The effect of crystalline phase morphology on the structure and properties of polypropylene impact copolymers
Motsoeneng, Teboho Simon
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The present study covers the preparation and the characterisation of β-nucleated impact polypropylene copolymer (NA-IPC), and its fractions prepared through temperature rising elution fractionation (TREF). Calcium stearate (CaSt), as well as pimelic (Pim) and adipic (Adi) acids, were doped into IPC as a mono- or bi-component nucleating agents (NAs) at varying mass ratios. The non-isothermal crystallisation kinetics, and the effect of the NAs on the morphology, thermal and mechanical properties were investigated. However, only thermal properties of the fractions were investigated on. DSC and XRD results revealed that IPC nucleated with Pim and Pim-CaSt nucleants induced up to 90% β-crystals, while Adi and Adi-CaSt formed only about 17% β-crystals. This was associated with the strong nucleation efficiency (NE) of Pim. The 110C and 120C fractions treated with Pim-based NAs were the only fractions that formed β-crystals, probably due to their higher isotacticity. The nonisothermal crystallisation kinetics showed that the crystallization of IPC and NA-IPC followed a three-dimensional growth with athermal nucleation mechanism. The SEM images showed no changes in the distribution and size of the rubber phase after treatment with NAs. FTIR showed that none of the NAs chemically reacted with IPC, and the chemical structure of the polymer was thus intact during the treatment. Formation of β-crystals in the samples with Pim and Pim-CaSt improved the impact strength by more than 50%. However, incorporation of Adi, CaSt, and Adi-CaSt nucleants had little effect on the impact resistance of IPC. The tensile properties such as Young’s modulus, yield stress and stress at break changed very little for the nucleated samples. On the other hand, the elongation at yield and at break increased. This is an indication of the strong ductility of IPC caused by the formation of β-crystals. The glass transition temperatures shifted slightly to higher temperatures with increasing β-crystal contents, due to the immobilization of the chains in the amorphous phase in the vicinity of the β-lamellae.