Fabrication Of High-performance Crystalline Polymer Foams Using Supercritical Carbon Dioxide And Their Applications Research

Polymeric foams have been widely used in a variety of applications,including food packaging,thermal building insulation,family product design,automobile,tissue engineering,sound absorption,conducting and electromagnetic shielding,etc,because of their lightweight,low-cost,low heat permeability,insulation,and impact resistance.In recent decades,amorphous thermoplastic polymeric foams have received significant development and became popular.However,the problems of amorphous thermoplastic polymeric foams in practical application limited their further development,for example,associated with environmental pollution and the shortage of oil resource;thereby the crystalline polymeric foams have begun to notice.Nevertheless,due to low melt strength of crystalline polymers,it is hard to use supercritical carbon dioxide(Sc-CO2)foaming technology to prepare high-quality crystalline polymeric foams.Therefore,the approach to prepare high-quality crystalline polymeric foams has been an ongoing challenge in polymeric foams area.Based on these backgrounds,this thesis mainly studies the preparation of high-quality and high-performance crystalline polymeric foams(e.g.,PE,PLA)using a combinational technology of pressure-induced flow(PIF)processing and Sc-CO2 foaming.The effect of PIF processing on the properties(e.g.,morphological,mechanical,crystallization,foaming,etc.)of crystalline polymers was discussed in detail.Moreover,the investigation on the applied application of as-received crystalline polymeric foams was also employed.The purpose of this thesis is to develop an effective way to fabricate high-performance crystalline polymeric foams,and intend to promote the development of crystalline polymeric foams in practical application.The detailed research contents and results are as follows:1.Fabrication of high-strength HDPE materials and lightweight high-strength HDPE foams.The section has studied the effect of PIF processing on the macro-mechanical performances and crystallization properties,and found that the PIF processing could effectively enhance the mechanical properties of HDPE materials.After PIF processing,a highly-orientated morphology was achieved,which is beneficial for fabricating the high-performance materials.To further introduce the crosslinking effect into the PIF processing,the HDPE material can be achieved the property of super strength which is close to the strength of engineering plastics.The foamability of various PE samples were investigated and discussed.The introduction of PIF processing was able to improve the foamability of HDPE.When the PIF process was combined with adequate crosslinking effect,a dual-mode foam structure with a combination of few large bubbles and many smaller and denser bubbles was obtained.Furthermore,the density of PIF XLPE foams was further decreased(~15%),and had the highest compressive strength.These results suggest that the synergistic effect of PIF process and crosslinking not only improve the mechanical performance of HDPE materials,but also to be an effective way to prepare lightweight high-strength HDPE foams for practical application.2.Fabrication of open-cellular porous PLLA or PLLA-based scaffold for tissue engineering application.The first section has studied the PIF processing-assisted Sc-CO2 foaming technology to fabricate high-quality PLLA or PLLA-based foams.The effect of PIF processing on the macro-mechanical performances and crystallization properties were investigated firstly,and also found that the PIF processing could effectively enhance the mechanical and crystallization properties of PLLA materials.During the Sc-CO2 foaming process,the introduction of PIF process could improve the foamability of PLLA materials due to the formed “brick and mud” structure.Further,we discussed the effect of foaming temperature on the morphology of PLLA materials.Through the control of the crystallization rate in Sc-CO2 foaming process,the open pore or closed pore structure can be obtained at different foaming temperature.The mechanism of the formation of open/closed pore structure was studied in detail,and found that the change of crystallization rate plays a key role in the formation of final foams structure.At last,the biocompatibility results of as-received PLLA foams showed that the porous scaffold could provide a promising cellular micro-environment for cell adhesion and proliferation.In view of the above-mentioned method has shown easy control,low-cost and non-toxic,the approach to prepare open-cellular PLLA scaffold has great significance in tissue engineering.In consideration of the poor hydrophilic and slow degradation rate of PLLA may cause delayed inflammation or affect the biocompatibility,PEG was added into the PLLA matrix to prepare PLLA/PEG composites.The effect of PIF process on the macro properties,micro morphology,and foaming properties of composites was discussed in detail,and has come up with similar results with previous studies.Meanwhile,the hydrophilic property,biocompatibility,and degradable properties were studied to explore the effect of PEG in PLLA matrix.The results presented that the porous PLLA/PEG scaffold not only provides a suitable micro-environment for cell adhesion and proliferation,but also improves the hydrophilicity and degradation rate of PLLA materials efficiently.3.Fabrication of lightweight and high-strength PLLA/MWCNT nanocomposites foams for electromagnetic interference shielding.Firstly,we studied the effect of PIF processing on the microstructure of PLLA/MWCNT nanocomposites.Meanwhile,the dispersion of MWCNT in the PLLA matrix was also investigated.The results showed that the MWCNTs were closely covered in the PLLA matrix,forming a strong conductive network.After PIF processing,a highly orientated structure was formed.Then,we utilized a combinational technology of PIF processing and Sc-CO2 foaming to prepare a lightweight and open-cellular nanocomposite foams.The nanocomposite foams presented high electric conductivity and excellent EMI shielding properties with less reflection in the measured X-band frequency region.The mechanism of the enhancement of electric conductivity and EMI shielding property was discussed in detail.Results showed that the presence of microcellular structure could decrease the percolation threshold of the composite foams,and the absorption is the dominant EMI shielding mechanism for the PLLA/MWCNT composite foams.Considering the simple,low-cost and eco-friendly fabrication process,the lightweight,high-strength and highly conductive biodegradable polymer composite foams are expected to be used as high-performance EMI shielding materials in areas such as electronics,automobiles and packaging.