Carbon Nanotube-Induced Changes of Crystal Growth In Polymer Films
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Carbon Nanotube-Induced Changes of Crystal Growth In Polymer Films Georgi Georgiev1,2, Yaniel Cabrera2, Lauren Wielgus2, Zarnab Iftikhar1, Michael Mattera1, Peter Gati1, Austin Potter1 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 Isotactic Polypropylene (iPP) nanocomposites with low concentrations of multiwall carbon nanotubes (CNTs) 0-1% were studied, using differential scanning calorimetry and Avrami analysis. The nanocomposites were isothermally crystallized at 135°C, in order to measure the effect of nanotubes on the kinetics of crystallization. In our study there is a great effect of the CNTs on the iPP crystallization kinetics. The Avrami analysis shows increase in the crystallization rate constant and constancy 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. For iPP, the carbon nanotubes serve as nucleation agents to speed up the crystallization process. INTRODUCTION Crystal nucleation processes and rates of growth in polymers are modified 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 135°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 τ1/2 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 one phase to another, f(t), is described by:
f(t)=1-exp(-Ktn)
[1]
where t is the time, K is the temperature dependent crystallization rate constant and n – the Avrami exponent - represents growth and nucleation behavior. This equation has been applie
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