A satisfactory elastomer can quantitatively store and release mechanical energy without loss, that is, it can immediately return to its original shape after deformation. A sympathetic elastic body is measured by its modulus, which represents its resistance to deformation. A satisfactory viscous body will consume all the received mechanical energy in the form of heat energy. A substance that has both elastic and viscous properties is called viscoelastic. Polymer is a typical viscoelastic body, and the paint film composed of polymer is also viscoelastic.
The polymer molecules are subjected to external force in an instant or in a short period of time, and there is not enough time for relative movement, so they can only change the bond length and bond angle to elongate or deform. Over time, the chain segments of some polymer molecules can move relative to each other and change their relative positions. When the external force acts for a short time, the original shape can be restored after the external force is removed, and the stored energy can be released at the same time, just like an elastic body. If the external force acts for a long time, the external force will gradually dissipate and the relative movement of the polymer chain segments will no longer return to the original shape. The larger the molecule of the polymer, the longer the chain, the greater the degree of intertwining, and the greater the hindrance to the relative movement of the chains.
After the viscoelastic body is deformed by external force, part of the mechanical energy is stored in the body as potential energy, and part of it is dissipated as thermal energy (such as the relative movement of polymer chain segments). The stress-strain curve of viscoelastic body is related to temperature and time, such as creep and stress relaxation. Creep refers to the gradual increase in deformation with time under constant stress. Stress relaxation refers to the gradual decrease of stress with time under constant strain. The creep or stress relaxation time of viscoelastic body is shortened at high temperature, and prolonged at low temperature, and the effect of time and temperature is the same.
