5.6.1 Morphology of polymer crystallites
The fundamental unit of structure formed by crystalline polymers which is accessible using the optical microscope is the spherulite. Isolated spherulites are formed easily at relatively slow spherulite growth rates such as those exhibited by polypropylene and isotactic polystyrene. Unlike aramid fibres where the degree of crystallisation is close to 100 per cent (Figure 54), most crystalline polymers contain significant amounts of amorphous polymer either between spherulites or present at crystallite boundaries within individual spherulites.
By cooling solutions of polymers in organic solvents, it is possible to obtain minute platelets (lamellae) which represent the smallest crystal elements within much more complex structures like spherulites (Figure 55). Such lamellae vary in thickness between 10 and 40 nm and may be several micrometres (μm) wide. The chains in each crystal, which in polyethylene are in a zig-zag conformation (Figure 55(a)), are folded one upon the other as shown schematically in Figure 55(b). It is thought that similar folds occur in bulk polymers, although there is probably more disorder at the fold surface where chains interconnect with other lamellae (Figure 55(c)).Clearly, such chain segments represent non-crystalline polymer; although their modulus is lower they provide the essential links between crystals and so bind the material together like a composite.
A spherulite can grow from a single crystal nucleus very much in the way suggested by the sequence shown in Figure 56(a). Growth occurs when chains continue the folding action and crystal defects lead to lamellar twisting and branching. Fibrillar structures are thus formed which successively twist round to form a ‘wheatsheaf’. Continued growth ultimately yields a spherulite which then grows uniformly as a sphere. For HDPE and nylon 6,6, nucleation and growth are very rapid, radial growth rate for HDPE is about 5000 μm min−1), so that spherulite impingement is the norm (Figure 56(b)). In polymers with lower growth rates, such as polypropylene (growth rate about 20 μm min−1, specific nucleating agents are often added to increase the degree of crystallinity developed in objects during processing. Such agents are usually organic salts like calcium stearate.