Study on the crystal morphology and melting behavior of isothermally crystallized composites of short carbon fiber and poly(trimethylene terephthalate)

The spherulites of the short carbon fiber(SCF)/poly(trimethyleneterephthalate)(PTT)compositesformedin limited space at designed temperatures,and their melting behaviors were studied by the polarized optical microscopy,atomic force microscopy(AFM),and scanning electron microscopy(SEM),respectively.The results suggest that SCF content,isothermal crystallization temperatures,and the film thicknesses influence the crystal morphology of the composites.The dimension of the spherulites is decreased with increasing SCF content,but whether banded or nonbanded spherulites will form in the composites is not dependent on SCF content.However,the crystal morphology of the composites depends strongly on the temperature.When the isothermal crystallization temperatures increase from 180℃ to 230℃,the crystal morphology of SCF/PTT composites continuously changes in the following order:nonbanded→banded→nonbanded spherulites.Disconti-nuous circle lines form in the film when the film thickness increases from 30 to 60μm.Basing on the SEM observation,it is found that these circle lines are cracks formed due to the constriction difference of the different parts of the spherulites.These cracks are formed when the film is cooled from the isothermal crystallization temperature to the room tempera-ture at a slow cooling rate;while they will disappear gradually at different temperatures in the heating process.The crack will appear/disappear first around the center of the spherulite when the film was cooled/heated.The nontwisted or slightly twisted lamellas will reorganize to form highly twisted lamellas inducing apparent banded texture of the spherulites.