Cold Crystallization of Polytrimethylene Terephthalate and Copolymers

The melt of crystalline polymers normally crystalize on cooling and the polymer melts with low critical cooling rate would easily solidify as glass during cooling to their glass transition temperature(Tg). The glassy process exhibits variety within each kind of polymer melts in the slope of the viscosity-temperature visibility, and the rate of the excess conformational entropy in the polymer melt over the crystalline phase changes in the Tg approaching. Polytrimethylene terephthalate (PTT) and a series of polytrimethylene terephthalate copolyesters (PTTG) with different molar ratios of 1,3-propylene glycol (1,3-PDO) to 1,4-cyclohexanedimethanol (1,4-CHDM) were synthesized to modify the main chain flexibility and the critical cooling rate. The glassy samples were obtained via quenching the melts of PTT and copolymers below the Tg to characterize the cold crystallization behaviors. The evolutions of the crystalline structure of the amorphous samples were in situ investigated via DSC and simultaneous SAXS/WAXS approaches during heating. The polymer chain dynamics and the critical cooling rates of the PTT and copolymers were estimated by rheological testing and DSC characterization. The results indicated that the chain conformation adjustment of PTT and copolymers is proposed relate to the comonomers in the copolymers, and their cold crystallization behaviors can be divided into three stages based on the heating temperature. The first involves rapid transition from a pre-ordered phase to a crystalline phase, the second is the adjustment of defects in an imperfect crystalline phase, and the third may refer crystal reorganization before melting. The poor crystallization ability is proposed due to the difficulty of chain conformation adjustment with high comonomer content and the crystallization is no longer obvious during heating. The cold crystallization behaviors of the PTT and copolymers were discussed based on the polymer chain dynamics and it is convinced that would be helpful to understand the crystallization and processing of polymers.