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Application of ultrasonic wave propagation to polymer characterization

Involved People: Alfonso Maffezzoli, Francesca Lionetto, FrancescoMontagna
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 glass transition, crystallization, physical or chemical gelation, 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 strong potential as a sensor for on-line and in-situ monitoring of production processes for polymers and polymer matrix composites.
A custom made ultrasonic device for polymer characterization, even at high temperatures, has been developed (Fig.1). It is able to propagate and receive low intensity longitudinal waves in the 2-10 MHz frequency range up to 250 °C and to operate both in pulse-echo and in through-transmission mode. The ultrasonic equipment is coupled with a rotational rheometer (Fig. 2). 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. From the ultrasonic velocity and attenuation measurements, the viscoelastic properties of the tested polymers are evaluated in terms of complex longitudinal modulus and correlated with the results of conventional dynamic mechanical analysis, carried out at low frequency (0.1-16 Hz).
Several studies have been carried out on different polymeric polymeric materials:
   - curing of thermosetting resins (both epoxy and unsaturated polyesters), whose structural changes occurring at gelation and vitrification were detected from the evolution of viscoelastic properties and the correlation between ultrasonic velocity and degree of reaction;
   - superabsorbent hydrogel cross-linking, whose degree of crosslinking was evaluated by NMR and DMA analysis;
   - hydrogel swelling kinetics of dry PVA hydrogels;
   - retrogradation of wheat starch gels on ageing in the rubbery state, in collaboration with the University of Nottingham, U.K. (Prof. J.R. Mitchell);
   - crystallization kinetics of poly-lactic acid (PLLA) and linear low-density polyethylene (LDPE);
  - gelation of waxy crude oils on cooling (Fig. 3) due to the precipitation of paraffin crystals (in collaboration with EniTecnologie S.p.A.).
Recently, the interest of the research group are being oriented towards air-coupled ultrasound through the development of an air-coupled ultrasonic device for the cure monitoring of composite matrices during manufacturing processes. The ultrasonic transducers are placed on the same side of the tested materials, making this technique very suitable for those processes where only one side of the tested material is accessible.
The possibility to eliminate the physical contact between the ultrasonic transducer and the resin can extend the application of the ultrasonic wave propagation to several manufacturing processes, where direct contact between transducers and component or contact with fluid would adversely affect the process or the part quality.
Figure 1 - Sketch of the custom made ultrasonic device for contact measurements. (From: F. Lionetto, R. Rizzo, V.A.M. Luprano, A. Maffezzoli (2004), "Phase transformations in unsaturated polyester resins during the cure", Materials Science and Engineering: A-Structural materials properties microstructure and processing, vol. 370, no. 1-2, pp. 284-287)
Figure 2 Ultrasonic probes fitted into the fixtures of a parallel plate rheometer (Rheometric Scientific): pulse-echo (left) and through-transmission (right) configuration. (From: F. Lionetto, F. Montagna, A. Maffezzoli (2005), "Ultrasonic dynamic mechanical analysis of polymers", Applied Rheology, vol.15, no. 5, pp.326-335)

Figure 3 Evolution of the longitudinal velocity and attenuation in a waxy crude oil due to the precipitation of paraffin crystals upon cooling at 2°C/min
(From: F. Lionetto, G. Coluccia, P. D'Antona, A. Maffezzoli (2007), "Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis", Rheologica Acta, vol. 46, no.5)
Figure 4 Effect of the promoter content on the longitudinal velocity during cure at room temperature of unsaturated polyester resins monitored by air-coupled ultrasound
(From: F. Lionetto, A. Tarzia, M. Coluccia, A. Maffezzoli (2007), "Air-coupled ultrasonic cure monitoring of unsaturated polyester resins", Macromolecular Symposia, vol. 247, no.1, pp. 50-58)
Recently, a research project on the ultrasonic wave propagation has gained a special mention by the Italian Minister for University and Research (MIUR). The project has been selected among those funded by the MIUR and has been classified as an excellent research project both for the addressed problem and the obtained results, which will be disseminated in a specific publication.
The project, entitled “Development of a technique for the cure monitoring of composite matrices during filament winding processes”, was aimed at developing a novel application of the non-destructive control by means of air-coupled ultrasound, i.e. ultrasonic wave propagation without any coupling medium between the transducer and the sample. The obtained results will impact on the quality control of manufacturing processes of polymer composites for aerospace and other applications.
The results of the research activity have been presented in international conferences and published on international scientific journals.


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University of Salento   Facoltà di Ingengeria    Department of Engineering for Innovation