Effects Of Melt Thermal History And Extrusion Conditions On Crystallization Of Poly(L-lactide)

The crystallization morphology and structure of polymer have great influence on the performance of materials, the crystallization morphology and structure are dependent on processing conditions, so the performance of materials can be adjusted by controlling processing conditions. The crystallization rate of poly(lactic acid)(PLA) is slow, which makes it easier to control the crystallization morphology, structure, and crystallinity during processing, and it also will be easier for in-situ characterization, so we choose PLA to study the effect of melt thermal history, extrusion conditions and crystallization conditions(shear stress, extrusion temperature and isothermal crystallization temperature) on crystallization morphology and structure of poly(?-lactide)(PLLA), the main research contents are as follows.1. The influence of melt thermal history on the crystallization morphology and structure of PLLA was investigated. Near melting point melt was obtained by heating PLLA samples to near melting point temperature which was between nominal melting point and equilibrium melting point, and supercooled melt was obtained by cooling blank melt(temperature of the melt is higher than the melt of equilibrium melting point) to supercooled temperature. Near melting point samples and supercooled samples were extruded under the same extrusion and crystallization conditions. It was found that, the crystal morphology and structure of extrusion samples were of great different because of the different melt thermal history. A few small spherulites which were induced by crystal nucleus formed during the process of cooling after extrusion were found in the supercooled extrusion samples. While lots of cylindrites were found in the near melting point extrusion samples, the cylindrites come from the ordered structure which oriented under the action of shear stress existed in the melt before extrusion.2. The effects of near melting point extrusion conditions on crystallization of PLLA were investigated. It was found that,(1) The number of cylindrites increased, the size of the spherulites decreased and nucleation density increased gradually with the increasing of shear stress.(2) the quantity of cylindrites decreased gradually with the increasing of extrusion temperature; and the crystallinity elevated.(3) Amorphous region existed at low isothermal crystallization temperature, while the quantity of cylindrites and amorphous decreased with the increasing of crystallization temperature, with the oriented structure relaxation during crystallization; the size of the spherulites increased gradually with the increasing of crystallization temperature; and the crystallinity elevated.3. The effects of supercooled extrusion conditions on crystallization of PLLA were investigated, it was found that,(1) Nucleation density of cylindrites of the wall increased with the increasing of shear stress; the crystallinity and melting point were invariable in principle.(2) Amorphous region increased with the increasing of extrusion temperature; the oriented structure decreased and nucleation density of cylindrites of the wall decreased gradually with the increasing of extrusion temperature; the crystallinity decreased.(3) The number of liner crystal nucleus decreased, nucleation density of cylindrites of the wall decreased and the size of the spherulites increased gradually with the increasing of crystallization temperature; the crystallinity elevated with the increasing of crystallization temperature.