Visible dichroism of polypropylene–disperse dye system at high temperatures

The dichroism of polypropylene film dyed with C. I. Disperse Yellow 7 was investigated at various temperatures up to 160°C. The dichroic value D drops as the temperature is raised. So long as the amorphous structure does not change irreversibly, D changes reversibly with temperature. The experimental results agree qualitatively with those obtained on poly(ethylene terephthalate) in our previous paper, although the effect of temperature on the extent of the reversible change in D is larger in PP than in PET. The plot of D versus temperature exhibits breaks at 40°–50°C (T₀), 70°–80°C (T₁), and 115°–120°C (T₃). These temperatures agree with the transition points of polypropylene in the literature. From the change in the intrinsic dichroism D₀ with temperature, it is concluded that the decrease in D at high temperatures is due to the drop of D₀ caused by the disorientation of dye molecules in the amorphous region, while the amorphous polymer chain is not disoriented. Such a conclusion is supported by the fact that Δn of a heat-set specimen is kept constant during heating, in contrast to D.