Surface-assisted Crystallization In Polymer Thin Films

Polymer material is an important material,which is widely used in many fields,indicating that polymer material can meet the requirements of different fields.This is mainly due to the property dependence on different level structure and control of different structures.Therefore,the mechanism for the formation of different level structure of polymers,influence factors and evolution laws have been the focus of scientists.There are series of methods for regulating polymer structure at different level,surface-assisted(induced)crystallization can effectively control the molecular chain orientation arrangement,and then regulate the crystal structure of polymers.It provides an effective method for the preparation of special structures,specific properties of the orientation polymer composite film system and become a hot spot in the field of polymer research.Many studies show that during surface-assisted crystallization,there exists a special structural(lattice)matching between the substrate and the epitaxial polymer.However,the cross-nucleation behavior in polymer,that is,one type of crystal of the same polymer induces the growth of another type of crystal,indicates that lattice matching is not the only controlling factor of surface-assisted crystallization.Therefore,it is great academic value and practical significance to reveal the essence of surfaceassisted crystallization and to develop effective methods to regulate polymer structure at different levels.With the help of transmission electron microscope(TEM),atomic force microscope(AFM),optical microscope(OM),fourier transform infrared spectroscopy(FTIR),differential scanning calorimeter(DSC),etc.,this PhD thesis have carried out systematic research work on surfaceassisted reversible crystallization,the effect of polymer initial state(melt and glass state)and the influence of different lamellae surface structure on surface-assisted crystallization.We reveal that the motion of polymer chains,crystallization kinetics,crystallization temperature,interchain interaction and self-induced nucleation can influence the surface-assisted crystallization in polymer thin films.Brief introduction of research contents of this paper are as follows:1.Vacuum evaporated carbon thin film can fix the oriented structure of polyethylene(PE)after melt-crystallization.The melt-crystallization of isotactic poly(methyl methacrylate)(iPMMA)on oriented PE thin films is conducted,oriented structure of iPMMA is the same as iPMMA coldcrystallization on oriented PE.Polymer chains of iPMMA always align parallel with oriented PE chains,indicating that the initial state before iPMMA crystallization does not affect epitaxial crystallization behavior of iPMMA on PE.However,crystallization kinetics show that the crystallization induction period of iPMMA cold crystallization on oriented PE is shorter than that of the melt crystallization.But the halfcrystallization time of the cold-crystallization process is longer than that of the melt-crystallization,which indicates that the rate of iPMMA crystal growth rate in cold-crystallization is slower than that of iPMMA meltcrystallization.We explain this phenomenon from differences in molecular chain movement capabilities.2.The melt-crystallization of isotactic poly(1-butene)(iPBu)in the melt-drawn oriented poly(1-butene)/isotactic polypropylene(iPBu/iPP)blend thin films is studied.We confirm that iPBu in the melt-drawn iPBu/iPP blend films is form I shish crystals which is different with the pure iPBu melt-drawn thin films with form I folding crystal.Selected meltcrystallization of iPBu in blended film does not change crystal form and orientation state.In order to prove that the melt-crystallization of iPBu in oriented iPBu/iPP blended film is not caused by nano-confinement effect,the melt-crystallization behavior of iPBu on oriented iPP film is studied.Oriented form-I folding lamellae crystal structure is obtained after meltcrystallization on the surface of iPP,rather than the form-? that normally obtained.All experiments indicate that oriented iPP can induce iPBu form? crystal.We explain the direct formation of iPBu form-? crystal on oriented iPP from the molecular chain interaction,lattice matching,and crystallization kinetics.3.Oriented isotactic polystyrene(iPS)thin film is obtained by epitaxial cold-crystallization of iPS on iPP orientation film,then the meltcrystallization of iPP on its own induced oriented iPS film was realized through selective melting iPP,which proved the reversibility of epitaxial crystallization.However,epitaxial crystallization of iPP on oriented iPS have temperature dependence,only when the crystallization temperature is lower than 30?,the oriented iPS can induce iPP epitaxial crystallization.It shows that lattice matching is not the only condition for the occurrence of epitaxial crystallization,crystallization temperature would also play an essential role in this process,we explain this phenomenon through the Hoffmann-Lauritzen theory.4.By observing the crystalline morphological structure of iPS film at high temperature,the folding surface induction effect of iPS lamellae on the stacking growth of iPS single crystal is explored.IPS "threedimensional single crystal" is successfully obtained,it reflects the symmetry of iPS cells.We prove that in "three-dimensional single crystals",crystals at different layers not only have parallel crystalline c-axis,but also have parallel a-and b-axes.From a morphological point,crystals at different layers have parallel(110)exposed growth front.We propose a mechanism named as "self-induced nucleation" to explain the lamellae stacking on the folding surface with specific rules.