied to accomplish this work. There have been many structural studies of hiPP that have investigated the composition of blends with different fractionation methods, such as temperature- rising elution fractionation (TREF) 16,17 and tempera- ture-gradient extraction fractionation (TGEF). 18,19 Structural studies also have analyzed the chain struc- ture of the resin (mostly of its fractions) with infrared spectroscopy, 20,21 NMR spectroscopy, 22,23 thermal analysis, and other common methods. 24,25 Morphological studies of hiPP have mainly aimed to clarify two aspects: (1) the architecture of the iPP Correspondence to: H. Mahdavi (hmahdavi@khayam.ut.ac.ir). Journal of Applied Polymer Science, Vol. 125, 1606–1615 (2012) V C 2012 Wiley Periodicals, Inc.