The effect of orientation of global chain on crystallization behavior of amorphous polyethylene terephthalate

In the rubbery state of amorphous polymers under uni-axial drawing, the glolal chain orientation will relax in orders of magnitude slower than the relaxation of local segmental orientation. When this state of hot drawn sample is frozen at temperatures lower than its glass transition, T _g, an amorphous state with high global chain orientation but nearly random segmental orientation (GOLR) would result. The characteristic features of GOLR state are large recovery of the elastic deformation β′ of global chain on being heated to temperatures above T _g(β′ = 70%-90%) and very small birefringence (Δn -10 ^-4). The effect of global chain orientation (GCO) on crystallization behavior of amorphous polyethylene terephthalate (PET) fibres is studied by differential scanning calorimetry (DSC) performed at different heating rates and solvent induced crystallization (SINC) method by comparing the crystallization behavior of PET as-spun fiber in GOLR state (β′= 34.3%, Δn = 1 × 10 ^-3, p=1.3362g/cm ³ with that of its thermally relaxed (105°C, 3min in the air) PET sample (β′ = 5.4%, p = 1.3362g/cm ₃). The results indicate that the cold crystallization temperature, T _cc, is effectively depressed by the GCO. That means the global chain orientation accelerates the development of crystallization.