Ultrasonic Dynamic Mechanical Analysis of Polymers

The propagation of ultrasonic waves in polymers depends on their viscoelastic behaviour and density, resulting significantly affected by phase transitions occurring with changing temperature and pressure or during chemical reactions. Therefore, the application of low intensity ultrasound, acting as a high frequency dynamic mechanical deformation applied to a polymer, can monitor the changes of viscoelastic properties associated with the glass transition, the crystallization, the physical or chemical gelation, the crosslinking. Thanks to the non-destructive character (due to the very small deformation amplitude), low intensity ultrasound can be successfully used for polymer characterization. Moreover, this technique has a big potential as a sensor for on-line and in-situ monitoring of production processes for polymers or polymer matrix composites. Recently, in the laboratory of Polymeric Materials of Lecce University a custom made ultrasonic set-up for the characterization of polymeric material, even at high temperatures, has been developed. The ultrasonic equipment is coupled with a rotational rheometer. Ultrasonic waves and shear oscillations at low frequency can be applied simultaneously on the sample, getting information on its viscoelastic behaviour over a wide frequency range. The aim of this paper is to present the potential and reliability of the ultrasonic equipment for the ultrasonic dynamic mechanical analysis (UDMA) of both thermosetting and thermoplastic polymers. Three applications of UDMA to different polymeric systems will be reviewed, concerning the cross-linking of a thermosetting resin, the crystallisation from melt of a semicrystalline polymer and the water sorption in a dry hydrogel film. From the ultrasonic velocity and attenuation measurements, the viscoelastic properties of the tested polymers are evaluated in terms of complex longitudinal modulus and compared with the results of conventional dynamic mechanical analysis, carried out at low frequency