Linear viscoelastic behavior of poly(ethylene therephtalate) above T g amorphous visco-elastic property Vs crystallinity: Experimental and micromechanical modeling

Abstract

Linear viscoelastic behavior of amorphous and semi-crystalline Pol(ethylene teraphtalate), PET, was experimentally investigated. Micromechanics models were used to predict the elastic and plastic behavior of semi-crystalline polymers successfully, however the viscoelastic behavior still difficult to be predicted with such models. Difficulties lie on the used numerical methods and also on the understanding of the properties of the amorphous phase. In this paper we tried to first simplify the Laplace Carson-based method by using a pseudo-elastic method that ovoid the numerical difficulties encountered before. The time-dependant problem is so replaced by a frequency-dependant set of elastic equations. To validate the new approach, Pol(ethylene threphtalate) (PET) with different crystallinity fraction (Xc) were prepared and characterized. Based on our experimental results (properties of the amorphous PET and semi-crystalline polymers) micromechanical model were used to first predict the viscoelastic properties of the semi-crystalline polymers and also to predict the changes on the viscoelastic properties of the amorphous phase when the crystallinity fraction increases. Good agreement between the predicted and experimental results of the low crystallinty fraction sample (X c=17%). However for high crystallinity (Xc=24% and Xc=35%) prediction based on the identified amorphous phase as an input for the micromechanical properties failed to fit the viscoelastic behavior of the semi-crystalline polymer. Based on the dynamic mechanical analysis (DMA) experimental data, changes on the glass transition temperature of the amorphous phase were observed, which could confirm the change of the viscoelastic properties of the amorphous phase upon crystallization. This result confirms the effect of confinement due to the presence of the crystalline phase on the prediction of the viscolelastic behavior of semi-crystalline polymer using micromechanical models. ©2010 Society for Experimental Mechanics Inc.