Mechanical Properties of Sorbitol-Clarified Isotactic Polypropylene: Influence of Additive Concentration on Polymer Structure and Yield Behavior

The solid-state structure and yield behavior of isotactic polypropylene (i-PP) containing the nucleating/clarifying agent 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) was investigated in the additive concentration range 0-1 wt %. The raise of the crystallization temperature of i-PP caused by addition of 0.2 wt % and more of the additive led to an increase in the polymer lamellar thickness, which, in turn, resulted in an increase of the yield stress at constant aging time. Both the strain-rate dependence of the yield stress and the rate of increase of the yield stress upon physical aging of compression-molded samples were not influenced by the addition of DMDBS. Analysis of the experimental data by means of the Eyring theory for thermally activated processes resulted in an average shear activation volume of 2.21 nm3. The dependence of the yield stress on the additive concentration was in gratifying agreement with the previously reported monotectic phase behavior of the i-PP/DMDBS system.