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Two Competing Crystallization Modes in a Smectogenic Polyester

Barbara Heck, Ernesto Perez and Gert Strobl. Macromolecules 43, 4172-4183 (2010)


two competing crystallization modes in a smectogenic polyesterThe work concerns a polyester composed of heptamethylene-bibenzoate units and hepta-methylene-terephthalate co-units that has unusual crystallization properties. Nucleation and growth of spherulites competes with a crystal formation within first formed smectic domains. X-ray scattering experiments carried out simultaneously in the ranges of small, intermediate, and wide angles provide detailed information about the structural development in the two different modes. Varying the temperature in time-resolved measurements shows a change in dominance: At high temperatures, only spherulite growth is observed; at low temperatures, the system uses the two-step process via the smectic phase. Sample heating is usually accompanied by extensive reorganization processes, which all end with a final melting at 140 °C; higher melting points are only obtained for crystallization temperatures above 130 °C. Results are presented in the framework of a crystal thickness−temperature nanophase diagram. Data evaluation yields the latent heats of the transitions between the three phases and the macroscopic equilibrium phase transition temperatures. Sizes and annealing properties of crystallites grown in the spherulitic mode obey the laws established for common polymers. The size of the crystallites formed out of the smectic phase varies inversely with the supercooling below the temperature of the smectic−crystalline phase transition.

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