Stress-induced phase changes, detection

If we take a value of Ha — 30 MPa (Martmez-Salazar et al, 1988) for the amorphous phase of i-PP and solve eqs. (4.15) and (4.16), values of // = 143 MPa and He = 119 MPa are obtained. The former value of 143 MPa fits well with the ab initio calculation for the a phase (Balta Calleja et al, 1988). In conclusion, the determination of microhardness is shown to be a technique capable of detecting polymorphic changes in polymers. Further examples of polymorphic crystal-crystal transitions induced by external field (stress or strain) are given in Chapter 6. 

Polymeric materials composed of flexible molecular chains respond very sensitively to changes in vanous surrounding conditions such as temperature, stress, pressure, and so on For example, temperature variation induces in polymeric materials drastic changes m mechamcal and electrical properties as well as the phase transitions of melting and crystallization. Since these phenomena are accompanied by considerable vanation in viscoelastic characteristics and thus in density (p) and elastic modulus (E) of the materials, the ultrasonic method which can detect quite sensitively these viscoelastic properties becomes one of the most valuable and important methods for investigating the macro- as well as microscopic structural changes occurring in the materials. 

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