Ultrasonic shear wave reflection method for measurements of the viscoelastic properties of polymer films

Abstract

We report a shear wave reflection technique which allows time- and temperature-dependent measurements of the complex dynamic shear modulus (G*=G′+iG′′) or the dynamic shear viscosity (ηs′) in the ultrasonic frequency range (3–40 MHz). The shear wave reflection method developed by Mason et al. [Phys. Rev. 75, 936 (1949)] was adapted to the possibilities of the high-frequency digital technique. A conventional single-frequency pulse-burst system and a broadband pulsed ultrasonic spectroscopic system using Fourier analysis for detection of phase and amplitude changes of the reflected signals are described alternatively. Due to digital analysis of the received signal it was possible to achieve a simplification of the experimental setup and to extend the limits of the shear wave reflection technique to about one Gigapascal (100 μm film thickness) or to minimum film thickness of about 10 μm (G′<10 MPa). Furthermore, it is possible to monitor time-dependent isothermal processes or reactions with the setup. The basic features of the two different techniques are presented together with brief experimental results on film formation from aqueous polychloroprene dispersions and of the crystallization kinetics in the films. Furthermore, temperature-dependent studies of melting and crystallization on semicrystalline polychloroprene and of the glass transition behavior of amorphous butyl acrylate/styrene copolymer films are reported. The importance of the method is discussed for both investigations of fundamental questions (e.g., film formation, reaction kinetics, or phase transitions) and technological assessment (glues, paints, etc.).