Solid‐state coextrusion of poly(ethylene terephthalate). I. Drawing of amorphous PET

Films of uniaxially oriented poly(ethylene terephthalate) (PET), M v = 81,000, have been drawn by solid‐state coextrusion in the range 40–100°C surrounded by polyethylene. This is well below the PET melting temperature and in some cases below its glass transition temperature. Properties of the extrudates, such as degree of crystallinity, mechanical and thermal properties, were investigated as a function of coextrusion temperature and draw ratio (EDR ≤ 4.4). The results show that the percent crystallinity depends strongly on draw ratio, whereas its sensitivity to extrusion temperature is limited only to the highest draw ratio (4.4). On the other hand, Young's modulus was sensitive to both extrusion temperature and draw ratio, exhibiting a maximum at EDR = 4.4 and T ext = 65°C. Above this temperature, moduli decrease apparently because of increased chain mobility, resulting in dissipation of chain orientation. Furthermore, changes in yield and tensile strength followed the changes in mechanical properties, suggesting that they are dominated by the same factors. The cold‐crystallization temperature T CC also revealed information about the morphological changes occurring during the extrusion drawing. For samples of EDR = 4.4, T CC increased with extrusion temperature, suggesting again dissipation of orientation by thermal motions. On the other hand, T CC decreases with EDR, and a Δ T CC as high as 73°C was found. Conventional drawing of amorphous PET has been widely reported. To our knowledge this is the first time oriented PET has been prepared using the advantages of solid‐state coextrusion.