Solid‐state ¹⁹F MAS NMR study on the conformation and molecular mobility of poly(chlorotrifluoroethylene)

Abstract

The temperature dependence of molecular mobility and conformational changes of poly(chlorotrifluoro- ethylene) (PCTFE) have been investigated by solid-state (19)F magic angle spinning (MAS) NMR spectroscopy. The pulse techniques of dipolar-filter and T(1rho)-filter allow selective observation of the amorphous and crystalline domains, respectively. The temperature dependence of T(1rho) (F) revealed that the segmental motion in the amorphous domain becomes vigorous above ca 80 degrees C, which is well above the glass transition (T(g)) temperature (52 degrees C) and more close to the beta-relaxation temperature (95 degrees C). On the other hand, vigorous molecular motions in the crystalline domain occur above 120 degrees C, which is much below the melting temperature (212 degrees C). This indicates that the polymer chains in the PCTFE crystallites are more mobile than those of typical semicrystalline fluoropolymers like poly(vinylidene fluoride) (PVDF), which can be associated with structural imperfections in the crystallites. In addition, the density functional theory (DFT) calculations of (19)F magnetic shielding suggest that the high-frequency shifts observed for the crystalline signals above 80 degrees C can be ascribed to the conformational change around meso diads toward more twisted and/or helical conformations in the main chain.

Computational 19F NMR. 1. General features

“…Some early computational work (aimed at benchmarking and validation) included F 2 as test molecule, which proved to be challenging [26][27][28]. Interest in modeling 19 F shifts has steadily continued, and several papers have appeared in the last decade aimed at general structural issues, solution chemistry and physical organic chemistry, [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] solid-state issues (especially the prediction of chemical shift tensors), [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] J-couplings [65][66][67][68][69][70][71][72]…”

Emulating Natural Product Conformation by Cooperative, Non‐Covalent Fluorine Interactions

“…From previous studies on fluorinated and perfluorinated polymers, including polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PTFCE), and polyvinylidene fluoride (PVDF), a correlation between the isotropic 19 F chemical shift and the specific polymer chain conformation has been established . This conformational effect, referred to as the γ‐ gauche effect, was shown to be ≈10–15 ppm when comparing the various crystalline forms of PVDF, whereas molecular packing effects are ≈3–8 ppm . These differences show that the isotropic 19 F chemical shifts observed in 19 F MAS NMR directly offer additional information on the specific conformation of various diastereomers in their crystalline form.…”

“…[26][27][28] This conformational effect, referred to as the g-gauche effect, was shown to be % 10-15 ppm when comparing the variousc rystalline forms of PVDF,w hereas molecular packing effects are % 3-8 ppm. [28][29][30] These differences show that the isotropic 19 Fc hemical shifts observed in 19 FM AS NMR directly offer additional information on the specific conformation of various diastereomers in their crystalline form. Thus, to furtherc haracterise the conformational differences between the trifluoride motifs in 3c and 3d,s olid-state 19 FM AS NMR spectra were recorded as summarised in Figure 11 ba nd d, respectively.T he 19 FM AS NMR spectrum for 3c in Figure 11 bi ncludesf our 19 Fr esonances, which can be deconvolutedi nto six distinct 19 Fc omponents.F or 3d,t he 19 FM AS NMR spectrumi n Figure 11 di ncludes three well-resolved 19 Fr esonances.T hese observations are in agreement with the two and one molecules per asymmetric unit determined from X-ray crystallography for 3c and 3d,r espectively.T oa ssign the experimentally observed solid-state 19 Fc hemical shifts to the specific fluorine atomsi nt he unit cells we have performed GIPAW calculations using CASTEP [31] as summarised in Figure 11 cf or 3c.F rom ad irect comparison of the experimental 19 FM AS NMR spectra of 3c and d,a nd the calculated 19 Fc hemicals shifts displayed in Figure 11 b-d, as trong shift to higherp pm values for F-3/F-3' in 3c by about 14.5 ppm can be observed.…”

“…Solid‐state NMR analysis : In an attempt to reconcile the solution‐phase NMR analysis with the crystallographic data, a solid‐state NMR analysis was performed to compare candidate 3 c with 3 d as a control. From previous studies on fluorinated and perfluorinated polymers, including polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PTFCE), and polyvinylidene fluoride (PVDF), a correlation between the isotropic 19 F chemical shift and the specific polymer chain conformation has been established . This conformational effect, referred to as the γ‐ gauche effect, was shown to be ≈10–15 ppm when comparing the various crystalline forms of PVDF, whereas molecular packing effects are ≈3–8 ppm .…”

A New Methodological Approach to Correlate Protective and Microscopic Properties by Soft X-ray Microscopy and Solid State NMR Spectroscopy: The Case of Cusa’s Stone

“…T 1 F values for the two polymers in contact with the stone are longer than the relaxation times of the pure materials. This T 1 F increase suggests that the local molecular motions in the order of MHz decrease after the application [34].…”