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Center for Polymer Microfabrication (CPM)

MIT Laboratory for Manufacturing Productivity

NTU Biochemical Process Engineering Laboratory

Research:
materials and process modeling

Behavior of amorphous polymers above and below their glass-transition temperatures.

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We have completed the characterization of one of the key material for embossing, PMMA, and begun a similar effort on higher temperature COP COC materials. A Ph.D. thesis was recently completed that developed a fully coupled, high strain thermo-mechanical constitutive model for this material BWATODO_LINK. It captures all the details of the material behavior including strain rate effects, internal heating, and the deformation behavior below, at and above the glass transition temperature. A typical plot of experimental and model results is shown in Figure 1.

PMMA Experiment

Fig 1. A typical constitutive relationship for PMMA, experimental (solid line) and constitutive model (dashed line).

This model has been calibrated for temperatures ranging from 22C up to 170C (which is well above this polymerís glass transition of about 105C), and strain-rates spanning nearly 4 decades from 3x10-4 1/s to 1x10-1 1/s. This represents the first such model and attendant data for this material under these conditions.

Additional experimental work on amorphous cyclo-olefin polymers (grades of Zeonex) for hot-embossing has been initiated. These polymers are believed to possess characteristics that make them more favorable for microfluidic device applications. A medical grade of Zeonex (Zeonex 690R) was also obtained in pellet as well as in extruded bulk forms. Dynamic Mechanical Analysis tests have been performed on Zeonex 690R to characterize temperature and frequency dependence of storage and loss moduli of these materials. An experimental program has been conducted to measure the mechanical response of Zeonex 690R in large-strain compression in the temperature and strain rate ranges of interest for micro-embossing. This includes temperatures ranging from 22C up to 180C (which is well above this polymerís glass transition of about 135C), and strain-rates spanning nearly 4 decades from 3x10-4 1/s to 1x10-1 1/s. A typical result is shown in Figure 2.

Figure 2. Constitutive relationship for COP polymer (Zeonex), empirical data and constitutive model.

Based on these models new three-dimensional, coupled thermo-mechanical, large deformation  elasto-viscoplastic constitutive  model to represent the response of amorphous polymeric materials through their glass transition temperature has been developed.  Initial simulation using the PMMA model and ABAQUS/Explicit has been able to replicate experimental results well, see Figure 3.

Figure 3. Isothermal simulation of micro-channel forming and empirical comparison

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