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Poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), is a ferroelectric polymer whose vibrational spectra depend on crystalline structure and chemical composition. Interpreting these spectra requires distinguishing phase-dependent features from changes associated with TrFE content. This study compares density functional theory (DFT) predictions with published infrared and Raman measurements to evaluate the experimental relevance of calculated phase and composition sensitivity.
Calculations were performed using CASTEP within the generalized gradient approximation and the PBE exchange-correlation functional. Three $\alpha$-phase structural models with fluorine substitution levels of $2.1\%$, $4.2\%$, and $6.25\%$ were investigated alongside a $\beta$-phase model with a $4.2\%$ substitution level. With one H-to-F substitution per converted repeat unit, the adopted model compositions correspond to nominal TrFE contents of $12.5$, $25$, and $37.5$ mol.% for the $\alpha$ phase and $25$ mol.% for the $\beta$ phase. Published infrared [1] and Raman [2,3] spectra were digitized and compared using reference-band normalization, without empirical frequency shifts or rescaling.
The infrared comparison reveals local correspondence in band positions and phase-dependent intensity distributions. The experimental profile is dominated by absorption near $1183\,\mathrm{cm}^{-1}$, corresponding closely to the $\beta$-model band near $1181\,\mathrm{cm}^{-1}$. The $\alpha$ model predicts a neighboring band near $1171\,\mathrm{cm}^{-1}$. These features provide reference points for comparing the calculated structures with experiment.
The Raman comparison at matched nominal composition distinguishes the two calculated phases more directly. The models exhibit different dominant-band positions and intensity distributions in the $780$–$900\,\mathrm{cm}^{-1}$ region, with the $\beta$-phase maximum closer in frequency to experiment. Peak positions and relative intensities thus provide complementary criteria for assessing phase-dependent spectral patterns.
Analysis of the wider concentration series shows that these signatures also depend on TrFE content. Across the $\alpha$ models, the dominant Raman band moves towards higher wavenumbers with increasing substitution, following the direction observed experimentally [2]. The experimental contribution in the $1340$–$1380\,\mathrm{cm}^{-1}$ region strengthens, whereas bands near $1280$ and $1430\,\mathrm{cm}^{-1}$ generally weaken across the copolymer series [3]. The $\alpha$ models exhibit composition-dependent intensity redistribution near $1280$–$1300$ and $1430\,\mathrm{cm}^{-1}$, providing additional spectral regions for comparison.
Combined infrared and Raman analysis therefore provides complementary information on phase and composition sensitivity. The comparison establishes band-dependent agreement between theory and experiment and supports the combined use of Raman and infrared spectroscopy for evaluating composition and local structure. Reliable interpretation requires multiple spectral markers, consideration of composition and measurement conditions, and independent structural information.
References:
[1] Arrigoni, A.; Brambilla, L.; Bertarelli, C.; Serra, G.; Tommasini, M.; Castiglioni, C. P(VDF-TrFE) nanofibers: structure of the ferroelectric and paraelectric phases through IR and Raman spectroscopies. RSC Advances 2020, 10(62), 37779–37796. https://doi.org/10.1039/D0RA05478J
[2] Resende, P. M.; Isasa, J.-D.; Hadziioannou, G.; Fleury, G. Deciphering TrFE Fingerprints in P(VDF-TrFE) by Raman Spectroscopy: Defect Quantification and Morphotropic Phase Boundary. Macromolecules 2023, 56(23), 9673–9684. https://doi.org/10.1021/acs.macromol.3c01700
[3] Tashiro, K.; Kobayashi, M. Vibrational spectroscopic study of the ferroelectric phase transition in vinylidene fluoride-trifluoroethylene copolymers: 1. Temperature dependence of the Raman spectra. Polymer 1988, 29(3), 426–436. https://doi.org/10.1016/0032-3861(88)90359-X
| Type of presence | Presence at Taras Shevchenko National University |
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| Figure caption | Digitized P(VDF-TrFE) IR (a: $20$ mol.% TrFE [1]) and Raman (b: $25$ mol.% [2]) spectra are compared with nominal $25$ mol.% $\alpha/\beta$ DFT spectra. Solid/dashed: experiment/theory; each is normalized to its IR band near $1183,\mathrm{cm}^{-1}$ or Raman maximum within $780$–$900,\mathrm{cm}^{-1}$. |