Competition between α, β, and γ Polymorphs in a β-Nucleated Metallocenic Isotactic Polypropylene

The competition between α, β, and γ polymorphs has been studied in a β-nucleated metallocenic isotactic polypropylene, iPP, as a function of the cooling rate and of the isothermal crystallization temperature, by performing X-ray diffraction and DSC experiments. It was found that the addition of a 1% by weight of a typical β-nucleating agent is not enough to develop any appreciable amount of β modification, at least under the crystallization conditions used, which cover a wide range of cooling rates. In comparison, the same amount of nucleating agent added to a Ziegler−Natta iPP leads to almost 100% of β form at low cooling rates. It seems that such amount of β nucleating agent is not enough to counterbalance the well-known γ nucleation ability of the relatively high content of defects (stereo- and regioerrors) which are present in the studied metallocenic iPP, and only different proportions of γ and α modifications are obtained in this sample, the relative amount of them depending on the cooling rate. On the contrary, if a 5% nucleating agent is added, the β modification is also obtained, in addition to the γ and α polymorphs. However, now the amount of β crystals as a function of the cooling rate follows a trend opposite to that for the Ziegler−Natta iPP: the higher are the cooling rates (or the lower are the isothermal crystallization temperatures) the larger proportions of β modification are obtained. It is deduced, therefore, that the nucleation ability of the chain errors which leads to the development of the γ form predominates over that one of the β nucleating agent. The enthalpies for the 100% crystalline modifications, estimated from the enthalpies of melting and from the X-ray determined proportions of the different polymorphs, are rather similar: 162, 159, and 158 J/g for the α, β and γ phases, respectively. These values are inside the experimental error.