Polymer crystallization spherulites

Fig. 8. Schematic design of a spherulite which is formed in the course of polymer crystallization. Spherulites contain blocks of crystalline zones, embedded in an amorphous matrix. The size can be in the pm range and larger...

Morphology. Observations with the light microscope, under polarized light, showed that the end blocks in the case of both types of polymers crystallized in the form of the usual spheru-lites, but not as well as the analogous homopolymer, H2-l,4-polybutadiene. The formation of the spherulites was improved with increasing end-block content and/or higher molecular weight of the end blocks. 

Most polymers consist of a combination of crystalline and amorphous regions. Even within polymer crystals such as spherulites (Figures 2.15 through 2.17), the regions between the ordered folded crystalline lamellae are less ordered, approximating amorphous regions. 

The density and n value of a polymer crystal are greater than those of an amorphous polymer. Many polymers are opaque because of the presence of ordered clusters of crystals called spherulites which have different n values. ptfe, which is highly crystalline, is opaque but amorphous polycarbonate (PC), PMMA, and PS are noncrystalline and clear. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

The discussion above shows that structural problems in CPs are very different from those in organic conductor crystals. They also differ somewhat from those in other polymers the dominant polymer crystal morphologies, lamellae and spherulites [12] are not found. Rather, fibrils are often observed. 

When a polymer crystallizes from the melt without disturbance, it normally forms spherical structures that are called spherulites [1,2]. The dimensions of spherulites range from micrometers to millimeters, depending on the structure of the polymer chain and the crystallization conditions, such as cooling rate, crystallization temperature, and the content of the nucleating agent. The structure of spherulites is similar regardless of their size they are aggregates of crystallites [1-6]. 

Once primary nuclei are formed the ensuing spherulites grow radially at a constant rate. Primary crystallization, which occurs initially on the surface of the primary nucleus and then on the surface of the growing lamellar, also involves a nucleation step, secondary nucleation. It is this step that largely governs the ultimate crystal thickness and which forms the focus of most kinetic theories of polymer crystallization. 

Many polymers crystallize in the form of spherulites. Hedrites, as early stages of development of spherulites, can reveal information about the growth and development of spherulites in time. As shown below, the morphological analysis is often... 

The morphology of crystalline isotactic polystyrene, i-PS, has been investigated by others, and they have concluded that i-PS normally crystallizes as stacks of folded chain lamellae which are arranged in volume filling spherulites. The melting point of lamellar polymer crystals depends on the lamella thickness, L, as follows (28 )... 

The Avrami equation [Eq. (2.15)], which was originally proposed in the general context of phase changes, has provided the starting point for many studies of polymer crystallization and spherulitic growth. It relates the fraction of a sample still molten, 9, to the time, t, which has elapsed since crystallization began. The temperature must be held constant. 

Spherulitic morphology (commonly developed when a polymer crystallizes from an unstressed melt) (8)... 

In previous sections we have shown that the redistribution of additives at the spherulite boundaries during polymer crystallization leads to the additives uneven distribution, whose form is determined by the kinetics of the growth rejection process. In time, this initial dynamic distribution should relax to an equilibrium form in which the noncrystalline polymer is uniformly permeated by the additive, whose distribution reflects that of the noncrystalline polymer. The relevanoe of these observations to oxidative degradation processes in semi-crystalline polyolefins is discussed in this section. 

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