A method of forming an amorphous crystallizable thermoplastic polymer composition. The polymer composition has a modulus of rigidity which decreases to a minimum value with increasing temperature and then begins to increase. The process includes heating polymer to below a temperature at which crystal formation occurs and form the polymer. The polymer composition is then heated to a temperature at which crystal formation can occur and be annealed. This process is useful to make articles having a thickness of from 0.127 cm (0.050 inches) to 1.27 cm (0.50 inches) such as sheets and tubes of the amorphous polymer and then reforming and annealing them.