Dichroic polyethylene terephthalate films based upon compounds containing dyestuffs

An experimental study of the dichroism of uniaxial and biaxial orientation of polyethylene terephthalate films containing dichroic dyestuffs is described. It is found that hydrophobic dyestuffs orient more and cause greater dichroism than do hydrophilic dyes. Dyestuffs which can induce dichroism in cellulosic films are not effective with PET films. The more anisotropic the hydrophobic dyestuff molecule, the more effective it is in inducing dichroism. At low stretch ratios, the dichroism (D ₁₂–1)/(D ₁₂+2) (whereD ₁₂ isA ₁/A₂) increases linearly with birefringence, but at higher stretch ratios the rate of increase is slower. This may be attributed to crystallization and reduced orientations in the amorphous regions. Biaxial stretching generally lowersD ₁₂.