Relationship between microstructure and elastic properties of semi-crystalline polymers
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EE7.9.1
Relationship between microstructure and elastic properties of semi-crystalline polymers * F. Bédoui , J. Diani, G. Régnier Laboratoire d’Ingénierie des Matériaux (LIM – UMR CNRS), ENSAM Paris, 151 bd de l’Hôpital 75013, Paris, France. * Corresponding author. E-mail address: [email protected]
Phone: (33) 1 44 24 61 06, Fax: (33) 1 44 24 63 82 1. Introduction Even though semi-crystalline polymers are closer to composite, even nano-composite material, little work has been done to predict their properties as in the case of composite or filled polymer. In their work Halpin and Kardos [1] proposed to determine the elastic modulus of semi-crystalline polymers. The lamellae are supposed to be like fibers. An adjustable parameter in this model was linked to the crystallite shape ratio. However, this model is well adapted for low volume fraction, which is not the case of semi-crystalline materials like Polypropylene (PP) and Polyethylene (PE). Other authors were interested in the evolution of the microstructure and large deformation of the semi-crystalline polymer using a new developed model [2]. The main ideas of the model were adapted by other authors to predict the elasto-visco-plastic behavior [3,4]. The predicted initial modulus variation of PE versus the crystalline fraction does not fit the experimental values. The aim of our work is to establish a relationship between the microstructure and elastic properties of the semicrystalline polymer. To fulfill this goal, experimental investigations of two kinds of polymers, Polypropylene and Polyethylene, are done. Micro-mechanical models are tested in order to choose which model is better adapted for these materials. A comparison of modeling and experiments will be presented. 2. Material description and modeling At a microscopic scale, semi-crystalline polymers are heterogeneous materials consisting of co-existing amorphous and crystalline phases. To investigate the ability of micromechanics modeling to predict homogeneous elastic behavior of semi-crystalline polymers, two materials are considered, Polypropylene and Polyethylene. 2.1. Material description Two commercial homopolymers were supplied by Solvay: Polypropylene PP ELTEX HV 252 and high density Polyethylene PE HD6070 EA. The Young’s modulus and the Poisson’s ratio were measured at ambient temperature on 4mm-thick injection-molded samples by tensile tests on an Instron 4502 machine equipped with a mechanical Instron bi-axial extensometer. The test conditions were set according to the standard ISO 527. The mean crystallinity was determined from density measurement considering a single crystalline phase in each polymer: α phase for PP and the orthorhombic phase for HDPE. The Young’s modulus of other quoted PE versus density or crystallinity is extracted from BP-Solvay Polyethylene database. 2.1.1. Characterization of the amorphous phase For both polymers, the glass transition temperature of the amorphous phase is lower than ambient temperature; therefore the amorphous phase is in the rubbery state at
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