Positive and Negative Spherulites

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]. 

The a axis (hydrogen bonded sheets) // Growth direction 

---

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).

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). 

In some cases, the enzymatic polymerization afforded spherulites of artificial cellulose II, composed of single crystals with the molecular axis orientated perpendicular to the plane (29). Both positive- and negative-type spherulites were observed by polarization optical microscopy. By changing the reaction parameters. 

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. 

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. 

The optical character of a-spherulites is controlled by the ratio of radial to tangential fibrils. Raising the temperature of crystallization leads to a reduction in the proportion of tangential ones [181] and, simultaneously, a positive to negative character transformation. No tangential fibrils form above 155°C [172]. It was also revealed that the thickness of tangential fibrils was lower than that of the radial ones. 

Formation of some new types of spherulite was also detected with a high-temperature crystallization of iPP (Tc = 150—160°C). Awaya [171] observed several new types of a-spherulites, designated pseudopositive, pseudo-negative, neo-mixed, high-tempera-ture positive, and flower-like ones. Varga observed for highly degraded iPP positive a-spherulites independently of the crystallization temperature, where... 

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

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. 

Calcitrm carbonate in polyethylene has two mutually opposite effects the reinforcement and the nucleating effect. The reinforcement effect increases the btrlk crystalhnity and modttlus. The nucleating effect decreases the spherulite size and has a negative influence on yield stress, neutralizing the positive irtfluence of increase in percentage crystallinity. ... 

With increasing neat PVAc content, the heat of fusion decreases and die melting peaks shift to lower temperature in PLA/PVAc blends. The interaction parameters exhibit negative values for up to 10% hydrolyzed PVAc copolymer, but the values increase to positive ones with increasing the degree of hydrolysis. SAXS analysis and polarized optical microscopy observation indicate that a considerable amoimt of PVAc components is located in the interlamellar region. But P(VAc-co-VA) component is expelled out of the interfibrillar regions of the PLA spherulites in PLA/P (VAc-co-VA) blends. 

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

Early studies by Padden and Keith [180] suggested the formation of five different types of spherulites eon-sisting of a and p modifieations (ot- and -spherulites formed during the melt erystallization of iPP). They demonstrated that three types of ot-spherulite might be formed depending on the temperature of erystallization positive radial (oti) below 134°C, negative radial above 137°C (otn), and mixed-type spherulites in the intermediate range (atm). 

---