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Poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), is a ferroelectric polymer of interest for flexible electronics, sensors, and energy-harvesting devices. Its optical response depends on both crystalline structure and chemical composition, making spectroscopy a potentially useful tool for identifying phase composition and fluorination-related structural changes. However, the sensitivity of different optical parameters to these changes is not uniform. This study uses density functional theory (DFT) to compare the optical properties of α- and β-phase P(VDF-TrFE) crystals and assess their suitability for spectroscopic characterization.
The calculations were performed using the CASTEP package within the generalized gradient approximation and the PBE exchange-correlation functional. Three α-phase structural models containing 2.1%, 4.2%, and 6.25% fluorine substitutions were investigated, alongside a β-phase model with 4.2% fluorine content. Optical absorption, reflectance, refractive indices, and Raman spectra were analyzed to evaluate the sensitivity of these observables to fluorination and crystalline phase.
The calculated absorption spectra reveal a systematic increase in absorption in the 7–9 eV region as fluorination increases in the α-phase models. The absorption onsets also differ by more than 1 eV between these models, suggesting that absorption spectroscopy may help distinguish structures with different fluorination levels. For α- and β-phase models with the same fluorine content, the absorption edges exhibit an approximately 2 eV displacement, providing a potential spectral indicator of crystalline phase. In contrast, reflectance spectra show relatively similar profiles: the α-phase reaches a maximum of approximately 0.40 near 23 eV, while the β-phase reaches approximately 0.31 near 25 eV. Such similarities may complicate reliable phase identification based on reflectance alone. Differences in refractive indices are likewise limited over parts of the spectrum, reducing the expected discriminative power of ellipsometric measurements.
Raman spectra provide complementary structural information, particularly through phase-dependent peak positions and intensity redistribution in the 500–1500 cm⁻¹ and 3000–3200 cm⁻¹ regions. The combined results indicate that optical absorption and Raman spectroscopy offer complementary means of distinguishing fluorination levels and crystalline structures, whereas reflectance and refractive-index measurements may be less diagnostic when used independently.
This computational comparison establishes a basis for selecting spectroscopic observables for the characterization of P(VDF-TrFE) and highlights the importance of combining complementary techniques when analyzing structurally complex polymeric materials.
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