Preparation And Properties Of Polylactic Acid/Nano-Cellulose/Layered Double Hydroxide Composite Materials

Polylactic acid?PLA?is a kind of biodegradable polyester produced from renewable plant resources.It can be used in many fields such as automotive,electronics,food packaging,biomedicine and so on.It is one of the most promising compounds which plays an important role in the biopolymer field.Unfortunately,PLA has high brittleness,poor heat resistance and poor barrier properties,which limit its application in specific scenarios.Therefore,it is necessary that adding functional particles and reinforcing fibers to PLA matrix to obtain PLA-based composite materials with specific properties.Considering the enhancement properties of nanocellulose and the functional characteristics of Layered Double Hydroxides?LDHs?,PLA/nanocellulose/LDHs composite materials were prepared by using PLA as matrix,nanocellulose and LDHs as filler particles.The effects of nanocellulose and LDHs on the properties of PLA materials were systematically studied,provided theoretical basis and technical support to the performance enhancement and functional utilization of PLA.In this thesis,the fluorescence of different types of LDHs was qualitatively analyzed by laser confocal microscopy.And the fluorescence was used to trace the distribution of LDHs in composites.The results showed that LDHs could fluoresce in the absence of intercalation or adsorption of fluorescent substances,and the fluorescence of LDHs could be used to achieve intuitive and accurate localization of LDHs in composite materials.The PLA membranes were aminolyzed with 1,6-hexanediamine to obtain PLA membranes with free amino groups on the surface.Mg-Al-Eu LDHs and Mg-Al LDHs were exfoliation in formamide solution to obtain positively charged LDHs nanosheets.PLA/?CNCs/LDHs?_n composite films were prepared by layer-by-layer assembly using aminolyzed PLA films as substrates based on electrostatic interactions between LDHs nanosheets and negatively charged sulfuric acid hydrolyzed cellulose nanocrystals?CNCs?.The assembly process,surface morphology,mechanical and fluorescence properties of PLA/?CNCs/LDHs?_n composite films were investigated.The results showed that the assembly process is controllable and the surface of composite membranes were smooth,dense and continuous.The composite materials showed tunable fluorescence performance and better mechanical properties,expand the application of PLA in optoelectronic devices and anti-counterfeiting packaging.Which also provided new ideas for the construction of biopolymer composite materials.The TEMPO oxidized cellulose nanofibers?TOCNs?and LDHs were used as dispersing agents to prepare pickering emulsion,and then PLA/TOCNs/LDHs composite membranes were prepared by hot pressing.And the chemical and crystal structures,the thermodynamic and barrier properties of the composite membranes were analyzed.The results showed that the pickering emulsion prepared with TOCNs and LDHs as dispersing agents had uniform droplet and particle size.Compared with pure PLA films,the PLA/TOCNs/LDHs composite films prepared by drying and hot pressing with pickering emulsion have improved thermal stability,mechanical and barrier properties.And sodium borohydride was used as a reducing agent to convert the chromophore groups into colorless groups,which increased the UV transmittance of TOCNs.Then,PLA-based composite membranes with reduced TOCNs were prepared,the coking yellowing condition was improved after hot pressing treatment.2,3-dihydroxy-6-sulfonate?DNSA?and sodium 2,7-dihydroxynaphthalene-3,6-disulfonate?DNDA?were introduced into MgAl-NO₃-LDHs by intercalation assembly to prepare MgAl-DNSA-LDHs and MgAl-DNDA-LDHs.And the crystal structure and thermal stability of MgAl-DNSA-LDHs and MgAl-DNDA-LDHs were investigated.Then,PLA/TOCNs/MgAl-DNSA-LDHs and PLA/TOCNs/MgAl-DNDA-LDHs composite membranes were producted by spin-coating,and the UV resistance of the composites were investigated.The results showed that both DNSA and DNDA anions were successfully inserted into MgAl-NO₃-LDHs,and a strong interaction with LDHs layers was established,which enhanced the thermal stability and UV absorption propertiesof LDHs.PLA/TOCNs/MgAl-DNSA-LDHs,PLA/TOCNs/MgAl-DNDA-LDHs and PLA/TOCNs/MgAl-DNSA-LDHs/MgAl-DNDA-LDHs composite membranes obtained by spin-coating exhibit excellent UV photostability and prevent the UV degradation of PLA.