The Banded Spherulites of iPP Induced by Pressure Vibration Injection Molding

Isotactic polypropylene（i PP） samples obtained by pressure vibration injection molding（PVIM） and conventional injection molding（CIM） were studied by polarized-light microscopy（PLM）, respectively. It was found that the alternating bright and dark banded spherulites were generated in the transitional region of PVIM parts. It is the first time that the banded spherulites of isotactic polypropylene were observed in polymer processing. What＇s more, the banded spherulites were proved to be constituted of α-form crystal by hot stage polarized-light microscopy（HT-PLM） and wide angle X-ray diffraction（WAXD）. Morphology of the banded spherulites was also studied by scaning electronical microscopy（SEM）.

Replicated Banded Spherulite:Microscopic Lamellar-assembly of Poly(L-lactic acid)Crystals in the Poly(oxymethylene)Crystal Framework

The morphologies of poly（L-lactic acid） （PLLA） spherulites, when crystallized within the pre-existed poly（oxymethylene） （POM） crystal frameworks, have been investigated. PLLA/POM blend is a melt-miscible crystalline/crystalline blend system. Owing to the lower melting point but much faster crystallization rate than PLEA, POM crystallized first upon cooling from the melt state and then melted first during the subsequent heating process in this blend system. Lamellar assembly of PLLA crystals within the pre-existed POM spherulitic frameworks was directly observed with the polarized light microscopy by selectively melting the POM frameworks. The investigation indicated that PLLA crystals fully replicated the spherulitic morphology and optical birefringence of the POM crystal frameworks, which was independent of To. On the other hand, POM could also duplicate the pre-existed PLLA morphologies. The result obtained provides us a possibility to design the lamellar assembly and crystal structures of polymer crystals in miscible crystalline/crystalline polymer blends.

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.