Glass transition and relaxation in the amorphous phase of isotactic polypropylene

Two breaks are observed in the temperature dependence of both specific volume and logarithmic retardation time in isotactic polypropylene, indicating two glass transitions. In correspondence with two transitions, two loss peaks are found in isochronal measurement of dynamic mechanical loss at 3 Mc./sec. The lower transition T g1 is assigned to the amorphous phase, which is rich in atactic chains (A 1 phase); the upper transition T g2 , to the amorphous phase, rich in isotactic chains (A 2 phase). The lower transition shifts to high temperature with room temperature aging time owing to recrystallization in the A 1 phase, accompanying the increase in flexural stiffness. The upper transition shifts to high temperature through annealing above 100°C. on account of recrystallization in the A 2 phase. The situations of the two phases in the spherulite of polypropylene are suggested by variations of long period with annealing temperature and birefringence with aging time. The effects of molecular weight on T g1 and T g2 and on impact strength and brittle point are discussed.