The molecular mobility in copolymers of vinylidene fluoride-hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10 À1 -10 7 Hz), differential scanning calorimetry DSC (À100 to 1508C), and of wide angle X-ray diffraction WAXS. Four relaxation processes and one ferroelectric-paraelectric phase transition have been detected. The process of the local mobility b-(at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (a a -relaxation) depend on the ratio of comonomer with lower kinetic flexibility. a c -relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of a-(a p -) phase. Diffuse order-disorder relaxor type ferroelectric transition connected with the destruction of the domains in low-perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (a-process) was observed for both copolymers in high-temperature region. DSC data shows that it falls on the broad temperature region of aphase crystals melting. It is considered to be connected with the space charge relaxation.

The hydrosilylation of four isomers of tetra(dimethylsiloxy)tetraphenylcyclotetrasiloxane with (4´ cyanobiphenyl 4 yl)undec 10 enoate in the presence of a platinum catalyst afforded four liquid crystalline stereoisomers of phenylcyclotetrasiloxane with mesogenic cyanobiphenyl groups. The reaction was performed for the first time. The influence of the spatial structure of the stereoisomers of tetraphenylcyclotetrasiloxanes with mesogenic groups on the formation of the liquid crystalline state by thermooptical, X ray diffraction, and calorimetric methods. The temperatures and enthalpies of phase transitions were determined, and their reversibility was shown. Differences in both the textures of melts formed on cooling and the phase state and in interplanar distances were found by optical microscopy and X ray diffraction methods. Based on these data, the spatial structure of the isomers was concluded to affect the phase composi tion and type of packing in the liquid crystalline state.Data on the influence of the structure of stereoisomers of cyclosiloxanes on the properties and type of packing in the liquid crystalline (LC) state are lacking in literature. This is related, first of all, to the fact that methyl(hydri do)cyclosiloxanes used for the synthesis of LC compounds usually represent mixtures of isomers (regardless of the cycle size). The influence of the flexibility of the central core and spacer on the type of the forming LC phase has been revealed 1,2 to the present time by computer simula tion. Weak interactions between mesogens, a sufficiently long spacer, and a flexible core favor the formation of nematic phases, whereas discotic molecules with a rigid core, a short spacer, and strongly interacting mesogenic groups form smectic or columnar phases. For example, studying methylcyclosiloxanes, the authors assumed the existence of several types of packing: cylindrical, 3-5 sheaf like, and disk like. 6 Since several stereoisomers of methyl cyclosiloxanes can exist, the types of packing in the LC state for the cis isomer and r 2,t 4,t 6,t 8 isomer should differ due to different intramolecular interactions of mesogenic groups. Unfortunately, none of the published works discusses the influence of the spatial structure of stereoisomers or the composition of a mixture of isomers of methylcyclosiloxanes on the type of packing in the LC state. 3- 10 We earlier 11 attempted to study this influence for cyclohexasiloxane. It was found that methylcyclosiloxanes enriched in the trans isomer form different LC textures on cooling of the isotropic melt. 11 cis Isomers of cyclo tetra and cyclohexasiloxanes with the same mesogenic groups also form different types of packing in the LC state. 12 The purpose of this work is to synthesize structural isomers of cyclosiloxanes with a certain position of mesogenic cyanobiphenyl groups and to identify phase transitions and the type of packing in the LC state. Results and DiscussionSynthesis of LC stereoisomers of tetraphenylcyclo tetrasiloxanes with cyanobiphenyl mesogenic groups. A...

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Articles you may be interested inStructural and dielectric properties of poly(vinylidene fluoride)-based terpolymer/copolymer blends developed on aluminum foil J. Appl. Phys. 112, 053505 (2012); 10.1063/1.4748181 Interacting and noninteracting dipole systems in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer J. Appl. Phys. 108, 084109 (2010); 10.1063/1.3499614 Structural evolution and dielectric relaxation behavior of electron-irradiated poly(vinylidene fluoride-trifluoroethylene) 80/20 mol % copolymers J. Appl. Phys. 94, 5566 (2003); 10.1063/1.1606853 Infrared spectroscopic ellipsometry study of vinylidene fluoride (70%)-trifluoroethylene (30%) copolymer Langmuir-Blodgett films J. Appl. Phys. 94, 195 (2003); 10.1063/1.1578697Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers: High-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) copolymersThe low-temperature molecular mobility has been studied by dielectric relaxation spectroscopy on the bulk films of statistic copolymers of vinylidene fluoride (VDF) with tetra-(TFE) and trifluoroethylene obtained by crystallization from a solution in acetone. The results show that activation energy of the kinetic units in the glassy state depends significantly on the concentration of polar groups. The cooperative mobility above the glass transition temperature is described by Vogel-Tamman-Fulcher equation; the lowest value of the effective activation energy was found in the copolymer with highest amount of non-polar TFE content. It has been shown that the parameters of the dynamics are related to the structural parameters obtained by infrared spectroscopy, wide-angle X-ray Spectroscopy, and small-angle X-ray scattering. In particular, the largest crystals and the highest value of the "large" period have been detected in VDF/TFE copolymer with lowest activation parameters of microbrownian and local mobility. Number of lamellar crystals in stacks is determined by the concentration of VDF polar groups in copolymer chain. V C 2015 AIP Publishing LLC.