Molecular Orientation and Birefringence
topic
Birefringence (Δn = n∥ − n⊥) quantifies optical anisotropy arising from molecular orientation. Measured by polarised light microscopy with tilting compensator or by retardation measurement. PET birefringence: amorphous UDY <0.01; POY 0.04–0.07; FDY >0.1. Orientation function f = (3⟨cos²θ⟩ − 1)/2 (Hermans orientation factor). Both crystalline and amorphous orientation fractions contribute.
Role
Birefringence is the primary analytical measurement for quantifying molecular orientation in polyester fibres, correlating directly with tenacity, modulus, shrinkage, and dyeability. It is a fundamental measurement in fibre structure–property analysis.
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