Mechanical properties of isotactic polypropylene with nodular or spherulite morphologies

alpha-form crystalline isotactic polypropylene (PP) was prepared by two methods: quenching from the melt state at slow or high crystallization rates. The mechanical properties of the resulting PP samples were evaluated using tensile and stress-relaxation tests to investigate the influence of nodular or spherulite morphologies. PP samples with different morphologies had almost the same crystallinity after annealing at 100 degrees C for 5 h. The yielding stress, strain-hardening modulus, and strength of the nodular PP were higher than those of the spherulite PP, although the crystallinity was almost the same for both PP samples. Furthermore, the relaxation time during the stress-relaxation process at epsilon = 4.0 was longer for the nodular PP than for the spherulite PP. The lamellar structure of a spherulite morphology comprises parent and daughter lamellar crystals, whereas that of a nodular morphology prepared via a mesophase structure comprises only parent lamellar crystals. The formation of the daughter lamellae reduces the number of tie chains. Therefore, the nodular PP has more tie chains than the spherulite PP. This transformation from a spherulite to a nodular morphology improved the mechanical properties of the PP.HighlightsInfluences of morphology on mechanical properties of polypropylene (PP).Tensile properties were higher for nodular PP than spherulite PP.Relaxation time of nodular PP was longer than spherulite PP.Nodular PP has more tie chains, resulting in improvements of tensile properties. The tensile properties such as the yielding stress, strength, and strain-hardening modulus were higher for polypropylene (PP) with a nodular morphology than PP with a spherulite morphology because the tie-chain fraction of the nodular PP should be larger than the spherulite PP.image