Crystallinity dipole orientation

Although it is a polar polymer, its electrical insulating properties at room temperature are good even at high frequencies owing to the fact that since room temperature is well below the transition temperature dipole orientation is severely restricted. Some data on the crystallinity of poly(ethylene terephthalate) are presented in Table 25.5. 

Polarization can be classified as electronic (electron cloud distortion), atomic, molecular, ionic, and crystalline. The point of maximum polarization in a system would occur when all dipoles reacted to the applied field and aligned. This is difficult to obtain even in a static situation. In an alternating field situation, the dielectric remains the same or decreases as the frequency increases past the microwave region (11). In the microwave region, attainment of equilibrium is more difficult, and there is an observable lag in the dipole orientation which is commonly called relaxation. The polarization then acquires a component out of phase with the field thermal dissipation of some of the energy of the field. This dissipation and its relation to the normal charging current can be related by Equation 1 where c is the measured dielectric constant of the material and e"... 

Typical lattice defects include cation vacancies substitutional or interstitial ions are other types of more complicated structural defects. A cation vacancy behaves like a negative charge. If the temperature is high, ions are sufficiently mobile that an anion could be expelled from the lattice by the Coulomb potential of the cation vacancy. Cation and anion vacancies could form a dipole oriented along one of the six crystallographic axes. This vacancy coupling is then able to induce a crystalline dipole. Similar dipoles can also appear when an ion is substituted for the host ion. 

Asahina proposed first a model based on the orientation of PVDF polar crystal.[1] Nakamura and Wada[4], and Bergman et al.[5] have speculated that PVDF is ferroelectric. Since then, many people explained the phenomena on the orientation of polar crystals or its ferroelectricity. Some evidence of the dipole orientation in the crystalline phase has been reported recently by means of infrared absorption[6,7,8,9] and X-ray diffrac-tion[10,ll]. Takahashi and Odajima reported the X-ray diffraction studies in the most detailed manner for a stretched PVDF samples, in which they have shown that the orientational change of crystallites about their c-axis on polarizing. They have proved that the reorientation induced by polarizing is dependent on the successive 60 rotation as proposed by Kepler[10]. Some... 

In contrast to charge electrets with their stored real charges, dipole electrets are characterized by a preferential orientation of their molecular dipoles. Here amorphous and semicrystalline dipole electrets are distinguished. Amorphous dipole electrets are characterized by a quasi-permanent dipole polarization in the amorphous phases (Fig. 3a), whereas semicrystalline ones show a permanent dipole orientation in the crystalline phases and additionally contain Maxwell-Wagner compensation charges at the interfaces between amorphous and crystalline regions for stabilizing the polarization (Fig. 3b) (Mopsik and Broadhurst 1975 Broadhurst et al. 1978 Sessler et al. 1992). 

D. Naegele and D. Y. Yoon, Orientation of crystalline dipoles in poly(vinylideae fluoride) films under electric field. Appl Phys. Lett. 33 132 (1978X... 

Only the polar crystalline a, fi, and y phases (49] can carry permanent polarization and contribute to an electric field-induced macroscopic remanent polarization in pure PVDF at ambient temperature, since the glass transition temperature 7, is about -40 Electric field-induced dipole orientations in the amorphous phase would therefore randomize after short times. This bolds also for PVDF/PMl blends with a detectable degree of crystallization (cf. Figs. 1 and 2), where 7, is accidently also below room lemperalure. In... 

The values of some properties of various forms of poly(ethylene terephthalate) which are given in Table 11.3 illustrate the influence of crystallinity. The crystalline melting point of poly(ethylene terephthalate) is 265°C. Although a polar polymer, poly(ethylene terephthalate) has good electrical insulating properties at room temperature (even at high frequencies) since dipole orientation is restricted at temperatures below the glass-transition temperature (80°C). The principal application of poly(ethylene terephthalate) film is for electrical insulation. 

In the present work, to find the experimental evidence for preferential dipole orientation during electrospinning, the P(VDF-TrFE) copolymer was selected instead of PVDF as it has a definite Curie transition temperature (T ) and orJy one ferroelectric crystalline stmcture (P-phase) at ambient temperature far below On the other hand, PVDF has at least four crystal modifications at ambient temperature depending on sample preparation methods [1], which makes it much more difficult to find experimental evidence of dipole orientation during electrospinning using... 

Observed wavenum ber (cm- ) Approximately normal mode assignment Optical anisotropy in terms of molecular chain orientation Symmetry species Direction of vibrational transition moment jt Sensitivity to ferroelectric crystallinity Sensitivity to dipole orientation toward the applied electric field... 

In the present book, we aim at the unified description of ground states and collective excitations in orientationally structured adsorbates based on the theory of two-dimensional dipole systems. Chapter 2 is concerned with the discussion of orientation ordering in the systems of adsorbed molecules. In Section 2.1, we present a concise review on basic experimental evidence to date which demonstrate a variety of structures occurring in two-dimensional molecular lattices on crystalline dielectric substrates and interactions governing this occurrence. 

One may consider a series of physical states ranging from the crystalline, where molecular aggregation and orientation are large, to the dilute gaseous state, where there are no significant orientational limits. States of intermediate order are represented by micelles, liquid crystals, monolayers, ion pairs, and dipole-dipole complexes. In the crystalline state, the differences between pure enantiomers, racemic modifications, and diastereomeric complexes are clearly defined both structurally and energetically (32,33). At the other extreme, stereospecific interactions between diastereomerically related solvents and solutes, ion pairs, and other partially oriented systems are much less clearly resolved. 

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