Orientation and Structure Development in Poly(trimethylene terephthalate) Tensile Drawing

Abstract

Tensile drawing of poly(trimethylene terephthalate) (PTT) was studied at room temperature, 35, 50, and 75 °C. Instead of a typical increase in draw ratio with increasing temperature, the PTT draw ratio first increased, went through a maximum, and decreased; all occurred within a narrow range of temperature between Tg and Tg + 30 °C. This unusual drawing behavior was due to the onset of cold crystallization during hot drawing. The fast cold crystallization rate could become dominating at high draw temperature, impeding PTT drawability to the extent that it caused in situ ductile−brittle transition and draw failure. The crystal orientation function, fc, measured with wide-angle X-ray diffraction (WAXD), increased rapidly with draw, and saturated at fc ≈ 0.96. Four IR vibration modes at 933, 1037, 1358, and 1505 cm-1 wavenumbers were found to be sensitive to draw for dichroic ratio characterization. By combining WAXD and IR orientation measurements, transition moment vectors of three crystalline 933, 1358, and 1505 cm-1 vibrations were found to make 54°, 29°, and 45° angles to the c-chain axis. The 933 and 811 cm-1 CH2 rocking modes were used to estimate changes in PTT's methylene gauche conformation in drawing. The gauche content increased with increasing draw ratio; however, it is a unique function of the polymer's final crystallinity obtained through a combination of strain-induced and cold crystallizations irrespective of the draw temperatures.