Quantitative Monitoring of Crystalline Phase Transformation and Orientation Evolution in Poly(vinylidene fluoride) Using Terahertz Spectral Imaging

Crystalline phase transformation and orientation evolution in crystalline polymers critically influence processing performance, yet real-time and spatially resolved characterization remains challenging. Conventional crystal characterization techniques such as infrared spectroscopy and X-ray technology provide accurate measurements but are limited by slow acquisition rates and difficulties in mapping spatial distributions. Leveraging the crystal structure sensitivity of terahertz time-domain spectroscopy (THz-TDS), this study reports a dynamic and nondestructive method for monitoring dynamic crystalline phase transformations and orientation evolution in poly(vinylidene fluoride) (PVDF) during uniaxial stretching. The THz anisotropic absorption exhibited a strong linear correlation with both the β-phase fraction and orientation parameter, indicating that THz-TDS can quantitatively capture structural anisotropy. Semiquantitative models between THz anisotropic absorption and the fraction and orientation of the crystal were developed and experimentally validated. The measured β-phase fraction showed good agreement with FTIR and 2D-WAXD results, with RMSE values of 0.019 and 0.005, respectively. THz spectral imaging further enabled the visualization of spatial distributions of both the phase content and orientation, revealing the orientation and phase evolution during stretching. Spatial mapping showed that regions of increased β-phase content and orientation coincided with the necking zone, suggesting that localized deformation during necking drives the crystalline phase transformation. This work establishes THz-TDS as a rapid, noninvasive, and distribution-resolved tool for monitoring structural evolution in crystalline polymers, with potential applications in in-line process control and polymer physics studies.