For the sake of thread self-sufficiency, I’ve copied a passage from Wiki on natural rubber that complexifused me:
In most elastic materials, such as metals used in springs, the elastic behavior is caused by bond distortions. When force is applied, bond lengths deviate from the (minimum energy) equilibrium and strain energy is stored electrostatically. Rubber is often assumed to behave in the same way, but it turns out this is a poor description. Rubber is a curious material because, unlike metals, strain energy is stored thermally.
In its relaxed state, rubber consists of long, coiled-up polymer chains that are interlinked at a few points. Between a pair of links, each monomer can rotate freely about its neighbour, thus giving each section of chain leeway to assume a large number of geometries, like a very loose rope attached to a pair of fixed points. At room temperature, rubber stores enough kinetic energy so that each section of chain oscillates chaotically, like the above piece of rope being shaken violently. The entropy model of rubber was developed in 1934 by Werner Kuhn.
When rubber is stretched, the “loose pieces of rope” are taut and thus no longer able to oscillate. Their kinetic energy is given off as excess heat. Therefore, the entropy decreases when going from the relaxed to the stretched state, and it increases during relaxation. This change in entropy can also be explained by the fact that a tight section of chain can fold in fewer ways (W) than a loose section of chain, at a given temperature (nb. entropy is defined as S=k*ln(W)). Relaxation of a stretched rubber band is thus driven by an increase in entropy, and the force experienced is not electrostatic, rather it is a result of the thermal energy of the material being converted to kinetic energy. Rubber relaxation is endothermic, and for this reason the force exerted by a stretched piece of rubber increases with temperature. (Metals, for example, become softer as temperature increases). The material undergoes adiabatic cooling during contraction. This property of rubber can easily be verified by holding a stretched rubber band to your lips and relaxing it. Stretching of a rubber band is in some ways equivalent to the compression of an ideal gas, and relaxation is equivalent to its expansion. Note that a compressed gas also exhibits “elastic” properties, for instance inside an inflated car tire. The fact that stretching is equivalent to compression may seem somewhat counterintuitive, but it makes sense if rubber is viewed as a one-dimensional gas. Stretching reduces the “space” available to each section of chain.
If the rubber were stretched say underwater what would be the volumetric change if say stretched 500%. Also how my hysterisis is measured when a stretched rubber returns to it’s relaxed state? And does the speed of relaxing it or how long it is held in the stretched state affect the hysterisis?
Perhaps I can point you in the right direction. The property you are looking for is the “loss modulus”. It’s a measure of how much energy is lost during deformation. Also known as the imaginary modulus, I think it’s similar to resistance and impedance (both only show up under oscillating conditions).
It also depends on the type of rubber. Eg for tyres you want a low loss modulus, since they’re constantly being deformed, otherwise they’d waste fuel and heat up.
I have noticed that tensile test specimens of high strength metal alloys became rather warm during ‘stretching’, i.e. axial loading applied and increased until fracture. I assume that this is a result of friction induced heat that’s generated between the microscope grain boundaries as the grains deform. It typically takes 20,000 to 60,000 pounds to pull a high strength steel tensile specimen to failure depending on the alloy, heat treatment history, and specimen size. So, this metal stretching requires far more power (heavy machinery) compared to that required to stretch rubber bands.
Heat given off by a stretched rubber band is probably too negligible to be detected my human hands, but it would not surprise me if heat is given off.
Interestingly, stretched rubber cools down when it is allowed to unstretch.
So if you stretch it, then hold against your lip, you will feel the warmth. If you keep it stretched and allow it to equalise with ambient temperature, then let it relax, aand put it against your lip, it will feel cold.
One of the items on my project list is to try to make a working refrigeration system using rubber bands as the cooling cycle.
I don’t know if you can get useful refrigeration out of stretched rubber bands (I’ve done the experiment of cooling down the band many, many times), but you can work things the other way, and build a candle-powered (or lamp-powered) 'rubber band motor" that uses the temperature change to change the band tension, making a wheel revolve
I just watched Feynman on rubber bands. He uses the analogy of a stretched string being constantly “bombarded with jiggly things,” to which the stretched part (which is what?) resists, producing heat. These are molecular or atomic? And why do they jiggle?
