Spherulite negative

The differences in the speed of the light result from differences in the refractive index. If the highest refractive index is in the radial direction, one talks of positive spherulites. Negative spherulites show the highest refractive index in the tangential direction. Thus, information about the microstructure of the spherulites can be gained from their optical properties. 

By inserting a A plate at an angle of 45° to the polarizer/analyser pair, quadrants 1 and 3 of the spherulites appear either blue or yellow, and quadrants 2 and 4 of the spherulites appear with the reverse colour. This colour combination shows the sign of the spherulites. Negative spherulites, the common case, have a higher refractive index in the tangential plane than... 

Figure 17 Isothermal melting of Ziegler-Natta isotactic poly(propylene). (a) Spherulites with mixed birefringence at Tc = 148°C. The top middle figure displays the melting for the same thermal history, (b) Subsequent to crystallization, the temperature was raised to 171°C spherulites acquire negative birefringence, (c), (d) and (e) Isothermal melting at 171°C for 80, 200 and 300 min, respectively. Reproduced with permission from W.T. Huang, Dissertation, Florida State University, 2005. (See Color Plate Section at the end of this book.)...

Crosspolarized photomicrographs of PEO 1, EO-Is-EO 2, EO-Is-EO 3, and EO-Is-EO 4 films cast from 1% benzene solutions at 30° C are presented in Figure 1. The spherulitic texture with negative birefringence became less perfect and led to a less clear Maltese cross as the fraction of amorphous Is segment increased. When the EO fraction constituted less than 50%, the texture was not clearly resolved by light microscopy. 

Spherulites are classified as positive when the refractive index of the polymCT chain is greater across the chain than along the axis, and negative whrai the greater refractive index is in the axial direction. They also show various other features sueh as zigzag patterns, concentric rings, and dendritic strucfirres. 

Figure 5-25. Narrow-angle light scattering of positive and negative spherulites. Photographs from incident and scattered light. Left, experimental right, theoretical. Top, negative spherulites bottom, positive spherulites (after R. J. Samuels).

The surface and crystallization energies are of opposite sign, and, so, the energy of nucleus formation only becomes negative above a critical nucleus size of Tcrit ory crit (Figure 10-6). The embryons convert to stable nuclei above the critical nucleus size, and can then grow further, into, for example, spherulites. 

To illustrate In drawn poly ethylene) fibers, the speed of light is less in the direction of the fibers than in the direction perpendicular to this. Here, light parallel to the fiber direction shows a higher refractive index. In drawn poly(ethylene) fibers, the molecular axes are largely parallel to the fiber axis. Since poly(ethylene) forms negative spherulites, the molecular axes must be at right angles to the spherulite radius. 

The origin of the black cross is due to the extinction arising from the coincidence of the transmission direction with the direction of the polarizer or analyzer. A plane equatorial section of a spherulite is shown in Figure 3.2. If the larger refractive index is radial, the spherulite is optically positive if it is tangential, it is optically negative [13]. 

Very similar results have been recently obtained by Shin el al. [36] in PLLA-6-PEO-6-PLLA samples Ain values were in the range of 30-50 kg/moi). They found that PLLA and PEO blocks were miscible in the melt upon cooling from the melt at 2°C/min, sequential crystallization of PLLA and PEO occurred. The PLLA formed negative spherulites at higher temperatures then, at lower temperatures, the PEO crystallized within the PLLA spherulites, and the sign of the birefringence was maintained, only the retardation increased in an additive fashion. 

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