Quantitative characterization of crystallinity in semi-crystalline fluoropolymer through ¹⁹F LF-NMR relaxometry

Crystallinity is important to the properties of a semi-crystalline fluoropolymer, such as solubility, mechanical property, bonding strength, etc. This study extended the use of ¹⁹F Low-Field Nuclear Magnetic Resonance (¹⁹F LF-NMR) to the measurement of crystallinity in the semi-crystalline fluoropolymer F2314, which is the copolymer of vinylidene fluoride (VDF) and chlorotrifluoroethylene (CTFE) with a molar ratio of 1:4 and commonly used as binder in polymer-bonded explosives (PBXs). Based on the difference between spin-spin relaxometry of the ¹⁹F in crystalline region and in amorphous region, the crystallinity of F2314 can be qualitatively characterized. The obtained crystallinity exhibited a positive correlation to the time duration of thermal treatment, which is in consistent to the result of Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD). Moreover, ¹⁹F LF-NMR detected the slight decrease in segmental motion of crystalline region with the increase of crystallinity, and on-line monitored the evolution of segmental motion during heating. This study not only demonstrates the practicality and reliability of ¹⁹F LF-NMR in quantification of crystallinity, but also laid the foundation for in-situ characterization of fluoropolymers in PBXs in future work.