Crystallization Of Ultrathin Films And The Form Ⅱ-Ⅰ Transformation In Polybutene-1

Whether there exists ordered structure or phase at crystal growth fronts is theintrinsic distinction between the one-stage and the multistage growth models.Insight into the crystal growth mechanism in phase transition is crucial to regulating the aggregation structure and thereby controlling the ultima property,which is also a key research topic in polymer physics and polymer processing.The interface layer of the growth front of lamella may consist of the mesomorphic phase（whose ordered degree is between the crystalline phase and amorphous）,the melt which is ready to form the mesomorphic phase and the native crystals that is formed by the crystallization of the mesomorphic phase.The existed characterization methods cannot meet requirements in resolution and sensitivity to directly distinguish the location and structure of the interface layer due to the rapid evolutions and the heterogeneity in the spatial distribution of the structures in the interface layer.In fact,during the solid-solid phase transition,if the ordered structures exist between the two crystalline phases,the obvious differences in mechanical properties between the mesophase and crystalline phase make it possible to characterize directly the morphology and structure of interface layer.The melt-crystallization of polybutene-1（PB-1）commonly generates metastable Form II crystals which can transform into stable Form I crystals upon aging at room temperature.Such a polymorphic polymer is a model system to investigate the mechanism of phase transition growth.In this study,the crystalline morphology and growth kinetics of monolayer lamella in the confined condition by using optical microscopy（OM）and atom force microscopy（AFM）.The Ⅱ- Ⅰ phase transition of Form II monolayer lamella is characterized by in-situ AFM,in order to further understand the evolutions of structures in the interface zone.The role of the mesomorphic phase in the multistage growth mechanism of polymer phase transition has been thorough discussed.This study can not only deepen the understanding of the lamellar growth process of the polymer phase transition but also provide a new thinking for regulating the aggregation structure and properties.1 The solid-solid phase transition can be significant influenced by the initial crystalline morphology and structure of crystals.The crystallization of ultrathin films is a feasible method to prepare the monolayer lamella with fewer defects and better flatness.The results suggest that the crystalline morphology and structure of PB-1 in the ultrathin films are related to crystallization conditions.Generally,the Form II and I′lamellae with a flat-on orientation are usually crystallized from the ultrathin films.By increasing the crystallization temperature,the branch number density of dendrite crystals of Form II is decreased,while the lamella thickness and lateral branch width present opposing laws.For Form I’crystals,as elevating crystallization temperature,the irregular branching seaweed dendritic evolves into a single crystal with a regular growth plane,and the lamella thickness increases.The growth rates of Form II and I’are both reduced with increasing the crystallization temperature,and the former is much larger than the latter.Upon shear flow,the Form II lamella with an edge-on orientation prefers to be produced,which can be altered to the flat-on orientation during the further growing process.With the increase of crystallization temperature,the number of transformed flat-on lamellae increases.The number density of edge-on lamellae decreases with the increasing shear temperature.The transition site of the edge-on to flat-on lamella is located at the lateral surface of the growth front of the edge-on lamella,which has been uncovered by selective dissolving.2 The Ⅱ- Ⅰ phase transition kinetics and its effect factors for monolayer lamellae in ultrathin films have been investigated by using AFM with tapping mode.The increase in phase contrast and the decrease in height can be found in the monolayer lamella accompanied by the Ⅱ- Ⅰ transition.The linear growth rate of Form I lamella is 0.46μm/day which is obtained by the evolutions of the AFM phase and height images.Compared to the washed sample,the lower linear growth rate of Form I is found for the Form II monolayer lamellae within the unwashed sample,which is maybe due to the decrease in the mobility of the segments distributed at the end surfaces of the folded lamellae.The dependency of the linear growth rate of Form I on the annealing temperature presents a bell shape,and the growth rate reaches maximum when the annealing temperature is 40℃.Annealing at the same temperature,the linear growth rate of Form I increases with the lamellar thickness of the initial Form II crystals.3 The evolutions of the morphology and structure of the Ⅱ- Ⅰ interface zone within the monolayer lamellae in ultrathin films have been investigated by in situ AFM.The understanding of the effect of mesomorphic phase on the lamellar growth during solid-solid phase transition is further deepened.It is found that there is an interface layer with the thickness of 290 nm at the growth front of Form I crystals.Different from the thermal expansion behavior of Form I and II crystals,and based on the results about the morphology characterization with a high resolution and the selective melting behavior occuring at the interface layer,we estimate that the interface layer consists of two parts,namely,Form I nanocrystals（with a diameter of 20-40 nm）and the mesomorphic phase.The induction period and growth period of Form I nanocrystals are 170 s and 510 s,respectively,by tracing the nucleation and growth process of Form I nanocrystals in the interface layer.The shift of the interface layer is caused by the concurrent nucleation and growth of multitudinous Form I nanocrystals.Compared with the nucleated in Form II crystals directly,the mesomorphic phase in the interface layer can significantly lower the interfacial energy and nucleation barrier for Form I nucleation.Finally,a stable Form I lamellae is formed by the solidification process for the Form I nanocrystals.The growth process of Form I crystals within Form II crystals can be described by multistage growth model of lamellae.4 In the bulk system,the initial Form II lamellae with different orientations are prepared by the pre-shearing.And the effects of the multilayered oriented Form II lamellae on the Ⅱ- Ⅰ phase transition kinetics have been studied.It is found that the oriented Form II lamellae can significantly accelerate Ⅱ- Ⅰ phase transition kinetics.The correlation between the induction period of the Form I nucleus（t_i）and the degree of orientation related with the lattice planes（200）of Form II is established with a slop of-1:t_i～f_200^-1.The growth of Form I crystals with orientation consumes from oriented Form II crystals.Further studies show that when the lamellar thicknesses of initial Form II lamellae are identical,the well-defined oriented lamellae are in favor of shortening the t_i,The acceleration of this phase transition may be due to more tie molecules distributed in the oriented lamellae increasing the probability of the formation of Form I nuclei.