Shish-kebab structure, polypropylene

The subject of crystallization from stressed melts has been reviewed (Kumaraswamy 2005) and one frequent occurrence in the time sequence of this crystallization is the formation of row nuclei which give rise to structures known as shish-kebabs . Polypropylene is the polymer best known for its remarkable propensity for forming row structures, which often contain considerable amounts of P-phase material (Olley et al. 2014), and are a major feature of the skin-core stmcture of injection-moulded polypropylene (Shinohara et al. 2012), but they are also found in polyethylene (An et al. 2006), isotactic polystyrene (Azzurri and Alfonso 2008), poly(phenylene sulfide) (Zhang et al. 2008) and polylactide (Xu et al. 2013). 

Polypropylene can be easily induced to form row structures by extension of the melt. An example of the resultant morphology is shown below. Closely spaced shish-kebab lamellae have grown directly out from the central row nucleus, but as growth proceeds outwards, increasing numbers of cross-hatching lamellae are found. The cross-hatching lamellae are found almost perpendicular to the lamellae growing out from the initial nucleus (Fig. 3.3). 

Somani RH, Yang L, Zhu L, Hsiao BS (2005) Flow-induced shish-kebab precursor structures in entangled polymer melts. Polymer 46 8587-8623 Sorrentino A, De Santis F, Titomanlio G (2007) Polymer crystallization under high cooling rate and pressure a step towards polymer processing conditions. Lect Notes Phys 714 329-344 Stocker W, Magonov SN, Cantow HI, Wittmann JC, Lotz B (1993) Contact faces of epitaxially crystallized a- and y-phase isotactic polypropylene observed by atomic force microscopy. Macromolecules 26 5915-5923... 

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