The Influence Of Annealing On Crystal Structures And Mechanical Properties Of β-form Isotactic Polypropylene

Isotactic polypropylene is one of semi-crystalline polymer, whose crystallinestructure and perfection largely depend on crystallization and post-treatmentconditions. It is easy to form defective crystalline structure for semi-crystallinepolymer, which is due to the influence of external stress and cooling fast from meltduring forming process. Annealing is a common method to regulate the crystalstructure and morphology of polymer materials. It can not only eliminate the internalstress of material, but also promote the rearrangement of the molecular chain toreduce the defects, which results in improvement of structure and properties.In this work, the influence of annealing on the crystal structures and mechanicalproperties of β-iPP was investigated by small-angle X-ray scattering (SAXS),wide-angle X-ray scattering (WAXS), tensile measurements and DSC. Theexperiment results indicate that the increase of annealing temperature can induce thetransformation of β–iPP to α-iPP, resulting in a decline in the relative content of betacrystal. Meanwhile, the lamellar thickness and long period of β-iPP are increased withthe annealing temperature elevated, and the crystalline structure also become moreperfect. These changes of structure lead to the yield strength of β-iPP increased withthe annealing temperature increased, but the yield strain and fracture toughness aredecreased. When the annealing temperature is130℃, the crystalline form of β-iPP isnot changed with the annealing time extension, but the long period is increased. Andfurthermore, the increase of long period is the results of lamellar thickness andamorphous layer thickness increased together.In order to explore the microstructure changes of iPP with different crystallineforms during drawing process, the in-situ synchrotron SAXS and WAXS wereemployed to observe the tensile process of β-iPP after annealed for12h at130℃,140℃and150℃. When deformation measurements were carried out at30℃, theresults indicate that β-iPP and β/α mixed iPP are transformed into smectic mesophase,which is parallel to the tensile direction. And the long period in the direction parallelto tensile direction is increased, but the long period perpendicular to the tensile direction is almost constant. It is worth noting that we can only obtain the long periodbefore yield point, due to the interference of cavitations after yield point.