Experiments, Constitutive Modeling, and Multi-Scale Simulations of Large Strain Thermomechanical Behavior of Poly(methyl methacrylate) (PMMA)

Poly(methyl methacrylate) (PMMA) is a transparent, biocompatible, amorphous thermoplastic with a wide variety of applications. With its relatively low glass transition temperature, Tg, and ability to maintain high aspect ratio features after processing, it is an ideal candidate for the polymer processing technique of hot embossing. Hot embossing takes advantage of the thermomechanical properties of PMMA by applying the die at temperatures greater than Tg. The highly viscous behavior of PMMA at temperatures greater than Tg reduces the required pressure to fully fill the features of the die. After cooling to a temperature less than Tg, PMMA behaves as a solid and maintains the relief shape of the stamp. Despite cooling to temperatures less than Tg, the highly viscoelastic nature of PMMA allows it to recover some of its original shape, or spring-back, after load removal. While these drastic temperature dependent material behaviors are ideal for hot embossing, they make prediction of the final feature shape extremely difficult. Previous simulations have focused on die filling without considering demolding to avoid having to capture these effects. However, without consideration of the spring-back behavior during demolding, it is impossible to know the extent of feature preservation. Therefore, the aim of this work is to develop a large strain constitutive model spanning the glass transition temperature capable of capturing the highly temperature and strain dependent spring-back behavior of PMMA with application in a micro hot ii embossing simulation. Large strain stress relaxation experiments are used to probe previous constitutive models to find their weaknesses. From this, a new constitutive model is developed to capture the highly temperature and strain dependent relaxation effects. Temperature and strain dependence of spring-back is investigated through modified unconfined recovery tests. Cooling is incorporated into the constitutive model using these results and then implemented into a finite element simulation to predict spring-back. Temperature dependence of PMMA is further investigated through simulations to determine its effect on spring-back. After showing the constitutive model is capable of capturing the held strain and temperature dependence of spring-back, it is used to predict the final feature shape of microchannel hot embossing. Large scale differences between the substrate and feature size require development of a multi-scale simulation technique. Boundary conditions are extracted from a representative volume element on the macro scale utilizing simplified die geometry and are applied to a micro scale simulation using actual die geometry. High feature resolution is maintained using this method while minimizing the number of elements required and reducing computation time. Simulations are verified to microchannels embossed into PMMA for use in directed cell growth experiments. By capturing the stress relaxation behavior in the constitutive model, simulations are better able to capture the temperature and held strain dependence of spring-back. Use of this constitutive model in micro hot embossing simulations shows its ability to predict final feature shape after die removal. Additionally, the multi-scale simulation technique has potential to be used in a variety of future simulations.