Crystallization of Copolyesters

Abstract

Studies were made on the crystallization of polyethylene terephthalate adipate (PET/PEA), polyethylene terephthalate sebacate (PET/PES), and polyethylene terephthalate isophthalate (PET/PEI) copolymers containing more than 90 mole-percent polyethylene terephthalate (PET) over a temperature range from the glass transition point Tg to the crystalline melting point T_m.
One has the following equation for the effect of foreign monomer units on nucleus formation which includes Flory's equation.
Here ΔF is the free energy of nucleus formation; T is the crystallization temperature, and N is an integer (1 for plate like growth and 2 for spherulite growth) depending upon the nucleation and growth process.
Data obtained for PET/PEA, PET/PES and PET/PEI have shown that
(a) The addition of foreign monomer units decreases the lowest temperature of crystallization T_c, the limiting density, the induction time of crystallization, and the crystallization rate.
(b) The effects are more remarkable for PET/PES than for PET/PEA. The effect is hardly observable for PET/PEI.
(c) In PET/PEI series, the depression of degree of crystallinity resulting from the addition of foreign monomer units is comparatively small.
The minimum crystallization temperature of copolymer containing more than 5 mole-percent adipic or sebatic acid is at least 20°C lower than that of PET.
All these phenomena can be explained in relation to the depression of T_m and T_g.