Morphological And Structural Evolution Of Polyethylene Ultrathin Films During Annealing Progress

In this study,the evolution of morphology of polyethylene ultrathin films during annealing was investigated by optical microscopy(OM),atomic force microscopy(AFM)and transmission electron microscopy(TEM).The results show that when the oriented polyethylene ultrathin films are annealed near the melting point,a large number of regions with local orientation are formed,which is completely different from the morphology from the isotropic melt.During the cooling process,a number of leaf-like crystals appeared within the film rapidly.The leaf-like crystals are actually composed of asymmetric two parts,separated by a ridge composed of scrolled lamellar layer.One part is mainly composed of orderly arranged lamellae,and the other one is less ordered,interweaved net structure with rope-like crystals.Different crystalline morphologies can be seen in the ordered lamellae,such as the lamellar twisting.That is,crystals with a certain inclination angle to the base surface normal are alternated with edge-on crystals,and the torsional period is not constant.In addition,when the two groups of lamellea approach each other from the opposite direction,the partial lamellar twisting appears at the terminal of the crystals.The possible reason is that the lamellae shift from edge-on to flat-on in order to absorb more melt to maintain crystal growth.It has been found that the the parallelly arranged lamellae grows along the b axis by TEM.In the same heat treatment,the lamellar thickening occurs on the hydrophobic silicon substrate,and there is no rearrangement and recrystallization process of the segment.We speculate that the PE segment and the hydrophobic substrate have a strong interaction that affects the relaxation of its molecular segment.AFM and OM were used to observe the effect of crystallization temperature on the morphology of PE ultrathin films.With the increase of the crystallization temperature,the distance among the individual crystalline region increases gradually.As to the morphology of each crystalline domain,its asymmetrical characteristics does not change.For the side with a high degree of order,the reclining crystals are arranged more closely.The relaxation process of the oriented film was explored.The morphological changes obtained by increasing the annealing temperature and extending the annealing time were investigated.It is speculated that during the disoriented process,the molecular chains in the oriented PE film change from the original orientation to the intermediate state of local orientation,and finally form an isotropic melt.