Material Characterization, Constitutive Modeling and Finite Element Simulation of Polymethyl methacrylate (PMMA) for Applications in Hot Embossing

Polymethyl methacrylate (PMMA) is an amorphous thermoplastic used in various industrial applications. PMMA is compatible with human tissues and allows high resolution features to be embossed onto a surface, thus making it highly desirable for use in bio-medical, micro-optics, micro-fluidic devices, electronics, micro-electro-mechanical systems (MEMS), etc. The processes used to fabricate these devices capitalize on the fact that the mechanical behaviour of the polymers changes drastically around the glass transition temperature (Tg). The polymer is deformed at temperatures above the Tg where the material is more fluid-like and then cooled below the Tg where it behaves more like a solid. The changes in physical properties make this temperature regime highly favourable for these warm-temperature deformation processes. The same rationale also makes it more difficult to develop a continuum model which accurately predicts the polymer behaviour with temperature and strain rate dependence across the glass transition temperature. Most of the existing constitutive models do not achieve this task; they either work below or above glass transition, but not in both these regions. Hence, there is a greater need to develop a constitutive model for the polymer that can capture the material behaviour across the glass transition temperature (T g - 20 to Tg + 60) relevant for hot embossing applications.;The aim of this thesis is to develop such a material model for PMMA. First, material characterizations experiments were conducted on PMMA well across its glass transition temperature (Tg). This experimental data along with the existing data in the Dupaix lab was used in developing the material model. In order to develop the new material model for application in hot embossing that will work across the wide range of temperature and strain rates, two existing constitutive models on the polymer PMMA were studied: the Dupaix-Boyce model and the Dooling-Buckley-Rostami-Zahlan model. From the aforementioned study, a new continuum model was developed to capture the mechanical behavior over a wider range of temperature across the glass transition. Experimental data was also collected from hot embossing experiments on the polymer PMMA across its glass transition temperature. This was done to better understand the process conditions of hot embossing and thus identify the vital parameters essential that the new developed material model must be able to capture. Finally, hot embossing simulations were performed on ABAQUS using the new material model. These results were used to validate the new material model. The new model worked extremely well for large strain deformations capturing the strain rate and temperature dependence, as well as stress relaxation of the material. The model was less accurate in capturing stress relaxation for small strain deformations. The strengths and weaknesses of the current model are discussed for future work improving the constitutive model.