Positive spherulites

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. 

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 7-modification of iPP (y-iPP) may form in degraded, low molecular weight iPP or in samples crystallized under high pressure [5, 6]. Certain propylene copolymers with low comonomer content (4-10 wt.%) crystallize preferentially in y-form, as well. y-iPP has a face-centred ortho-rombic unit cell with parameters a = 0.85 run, b = 0.993 nm and c = 4.241 nm containing isochiral helices. The cell structure proposed by Bruckner et al. [5] is unique in polymer crystallography the chain axes in adjacent crystal layers are not parallel. The angle between the chain stems is about 80°. y-iPP is not usually observed as an independent phase, but crystallizes with and within the a-spherulites. According to Lotz et al. [7], the positive spherulites observed in samples with mixed polymorphic composition of a- and y-iPP are probably made of a... 

In Fig. 2.9b, the specimen was crystallized at 120 °C, so that there is a preponderance of cross-hatching lamellae located roughly perpendicular to the dominant radially growing ones. This reverses the birefringence, giving positive spherulites... 

One hydrocarbon polymer that gives positive spherulites is isotactic polystyrene here the effect of aromatic rings with strong polarizability perpendicular to the molecular chains dominates over the polarizability of the chain. 

Polysters, polyamides and polycarbonates occasionally give rise to positive spherulites. All even polyamides (PA 6,6, PA 6,10 and PA 6,12) behave in a similar way. Positive spherulites are found at low temperatures below a certain temperature Tj (15-20 K below the melting point) and negative spherulites at temperatures between Ti and T2 (a temperature only a few kelvin below the melting point). The orientation of the chain axis [001] was preferentially tangential whereas the hydrogen bond planes were radial in both negative and positive spherulites. 

Negative and positive spherulites are readily distinguished by polarized light microscopy. How Also explain the differences in morphology between the two. 

Figure 6.28. Optical sign of spherulites. (a) Positive spherulite and (b) negative spherulite.

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