Epitaxial Crystallization Of Precisely Substituted Polyethylene Induced By Carbon Nanotube And Graphene

Substituent has a significant impact on the crystallizable sequence length and chain packing of polyethylene.Lattice matching between the substract and polymer generates strong interaction,which improves the orientation of the polymer chain and transformation of crystalline structure.The evoluation of crystallization sequence length and chain packing of substituented-polyethylene induced by substract were investigated.In the present paper,the precision ADMET polyethylene with substituents placed on each every 21 st backbone carbons was chosen as model.The well-defined primary structure expells the characterization of intra-or intermolecular heterogeneity of common polyethylene with random substituent,making ADMET polymers suitable model systems to study the effect of precise placement of substituent on the crystallization of polyethylene.The supercritical CO₂?SC CO₂?as antisolvent was also used to study the effect of crystallization condition on epitaxial crystallization.the polymers structure were characterized by 1HNMR.Transmission electron microscopy?TEM?was used to investigate the morphology of carbonaceous nanofiller-induced crystals.Thermal behavior of composites were shown by differential scanning calorimetry?DSC?and Flash-DSC.The crystal structure of composites were characterized by X-ray diffraction?XRD?and X-ray photoelectron spectra?XPS?.The main results are as following:?1?The precision ADMET polyethylene with substituents placed on each every 21 st backbone carbons has been studied,labeled as PE21X?X=F,Br,Cl?,where the number corresponds to the precise location of the side group in the PE backbone.The synthesis of these polymers required the preparation of ?-? diene monomers,followed by ADMET polymerization and subsequent exhaustive hydrogenation.Substituents as the defect of chain incorporated in the crystalline structure lattice impacts the chain ordering of branched polyethylene and cause the expansion of the orthorhombic lattice.As increasing the volume of substituent to Cl and Br,accommodation of the larger atoms distort the all-trans ordering of vicinal intra-and intermolecular carbons resulting in a more defective structure,transforming orthorhombic lattice of PE21 F to triclinic lattice and,as a consequence,significantly decreasing the meltingtemperatures and degrees of crystallinity.PE21X/CNT nanocomposites were prepared via solution crystallization.The kebab-like crystals formed on the CNT.Substituents as a defect of chain affect the lateral growth of crystal lamellae resulting in smaller lamellae size.Moreover,the influence of substituent is much more significant when PE21 Cl and PE21 Br crystal on CNT.Isothermal crystallization temperature plays a decisive role in thickness and interval of lamellae formed on CNT.Crystal lattice structure and crystallizable sequence length of PE21 X remained unchanged in PE21F/CNT and PE21Cl/CNT composites systems.CNT can induce triclinic form of PE21 Br to form ordered orthorhombic lattic,which causes melting temperature 20°C higher than bulk.?2?PE21X/RGO nanocomposites were prepared via solution crystallization.The rod-like crystals formed on the RGO.Selected area electron diffraction?SAED?pattern revealed that the c-axis of polymer chain is parallel to the surface of the reduced graphene oxide?RGO?.The thickness and density of lamellae formed on RGO is directly ralated to isothermal crystallization temperature.The size of rod-like crystals decreased with increasing of the volume of substituents.Crystal lattice structure and crystallizable sequence length of PE21 F remained unchanged in PE21F/RGO composites systems.RGO induces PE21 Cl to form orthorhombic lattic,which is of higher density than the triclinic form in the bulk.The melting point of PE21 Cl crystals grown on RGO is 75 °C higher.RGO also induce PE21 Br to form higher ordered and density orthorhombic lattic,which cause the melting point of PE21 Br crystals grown on RGO is 97 °C higher than bulk.PE21 Cl and PE21 Br can epitaxial crystallized on RGO only with orthorhombic form which have perfect lattic matching with RGO.?3?Supercritical CO₂ as antisolvent can deposite the polymer chain rapidly and largely which accelerate the lateral growth of lamellae,and improve the yield of lamellae formed on nanofillers.The regular and uniform kebab-like and rod-like crystals formed on the CNT and RGO,respectively.which are much larger than that without assistance of SC CO₂ under same isothermal crystallization.There exists an optimum SC CO₂ pressure at which the edge-on crystal lamellae size is the maximum,which is 15 MPa.Supercritical CO₂ can promote the lateral growth of lamellae on CNT and RGO,but it can not change the thickness of the crystal lamellae which is directly related to crystallization temperature.The melting peak temperatures and crystalline structures of PE21X/nanofiller composites produced with assistance ofsupercritical CO₂ are almost consistence with that prepared by traditional solution crystallization.