P form crystallization

Fig. 13 a, b. Possible stacking of macromolecular bilayers in the P form crystals of s-PS. The regular succession of bilayers ABAB. .. gives rise to the ordered P" modification (a) defects, corresponding to pairs of bilayers of the kind AA or BB, would characterize the disordered P modification an AA defect is reported in (b). The symbols (/) and ( ) indicate the orientation of the lines connecting the adjacent phenyl rings of each chain inside the macromolecular bilayers A and B, respectively [29]... 

Determination of the P-form crystal structure in the projection on the ad-plane... 

Stacking faults in syndiotactic polystyrene (s-PS) P-form crystals [3, 29-34]... 

Pr is reasonable because the x-axis of the film is taken as the direction of the poling field. On the other hand, the spontaneous polarization of the P-form crystal of PVDF is estimated from the crystal structure as Ps= 1.3 x 10 5 coul/cm2 = 4x 104 cgsesu. Considering the incompleteness in crystallinity ami orientation, the estimated value of PJN is reasonable compared with PF However, it would still be premature to conclude that the piezoelectricity of drawn and polarized PVDF film is due to the strain dependence of spontaneous polarization. 

Very sparingly sol in abs ale, ether, acetone sol in hot glacial acetic acid, pyridine, aniline, p Form crystals from hot H2O + ethanol, from dil acetic acid, or from pyridine mp 148-155°. 

Pressure of -form is lower than that of a -form as seen in Figure 1. However, when pressure was applied, only a-form crystals appeared and increased, but never p -form crystals were obtained. As the pressure was decreased gradually, the a -form crystals melted away, and then 5-form crystals appeared. As the pressure was increased again, only the -form crystals increased, but not the a -form, a -form crystals were powder-like, -form crystals were needle-like. These were the in-situ observations with an optical cell. 

By processing of fibres with high content of p form crystals transitions p—Ki and P mesophase are observed. The direction and range of transition depends on the processing parameters. 

Eling et al observed p-form crystals, whose melting temperature is 10 C lower than the melting temperature of the corresponding a-form. Cartier et al reported y-form crystals, which are obtained by epitaxial crystallization of PLA in hexamethylbenzene. In y-form, two chains are antiparallel to the crystal unit cell. 

Crystallization of triglycerides from a solvent system generally yields P-form crystals. 

Bu et al. [28] studied the crystallization of the a- and P-form crystals of SPS and found that it is determined by the crystallization temperature. Being crystallized at various temperatures from the melt, SPS forms the P-form crystal at high temperatures, above approximately 230 C, the a-form crystal at low temperatures, below approximately 170 C, and a mixture of the a and P forms at intermediate temperatures. 

Systematic investigation of the solid phases formed in the system SOj-HjO (Erametsa et al., 1972) has shown the composition of the major solid phase to correspond to the trisulfite complex found by Vickery in solution. When R is Sm, Gd, or Dy, (NH )3R(S03)3-HjO may be prepared as crystalline precipitate. X-ray powder diffraction studies indicated that this compound has two crystalline modifications the low-temperature a-form, best obtained around room temperature, and the high temperature or p-form, crystallized at temperatures 50°C higher. 

Figure 14 X-ray diffraction pattern of sample 6 (P form) crystallized at 298 K.

Due to the crystal memory effect, the starting structure, temperature (T and time of fusion prior to crystallization might influence the polymorphic behavior of polymers in the following cold or melt crystallization. An interesting example is s-PS. The a-form aystals of s-PS are exclusively obtained upon melt crystallization when traces of the a-form nuclei are present and not erased completely at low Tj (e.g., 280 °C). In contrast, once the crystal nuclei are completely removed at high (e.g., 380°C), only the thermodynamically stable p-form crystals are obtained under the same crystallization conditions [125]. 

Blending with a miscible component usually alters the thermodynamic and kinetic environments and thus affects the crystalline structure of polymorphic polymers. For example, formation of the s-PS p-form crystals is favored in blends of s-PS with miscible components such as a-PS, poly(2,6-dimethyl-p-phenylene oxide), and poly(styrene-co-a-methyl styrene) [125]. This is attributed to the lowered equilibrium melting temperature (T ) and decreased crystallization rate of s-PS upon blending. 

As stated by Lotz and co-workers [134,135], crystal morphology and structure can be regulated by epitaxial crystallization on the substrates of low-MW substances or polymers. The epitaxial crystallization of PLLA on the hexamethylbenzene surface induces the formation of a new crystal modification, i.e., y-form [135]. The epitaxial crystallization of PBA on the highly oriented PE or i-PP film favors the formation of the p-form crystals, rather than the thermally stable a-form [136,137]. 

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