Abstract
Stress relaxation and creep behavior of 16 commercial propylene‐ethylene block copolymers was measured in a molten state. The relaxation moduli of samples with monomodal molecular weight distribution show plateau or shoulder at long times. The bimodality of molecular weight distribution in the latter samples is due to the ethylene‐propylene copolymer component with high molecular weight. The plateau at long times is more noticeable as the bimodality is stronger. The height and width of the plateau is independent of the ethylene content. A PP‐HDPE‐EFR blend which was made as a model material of propylene‐ethylene block copolymer does not show the plateau. The relaxation modulus at high temperature is always lower than that at low temperature, the time‐temperature superposition can be performed well in all the measured range, and the shift factor obeys the Arrhenius equation. It was assumed from the above experimental results that appearance of the plateau at long times originates mainly from the bimodality of molecular weight distribution, not from the dispersion state of the ethylene‐propylene copolymer particles.
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