“Grafting From” Polymerization of Vinylidene Fluoride (VDF) from Silica to Achieve Original Silica–PVDF Core–Shells

A new method of surface modification based on the “grafting from” polymerization process enabled to prepare original silica nanoparticles covered with PDVF “hair”. This strategy involved two steps: (i) the radical addition of 1,4-diiodoperfluorobutane initiated by <i>tert</i>-butylperoxypivalate onto the double bonds of silica nanoparticles (specific area of 150 m² g^–1) led to silica that bore C₄F₈I end-groups on its surface, and (ii) its use as original macrochain transfer agent in the “grafting from” polymerization of vinylidene fluoride (VDF) initiated by bis(4-<i>tert</i>-butylcyclohexyl) peroxydicarbonate. The characterizations of both modified silica were achieved by elemental analysis, ¹H and ¹⁹F magic angle spinning solid state rotational-echo double resonance NMR spectroscopy, thermogravimetry, and water contact angle (WCA) assessments. The grafting efficiency was confirmed by three features: (i) the increase of weight percentage of fluorine atom (that ranged from 0 for the starting unsaturated silica to 2.50% for silica-bearing −C₄F₈I end-group and to 3.32% for original silica–PVDF core–shells), (ii) a high thermostability (that reached more than 400 °C under air), and (iii) a good hydrophobicity: the WCA values were ranging from 84° to 109° and to 126° for vinyl silica, for silica coated with −C₄F₈I, and for PVDF-covered silica nanoparticles, respectively

Radical Grafting of Tetrafluoroethylene and Vinylidene Fluoride Telomers onto Silica Bearing Vinyl Groups

Radical addition of ω-iodofluorinated telomers was used to modify silica (S50) nanoparticles bearing vinyl groups. These iodo terminated derivatives were either commercially available tetrafluoroethylene telomers, CnF2n+1I with n = 4 or 6, or vinylidene fluoride telomers, CnF2n+1[VDF]mI with m = 6 and 23. These latters were synthesized by radical telomerization of VDF with CnF2n+1I initiated by bis(4-tert-butylcyclohexyl) peroxydicarbonate in high yields (>85%). The resulting nanohybrids were characterized by solid state NMR spectroscopy, elemental analyses and thermogravimetry. A covalent grafting between double bonds and fluorinated iodotelomers was noted. The covering density of fluorinated chains was assessed and reported with respect to fluorinated chain lengths. These nanohybrids exhibited a high thermostability (higher than 400 °C under air), losing less than 10% by weight at 700 °C, and a low surface tension, γs, from around 15 mN·m–1 to about 44 mN·m–1 for silica

Influence of <i>trans</i>-1,3,3,3-Tetrafluoropropene on the Structure–Properties Relationship of VDF- and TrFE-Based Terpolymers

<i>trans</i>-1,3,3,3-Tetrafluoropropene (1234ze) was copolymerized under free radical conditions with vinylidene fluoride (VDF) and trifluoroethylene (TrFE), for the first time, leading to statistical poly­(VDF-<i>ter</i>-TrFE-<i>ter</i>-1234ze) electroactive terpolymers. The reactivity ratios of the three comonomer couples were determined (<i>r</i>_VDF = 0.77; <i>r</i>_TrFE = 0.32), (<i>r</i>_VDF = 1.67; <i>r</i>_1234ze = 0.00), and (<i>r</i>_TrFE = 7.56; <i>r</i>_1234ze = 0.00), at 48 °C, using the nonlinear fitting Mayo–Lewis method. 1234ze was shown to be regularly incorporated in the terpolymer chains over the entire course of the reaction providing terpolymer chains with statistical monomer distribution and almost constant composition. These new VDF/TrFE-based terpolymers were characterized by ¹H and ¹⁹F liquid state NMR spectroscopy. The characteristic NMR signals of the VDF–1234ze dyads were identified by comparing the NMR spectral signatures of a poly­(VDF₈₂-<i>co</i>-1234ze₁₈) copolymer and of a terpolymer. The thermal and electroactive properties of poly­(VDF-<i>ter</i>-TrFE-<i>ter</i>-1234ze) terpolymers, with 1234ze content ranging from 0 to 6 mol % and molar masses above 55 kg/mol, were assessed. The randomly distributed 1234ze termonomer units induced the decreases of both the Curie and the melting temperatures of the terpolymer even at low termonomer content (<i>T</i>_Curie = 70 °C and <i>T</i>_m = 126 °C and <i>T</i>_Curie = 72 °C and <i>T</i>_m = 150 °C; for a poly­(VDF₆₉-<i>ter</i>-TrFE₂₈-<i>ter</i>-1234ze₃) terpolymer and a poly­(VDF₆₅-<i>co</i>-TrFE₃₅) copolymer, respectively). Films of the terpolymers were cast, and their electroactive properties were examined by D–E loops measurements. They showed that the presence of 1234ze decreased the remnant polarization (<i>P</i>_r = 45 mC/m² for a poly­(VDF₆₅-<i>co</i>-TrFE₃₅) copolymer to 28 mC/m² for a poly­(VDF₆₉-<i>ter</i>-TrFE₂₅-<i>ter</i>-1234ze₆) terpolymer) probably because it also decreased the crystallinity of the terpolymer. The combination of the studies of the reactivity of the monomers, of the terpolymer microstructures, and of the assessment of their physical properties provides insights into their structure–property relationship

One-Dimensional Oxygen Diffusion Mechanism in Sr₂ScGaO₅ Electrolyte Explored by Neutron and Synchrotron Diffraction, ¹⁷O NMR, and Density Functional Theory Calculations

We investigated moderate-temperature oxygen diffusion mechanisms in Sr₂ScGaO₅ with Brownmillerite structure type. From oxygen isotope ¹⁸O–¹⁶O exchange experiments we determined that oxygen mobility sets in above 550 °C. Temperature-dependent neutron and X-ray (synchrotron) diffraction experiments allowed us to correlate the oxygen mobility with a subtle phase transition of the orthorhombic room-temperature structure with <i>I</i>2<i>mb</i> space group toward <i>Imma</i>, going along with a disorder of the (GaO₄)_∞-tetrahedral chains. From lattice dynamical simulations we could clearly evidence that dynamic switching of the (GaO₄)_∞-tetrahedral chains from its R to L configuration sets in at 600 °C, thus correlating oxygen diffusion with the dynamic disorder. Oxygen ion diffusion pathways are thus constrained along the one-dimensional oxygen vacancy channels, which is a different diffusion mechanism compared to that of the isostructural CaFeO_2.5, where diffusion of the apical oxygen atoms into the vacant lattice sites are equally involved in the diffusion pathway. The proposed ordered room-temperature structure in <i>I</i>2<i>mb</i> is strongly supported by ¹⁷O, ⁴⁵Sc, and ⁷¹Ga NMR measurements, which indicate the presence of crystallographically unique sites and the absence of local disordering effects below the phase transition. The electric field gradient tensor components measured at the nuclear sites are found to be in excellent agreement with calculated values using the WIEN2k program. The oxygen site assignment has been independently confirmed by ¹⁷O­{⁴⁵Sc} transfer of adiabatic populations double-resonance experiments