Work of adhesion/separation between soft elastomers of different mixing ratios
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Adhesion between soft matter is a universal mechanical problem in bio-engineering and bio-integration. The Johnson–Kendall–Roberts (JKR) method is widely used to measure the work of adhesion and work of separation between soft materials. In this study, the JKR theory is recaptured and three complementary dimensionless parameters are summarized to help design adhesion measurement experiments compatible with the JKR theory. The work of adhesion/separation between two commonly used soft elastomers, polydimethylsiloxane (PDMS, SylgardÒ 184) and EcoflexÒ 0300, is measured by the JKR method using a dynamical mechanical analyzer. Effects of base polymer to curing agent mixing ratio and solvent extraction are examined. A unified adhesion mechanism is proposed to explain the different adhesion behaviors. It is concluded that chain–matrix interaction is the most effective adhesion mechanism compared with chain–chain or matrix–matrix interactions. Chain–chain interaction obstructs chain–matrix interaction as it either blocks or entangles with surface chains which could have interacted with the matrix.
I. INTRODUCTION
Contributing Editor: Linda S. Schadler a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.242
literature. Three key factors have been identified to significantly affect the adhesion of soft elastomers: the mixing ratio of the base polymer to curing agent,15,16 the solvent extraction of free (i.e., uncross-linked) chains,9 and the ultraviolet ozone (UVO) treatment of the elastomer surface.13 Despite numerous measurements, the adhesion mechanism remains vague or even contradictory. In this study, we will explore various adhesion scenarios induced by the first two factors in a systematic manner. In addition, we will also investigate the adhesion between PDMS and EcoflexÒ. The goal is to propose a unified, consistent adhesion mechanism for soft elastomers based on a series of systematic studies. While double cantilever beam and four point bending tests are widely applied to quantify adhesion between stiff layers, we choose to perform Johnson–Kendall–Roberts (JKR) test17 to measure adhesion between soft elastomers because it minimizes the sample volume, which reduces bulk viscoelastic losses18 and makes it easy to implement for future adhesion measurements including bio-tissues. In fact, JKR test has been widely validated for measuring the adhesion between different types of soft materials.10,19,20 The three commonly measured parameters in a JKR experiment are load, displacement, and contact radius but only two are required to extract the adhesion energy and the combined elastic modulus. In the first method, the applied load and the contact radius are measured simultaneously and then fitted with the JKR equation to obtain the work of adhesion/separation and the combined elastic modulus.8,9,21,22 In the second method, the load penetration
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Ó Materials Research Society 2015
Adhesion between soft matter is a universal mechanical problem in bio-engineering and bio-integration.
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