| Résumé | The author studies the propagation of ultrasonic waves in polymers in relation to their state of structural organization in the transformation range from solid to liquid. Because of the disordered nature of the structure, the equation of state for polymers is determined by pressure (p), volume (V), temperature (T), and time (t), leading to the concept of thermodynamic history (p, V, T, t). A novel technique is presented that measures the velocity and attenuation and controls thermodynamic history from T=-150 degrees C to 400 degrees C, and up to p=200 MPa, and which incorporates simultaneous measurement of specific volume to characterize the thermodynamic state. The author investigates amorphous polymers with respect to anharmonicity, the glass transition phenomenon at the glass transition temperature evidencing irreversible structural rearrangements, and linear viscoelasticity associated with free-volume and configurational entropy. Semicrystalline polymers where collapse of the modulus correlates to melting of the crystalline phase and critical transformation to the liquid state are examined. |
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