Structure-Properties Relationships in Processed Poly(ethylene terephthalate)

Abstract

Abstract. Upon processing, polymeric products feature a complex microstructure. Besides evolving over the molded component, a through-the-thickness variation is also developed. This is the result of the thermo-mechanical environment (combined thermal and mechanical fields) applied during processing, which varies with the molding technique, the selected molding conditions and polymer properties (rheological, thermal, constitution). This complex microstructure makes rather intricate the establishment of structure-properties relationships in processed polymers. In fact, the basic identification of most relevant morphological parameters determining the behavior of the moldings is been revealed a difficult endeavor, further complicated by the multi-scale morphology presented by polymeric materials. This work follows an inductive approach for establishing the relationships between the structure and the properties (mechanical and barrier) of molded poly(ethylene terephthalate), PET. These relationships are investigated for specimens prepared by different methods, from “simple” to more “complex” stretching modes. Initially, PET specimens were prepared by stretching thin films at different high temperatures and strain rates, followed by quick cooling in a universal testing machine equipped with a thermal camera (uniaxial stretched specimens). More closely to processing, PET injection molded preforms were free blown without a mold with distinct conditions (free blown specimens). Finally, PET bottles were produced from the preforms also under different blown conditions. The morphology of all specimens was assessed by bi- and tri-refringence and DSC. The mechanical properties were evaluated by tensile tests at room temperature. Also, the oxygen transmission rate, OTR, was assessed for the PET bottles. For this low crystallinity and slowly crystallizable polymer, the initial modulus is mainly related to the amorphous phase (i.e., molecular mobility and orientation level). The yield stress appears to be determined by the degree of crystallinity and level of molecular orientation. In the case of free blown specimens (bi-axially stretched) the anisotropy of the initial modulus depends upon the induced anisotropy of the molecular orientation. OTR is influenced by the molecular orientation and the degree of crystallinity of the polymer. An attempt to interpret these types of relationships by molecular dynamics simulations is made.