Spherulite grow rate

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. 

Spherulite radial growth continues at a constant rate, even after other portions of the spherulite have impinged with its neighbors — indicating that lamellae within a given spherulite grow independently... 

The kinetic approach relies on the establishment of a relation between the densities of the crystalline and melt phases and the time. This provides a measure of the overall crystallization rate. It is assumed that the spherulites grow from nuclei whose... 

The Avrami equation represents only the initial portions of polymer crystallization correctly. The spherulites grow outward with a constant radial growth rate until impingement takes place when they stop growth at the intersection, as illustrated in Figures 6.13 and 6.22. Then a secondary crystallization process is often observed after the initial spherulite growth in the amorphous interstices (85). 

Much effort has gone into the determination of crystal growth rates in polymers. In practice, this involves determining how fast spherulites grow, so what is determined is the fast growth direction for the polymer in question, for example, the b-axis in... 

If we observe the crystallisation of a polymer melt under a polarised light microscope, we will see that the process begins with the formation of small nuclei somewhere in the melt.The nuclei then grow in size.This process is usually called nucleation and growth. The. spherulites grow with a constant rate up to a point when they touch each other. The area of contact between two spherulites will be planar if the two were nucleated at the same time, and bent if the times were different. Finally the whole volume is covered by bound spherulites. The frequency with which nucleation occurs, which is referred to as the nucleation density, determines the final size of the spherulites. The sizes can vary over a large range, from a few micrometres to several millimetres. 

The rate at which these spherulites grow is also affected by tacticity. Measurements were made of the Isothermal rate of spherulitic growth on a hot stage of a microscope. For this purpose, a sequence camera was used to photograph automatically the growing spherulites during crystallization. Dilatometric measurements were also made on these polymers in order to determine the rate of isothermal crystallization from the melt. The fraction of the polymer which had crystallized at any time was calculated from the measured density and the known values of the density of crystalline and amorphous poly(propylene oxide) (8,2,... 

Usually spherulites grow with a constant radial growth rate. The crystallization rate, as given by dp/dt, is then proportional to the total area of free, i.e. non-touching spherulite surfaces. For a growth rate u and a fixed number... 

Low nucleation densities also facilitate measurements of spherulitic growth rates (SGR). Typical plots of SGR as a function of crystallization temperature for several copolymers is shown in Fig 5.8. HV content has the effect of diminishing the overall rate profile at low HV contents but this trend is reversed at higher HV content [33]. This transition reflects the different crystal structures formed by the different copolymers. As already discussed, below 40% HV the crystal grows with a PHB lattice whilst above 40% HV a PHV lattice is formed. The variation in SGR with copolymer composition is due to the influence of... 

It was observed that the spherulites grow Unearly with time for all Tc. Calculating the growth rates G from the slope of the lines, it was found that G is increasing when Tc is lower than 200 °C however, G is decreasing when Tc is higher than 200 °C. This suggests that G is a convex function of Tc with the maximum near 200°C (Fig. 11.11). 

Spherulites grow from the center of their nucleus. Their growth rate can be taken as a function of the temperature. In the case considered by Rousset, the growth rate of the solid/liquid front, v, is isotropic, because temperature is assumed to be uniform in the sample. All the spherulites are spherical until there is some impingement. At each time step, a facet still in contact with the liquid moves forward at a radial increment... 

Various factors affect the growing rate of spherulites. For example, the rate decreases as the molecular weight increases, although the crystallization temperature giving the maximal G is almost the same. 

Primary nuclei, forming when thermal fluctuations overcome a free energy barrier, appear randomly in space and in time, at a rate fluctuating around the momentary mean value, dictated for a given polymer by its thermomechanical history and crystallization conditions. Quiescent crystallization of homopolymers from the melt occurs usually in the form of spherulites growing radially from primary nuclei, as illustrated by... 

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