Study On Melting And Recrystallization Behavior Of Syndiotactic Polypropylene In Ultrathin Films

Crystalline polymer can gain a better crystal structure by melt recrystallization,to improve the properties of products.In fact,during the heating process,the region with poor stability in the lamellar will first undergo melt recrystallization,thus forming the lamellar with higher stability,and eventually surviving during the selective melting as self-induced nuclei.The pattern of the self-seeding crystal can not only reflect the order difference and distribution within the lamellar but also be used as a method to reveal the orientation of the microregion crystal plane within the polycrystalline.The annealing thickening caused by chain slip makes it difficult to study the melting recrystallization behavior of polymers.Syndiotactic polypropylene(sPP)is often chosen as a model system for studying melt recrystallization of polymers because of the difficulty of its molecular chain movement along the chain axis and no chain slip thickening.In this paper,an optical microscope(OM)and atomic force microscope(AFM)were employed for the characterization of the transformation from single crystal to polycrystalline in sPP ultrathin films.The internal orientation of sPP polycrystalline was revealed through self-seeding,which realized the regulation of number density,distribution,and orientation and provide a new method for researching the formation mechanism of the orientation of the polycrystalline structure.It was found in a further study that the defect of sPP crystals could be self-repaired when annealing at a high temperature,in which process melt recrystallization exerted a significant influence.The main research contents and results are as follows:1.OM and AFM were employed to characterize the effects of crystallization temperature and film thickness on crystalline morphology and kinetics of sPP ultrathin films.It was found that the lamellae growth rate decreased obviously while the critical width of lamellae branching increased with the increase of crystallization temperature.The morphology of sPP lamellae evolved from compact dendrite polycrystalline to dendrite single crystal.With the increase of the thickness of ultrathin films,the sPP crystal have the growth transformation of flat-on and edge-on orientation.2.The annealing and melt recrystallization behavior of sPP lamellae were studied by AFM.The stable crystalline framework formed with sPP lamellae in the course of growth could be revealed by annealing.The sPP compact dendrite crystals were confirmed to be polycrystalline in the self-seeding experiment.The nucleation density decreased exponentially with the partial melting temperature increasing.At a low heating rate,the melt recrystallization of lamellae made the distribution of the metastable state in lamellae more uniform.With the increase of heating rate,the initial lamellae were not able to undergo melt recrystallization,which resulted in the self-seeding crystals showing the metastable state distribution in the initial lamellae.3.The effects of melt recrystallization on the evolution of sPP craze-like structure in the later healing period were studied by AFM.The results showed during the annealing process at a low temperature(92 ℃),the width of the craze-like decreased rapidly due to the thermal expansion of sPP lamellae.Subsequently,the unhealed craze-like was annealed by melt recrystallization.It was believed that the growth of craze-like followed the diffusion-limited(DL)mechanism in the early period of melt recrystallization(before 57 min)and subsequently changed into the nucleation-limited(NL)mechanism.