Deformation of spherulites

Fig. 29a and b. Structure of the shear zone at various temperatures in PP a T = —196 °C, two sets of discrete shear band A and B, b T = —40 °C, diffuse shear zone with quasi-homogeneous deformation of spherulites... 

In terms of macrostructure, studies on the deformation of spherulites during biaxial orientation elucidated that spherulitic break-up occurs not only at their boundaries, but also at their centers. In either case and preferentially at higher temperatures, a fibrillar morphology was observed to develop at higher levels of stretching. Less data are available on direct methods to quantify the behavior of amorphous segments as they take part in these deformations. 

The tensile plastic deformation of spherulitic polymers occurs in several stages as both amorphous tie chains and chain-folded block segments (which separate from the ribbon-like lamellae) become oriented with the tensile axis (Figure 15.13). 

Thomas, C., Seguela, R., Detrez, R, Miri, V., Vanmansart, C. Plastic deformation of spherulitic semi-crystalline polymers an in-situ AFM study of polybutene under tensile drawing. Polymer 50, 3714-3723 (2009)... 

Stage I Continuous deformation of the spherulitic structure before the neck ... 

At very small strains within the viscoelastic region (e<0.5%), deformation within the PP solid is confined to disordered amorphous regions [97,98) due to their inherently low stiffness at temperatures above their Tg (-10 °C). In this deformation region, spherulites undergo affine deformation on the whole. Inside the spherulites, rotation of lamellae occurs [101]. The resulting orientation of lamellae depends on their position within the spherulite in respect to the orientation of the external deformation [102-104],... 

Our findings in regard to the effects of deformation on PTFE accord in some respects with observations on the deformation of polyethylene " and polypropylene, although PE and PP usually have a spherulitic morphology before drawing. 

Rupture occurs without important plastic deformation when the ribbons of some neighboring spherulitic formations show a parallel orientation in the contact zone. Conversely, considerable elongation occurs when the orientation is normal at the contact limit because of plastic deformation of the ribbons (Figures 4 through 6). 

Deformed crystals. If a semi-crystalline polymer is deformed while undergoing crystallization, oriented lamellae form instead of spherulites. 

Deformation of the spherulites in the material between the crazes into a highly stretched, ellipsoidal shape (rig. 13b in a tensile test of a bulk PP specimen this internal deformation condition is found at nominal plastic strains of more than... 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

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