Spherulite Nucleation and Growth

The growth of the nuclei then occurs in one, two or three dimensions creating rods (fibrils), discs or spheres (spherulites). The development of crystallinity (Vc) under isothermal conditions with time ( ) is generally analysed according to 

Avrami coefficient, n Nucleation mode Growth dimensionality 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74], 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating 

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The effect of blending on the overall crystallization rate is the net combined effect of the nucleation and spherulite growth. Martuscelli (1984) observed that in blends of PP with LDPE, crystallized at a high enough to prevent any LDPE crystallization, the overall rate of crystallization of the PP matrix phase (thus in the presence of the LDPE molten droplets) was progressively depressed with increasing content of LDPE (Eig. 3.58). 

The effect of blending on the overall aystaUization rate is the net combination of the effect of nucleation and spherulite growth. Also, the presence of an amorphous dispersed phase in the crystal-lizable matrix can cause drastic variations of the important morphological and structural parameters of the crystalline phase, such as the shape, size regularity of spherulites and inter-spherulitic boundary. 

The miscibility of the components also affects the primary nucleation behavior of the crystallizing polymer. In iPP/iPB blends the presence of iPB influences both heterogeneous and homogeneous nucleation of iPP spherulites. The rates of homogeneous nucleation and spherulite growth of iPP were related to the variation of the average spherulite radius as a function of the crystallization temperature [61]. 

SEM micrographs of two members of these polymers (HB and HBIB-50) are shown in Figure 7 to provide further evidence for superstructure on the micron level within the solution cast films. One can directly observe the surface of the spherulitic structure of the HB homopolymer as well as in that of the copolymer HBIB-50. Clearly, the level of structure (-5 pm) is well above that of the individual domains of either HB or HI and reflects the possible primary nucleation and subsequent growth behavior common to spherulitic semicrystalline polymers. The Hv patterns shown in... 

Crystalhzation studies in blends of iPP/POE reveal that the crystallization process of iPP is affected by the addition of POE and vice versa. It has been demonstrated how the POE promotes the nucleation and crystal growth processes of iPP, this effect being more appreciable at low POE concentration (< 10 wt% POE). Analysis of the crystallization kinetics of the iPP crystals isothermally crystallized at different temperatures in blends of iPP/POE is supported by the morphological observations (lamellae, dendritic, and eventually spherulitic texmres) through optical microscopy. 

In the following part, a discussion on the crystallization behavior in immiscible polymer blends is given, including the nucleation behavior, spherulite growth, overall crystallization kinetics, and final semicrystalline morphology. Each topic is illustrated with several examples from the literature to allow the reader to find enough references on the discussed subject for further information. 

For example, it is possible to obtain selected-area X-ray diffraction patterns from parts of a spherulite, and deduce the orientation of the crystal axes within it. In fact the crystallographic a-axis is radial in the spherulite with the other two axes rotating about it to produce the typical banded appearance. The band spacing varies with the crystallization temperature but as yet there are no mathematical models relating band spacing to the fundamental parameters of nucleation and crystal growth. 

In order to bypass such limitations, the overall crystallization kinetics may be determined by DSC. However, in this case, both primary nucleation and crystal growth will make a contribution to the overall isothermal crystallization rate [6,7,12]. Ideally, it would be better to determine both spherulitic growth rate and overall isothermal crystallization kinetics in separate experiments, if possible. In the literature, the most commonly reported [30] type of isothermal crystallization kinetic data is that measured by DSC, because it is the easier to obtain. The DSC experimental approach can be very useful and in some cases, the DSC data thus obtained can provide not only the overall crystallization rate but also the separation of the individual contributions of the primary nucleation and growth rate (more details to follow). 

When the isothermal crystallization is determined by spherulitic growth experiments, the energy barrier determined by applying the LH model refers exclusively to secondary nucleation or crystal growth. Instead, when the inverse of half-crystallization time (1/t5q%) values obtained from DSC isothermal overall crystallization kinetic data is considered, both primary nucleation and crystal growth are considered. Therefore, the energetic parameters that we obtained after applying any classical kinetic crystallization theory to DSC data will include contributions from both processes. 

Similar reasoning, apphed by Johnson and Mehl [6] to a problem of nucleation and radial growth of spherical entities, resulted in Equation (7.5), with E expressed by Equation (7.10). The derivations of Tobin [27-29] were also based on a similar principle. However, over-simphfied reasoning, for instance, the incorrect assumption of the proportionality of an average spherulite volume (instead of a volume increment) to the unconverted fraction, led Tobin to the erroneous result [30]. 

The known theories dealing with the overall crystallization kinetics assume that the conversion of amorphous phase into crystalline phase occurs via radial growth of domains—spherulites—in the case of polymers. They do not apply to crystallization processes that do not occur via nucleation and radial growth of domains. 

When the noncrystallizable block in a diblock copolymer is rubber-like the isotherm shapes are very similar to those of the parent homopolymers.(55,56) This situation exists even when the crystallization occurs from a well-defined melt struc-ture.(55,57,58) However, at a fixed undercooling, there is a reduction in the overall crystallization and spherulite growth rates.(55) When the growth rates of ethylene oxide-butadiene block copolymers, and the corresponding homopolymer, are plotted against l/AT it is found, with the exception of the lowest content ethylene oxide polymer, that a set of parallel straight lines results irrespective of the iiutial melt domain structure.(55) This result implies that the products of interfacial free energies for nucleation are similar to one another. 

The properties of a given polymer will very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. It is thus useful to consider the factors affecting the formation of the initial nuclei for crystallisation (nucleation) and on those which affect growth. 

The rate of spherulitic growth is extremely temperature sensitive and seems to be independent of the nucleating agent. 

The very fast initial density increase due to nucleation and rapid spherulite growth as shown by the dotted lines, referred to as primary crystallization. 

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