Studies On Preparation And Properties Of Polylactide/Polydopamine Composites

Bio-based degradable polymer materials can be derived from animals and plants without relying on increasingly scarce petroleum resources,and can be decomposed by soil and enzymesto,which can reduce environmental pollution.These materials conform to the current trend of global environmental protection of products.Therefore,they have received extensive attention and research.Polylactide（PLA）is one of the most promising bio-based degradable polymer materials.It has the advantages of wide source,good biocompatibility and biodegradability,but it also has the defects of insufficient mechanical properties,poor thermal stability and long degradation cycle,which limits its application field.The preparation of PLA composites by blending and copolymerization is a relatively straightforward and simple method to solve the above limitations.Dopamine（DA）is a kind of multifunctional compound with biological origin and multiple reactivity.Under alkaline conditions,it can be oxidized,polymerize and self-assemble to form nano-scale controllable polydopamine nanoparticles（PDA NPs）,which is not only rereactive,but also can be used as an organic nano-filler to improve the comprehensive properties of the composites and endow the materials with other properties,such as UV-shielding property.Therefore,PLA was selected as the substrate in this paper,which was compounded with PDA NPs by casting to prepare a series of polylactide/polydopamine composites with different structures.The mechanical,thermal,degradation and UV-shielding properties of the composites were investigated.Firstly,PDA NPs with different particle sizes（140-550 nm）were successfully prepared by oxidizing,self-polymerization and self-assembly of dopamine in the medium with different pH values regulated by ammonia.And PDA NPs were introduced into PLA to prepare PLA/PDA nanocomposites by casting.By changing the amount of PDA NPs,the overall properties of the composites,especially the degradation properties,can be controlled.The degradation test results showed that the degradation rate of PLA/PDA nanocomposites were relatively slow at first.When the PDA NPs inside the composites were exposed to the surface due to the degradation of PLA on the surface,the degradation rate was obviously accelerated,and the degradation rate reached 95.3%at the highest after 20 days,which was much faster than that of pure PLA（the degradation rate was about 40%after 20 days）.At the same time,the addition of PDA NPs can improve the UV-shielding performance of the composites,and the shielding performance of the composites against UVA and UVB can be significantly improved at a small amount（0.5 wt%）,up to 65.98%and 71.49%,respectively,which is nearly double that of pure PLA.In order to fully understand the effect and mechanism of PDA NPs on the degradation of the composites,the unique degradation behavior of PDA NPs in alkaline environment was investigated in detail using PDA_0.5.5 as the model.The effects of pH and temperature on the degradation of PDA NPs were studied.It was found that PDA NPs was degraded to form small PDA nanosheets with a small amount of free monomers and oligomers.The degradation rate of PDA NPs would be accelerated by the increase of pH value and temperature,but it has little effect on the morphology of the degradation products.It is further indicated that the degradation of PDA NPs is a process of gradual depolymerization ofπ-πstacking,hydrogen bonding formed during self-polymerization and self-assembly,and the degraded PDA fragments will not undergo secondary polymerization.In order to improve the dispersion and compatibility of PDA NPs in the PLA matrix,PDA NPs were used to initiate ring-opening polymerization of lactide（LA）to prepare polylactide grafted polydopamine（PDA-PLA）composite nanoparticles.The graft length of PLA on the surface of PDA NPs was adjusted by adjusting the amount of LA,and PLA/polylactide grafted polydopamine（PLA/PDA-PLA）nanocomposites were prepared.The mechanical properties,thermal properties and UV-shielding properties of PLA/PDA-PLA nanocomposites were studied.The results showed that PDA-PLA composite nanoparticles reduced the cold crystallization temperature（T_c）of PLA and promoted the crystallization of PLA.And benefiting from the improved compatibility of PDA-PLA composite nanoparticles with PLA,the nanoparticles can be uniformly dispersed in the PLA matrix,which promoted the PDA-PLA composite nanoparticles to achieve better enhancement and toughening effects on PLA at the same time,with the tensile strength and elongation at break reaching up to 78.37±0.54 MPa and 58.33±4.50%.At the same time,the addition of PDA-PLA composite nanoparticles can also significantly improve the UV-shielding performance of the composites.Moreover,because the PLA chain segments on the surface can form ordered crystalline regions around PDA-PLA composite nanoparticles,the effective shielding regions of the composite nanoparticles were expanded,and the UV-shielding performance of PLA/PDA-PLA nanocomposites is obviously better than that of PLA/PDA nanocomposites.With very little addition（0.5 wt%）,the shielding property of the composites against UVA and UVB was more than twice that of pure PLA,and the highest shielding rate was 92.02%and 94.41%,respectively.After further increasing the content of PDA-PLA composite nanoparticles to 1 wt%,the shielding rate of the composites against UVA and UVB was greater than 99%.Therefore,the prepared PLA/PDA-PLA nanocomposites have potential applications in the fields of packaging materials and agricultural materials.In summary,by adding PDA NPs or PLA-grafted PDA NPs to the PLA substrate,the UV-shielding property of the composites can be imparted,and the degradability of the composites can be effectively controlled or the mechanical properties can be improved,which increasing the added value of PLA-based composites and broadening their application fields.And this provides a new direction for the preparation of PLA-based composites with excellent properties,which has certain theoretical and practical significance.