Evolution of Crystalline Morphology in Ultrathin Films of Copolyesters with Controlled Rigid Dihydric Alcohol Contents

Abstract

The isothermal crystalline morphology of random copolyesters, PTT_xG_y, prepared by polycondensation of terephthalic acid with mixtures of 1,3-propylene glycol (1,3-PDO, T) and 1,4-cyclohexanedimethanol (1,4-CHDM, G) in different molar ratios, was studied by atomic force microscopy (AFM). The results show that with an increasing molar ratio of the rigid component (G) in the dihydric alcohol, the size of the lamellar crystals in ultrathin films initially increased, followed by the formation of stacked lamellae, where the number of stacking layers progressively increased. The lamellar thickness also increased from 5.6 nm in PTT (0% G) to 12.2 nm in PTT₈₅G₁₅ (15% G) after crystallization at 195 °C. These phenomena are attributed to the inhibition of nucleation in the film with the increase of the G component. Consequently, the crystal size increased, while the crystal density decreased. This further makes the lamellar crystals to nucleate and grow on top of a basal lamellar crystal, forming correlated lamellar crystals. Exclusion of the G component from the crystal results in more tails and loops on the lamellar surface and forms thicker amorphous layers on two sides of the crystal; therefore, the lamellar thickness measured by AFM was increased. The effect of the number of nuclei on the formation of correlated lamellar crystals was further demonstrated through a two-step crystallization procedure. As the number of crystal nuclei increased, the degree of stacking and crystal size decreased until a monolayer of lamellae formed after crystallization at 195 °C. This paper highlights the role of the polymer chemical structure in the resulting crystalline morphology of ultrathin films.