A thermo-mechanical finite deformation theory of plasticity for amorphous polymers : application to micro-hot-embossing of poly(methyl methacrylate)

Abstract

Amorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-visco-plastic behavior has, until now, not been very well understood. The behavior has previously been modeled with mixed success by existing constitutive theories. As a result, there is currently no generally agreed upon theory to model the large-deformation, thermo-mechanically coupled, elasto-visco-plastic response of amorphous polymeric materials spanning their glass transition temperatures. What is needed is a unified constitutive framework that is capable of capturing the transition from a visco-elastic-plastic solidlike response below the glass transition temperature, to a rubbery-viscoelastic response above the glass transition temperature, to a fluid-like response at yet higher temperatures. We have developed a continuum-mechanical constitutive theory aimed to fill this need. The theory has been specialized to represent the salient features of the mechanical response of poly(methyl methacrylate) in a temperature range spanning room temperature to 60C above the glass transition temperature #g 110C of the material, in a strain-rate range of 10-4/s to 10-1/s, and under compressive stress states in which this material does not exhibit crazing. We have implemented our theory in the finite element program ABAQUS/Explicit. The numerical simulation capability of the theory is demonstrated with simulations of the micron-scale hot-embossing process for manufacture of microfluidic devices.