Study on the sequence length distribution of polypropylene by the successive self-nucleation and annealing (SSA) calorimetric technique

The effect of successive self-nucleation and annealing (SSA) parameters including the annealing time t(s), annealing temperature interval and heating and cooling rates on the crystallization properties of polypropylene were researched in detail. Based on those results, the crystallization behavior and the sequence length distribution of polypropylene with different isotacticity and propylene/1-butene copolymer with different 1-butene comonomer content prepared by Ziegler-Natta catalyst were studied through SSA. With the increase of the isotacticity of polypropylene, the sequence length of the polypropylene gradually increases. The polypropylene prepared by propylene bulk polymerization has stronger crystallization ability and longer sequence length than that prepared by propylene sully polymerization. For the propylene/1-butene copolymer, the crystallization ability gradually reduces and the sequence length of each chain segment gradually decreases with the increase of 1-butene content in copolymer. In addition, the polypropylene samples with similar isotactic index prepared by Ziegler-Natta catalyst and metallocene catalyst under similar polymerization condition were also characterized through the SSA technique. The results show that the sequence length of polypropylene prepared by Ziegler-Natta catalyst is far longer than that prepared by metallocene catalyst. The interesting phenomenon that polypropylene prepared by metallocene catalyst has multiple melting peaks after SSA treatment, which is similar to the polypropylene prepared by Ziegler-Natta catalyst, was first reported.