Hot Embossing Of Microchannels in Cyclic Olefin Copolymers
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Hot Embossing Of Microchannels in Cyclic Olefin Copolymer Patrick W. Leech1 1 CSIRO Materials Science and Engineering, Clayton, 3168, Victoria, Australia ABSTRACT The hot embossing properties of Cyclic Olefin Copolymer (COC) have been examined as a function of comonomer content. Six standard grades of COC with varying norbornene content (61-82 wt%) were used in these experiments in order to provide a range of glass transition temperatures, Tg. All grades of COC exhibited sharp increases in embossed depth over a critical range of temperature. The transition temperature in embossed depth increased linearly with norbornene content for both 35 and 70 µm deep structures. At temperatures above this transition, the dimensions of the embossed patterns were essentially independent of COC grade, the applied pressure and duration of loading. Channels formed above the transition in a regime of viscous liquid flow were extremely smooth in morphology for all grades. The average surface roughness, Ra, measured at the base of the channels decreased sharply at the transition temperature, with a levelling off at higher temperatures. Grades of COC with higher norbornene content exhibited extensive micro-cracking during embossing at temperatures close to the transition temperature.
INTRODUCTION Micro/nanofluidic systems based on polymers have become increasingly important in the realisation of portable devices for biomedical analysis. The ease of replication of polymers by hot embossing and injection moulding combined with the ability to thermally seal the channels has enabled the fabrication of disposable devices. Importantly, devices based on polymers are able to maintain the functionality of conventional substrates of silicon or silica. Heckele and Schomburg have reviewed the range of polymers which have been used in the molding of micro/nanofluidic systems [1]. In recent years, a new class of thermoplastic polymer, cyclic olefin copolymer (COC), has been synthesised with a unique range of optical, thermal and mechanical properties. COC copolymers have shown an optical transparency extending into the UV and deep-UV spectral ranges [2]. Another attribute of COC has been the ability to tailor it’s thermal and mechanical properties by variation in the percentage of norbornene incorporated during initial polymerisation [2]. COC has been synthesized by the copolymerisation of a cycloolefin (typically norbornene) with an olefin (ethylene). Increase in the norbornene content has acted to stiffen the main chain and prevent crystallisation of the amorphous structure. An increase in concentration of norbornene has correlated with an increase in the glass transition temperature, Tg, [3] and a decrease in the ductility of the copolymer [4]. The fabrication of microfluidic chips in COC has been reported using both hot embossing [5,6] and injection molding [7] techniques. However, little data exists on the effect of comonomer content and embossing parameters on the fabrication of micropatterns in COC. This paper examines for the fi
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