Spherulitic impingement

Upon lowering the temperature of the hot stage, crystallization may set in as shown in Fig. 4.33. Again, lamellae are observed in edge-on orientation in the area where two spherulites impinge. In such crystallization experiments, it has been revealed that the growth of lamellae does not proceed homogenously. 

This equation predicts that the degree of crystallinity increase as This is initially true, but obviously cannot continue since it would lead to Xc>l. The difficulty arises because the supply of amorphous material becomes exhausted and the rate of erystallization decreases toward zero as the amorphous material is nearly all consumed. This decrease appears in the theory because growing spherulites impinge so that their volume is then... 

The central part of the spherulites and spherulitic impinging lines is degraded first, followed by the other parts of the spherulites (Fig. 16). It was also found that a sample isothermally crystallized at 60°C, which had a spherulite composed of less densely packed fibrils, had a higher degradation rate compared with a melt-quenched sample, in spite of similar crystallinity. This result indicates that the internal stmcture of the spherulite also played an important role in hydrolytic degradation. 

The word spherulite means little sphere , but the spherical shape is maintained only in the initial stages of growth. In fact when neighboring spherulites impinge, they become polyhedral. In this case, if the spherulites are nucleated simultaneously, the common outline will be planar otherwise the boundary is going to be a hyperboloid of revolution. 

What defines a spherulite is its shape and structural symmetry. All radii of an ideal spherulite are equivalent, at least at lengths greater than the 0.1-pm resolution of optical microscopy. The spherical (or circular) envelope of growth fronts is established by advance at rate G of separate fibrillar crystals into the melt from a common nucleus. This ideal shape is altered by growth restrictions presented by other spherulites (impingement/ truncation) or by melt interfaces (two-dimensional spherulites). 

When neighboring spherulites impinge, the boundary is formed. It can be noticed that for any two spherulites nucleated at Ti and T2, a difference between their radii, expressed by Equation (7.1a), remains constant during the further growth ... 

Swaminarayan and Charbon presented two methods of simulating the growth of an isolated spherulite the arborescent method and the front-tracking method [44]. In a second article [45], the same authors coupled the front-tracking techniques with (1) a stochastic model for spherulite nucleation, (2) a cellular model for spherulite impingement and solid fraction evolution, and (3) a finite difference method for solving the energy equation. This multiscale approach predicts the final microstructure in a macroscopic part. 

The functions and describe the total volume and surface of all the spherulites,per unit of volume, neglecting, however, spherulite impingement and truncation while taking into account phantom spherulites. With the same assumptions, 02/8 rand 8 r represent the sum of radii and the number of spherulites in unit volume. 

These equations cannot be valid at long times, because they predict a physically meaningless imboimded increase m X. In real interactions, the spherulites impinge on each other growth slows and ultimately stops. The situation is easily remedied by assuming the following [19] ... 

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