Dendrites and Spherulites

Polyethylene Crystals Grown from Solution in o-Xylene 

The two sketches in Fig. 5.61 illustrate different paths to a spherulitic crystal morphology. These paths create a spherical overall appearance, despite the fact that the basic crystals are polyhedra, i.e., the spheruhtes are aggregates of polyhedral 

The Maltese crosses indicate the polarizer and analyzer orientations of the mioros-oope. The straight boundaries indioate athermal nuoleation. 

The subcrystalline habits may be fibrillar, lath-shaped, and lamellar 

During the preparation for electron microscopy the heterogeneous ( ) nucleus in the center was not replicated and is missing from the fracture surface. The molecules 

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The various morphologies, such as polyhedral, hopper, dendritic, and spherulitic, that are exhibited by single crystals and polycrystalline aggregates, have been discussed in relation to the driving force in the preceding chapters. 

It should be noted that dendrites and spherulitic aggregates are not completely crystalline. Certain amorphous regions remain even if macroscopically crystalline aggregates appear to have impinged. In some extreme cases of branched PE, the amorphous portions in spherulites can be as high as 90%. Two different types of amorphous regions can exist in these semicrystalline polymers,... 

Also dewetting and phase separation using crystalline polymers, such as poly(e-caprolactone) (PCL), has been described recently. Fu et al. [131] recently found that in PS/PCL blends a two-layer structure with PS at the upper layer and PCL at the bottom layer was formed during spin coating. The authors varied the solution concentration and as a consequence the thickness of the films formed and found that the crystallization is dependent of the final film thickness. So that, different kinds of crystal morphologies, such as fmger-like, dendritic, and spherulitic-like, could be obtained at the bottom PCL layer. 

Figures 3.24(a) and (b) show morphodroms of silicate crystals growing from silicate solutions Fig. 3.24(a) shows the results of observations on quenched products, and Fig. 3.24(b) summarizes the results obtained by a high-temperature in situ observation method of growth [23]. In these cases also, as apart from the liquidus of solid-solution component, it is seen that the morphology of crystals changes from polyhedral, through hopper, to dendritic, then spherulitic. The predictions described in Section 3.12 are thus confirmed by experiment.

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 case of iPP/POE blends, various types of structures developed in a manner dependent on blend compositions and temperatures of crystallization. With decreasing crystallization temperature, the morphology changed from the curved single crystals to sheafiike (dendritic) and eventually to spherulites. Another interesting... 

Crystal growth usually proceeds more rapidly in certain lattice directions and these may become exj erated as dendritic or spherulitic arms... 

Many polymers form more complex single crystals when crystalhzed from dilute solution including hollow pyramids that often collapse on drying. As the polymer concentration increases, other structures occur, including twins, spirals, and multilayer dendritic structures with the main structure being spherulites. 

We discussed iu Section 3.12 that there is a mutual relation among spherulitic, dendritic, hopper, and polyhedral crystals, with respect to the driving force. We will see how these mutual relatious appear in real systems, using, as representative examples, low-temperature suow crystals (vapor phase growth) and high-temperature silicate crystals growiug iu silicate solutiou phases. 

Calcium polyphosphate fibers were grown as dendritic spherulites. Sodium calcium polyphosphates were grown as simple crystals and then milled. 

Spherulites are classified as positive when the refractive index of the polymCT chain is greater across the chain than along the axis, and negative whrai the greater refractive index is in the axial direction. They also show various other features sueh as zigzag patterns, concentric rings, and dendritic strucfirres. 

Figure 10-12. Melting behavior of a poly(ethylene) as a function of the morphology and the rate of heating. L, Lamellar single crystals from solution D, dendrites obtained by shock cooling solutions Sq spherulites obtained by shock cooling melts under normal pressures Sc, spherulites produced by crystallization under normal pressures E, extended chain crystals obtained by crystallization from high molar mass Eh and low molar mass Em poly(ethylene) melts under high pressures (after B. Wunderlich).

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