Dielectrics monoclinic

The equimolar copolymer of ethylene and tetrafluoroethylene is isomeric with poly(vinyhdene fluoride) but has a higher melting point (16,17) and a lower dielectric loss (18,19) (see Fluorine compounds, organic-poly(VINYLIDENE fluoride)). A copolymer with the degree of alternation of about 0.88 was used to study the stmcture (20). Its unit cell was determined by x-ray diffraction. Despite irregularities in the chain stmcture and low crystallinity, a unit cell and stmcture was derived that gave a calculated crystalline density of 1.9 g/cm. The unit cell is befleved to be orthorhombic or monoclinic (a = 0.96 nm, b = 0.925 nm, c = 0.50 nm 7 = 96%. 

For the low-symmetry triclinic and monoclinic crystals the principal axes for the real and imaginary parts of the dielectric tensor are different. This makes life very complicated, and we—along with most other authors—will avoid such complications. 

In the monoclinic case, in which the dielectric tensor is not in the principal axis system ... 

In equation (2), C is the capacitance, is the capacitance without the dielectric and 5 is the angle by which current leads the voltage. Nitrobenzene was confined in porous silica (CPG and VYCOR), of pore widths H = 50 nm to 4 nm at 1 atm. pressure. The freezing temperature in the bulk is 5.6 °C (the liquid freezes to a monoclinic crystal). n = n —iK", the complex dielectric permittivity is measured as a function of temperature and frequency. 

The dielectric parameters are tensors, and consequently it is essential to use polarized radiation when recording the infrared absorption and reflection spectra of all but cubic crystals. Thus, with an orthorhombic crystal the reflection has to be measured with the electric vector parallel to the a, Z , and c axes. When obtaining the reflection spectra from the be plane of a monoclinic crystal, it is necessary to rotate the plane of polarization of the electric vector. The longitudinal optic (LO) frequency [see discussion of Equation (4)] can be found for q = 0 by noting that it is the frequency for which = 0. It can be measured directly in cubic crystals by a method due to Berriman [114]. 

In this work, we present results of dielectric measurements of two diacetylenes pTS (R = CH2-0-S02-< )-CH3) and pFBS (R = CH2-0 S02-< F). At room temperature, and down to ca. 200 K, both crystals are isomorphous (monoclinic P2i/c, Z = 2), with almost identical crystallographic paclangs [7-9]. The monomer of pTS exhibits two phase transitions (at ca. 200 K and ca. 160 K), the phase between them being incommensurate [10]. The low-T phase remains P2j/c, with four molecules per unit cell [8]. Poly-pTS displays only one phase transition, at ca. 190 K, directly to the low-T phase, which is isomorphous with that of monomer [11]. On the other hand, spectroscopic investigations of polymerized pFBS [12] did not show any evidence of a phase transition, from troom temperature down to 4 K. 

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