Determination of the elastic moduli of polymer films by a new ultrasonic reflection method

The development and first applications of a new ultrasonic reflection method for determination of the viscoelastic properties of polymer films are reported. The complex shear modulus G* and the complex longitudinal modulus L*=K*+ 4/3 G* of the samples are derived from the measured complex reflection coefficients of an ultrasonic shear and longitudinal wave, respectively. From G and L the Young's modulus E, the compression modulus K and the Poisson ratio ν can be calculated for isotropic materials. A LiNbO₃-transducer (10° rotated Y-cut) is used for the simultaneous excitation of longitudinal and transversal ultrasonic waves, which allows to determine different elastic constants by one measurement. A measuring cell with normal incidence of the ultrasonic waves is used. The equipment has been applied to study the time dependence of the moduli during film formation from an aqueous polymer dispersion and the isothermal curing of an epoxy resin. Furthermore, the temperature dependence of the elastic constants of a carbon-black filled rubber and during non-isothermal crystallization of a semi-crystalline polymer has been studied.