Huggins John Wiley & Sons: New York 1958 p 102. Physical chemistry of high polymers Maurice L. Certain rubber samples have, in fact, been found to conform to this relationship." It can readily be shown by thermodynamic reasoning that, if energy changes are negligible, the elastic modulus should be directly proportional to absolute temperature. Energy changes may also take place in the stretching and recovery processes, but they are not necessary for the phenomenon. Elastic recovery is a result of the tendency for the system to increase its entropy. This produces a decrease in the entropy of the system. Stretching produces molecular configurations in which the orientation about the single bonds of the structure is less random than in the unstretched specimen. The chains may be crosslinked into a network structure, but this is not essential. "Long-range elasticity has been observed only in substances and materials known to contain flexible molecular chains.
We infer, moreover, that these tangential stresses are not called into play so long as the fluid moves as a solid body, but only whilst a change of shape of some portion of the mass is going on, and that their tendency is to oppose this change of shape."Ī treatise on the mathematical theory of the motion of fluids Horace Lamb Cambridge Press: Cambridge 1879 pp 2-3. For if the mutual action were everywhere wholly normal, it is obvious that the moment of momentum, about the axis of the vessel, of any portion of fluid bounded by a surface of revolution about this axis, would be constant. These phenomena point to the existence of mutual actions between contiguous elements which are partly tangential to the common surface. If the vessel be now brought to rest, the motion of the fluid continues for some time, but gradually subsides, and at length ceases altogether and it is found that during this process the portions of fluid what are further from the axis lag behind those which are nearer, and have their motion more rapidly checked. If the motion of the vessel be uniform, the liquid is soon found to be rotating with the vessel as one solid body. Let us suppose for instance that a vessel in the form of a circular cylinder, containing water (or other liquid), is made to rotate about its axis, which is vertical. Very slight observation is enough, however, to convince us that oblique stresses may exist in fluids in motion. This property is the basis of Hydrostatics and is verified by the complete agreement of the deductions of that science with experiment. "The fundamental property of a fluid is that it cannot be in equilibrium in a state of stress such that the mutual action between two adjacent parts is oblique to the common surface.