Study On The Regulation And Properties Of Polylactic Acid Hybrid Crystals

With the rapid development of materials science,the demand for polymers is increasing,and polymers have become an irreplaceable part of industrial production and daily life.Poly（lactic acid）（PLA）,as a biodegradable plastic,has excellent physical,chemical,and biological properties.However,its application is greatly limited due to its low molecular chain interaction,low melt strength,and poor mechanical properties.In recent years,the development of high-performance PLA materials and the preparation of high-performance PLA products with excellent characteristics such as lightweight and high strength have become a research hotspot in the field of polymer materials.In this thesis,a high performance PLA material was prepared using a secondary convergent flow channel mold using flow induced changes in the aggregation structure and crystallization process of molecular chains during the molding process.The lightweight and high-strength PLA and its composite foam were prepared by supercritical foaming technology.The main research contents are as follows:（1）Taking PLA as the research object,high-strength PLA samples with hybrid crystal structure were prepared using a quadratic convergent runner mold through adjusting process parameters during injection molding.The effects of different injection molding conditions on the microstructure and properties of PLA were studied.Through experiments,it was found that the aggregation structure of PLA molecular chains changed under the flow induction of convergent channels,affecting their crystal morphology,and forming a hybrid crystal structure.The results show that secondary convergent flow induced medium speed high-pressure injection molding can significantly improve the crystallinity（7.1%）,tensile strength（71.1 MPa）,and impact strength（25.5 k J/cm²）of PLA polymers.（2）Using PLA as a matrix,PLA/PTFE in-situ fiber forming composites were prepared by extrusion molding,and high-performance PLA/PTFE products were prepared by injection molding using secondary convergent flow channels.It was found that using secondary convergent flow to induce the orientation of PTFE in situ microfibers can change the crystal structure of PLA,forming an ordered strand structure,and improving the strength and performance of PLA.The crystallization performance test found that under the high-speed high-pressure injection molding process,different PTFE content had a significant impact on the crystallization behavior.When the PTFE content was 3 wt%,the crystallinity increased by 1.9%.In addition,the mechanical properties of PLA/PTFE composites with a PTFE content of3 wt%were also significantly improved,with a tensile strength increase of 10.8%and an impact strength increase of 88.4%.However,with the further increase of PTFE content,PTFE agglomerates,resulting in a decline in the performance of the composite.（3）Microcellular foaming of PLA and PLA/PTFE in-situ fiber composites with different crystal structures was carried out using supercritical CO₂foaming method,and the effect of different crystal structures on the cellular morphology and structure of foam products was studied.Through the observation of cell morphology,it was found that the foamed products of PLA and PLA/PTFE with different crystal structures would produce an ordered string like microporous structure under low temperature（80℃）foaming.Under high temperature（110℃）foaming,the microporous structure disappears and turns into a circular bubble with a larger size.With the addition of PTFE,the cell size gradually decreases and the cell density gradually increases.