Hypersonic sound velocity in liquid and rubbery polymers

The hypersonic sound velocities in liquid and rubbery polymers were studied in connection with the degree of polymerization n and the cross‐link density ν of linear polymers by adopting lattice model for three congregation states. The elastic modulus M defined by the product of the density and the square of the sound velocity depends on 1/n in uniform mixing of molecular chains, on (1/n)1/3 in not uniform mixing of molecular chains, and on ν in uniform mixing of molecular chains with ν cross‐link points. The hypersonic sound velocity in linear polydimethylsiloxanes (PDMS) and chemically cross‐linked polydimethylsiloxanes (PDMSR) were measured by Brillouin spectroscopy. The modulus M of PDMS varied according to (1/n)2/3 in a region of low molecular weight and that of PDMSR depended linearly on ν. Since these treatments based on lattice model are thought to explain successfully behaviors of propagation of hypersonic sound, it is suggested that the energy elasticity plays a significant role here.