Crystallization Kinetics in Isotactic Polypropylene Films with Carbon Nanotubes
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Crystallization Kinetics in Isotactic Polypropylene Films with Carbon Nanotubes Georgi Georgiev1,2, Scott Schoen2, Devin Ivy2, Lauren Wielgus2, Yaniel Cabrera2, and Peggy Cebe2 1 2
Department of Natural Sciences, Assumption College, Worcester, MA 01609 Department of Physics and Astronomy, Tufts University, Medford, MA 02155
ABSTRACT Polymer nanocomposites are the largest commercial application for carbon nanotubes (CNTs) which determines the interest in their effect on crystallization processes of polymers. We chose Isotactic Polypropylene (iPP) as one of the most widely used polymers. Nanocomposites with multiwall carbon nanotubes (MWCNTs) 0-5% by weight were studied, using differential scanning Calorimetry to measure the crystal nucleation and kinetics effects of MWCNTs. Isothermal crystallization at 138°C was performed and the data were analyzed using Avrami analysis. We obtained results for the effect of MWCNTs on the crystallization kinetics. The Avrami analysis showed a dramatic increase in the crystallization rate constant and constancy of the Avrami exponent with increase of the CNTs concentration. The full width at half maximum (FWHM) of the heat flow exotherm and the peak time for crystallization (tp) change dramatically. The crystallinity shows a slight variation with the CNTs concentration dipping at 2% CNTs which can be explored further at higher concentrations. INTRODUCTION Kinetics of crystallization and the mechanisms of nucleation in polymers are influenced by the presence of carbon nanotubes (CNTs) [1]. We are exploring the effects of different concentrations of multiwall carbon nanotubes on the crystallization kinetics of isotactic polypropylene (iPP) isothermally crystallized at 138°C. The largest effect on kinetics that we observe is for concentrations of CNTs at or below 1% by mass, which is different than what other groups observe [2,3,4]. With increase of the concentration of nanotubes, we observe a decrease of the crystallization time during isothermal annealing, and an increase of the crystallization temperature during the nonisothermal crystallization. We analyze the nucleation and crystallization kinetics by using Kolmogorov-Johnson-Mehl-Avrami (JMAK) theory [5]. The Avrami exponent is not significantly affected by the addition of CNTs, but the rate constant K, the FWHM of the heat flow exotherm, the tp and the crystallization half-time IJ1/2 are affected greatly, results consistent with other reports [6]. THEORY The theory of phase transitions is extremely useful because it can be applied to phenomena from different disciplines exhibiting the same type of phase transition. Kolmogorov-JohnsonMehl-Avrami (JMAK) theory describes the decay of a metastable system to a unique equilibrium phase. This decay is driven by the difference between the free energy densities of the metastable and the equilibrium phase. The nucleation processes occur randomly in the metastable phase and the regions grow freely. The degree of phase transformation and in our case the fraction of volume transforming from
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