Micro-and macromechanics of single crystal and polygrannular lamellar block copolymers

Abstract

Block copolymers (BCPs) are a relatively new class of thermoplastic elastomers. Their macromolecular chain consists of covalently bonded repeating blocks of thermoplastic and elastomeric molecular chains. When given the thermodynamic freedom, the chain constituents phase separate into domains of various morphologies with sizes that can range between ten to hundreds of nanometers. BCPs are in essence nanocomposites with chemically bonded interfaces. As such, their mechanical behavior is consistent both with that of elastomers, and of thermoplastics. Due to this unique behavior, BCPs are among the most popular polymeric materials with diverse commercial applications that cover a number of industries. Furthermore, BCPs are emerging as instrumental for the future of nanotechnology as an increasing number of new techniques and applications seek to utilize their nanostructural features. BCPs, whether as polycrystalline configurations or as "highly" oriented single-crystals, attract an accumulating number of applications, and the increasing demand for efficient material design and product development extends over a range of length scales. Hence, there exists a need for continuum models that will predict both the oriented as well as the polycrystalline response of block-polymer materials to generic loading conditions. This thesis presents a general micromechanical framework for the derivation of large-strain continuum constitutive models for hyperelastic materials with layered micro- or macro-structures. The framework was implemented for the case of oriented (single-crystal) lamellar BCPs with Neo-Hookean phase behavior, and an analytical continuum model was derived for their large-strain hyperelastic response.

(cont.) The model was used to study the behavior of styrene-butadiene-styrene (SBS) triblock polymers, the behavior and micromechanics of which have been extensively investigated experimentally. Micromechanical unit-cell calculations were used as direct parallels to experimental (x-ray, microscopy, and stress-strain) data in order to verify the model's predictions. The presented continuum model describes the stress and deformation response of an oriented microstructure accurately, and was further implemented in multigranular numerical studies for the mechanical behavior of polycrystalline lamellar configurations. Simulations of the polycrystal structures reveal the manner in which the individual grains collectively deform and interact with each other to accommodate the macroscopic deformation. These results reveal the key roles of interlamellar shearing, lamellar dilation, rotation, and buckling.