Time-Resolved Synchrotron WAXD/SAXS Analysis of Mechanical Deformation and Viscoelastic Relaxation of the Crystalline Phase in Uniaxially Oriented Polymers

The crystallite modulus, commonly referred to as Young's modulus in the crystalline phase of a polymer, is determined by analyzing the strain and stress within the crystalline region through the X-ray method. Traditional experiments utilizing X-ray diffraction require several tens of hours to accurately assess the crystalline strain. In the present study, we have developed a synchrotron system that enables the collection of X-ray data in just a few minutes. However, since the crystalline stress is fundamentally unknown, it must be assumed that the crystal stress is equivalent to that of the bulk sample, which contradicts the conventional understanding of heterogeneous stress distribution. As a result, the crystallite modulus obtained is still an apparent value. To accurately ascertain the ‘true’ crystallite modulus, it is essential to create a sophisticated mechanical model that incorporates both crystalline and amorphous regions as elastic components, considering the heterogeneous stress distribution. Nevertheless, the current rapid-scan synchrotron system has provided new insights into the stress relaxation behavior of the crystalline component, which should be regarded as a viscoelastic entity rather than as a conventional simple elastic body. These two concepts—heterogeneous stress distribution and the viscoelastic nature of the crystalline (and amorphous) phase—are crucial for developing a new mechanical model, which has allowed us to estimate the ‘true’ crystallite moduli and relaxation behavior of various representative polymers such as polyethylene, nylon-6, cellulose, and silk. Throughout the extensive history of this research theme, establishing the true crystallite moduli has been challenging due to various unresolved factors. The present study has addressed these issues and introduced a new concept for treating the crystalline phase as a viscoelastic entity in the interpretation of semicrystalline polymers.