Abstract
An experimental study of the correlation between the development of orientation and extensional deformation during the melt spinning of polypropylene has been carried out. The variation of birefringence in the spinning line is divided into three regions. In the first region, the birefringence increases slowly with tensile stress and in the second, beyond a certain extensional viscosity independent of take up velocity, it increases rapidly under constant stress, reaching finally to a saturated value in the last region. The variation of birefringence is so similar to that of the orientation factor corrected for the density.
The relationship between birefringence and tensile stress is expressed by two different straight lines: in polymer melt Δn=8.3×10-11 σ and in taken up filament Δn=7×105σ0.3. Using the extrapolation technique, the point of intersection of two straight lines yields values of σ=4.4×108 dyne/cm, Δn=36×10-3. The former is very similar to the value for the breaking tensile stress during the melt spinning and the latter agrees favorably with the value for the maximum birefringence of polypropylene.
The birefringence results along with the x-ray results were used to separate the birefringence into two parts, one due to noncrystalline liquid orientation and the other due to crystalline orientation along the spinning way. Noncrystalline contribution is minimum at the beginning of crystallization. This may be interpreted in terms of the heat of crystallization.
