Cooperative effect of molecular orientation and β-form on toughening injection-molded isotactic polypropylene with β-nucleating agent

The toughness of conventional injection-molded isotactic polypropylene bars has been investigated with respect to the notch location and β-nucleating agent. Superior toughness is achieved in the β-nucleated bars with notches near the gate, but is absent in the near-gate bars without β-nucleating agent or in the β-nucleated ones far from the gate. With detailed structural analysis across the sample thickness, it is indicated that in these tough bars, extensive flow close to the surface suppresses the formation of β-form to a large extent but favors the generation of oriented α-form, whereas toward inner region, an opposite tendency is presented with the decay of flow strength and the activation of β-nucleating agent. Allowing that this peculiar hierarchical structure is absent in other two kinds of bars, it is deduced that the cooperative effect of molecular orientation of α-form in the skin layer and rich β-form in the inner region is responsible for the significant toughness enhancement in the β-nucleated bars near the gate. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012