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Rubber As an Aid to Teach Thermodynamics∗ the Discovery by a Blind Scientist

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Rubber As an Aid to Teach Thermodynamics∗ the Discovery by a Blind Scientist GENERAL ARTICLE Rubber as an Aid to Teach Thermodynamics∗ The Discovery by a Blind Scientist Geethamma V G and Sampath V The behaviour of rubber differs from that of conventional materials. Rubber heats up on stretching and cools on re- traction. Also, stretched rubber shrinks on heating (ther- moelastic shrinkage) while a stretched metal elongates. The elastic recovery of rubber is due to its tendency to maximize the entropy. The same property also causes the thermoe- lastic shrinkage. Metallic materials possess energy elastic- Geethamma V G was a ity, while ideal rubber possesses entropy elasticity. The ther- Fulbright Fellow at modynamic behaviour of rubber is similar to that of gaseous University of Illinois, USA materials. Hence rubber can be used as an aid for teaching and a Royal Society International Postdoctoral thermodynamics. Fellow at Cavendish Lab, University of Cambridge, UK. She was also a Young 1. Gough–Joule Effect Scientist Awardee. John Gough was not born blind. But by a sheer quirk of fate, he lost his eyesight due to smallpox before turning three. However, his senses of touch and hearing were intact, and he was bestowed with a sharp inquisitive mind and adequate skills for experimen- V Sampath is presently a tation. He tended to be philosophical too. John Dalton, a well Professor of Metallurgical known British scientist, who proposed the atomic theory, helped and Materials Engg. at IIT Gough by reading out to and writing for him. He had great admi- Madras. He holds a PhD from IISc, Bengaluru and has ration for Gough. three decades of research and In 1805, Gough observed two important properties of rubber [1]. teaching experience. His fields of interest are: shape Firstly, in a simple experiment, he sensed with his lips (lips being memory alloys, smart a temperature sensitive part of the human body), a rise in temper- materials, nano- and ature when a piece of uncrosslinked natural rubber was stretched composite materials, and rapidly. He also discovered that the rubber cooled down rapidly structure-property correlation in materials. on retraction. In another experiment, a rubber band (suspended ∗DOI: https://doi.org/10.1007/s12045-019-0772-x RESONANCE | February 2019 217 GENERAL ARTICLE Box 1. Gough–Joule Effect 1. Rubber warms on stretching and cools down on retraction. 2. Stretched rubber shrinks on heating and elongates on cooling. Experience Gough–Joule Effect! Take a piece of rubber with the dimensions 7 × 3 cm. Hold it loosely on the upper lip. Stretch it quickly to the maximum extent possible, taking care not to break it. Your lip will feel the warmth. Then hold the strip away from the lip for 5 seconds. Place it on the lip and release the force quickly; now your lip becomes cold. Keywords with a weight) was warmed. It was observed that the rubber band Gough–Joule effect, rubber, ther- contracted thus raising the weight. Instead, if the length of the moelastic, entropy elasticity, adi- abatic, isothermal, thermodynam- rubber band was kept constant, the tension in the rubber increased ics, teaching aid, strain-induced with temperature. So he observed that stretched rubber contracted crystallisation, elastomer. on heating and elongated on cooling. About fifty years later, another scientist named James Joule con- firmed these observations with crosslinked natural rubber [2]. This behaviour is observed in certain synthetic rubbers too. This phe- nomenon is known as Gough–Joule Effect (Box 1) [3]. Since the phenomenon involves a temperature change when a solid is sub- jected to stress, it is also known as thermoelastic (elastocaloric) property [4]. However, if the stress exceeds the elastic limit, it is called thermoplasticity. In other words, the thermal response of an elastic material is thermoelasticity while the thermal response of plastic is thermoplasticity. The phenomenon of Rapid deformation of metals also causes a change in tempera- shrinkage on heating and ture. But it is only in the range of one Kelvin. However, the expansion on cooling is phenomenon of shrinkage on heating and expansion on cooling is a unique property of stretched rubber. This a unique property of stretched rubber. This behaviour is in con- behaviour is in contrast trast to that observed in conventional materials which expand on to that observed in heating. conventional materials which expand on The increase in temperature during the deformation of rubber is heating. not fully offset by the decrease in temperature during its retrac- 218 RESONANCE | February 2019 GENERAL ARTICLE tion. So Gough–Joule effect is significant in certain practical ap- plications. Rubber products such as automobile tyres, bridge seg- ments, and vibration dampers are exposed to cyclic loading dur- ing their service life. Heat is generated in these rubber products due to the conversion of mechanical energy into heat (hysteresis). Hysteretic heat build-up represents a wastage of mechanical en- ergy. In addition, it leads to early damage to the product. The total heat developed in a product is the sum of hysteretic heat and heat due to the thermoelastic effect. In many products, heat generated due to thermoelasticity is crucial. A familiar example is the wear and tear of automobile tyre as it undergoes several deformation-recovery cycles during its rotation. 