Polypropylene random copolymer/MWCNT nanocomposites: Isothermal crystallization kinetics, structural, and morphological interpretations Pawan Verma, Veena Choudhary Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India Correspondence to: V. Choudhary (E- mail: veenach@hotmail.com or veenach@polymers.iitd.ac.in) ABSTRACT: This article describes the crystallization process of polypropylene random copolymer (PPCP) under isothermal conditions in presence of varying amounts of multiwalled carbon nanotubes (MWCNT) ranging from 0.5 to 4.0% w/w. Increase in the crystalli- zation temperature under dynamic conditions confirmed the nucleating behavior of MWCNTs, which was also corroborated by crys- tallization studies under isothermal conditions. The crystallization kinetics was analyzed using Avrami equation and the parameters such as Avrami exponent, the equilibrium melting temperature and fold surface energy for the crystallization of PPCP chains in nanocomposites were obtained from the calorimetric data in order to determine the effect of MWCNTs on these parameters. Spheru- litic growth of PPCP crystals was also investigated as a function of time and MWCNT content using hot stage polarizing microscope. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41734. KEYWORDS: composites; crystallization; graphene and fullerenes; kinetics; nanostructured polymers; nanotubes Received 18 June 2014; accepted 8 November 2014 DOI: 10.1002/app.41734 INTRODUCTION Copolymerization of propylene with ethylene or other olefins is an impressive way to produce high-impact polypropylene resins without using elastomers. 1–5 Random or block ethylene–propyl- ene copolymer with low ethylene content is commercially valua- ble because the ethylene improves the impact properties of polypropylene without seriously affecting the other desirable properties. In such copolymers, ethylene sequences also decrease the structural regularity which may also affect the crystallization behavior of polypropylene. Therefore, a comprehensive study of crystallization behavior is essential to optimize the processing parameters, such as mold temperature and holding time, which further help to achieve desired crystal morphology for specific applications at lowest possible processing time. It also helps to understand and control the crystallization behavior of materials. Basically crystallization is a two stage process: nucleation and growth. Usually differential scanning calorimetry (DSC) is used to determine such type of thermophysical properties of poly- mers whereas Avrami and Hoffman-Lauritzen analysis were used to evaluate the overall crystallization kinetics of polymer. Avrami analysis is based on the assumption that the heat released during crystallization is directly proportional to the rel- ative crystallinity whereas Hoffman-Lauritzen analysis suggests that the nucleation and movement of the macromolecules were controlled by the crystallization in the melt. 6–12 It is well reported in the literature that the crystallization kinetics of polymer is very much affected by the presence of external materials (filler). The most commonly used fillers include various grades of calcium carbonate, quartz, mica, silica flour, talc, various clays, fibers, and carbon nanotubes (CNTs). Among all, CNTs have become the most important nanofiller for polymer matrix because of its extraordinary thermal, mechanical, optical, and electrical properties. 13–15 Incorporation of CNTs disturbed the overall crystallinity and crystal structure of polymer which reveals the change in prop- erties of resulting composites. 16–18 A number of studies have explained the structure property relationship of CNT/polymer composites in the last few years. 19–21 It has been observed that CNTs act as a nucleating agent which enhances the rate of crystallization and properties of polymer matrix. 22–27 Jin et al. 28 reported the effect of modified and unmodified MWCNTs in poly(ethylene oxide). In this study, the number of nucleating sites reduced and the crystal size changed from spherical to disk like. Diez-Pascual et al. 29 also reported a decrease in the crystallization temperature of poly(ether ether ketone) upon incorporation of SWCNTs without affecting the melting temperature. Choudhary et al. reported that the addi- tion of MWCNTs did not affect the crystallinity of poly(tri- methylene terephthalate) (PTT); however, it restricted the spherulitic growth of PTT under isothermal and nonisothermal crystallization process. 30 V C 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2015, DOI: 10.1002/APP.41734 41734 (1 of 13)