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Polypropylene (PP)/ethylene butene copolymer (EBM) blends were melt-spun into fibers. The fiber orientation was determined by wide angle diffraction (WAXS) and birefringence. The birefringence of the blend fibers was much lower than that of the pure PP or EBM filaments spun at the same stresses. WAXS study showed that the PP phase had the same orientation versus stress relationship in its blends and in its pure state. However, EBM phase demonstrated much lower birefringence in its blends than in its pure state. The mechanism was explained as following. In melt spinning of the blends, the PP phase crystallized first. Before the PP phase crystallized, both PP and EBM phase had comparable modulus leading uniform stress distribution. After the PP phase crystallized, the modulus of PP phase became much larger than that of the EBM melt. Thus it would take most of the stress if it was the continuous phase leaving the EBM phase crystallizing at much lower stresses. This led to a large birefringence suppression of the EBM phase. In melt spinning of the blends with only 1 to 10 wt.% of PP, the dispersed PP phase could coalesce into a continuous surface layer. Thus small amount of PP could largely decrease the birefringence of EBM fibers.

Structure Development in the Melt Spinning of Polypropylene/Ethylene Butene Copolymer Blends and Birefringence Suppression Effect