Does Thermal Annealing Affect the Mechanical Properties of Silkworm ( Bombyx mori ) Cocoon Silk?
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Does Thermal Annealing Affect the Mechanical Properties of Silkworm (Bombyx mori) Cocoon Silk? Emily J. Reed and Christopher Viney School of Engineering, University of California at Merced, 5200 North Lake Road Merced, CA 95343, U.S.A. ABSTRACT It has been reported [1] that microwave radiation can enhance many of the mechanical properties of Bombyx mori silkworm cocoon silk, as measured in constant strain rate tensile tests to failure and in stress relaxation tests. The consequences of microwave radiation will affect decisions about the use of silk in settings subjected to significant microwave exposure – for example, as a reinforcing fiber in an epoxy matrix composite that may be microwave cured, or as a component in aircraft radomes. There are two possible mechanisms by which microwave radiation may affect a material [2]: (i) the radiation may enable chemical and/or microstructural changes – and therefore property changes – in the same way that conventional heating would, or (ii) the high heating rates that are achievable by microwaving may selectively favor changes that would be masked under conventional conditions, where heating rates are low enough to give preference to changes that have a lower activation energy. Here we explore the former possibility for silk. We characterized several mechanical properties of degummed and subsequently annealed B. mori silk, and compared them to the corresponding properties of degummed B. mori silk that was not annealed. The annealing treatment was carried out at 140 °C for 7 hours (conditions that optimally increased crystal size in an unrelated study of B. mori silk [3]), and then the fibers were allowed to cool gradually to room temperature over the course of an hour. Comparison of mechanical properties revealed no differences between the materials that we tested. Thus, for annealed silk, we do not observe the enhancements that can be achieved by microwaving. We conclude that in cases where microwaving affects the properties of silk, those changes are not a simple consequence of annealing by the microwaves. INTRODUCTION A previous study [1] has demonstrated that microwave irradiation of Bombyx mori silkworm cocoon silk and Nephila clavipes spider major ampullate silk can markedly enhance many of the tensile properties of those fibers (with the exception of ductility in silkworm silk and initial stiffness in spider silk) in constant strain rate tensile tests to breaking. Microwave treatment was also shown to reduce the rate of stress relaxation in these two materials. Repeating the microwave irradiation experiments in an alternative set of conditions yielded a different result: no statistically significant effect of microwave irradiation on tensile properties of B. mori silk fiber was observed [4]. Since the consequences of microwaving are sensitive to the conditions under which experiments are performed, we set out to investigate the possible mechanisms by which microwave irradiation can affect the mechanical properties of silk, subject to the appropriate (but
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