Ultrasonic properties of poly(4-methyl pentene-1)

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Abstract
Ultrasonic measurements of longitudinal and shear sound speeds and absorptions were made on poly(4-methyl pentene-1) or PMP. The measurements were made at 1.8 MHz and covered the temperature range from 0 to 70 °C. PMP is of interest because it has a lower density (0.835 g/cm3) than any other commercially available high polymer. PMP was found to have a major structural relaxation (the glass transition) at 25 °C. An analysis of the various Grüneisen parameters of the polymer leads to the conclusion that one-fifth of the normal modes of vibration of PMP are interchain rather than intrachain. PMP was evaluated as an acoustic window material and compared with poly(acrylonitrile butadiene styrene) or ABS, another rigid acoustic window material. For an infinitely thick layer of PMP, the reflected signal is 20 dB less than incident, while for ABS the reflected signal is 16 dB less. For plates of finite thickness, PMP is a better acoustic window than ABS at most, but not all, angles of incidence.