Injection molded hinges of polypropylene possess uniquely unlimited flex life. Polypropylene when crystallized from the melt under extensional flow (as in injection, molded hinges), shows a unique morphology characterized by a planar row structure of lamellae accompanied by a bimodal distribution of the unit cells having the alpha crystal phase. This bimodal orientation consists of two apparently independent orientations of the unit cells with respect to the flow direction: one with the c‐(chain) axis parallel to and one with the a′‐axis parallel to the flow direction. (The a′‐axis is a vector normal to (100) and thus parallel to a* in reciprocal space.) Experimental data on the morphology of polypropylene crystallized under extensional flow are interpreted in terms of two components. The c‐axis component corresponds to those molecules in the planar row structure. The a′‐component is attributed to secondary crystallization in which small crystallites are finely dispersed between the lamellae of the row structure. These small crystallites act as a “morphological plasticizer” protecting the load‐bearing row structure from destructive stress during flexure.
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Clark et al. (1976) studied this question.
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