The elastic constants of extruded polypropylene and polyethylene terephthalate

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Abstract
The authors have made ultrasonic measurements at 10 MHz and between 190 and 270K on extruded samples of polypropylene and polyethylene terephthalate which have a wide range of crystallinity (0-0.6) and extrusion ratio (1-10) by the use of quartz transducers directly bonded on to sample discs cut parallel, normal and at 45 degrees to the extrusion direction and have thus obtained all five elastic moduli. The results for polypropylene are considered in both the aggregate and the Takayanagi (1966) models. The aggregate model reproduces the development of mechanical anisotropy fairly well. The increase in the ultrasonic Young's modulus along the extrusion direction is much less than that previously observed in low-frequency-modulus data. The Takayanagi model shows that the stiffening effect of intercrystalline bridges is much less important for the ultrasonic moduli than for the room-temperature static moduli because of the much higher amorphous modulus for the glassy state.