Micromechanics of PS/PB/PS Triblock-Copolymer Films with Lamellar Morphology
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Micromechanics of PS/PB/PS Triblock-Copolymer Films with Lamellar Morphology Theodora Tzianetopoulou and Mary C.Boyce Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. ABSTRACT Highly oriented, near single-crystal lamellar triblock copolymer films with alternating rubbery-glassy layer morphology possess an interesting anisotropy in their mechanical behavior, which can prove to be attractive for various applications. Here, the deformation micromechanics of highly oriented styrene-butadiene-styrene (SBS) lamellar triblock copolymers are investigated. The competing roles of the glassy (PS) and rubbery (PB) phases in the layered configuration are studied with micromechanical finite element based representative volume element (RVE) models subjected to tensile loading at different directions to the lamellar plane. The calculated material response is compared to x-ray and microscopy data from the literature, and proves to be successful in linking the macroscopic material response to the underlying physics of the microstructural evolution. Moreover, the RVE simulations are successful in reproducing the mechanical behavior of highly oriented nano-clay block copolymer nanocomposites. The response of layered-silicate models having 2wt% clay content is compared to the behavior of the respective neat block copolymer (BCP) and to existing experimental data. The ultimate objective is to facilitate these micromechanical models to design hierarchically ordered materials with properties, which will be optimally tailored for different classes of applications. INTRODUCTION Thermoplastic elastomers (TPEs) are a relatively new class of materials that exhibit behavior consistent with both thermoplastics and elastomers. The most widely used class of TPEs are styrenic block copolymers (BCPs) which microphase separate into rubbery and glassy domains at a lengthscale of tens of nanometers. Their numerous commercial applications (automotive, wire and cable, coatings, footwear, medical) and their substantial promise for utilization in nanotechnologies [12] have raised the need to understand the link between their morphology and their mechanical properties, and how the latter can be altered with interventions on the microstructure, be those microdomain size/orientational changes or particle addition. Several studies in the literature, offering macroscopic stress-strain data as well as X-ray and microscopy observations [1,3,4,5,7,8,14], aim to do that. However, such in-situ monitoring techniques, being used on the material at bulk and at discrete instances, are not always definitive about the nature of the actual phenomena that take place. In the current research, we use finite element micromechanical models to reproduce the deformation process and continuously monitor the structural changes that take place during the mechanical loading of oriented SBS lamellar triblock copolymers. When possible, we correlate our numerical analysis results with existing data in the literature.
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