The Formation Of Shish-kebab Crystals Effect On Mechanical Properties Of Bimodal Polyethylene

Bimodal polyethylene?BPE?is a kind of polyethylene resin blend with high molecular weight and low molecular weight in molecular leve.Compared with ordinary polyethylene,BPE has both excellent mechanical properties and excellent processing performance,it has extensive applications of film and pipe in the fields.It is an effective way to regulate crystal structure and improve the properties of the polymer by preparing BPE nanocomposite materials.Under the flow field the structure evolution process of the injection BPE,shish-kebab,has played a very important role in improving the mechanical properties of products.As a highly oriented structure,shish-kebab crystals can obviously improve the stiffness?strength and other thermal properties of products.Therefore,it has attracted attention both the academic research and industrial application.In this paper,various characterization techniques,including Small Angle X-ray scattering,Wide Angle X-ray diffraction,Field emission Scanning Electron Microscope?SEM?,Differential Scanning calorimeter?DSC?,Mechanical performance test,Rheometer and other characterization methods were employed study the relationship between the formation process and mechanical properties of shish-kebab crystals in BPE during the injection molding process and foam process.The main results are as following:1.The influence of the formation of shish-kebab crystals in the mechanical properties of nanocomposite materials after the addition of chitin nanocrystals?CNC?was studied.The mechanical results of mechanical properties showed that the addition of CNC could enhance the mechanical properties of BPE.SAXS results showed that the addition of the proper amount of CNC made the molecular chain more orderly,inhibited the relaxation of the extended chain,and generated more shish crystals.WAXD results showed that adding CNC increased the orientation of products.At the same time,the flow field of injection molding make more short chain involved in the formation of shish-kebab crystals to improve the mechanical properties of BPE composite materials.In the process of rapid cooling of the melt,some short chain,far away from the CNC surface,was self-nucleating accumulation to grow into common lamella structure.2.BPE and BPE/CNC micro-cell foaming materials were prepared by supercritical carbon dioxide foaming at high temperature melt and low temperature foam method.As soon as the polymer/gas system forms a homogeneous system at a high temperature of 155?,and then cool to113-145?to release pressure in the moment to form foaming material with high expansion ratio.SEM results show that the average cell diameter was increases and the cell density was decreased with the increase of temperature in a certain range.With the increase of pressure,the average cell diameter was decreased,and the number of cell was increases in BPE and BPE/CNC foam materials and Open-cell structure foam material was achieved as pressure reached certain pressure.3.BPE and BPE/CNC foaming materials were prepared by supercritical CO₂ foaming at low temperature solid phase method.In 20 MPa steady pressure and 120-135?temperature ranges to isothermal 4.5 hours and then quickly release pressure to form foam materials.SEM results showed that nano-cell structure appeared in the foam material at a certain temperature range,and the shish-kebab structure could be clearly observed.After foaming,the peak of DSC curve moved to the right,indicating that the internal shish-kebab structure and ordinary lamella thickness increased.WAXD results showed that the crystallinity increased after foaming.The mechanical properties results showed that the maximum elongation of break of the foamed material increased by 98%.To sum up,the formation mechanism of shish-kebab structure was deepgoing studied by adding different content of CNC in the BPE system,and result showed that CNC promoted the generation of shish-kebab and improved the mechanical properties.In addition,by supercritical CO₂ high-temperature melting and then low-temperature foaming method,closed and through-cell foam materials of BPE with high foaming ratio were prepared and nano-cell structure layed in shish-kebab were prepared by solid phase foaming at low temperature.