P-phase crystallinity

The piezoelectric effect stems from hydrogen and fluorine atoms in the VDF, which are positioned perpendicularly to the polymer backbone. Fig. 5.7A shows a typical molecular structure of PVDF with different crystalline phases (Chang et al., 2012). The piezoelectric performance of PVDF is dependent on the nature of the crystalline phase (Crossley et al., 2014). Typically, PVDF has three crystalline phases, namely a, p, and y, and it is the a-phase that typically forms in most situations. While it is polar p-phase that shows the strongest piezoelectric behavior so this material needs to be electrically poled using an electric field with the order of 100 MV m or mechanically stretched. A higher P-phase crystalline can lead to a higher piezoelectric coefficient Note that the copolymer of P(VDF-TrFE) [(CH2-CF2) -(CHF-CF2)ml crystallizes more easily into the P-phase due to steric factors (Furukawa, 1989). So, the most applied material in piezoelectric generator is P(VDF-TrFE). 

Lund A, Hagstrom B. Melt spinning of poly (vinyUdene fluoride) fibers and the influence of spinning parameters on P-phase crystallinity. J Appl Polym Sci 2010 116(5) 2685-93. 

With respect to the samples A-D shown in Figure 8.5a, sample B distinguishes itself from others with the highest peaks of 2 = 20°, indicating more P-phase crystalline contents in the nanofibers for the applied voltage of 12 kV. For samples E-H, their diffraction patterns are compared in Figure 8.5b. Apparently, sample E, referring to the lowest flow rate of 0.01 mL min", displays the lowest diffraction... 

Copolymer compositions differ from PVDF in that the crystalline P phase is obtained without mechanical deformation. Thus, various thicknesses of the material can be readily produced. Unfortunately, a reproducible copolymer is not yet commercially available. 

Phthalocyanines exhibit high polymorphism, as well [7]. They are insoluble nano crystalline materials, which produce poor x-ray powder diagrams with high preferred orientation and have been intensely investigated by electron microscopy and diffraction [8]. From copper phthalocyanine (CuPc) (Fig.2) nine polymorphs (a, P, y, 5, s, n, p, (, a and R) are known, but only the most stable P phase could be solved by single crystal x-ray diffraction [9] (P2i/c a=14.628 A, b= 4.790 A, c= 19.07 A, b=120.93°). 

Various types of morphologies and crystalline modifications of s-PS have been reported [44], One of these modifications has an orthorhombic form (space group = P2i2i2i) with a planar zigzag conformation of back-bone chain. This form corresponds to the a-phase found by Kobayashi et al. [35] and to the P-phase named by Guerra et al. [36], The crystal structure of this form was analyzed and the plane of phenyl rings within this form is normal to the chain axis (c-axis) [37-39],... 

Trocellier, P. 2000. Immobilisation of radionuclides in single-phase crystalline waste forms a review on their intrinsic properties and long term behaviour. Ann. Chim. Sci. Mat., 25, 321-337. 

Figure 3 shows a freeze-fracture microphotograph of a 50 50 binary mixture of (II) and (IV) quenched from a temperature ( 25°C) where roughly equal proportions of solid and fluid phases are expected to coexist. The banded region corresponds to the crystalline, monoclinic P solid solution phase and the nonbanded region to the fluid phase. For a recent crystallographic study of the P phase of phosphatidylcholines, see Ref. 22. For a freeze-fracture study of this phase, see Ref. 28. 

The unique piezoelectric and pyroelectric properties of semicrystalline films of PVDF arise from changes in the polarization imparted to the overall film by the crystalline P-phase. The polar nature of the P-phase is, in turn, a direct result of the parallel alignment of the dipole moment of the repeat units in the unit cell (Figure 11.1). The crystal polarization is defined as the dipole moment density of the crystal ... 

In the first discussion of equilibrium (Ch. 5) we recognized that there may be states of a system that are actually metastable with respect to other states of the system but which appear to be stable and in equilibrium over a time period. Let us consider, then, a pure substance that can exist in two crystalline states, a and p, and let the a phase be metastable with respect to the p phase at normal temperatures and pressures. We assume that, on cooling the a. phase to the lowest experimental temperature, equilibrium can be maintained within the sample, so that on extrapolation the value of the entropy function becomes zero. If, now, it is possible to cool the p phase under the conditions of maintaining equilibrium with no conversion to the a phase, such that all molecules of the phase attain the same quantum state excluding the lattice vibrations, then the value of the entropy function of the p phase also becomes zero on the extrapolation. The molar absolute entropy of the a phase and of the p phase at the equilibrium transition temperature, Tlr, for the chosen... 

The main basic difference between these two wide classes of compounds is described as follows p-diketonate molecules are associated in the condensed phase (crystalline) and, in a number of cases, also in solution, while the pyrazolyl-borates keep themselves as monomers. In addition to boron, ligands 232 and 233 may contain aluminum (E = A1) and gallium (E — Ga) [443-445],... 

Finally, we have attempted to evaluate the possible impact of an intermediate liquid crystalline phase and the possibility of transfer of helical hand information from the melt to the crystal throughout this process. Assuming that the melt is structured, the melt of chiral but racemic polyolefins would be made of stretches of helical stems that are equally partitioned between left- and right-handed helices. Formation of antichiral structures (such as in a iPP) could be interpreted as indicating a possible transfer of information (but the problem of the sequence of helical hands would still remain). This analysis is, however, ruined by the observation that many of these polymers also form chiral structures (frustrated p phase of iPP, Form III of iPBul). For the achiral poly(5-methyl-pentene-l), the chiral, frustrated phase is actually the more stable one, and can be obtained by melting and recrystallization of a less stable antichiral phase. 

Figure 1 Different types of lipid bilayers. In the Lq and Lqj phase bilayers, the polar head groups are shown as squares with arrows to indicate that a crystalline order exists in the arrangement of the head groups. In the P bilayers, the saw-tooth ripple is indicated by the parallel lines, and only a few lipids are drawn in this structure to show the way in which the lipids are arranged in the different domains of this structure. The reader is referred to References (7) and (8) for the structural details of the P phase.

The fact that the abrupt drop in H (Fig. 6.2) coincides with the well documented a-/3 transition in the deformation interval between 4 and 10-12% suggests that the observed H changes (Fig. 6.2) are exclusively related with the stress-induced polymorphic transition. Thus, the starting value oi H = 155 MPa should be typical for PBT containing the crystalline a phase and the lower value oi H = 118 MPa can be assigned to PBT comprising mostly the crystalline p phase. The observation that is obviously related to the fact that the a modification is distinguished by... 

The microhardness, degree of crystallinity and the percentage of a and p phase obtained are summarized in Table 6.3. The dependence of the microhardness H on the deformation s for drawn and annealed bristles of PEE (PBT/PEO = 57/43 wt%) is plotted in Fig. 6.5. One can see that the H variation can be split into several regimes depending on the stress applied. For the lowest deformations (e up to 25%) H is nearly constant ( 33 MPa) and thereafter in a very narrow deformation interval ( = 2-3%) H suddenly drops by 30% reaching the value of 24 MPa which is maintained in the s range between 30 and 40%. With further increases in s... 

When compounds of this type are heated, the first crystalline phase to appear in the 700-1000°C temperature range is the so-called /.-Na20 XAI2O3 phase x varies from 3 to 12) with the mullite crystal structure . This phase is the low temperature precursor to the P and P intergrowth phases, which begin to form at 1100°C . At 1200°C all the 2 phase converts to a mixture of the p and P" phases. 

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