James V. Duffy

Molecular basis of the acoustic properties of polyurethanes

The acoustic properties of polyurethanes as a function of frequency are dominated by a very broad relaxation process in which the polymer makes a transition from the rubbery to the glassy state. It is convenient to use an analytic representation of the data that expresses the complex shear modulus in the form G* = G∞ − (G∞ − G0)/[1 + (iωτ)α], where G0 is the limiting low‐frequency modulus, G∞, is the limiting high‐frequency modulus, τ is the relaxation time, and α is a dimensionless parameter, with values between zero and one, that determines the width of the relaxation. It will be shown that τ is related to the glass transition temperature Tg of the soft segment in the polyurethane. In turn, Tg is found to depend on the degree of phase separation that occurs between the chemically incompatible hard and soft segments, as well as the degree of crystallinity developed. These same molecular factors determine G0, but G∞ is found to be essentially constant. Attempts to relate a to the width of the glass transi...