Polymer processing and rheological analysis near the glass transition temperature

The hot embossing process has been widely used in fabricating micro- and nano-structures for polymer based bio- and chemical-MEMS (MicroElectro Mechanical Systems) application. However, the relationships among the processing conditions, material properties and part quality have not been thoroughly investigated. In this study, we conducted a series of experiments to address these issues for both isothermal and non-isothermal embossing. Observation of flow patterns showed a substantial difference in polymer deformation between these two heating methods. In isothermal embossing, the flow pattern resembled a bi-axial extensional flow. As to non-isothermal embossing, the polymer flowed upward along the wall of the mold feature followed by being compressed downward and squeezed outward. Both the replication accuracy and birefringence pattern of molded parts in isothermal embossing depended strongly on the processing conditions while, in non-isothermal embossing, the replication was excellent as long as the feature transfer is completed.; Rheological characterization near the glass transition temperature (T _g) was carried out to better understand the flow behavior at this temperature region, which is the operating temperature range for the hot embossing process. Various characterizing techniques such as the dynamic shear viscosity measurement, tensile tests, transient shear and extensional viscosity measurement, were combined to assess the material properties since no single method was capable of providing the rheological information at temperatures slightly and far above T_g. A viscoelastic model was also proposed to describe the rheological behavior.; A finite element code, DEFORM, was used to simulate the process and compare with the experimental observation of the shape change of polymers during the embossing process. Furthermore, the flow was estimated through simulation results and the correlation between the temperature sensitivity of the displacement curves in the hot embossing process and material properties was provided.