2. Thermoelastic Inversion In Gough–Joule effect, the temperature of rubber increases con- The thermoelastic siderably when it is stretched to large extensions. But for small inversion occurs due to extensions (< 50%), a slight decrease in temperature is observed. the small but thermodynamically This phenomenon is called thermoelastic inversion [5]. However, significant changes of for small deformations, the temperature of rubber increases dur- volume occurring with a ing compression and does not even change during shear or tor- change of temperature or sion. Also, generally materials exhibit a positive coefficient of the application of a ffi force. This phenomenon linear expansion. But for rubber, this coe cient changes from is not shown by ideal positive to negative depending upon its extension. Stretched rub- rubbers where the ber shrinks on heating at fairly high extensions. At high elonga- change in volume is tion, the force developed in stretched rubber is proportional to the negligible. absolute temperature. But at low extensions (< 10%), stretched rubber elongates on heating. It indicates decreased tension at high temperature [6]. This phenomenon is also due to thermoelastic inversion. The thermoelastic inversion is due to the small but thermodynam- ically significant changes of volume occurring with a change of temperature or the application of a force. Hence this phenomenon is not shown by ideal rubbers where the change in volume is neg- ligible. In brief, the competition between energy effects and en- tropy effects in rubber at small and large extensions leads to ther- moelastic inversion. RESONANCE | February 2019 219 GENERAL ARTICLE Figure 1. Isothermal and adiabatic processes. 3. The Secret Behind Gough–Joule Effect In general, an object feels cold when heat flows from our skin to the object. Conversely, it feels warm when heat flows from the object to our skin. The stretched rubber strip feels warm because it liberates heat to our skin. Now let us see, how heat is developed in rubber during stretching. The reason of Gough–Joule effect remained elusive for many years until Herman Staudinger, Kuhn and others propounded theories in the 1920s and 1930s [7–9]. The two fundamental requirements for a material to exhibit thermoelasticity are temperature change and elasticity (reversibility). At first, consider the temperature effect. There are two reasons for this: adiabatic stretching and strain-induced crystallisation. In an isothermal process, The temperature of a system increases or decreases depending the system is in contact upon the nature of phenomenon occurring in the system. Let us with a reservoir at a see the difference between isothermal and adiabatic processes. In constant temperature, while in an adiabatic an isothermal process, the system is in contact with a reservoir process, the system is at a constant temperature. Thermodynamically speaking, a reser- thermally insulated from voir is a large entity which can transfer heat into or out of a system its surroundings. without undergoing a change in temperature. Hence in an isother- mal process, the system either receives heat or loses heat so that its temperature remains constant (Figure 1). The transformation 220 RESONANCE | February 2019 GENERAL ARTICLE is very slow in an isothermal process. But in an adiabatic process, the system is thermally insulated from its surroundings. The insulator can be rubber, plastic or wood. These materials have very low thermal conductivity and the system neither gains heat nor loses heat from or to the sur- roundings. Hence though the system undergoes a process which may increase (or decrease) its temperature, heat is not emitted (or absorbed). In this process, energy is exchanged with the sur- roundings only as work. When the process is fast, it does not have enough time for the transfer of energy as heat to or from the system. So when a material is deformed rapidly, the temperature of the system is changed. The experiment mentioned in Box 1 should be rapid so as to minimize heat transfer from the rubber strip to the surroundings, and thus make the process adiabatic. Normally rubber is amorphous. But certain types of rubbers crys- Normally rubber is tallise on fast stretching due to strain-induced crystallisation. This amorphous. But certain property is discussed in detail later. Simultaneously, the temper- types of rubbers latent heat of crystalli- crystallise on fast ature of rubber rises due to the release of stretching due to sation. This explains why the lip feels warm as discussed earlier. strain-induced The latent heat of crystallisation raises the temperature of rubber crystallisation. up to 10 K at 500% strain.
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