Chapter 17 Thermal Crystallinity and Mechanical Behavior of Polyethylene Terephthalate Sudheer Bandla, Masoud Allahkarami, and Jay C. Hanan Abstract Polyethylene Terephthalate’s (PET) properties are particularly sensitive to its processing history. Processing impacts the extent of crystallinity. Mechanical stretching and melt flow history influence the extent and structure of crystalline domains in the semi-crystalline polymer. Typical processing parameters include the rate of cooling and the amount of stretch. Both influence crystallization differently. Isolating the contribution from stretch or thermal crystallization is valuable for identifying the relationship to mechanical properties. In this work, the influence of thermal crystallinity on the mechanical behavior of PET was observed. Annealed injection molded samples with thermal crystallinity were tested Young’s modulus. Using the two-phase composite approach, mechanical behavior of injection molded semi-crystalline PET samples was modeled based on crystallinity from density. Crystallinity measured from different techniques did not always agree. Keywords Polyethylene terephthalate (PET) • Crystallinity • Young’s modulus • Density • Two-phase model 17.1 Introduction The interdependence of chemistry, processing conditions, microstructure and properties is an important aspect in the engineering of polymers [1, 2]. The ability to control the microstructure through changing the process provides a means for selecting microstructure, and thereby affecting the properties. With wide spread application of semi-crystalline polymers (e.g. PE, PP, and PET), establishing the relationship between microstructure and properties is essential for efficient material use. Modeling the behavior of polymers, in particular for semi-crystalline polymers has received considerable attention. Numerous micromechanical models were suggested considering the semi-crystalline polymer as a composite material, with amorphous and crystalline domains as different phases [3–5]. A number of intrinsic properties like: Young’s modulus, elastic and plastic deformations, and texture evolution were studied through this approach [6, 7]. Hitherto there is no single model that is applicable in general for polymers and describes their mechanical properties, due to factors like chemistry, crystallization kinetics, and type of crystallization. For example, Patel and Phillips [8] found that microstructure parameters including the spherulitic radius and lamellar thickness affect the modulus of polyethylene (PE). However, the increase in modulus was observed only up to an aspect ratio of 1,000 (ratio of length to the thickness of the crystal), from controlled crystallization (spherulitic) studies. Polyethylene terephthalate (PET) is an aromatic semi-crystalline polymer with applications in the field of textiles, packaging, and engineering molding. Crystallinity and molecular orientation affect the modulus of PET [9]. On account of its slow crystallization rate and strain sensitive characteristics, PET can be engineered to have a distinctive microstructure, and thereby distinctive properties. Therefore, correlating the mechanical behavior of PET with its microstructure will help in effectively designing the manufacturing processes. In 1958, Lyons [10] estimated the theoretical modulus for PET at 146 GPa along the molecular axis, based on bond stretching. Nevertheless, the maximum modulus reported was found to be S. Bandla • M. Allahkarami • J.C. Hanan (*) School of Mechanical and Aerospace Engineering, Oklahoma State University, Tulsa, OK 74106, USA e-mail: Jay.Hanan@okstate.edu H.J. Qi et al. (eds.), Challenges in Mechanics of Time-Dependent Materials, Volume 2: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06980-7_17, #The Society for Experimental Mechanics, Inc. 2015 141
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