Crystallization Behavior Of Precise Polymer

Polyethylene(PE)shows the samplest molecular chain among polymers.The ideal PE,especially HDPE,has only CHZ group in its main chain without any other functional groups and side chains.But reactions such as braching and copolymerization with other comonomers would induce defects into the molecular chain which disrupt the crystallization.Besides,functionality on PE chain induces functional groups as the defects which influence crystallization as well.However,many defects distribute randomly on the chain,making it difficult to understand in detail their effect on crystallization.Thus,two kinds of presice polymer were studied in this work.One is based on ADhMET polymerization which ensures exact placement of chain defects.The defects groups were all separated by 20 CHZ units.Another type is one general polymer PVDF,short name of poly(vinylidene fluoride),with two fluorine atoms placed in every two CHZ in the main chain.Due to electronegativity of fluorine,PVDF shows different behavior during crystallization compared with PE.Three conformations and five crystalline phases were observed under different conditions.Based on the two kinds of presice polymer,the work concerned about their crystalline behaviors have carried out as follows:1.Morphology and thermal properties of precision polyethyleneChemical irregularities such as side chain branches or comonomers in an otherwise regular polymer influence its crystallization.In most polymer systems,these chemical defects are randomly distributed along the chain,and it is difficult to understand in detail their effect on crystallization.Three precision polymers prepared by acyclic diene metathesis(ADMET)polymerization.The synthesis ensures exact placement of the chain defects;here they are all separated by 20 CHZ units.The polymers are two polyphosphoesters with a phosphate or phosphonate group in the main chain and one polyethylene with butyl branches.Although the alkyl part is identical for all three polymers,their thermal and crystal properties differ noticeably.Compared with ADMET PE,their melt point decrease dramatically.By means of differential scanning calorimetry,X-ray scattering,and transmission electron microscopy,we characterize the lamellar crystals and correlate our findings to the observed difference in thermal behavior.Due to difference of the defects volume,the defect groups of PPE-Ph and PE21-butyl were excluded from lamellae.Fold lamellae were formed during crystallization of PEZI-butyl and decrease of surface free energy was found in PPE-Ph due to exclusion of benzene ring.PPE-m formed lamellae stacks with its defects form layers within one crystal which induce high defect energy.2.Transformation from PVDF ? phase to ?' phasePVDF ? phase is electrically active which can make it strong interest for the application in sensors and memory technology et al.Thus,promotion of y content in PVDF film is an on-going pursuit.Contaction effect with ? phase can facilitate the transformation from ? phase to ?' phase.Besides,?' phase has the same crystalline structure as ? phase.Thus,increasing the transformation effect to ?' phase is a promising way to obtain high ?-content film.However,there is a big energy barrier for the occurance of transformation when annealing at high supercooling temperature.The nucleation efficiency is rather low when the supercooling temperature is low leading the ?-content in the sample is low.Here fractional crystallization was adopted to increase the ?-content in the film.Firstly,the sample was crystallized at high supercooling tempreture leading coexistence of both a and ? phase.Secondly,increasing the annealing temperature to achieve the energy barrier of transformation.Using this method,?-content in PVDF film was highly increased compared with one step annealing.3.Microstructure control of ferroelectric PVDF via homoepitaxy and self-nucleation process.PVDF is a known polymer with strongest piezoelectric and pyroelectric activity.However,its piezo-and pyro-electricities encountered only in certain crystal structures,e.g.,its ? and ? forms.We report here a simple melt-recrystallization approach for producing ?-phase rich PVDF thin films through selective melting and subsequent recrystallization.The proposed approach comprises the selective melting of the non-polar ?-PVDF crystals and their subsequent isothermal recrystallization at 160 ?.During the recrystallization process,the non-melting stable y-PVDF crystals induce y-PVDF crystallization of the melt through homoepitaxial mechanism.Moreover,in the molten ?-PVDF melt,the insufficient melting leads to the existence of locally ordered domains previously included in the original ?-PVDF crystal lattice.These locally ordered domains are confirmed to adjust their molecular chain conformation more easily and adopt a conformation required for the y-PVDF crystals,which results also in the formation of y-PVDF crystals through a self-nucleation mechanism.This is of great importance to fabricate high quality thin films of y-PVDF crystals.for advanced applications.4.The morphology and structure of a and ? phase in PVDFPVDF as an industrialized polymer with precise structure was used in varied field.Crystallized from melt,PVDF forms a phase and ? phase.Conformation difference between the two phases leads to difference in their morphologies.A spherulites show banded morphology whereas ? phase shows two kinds of morphologies in its spherulites,namely,flat-on and scroll lamellae.Transcrystallization of ? phase was induced by a tip scratch in PVDF film.Transformation process from ? phase to ?' phase was observed in the transcrystal.Transformed ?' phase was found weaker birefringence by polarized optical microscope.Combined with atomic force microscope,morphology of ?'phase was characterized.