Negative 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).

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. 

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. 

Relative values measured on stepwise (a) heating/ (b) cooling of thin solution-crystallized negative spherulites ... 

A spherullite is usually pictured as an array of lamellae radially disposed to one another and as a result is spherically birefringent. A spherullite has two unique refractive indices the radial (Ht) and the tangential ( t)- The refractive index ellipse can represent the variation in refractive index in the plane, where the length of the major axis of the ellipse is proportional to the maximum refractive index in the plane and the length of the minor axis is proportional to the minimum refractive index. If the larger refractive index is in the tangential direction, i.e. < rit, the spherulite is termed negative. Spherulites show a characteristic Maltese cross pattern with a maximum in the intensity in the direction at 45 ° to the polarizer/analyser pair. 

Spherulites of aliphatic hydrocarbon polymers such as polyethylene generally have their molecular chains with the highest refractive index oriented perpendicular to the radial growth direction. This is the case with the polypropylene spherulites shown in Fig. 2.9a, which were crystallized at 140 °C and consequently contain very few cross-hatched lamellae, so in this respect they are like polyethylene spherulites. The lower refractive index in this view is radial in direction, giving a yellow colour where the radius is parallel to the slow direction of the tint plate these are therefore termed negative spherulites. 

After an NHSK crystallization experiment during which the solution was still at a predetermined crystallization temperature (185°C in glycerol), additional Nylon 6,6/glycerol solution was added to the mixture, then allowed additional time to crystallize, and finally cooled to room temperature. Negative spherulites were formed by this process. Figure 5.10c shows a diagram of the process used to produce PA spherulites with fully incorporated MWCNT. 

FIGURE 5.10 (a) and (b) Spherulites formed by the addition of Nylon 6,6 to Nylon 6,6 shish kebabs showing negative spherulites. (c) Schematic representation of the production method. (From L. Li et al.. Polymer, 48, 2007, 3452-3460.)... 

When the spherulites were observed between crossed pola-roids using a 1st order red plate in the usual 45 orientation, it was found that both types of spherulites are negatively birefring-ent. This is not surprising since negative spherulites have also been observed in polypropyleneoxide films (10). Nevertheless, the spherulites are not identical when viewed with the 1st order red plate. Type I is blue in the second and fourth quadrants and orange in the first and third. Type II is similar but the quadrants are separated by a red cross. 

Isotactic polypropylene displays a highly unusual ability to induce epitaxial crystallization of a number of different polymers with their chain axes tilted at large angles, 40—80°, relative to the helix axis direction of the polypropylene substrate. The same polymer shows an epitaxy, i.e. homoepitaxy, at an angle of 80° (Fig. 7.30). The homoepitaxy is responsible for crosshatching, a structure typical of the monoclinic a structure. This feature causes the lamellar branching and the optical complexity, with both positive and negative spherulites, typical of isotactic polypropylene (section 7.4). 

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. 

Figure 7.40 Origin of Maltese cross pattern from a negative spherulite. The orientations of the polarizer (P), the analyser (A) and the refractive index ellipses are shown.

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... 

The sign of birefringence of a sample is given with reference to a distinct direction in the sample. For a fiber this is the fiber axis, for a spherulite it is the radius. Thus polyethylene, which has a positive birefringence as a material, forms positive fibers (chains along the fiber axis) but negative spherulites (chains perpendicular to the radial direction. Fig. 3.15). 

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

Figure 5.5 (a) A negative spherulite of poly(ethylene oxide) and (b) a spheruiite of poly(ethylene adipate). (See color insert.)... 

Occasionally, the positive and negative spherulites can coexist in one sample. For example, syndiotactic polystyrene and its blends with atactic polystyrene show the coexistence of the spheruhtes of the opposite optical properties when cooled from the melt [25]. 

Cooling on the Roll The microscopic observation of microtomed sections reveals two types of spherulites a background of positively and weakly birefringent spherulites, crystallized in the a phase, and highly negative spherulites crystallized in the phase. 

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