Nucleation and crystallization of random aliphatic-butylene terephtalate copolyester

• Nucleation and crystallization of random aliphatic-aromatic copolyester were studied.
• Strong self-nucleation was discovered enabling to control copolyester crystallization.
• Copolyester crystallized in the form of long and thin fibers arranged in spherulites.
• Several agents nucleating crystallization of copolyester were found.

Aromatic component of biodegradable aliphatic–aromatic random copolyester crystallizes. A strong self-nucleation is active with self-nuclei stable below 160 °C. Without self-nuclei the peak crystallization temperature decreases by 25 °C during subsequent cooling. The activity of self-nuclei was incentive to find artificial nucleating agents. Green phthalocyanine, blue phthalocyanine and talc were found to be effective nucleating agents for aliphatic–aromatic copolyester in absence of self-nuclei, increasing both the peak crystallization temperature and the density of nucleation. Crystallization of aliphatic–aromatic copolyester led to the formation of spherulites filling the entire volume of the material although aromatic crystal amounted to 10% of entire volume. SAXS analysis and AFM imaging showed that crystals are neither lamellar nor aligned in stacks. Instead, the crystals are formed as narrow, 3–4 nm thick fibrils radiating from spherulite centers. Crystallization of nucleated copolyester proceeds during cooling in a very peculiar way: first long fibrils are formed that partially immobilize the rest of material. During further cooling partially confined melt crystallizes in the form of short fibrils, however, in significantly smaller amount. The strangest outcome of artificial nucleation is that the crystallinity degree of nucleated copolyester is lower than nonnucleated. By short annealing at 70 °C long fibrils decompose by partial melting and capillary instability freeing the material from constraints.

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