Hot Embossing of Microfluidic Channel Structures in Cyclic Olefin Copolymers
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Hot Embossing of Microfluidic Channel Structures in Cyclic Olefin Copolymers Patrick W. Leech1, Xiaoqing Zhang1 and Yonggang Zhu2 1 CSIRO Materials Science and Engineering, Clayton, 3168, Victoria, Australia 2 CSIRO Materials Science and Engineering, Highett, 3193, Victoria, Australia
ABSTRACT The dynamic mechanical behaviour of a series of cyclic olefin copolymers (COCs) with varying norbornene content has been examined in the vicinity of the glass transition temperature, Tg. The temperature of the transition has been shown to increase linearly with increase in norbornene content. Measurements of both the elastic storage modulus, E′, and loss modulus, E″, have decreased exponentially with rise in temperature above Tg . A levelling-off in E″ occurred at >20 °C above Tg for all copolymers. The results of Dynamic Mechanical Thermal Analysis (DMTA) have been used in the identification of optimum conditions for hot embossing. At >20 °C above Tg in a region of viscous liquid flow, the hot embossing of COC has resulted in a full replication of channel depth without cracking or distortion.
INTRODUCTION Cyclic olefin copolymer (COC) has emerged in recent years as an attractive substrate for use in microfluidic devices. COC has exhibited a unique combination of properties including an optical transparency extending into the DUV range, chemical inertness and negligible moisture absorption. Another attribute of COC has been the ability to tailor it’s thermal and mechanical properties by variation in the ratio of cyclic monomer (typically norbornene) to olefin (ethylene) [1]. In copolymers with < 40% norbornene, the chemical structure of COC has been identified by nuclear magnetic resonance as comprising sequences of norbornene and ethylene units within the backbone of the main polymer chain [2]. Higher norbornene contents have acted to stiffen the main chain through the substitution of ethylene units by the bulky ring structure. As a consequence, an increase in norbornene content in COC has correlated with a linear rise in glass transition temperature, Tg [3], an increase in microhardness [4] and tensile strength [4] and decrease in ductility [5]. The linear dependence of Tg on composition was shown by Forsyth et.al. [6] as sensitive to the type of microstructure within the copolymer at high norbornene content. COC has recently been used in the replication of microfluidic devices by processes of hot embossing [7,8] and injection molding [9,10]. In these studies, a number of different compositions of copolymer have been applied in the fabrication of channel structures. However, little systematic data exists on the effect of comonomer content in the replication of micro-patterns in COC. This paper has examined both the dynamic mechanical behaviour and the hot embossing properties of a series of standard grades of COC. The norbornene/ ethylene compositions of the copolymers were selected as providing a wide range of Tg . All grades have been synthesised using a common metallocene catalyst. Measurements of thermal
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