Study On Structure And Properties Of Natural Polysaccharide Nanocrystals Modified Polymer Materials

Bio-based and biodegradable materials have raised a tremendouslevel of attention in the recent years, since sustainable developmentpolicies tend to expand with the decreasing reserve of fossil fueland the growing concern for the environment. Good enhancement effectswere achieved when polysaccharide nanocrystals were added into natural andsynthetic polymers, mechanical and thermal properties were improved to some extent.There are many hydroxyl groups on the surface of polysaccharide nanocrystals, whichweaken the interface interactions between polymers, restrain the active role innon-polar and weak polar polymer matrix. They can be modified by chemicalmodification to improve the interaction and compatibility with polymer matrix. Thusstudy and development of polysaccharide nanocrystals have raised a great attention ofmany sectors. The studies focus on modification of them and make biodegradablenanocomposites with excellent performance.The main contents of this thesis are as follows:(1) Chitin nanocrystals werederived from chitin, and were surface acetylated, surfaced acetylated chitinnanocrystals were made.(2) Surface acetylated chitin nanocrystals were introducedinto poly (lactic acid), mechanical properties of the nanocomposites were improvedby the interaction between acetylated chitin nanocrystals and the poly(lactic acid)matrix.(3) Polyurethane prepolymer was synthesized, and surface acetylated chitinnanocrystals were introduced into the prepolymer, nanocomposites with goodperformance were made. The creative points of this thesis were as follows:(1) Chitinnanocrystals were surface acetylated, and dispersion of acetylated chitin nanocrystalsin polar solvent were improved, morphology and structure of chitin nanocrystals werereserved after modification.(2) Good dispersion of acetylated chitin nanocrystals inpoly (lactic acid) and polyurethane matrix and the strong interfacial adhesion functionare good for the improvement of mechanical properties of nanocomposites.The main conclusions were as follows:1. Chitin nanocrystals were surface acetylated by acetic anhydride, and testresults showed that chitin nanocrystals were successfully acetylated and morphologyand structure of chitin nanocrystals were reserved after surface acetylation.2. Surface acetylated chitin nanocrystals were used as the filler to modify poly(lactic acid), new nanocomposites were made. Contents of surface acetylated chitin nanocrystals affected the mechanical properties of nanocomposites. Surface acetylatedchitin nanocrystals dispersed well in poly (lactic acid) matrix and formed stronginterfacial adhesion between poly (lactic acid) matrixes, which are good for theimprovement of tensile strength and Young’s modulus, while the existence of rigidnanocrystals caused the decrease of elongation at break of nanocomposites. When thecontents of nanocrystals continue to increase, the existence of rigid nanocrystals andformation of percolation network slightly damage the structure of poly (lactic acid),thus tensile strength and Young’s modulus decreased.3. Surface acetylated chitin nanocrystals were used as the filler to modifypolyurethane, new nanocomposites were made. Surface acetylated chitin nanocrystalsdispersed well in polyurethane matrix and formed strong interfacial adhesion betweenpolyurethane matrixes and the introduction of acetylated chitin nanocrystals promotedthe separation of soft and rigid segment, they all promoted the increase of tensilestrength、Young’s modulus and elongation at break of nanocomposites. With thecontent of acetylated chitin nanocrystals continuing to increase, percolation networkwas formed, which caused the decrease of tensile strength and elongation at break.In conclusion, chitin nanocrystals were surface acetylated by a simple methodin this thesis, and surface acetylated were used to modify the poly (lactic acid) andpolyurethane matrix, and biodegradable material with good mechanical propertieswere made. Beside, relationship between structure and properties of nanocompositeswere analyzed. Not only broaden the modification method of poly (lactic acid) andpolyurethane, but also broaden the application of natural polymers, correspond tonational sustainable objectives and has some practical value.