Five samples of isotactic polypropylene, iPP, and two copolymers of iPP with I-hexene, synthesized with different catalyst systems, both heterogeneous and homogeneous, have been studied in order to analyse the effect of the catalyst system, the presence of comonomer units and the crystallization conditions on the phase structure of iPP and, in particular, on the amount of y modification obtained. Minor amounts, if any, of y modification are present in iPP samples synthesized with highly isospecific Ziegler-Natta catalysts and crystallized from the melt at different cooling rates, ranging from 100 to 3Wmin. On the contrary, considerable amounts of the y form have been obtained both in samples prepared with catalysts of very low isospecificity or in those prepared with homogeneous metallocene catalysts. It has been shown that the relative proportion of the cxand y modifications can be controlled just by changing the crystallization conditions. A clear influence of the presence of comonomer units on favouring the formation of the y phase has not been ascertained. Moreover, attempts to deduce the y content from the d.s.c. melting patterns have been unsuccessful, since the two modifications exhibit very similar melting temperatures.
Посилання на статтю:
Obtaining the y phase in isotactic polypropylene: effect of catalyst system and crystallization conditions / Ernesto Pieza, Debora Zucchib, Maria Carmela Sacchib, Fabricio Forlinib, Antonio Belle // Polymer. – 1999. – N 40. – P. 675–681.