| Due to the increasing environmental pollution problems caused by petroleum-based plastics,biodegradable plastics are gradually attracting widespread attention.Among the biodegradable plastics,polylactic acid(PLA)is often used in medical,packaging,textile and other fields because of its biodegradability,biocompatibility and good processing properties.However,PLA has the disadvantages of slow crystallization rate,unsatisfactory mechanical properties and high production cost,which limit its application scope.Therefore,it was considered to combine it with high crystallinity,high modulus and high strength starch nanocrystals(SNC)to prepare fully biodegradable composites for the purpose of improving PLA properties.In order to improve the weak interfacial compatibility between PLA and SNC,PLA/SNC composites were prepared by adding polyethylene glycol(PEG)and hydrophobically modifying SNC,and the effects of both methods on the composite properties were investigated.The main findings are as follows:(1)SNC was prepared by acid digestion using waxy corn starch as raw material,and the effect of sulfuric acid concentration on the properties of SNC was investigated.Fourier transform infrared spectroscopy(FT-IR)and X-ray diffraction(XRD)showed that the chemical and crystalline structures of starch remained basically unchanged after acid digestion,and the crystallinity gradually increased with the increase of sulfuric acid concentration.Thermogravimetric analysis(TG)showed that the thermal decomposition temperature of starch broadened after acid-catalyzed hydrolysis,the maximum thermal decomposition temperature increased,the amount of residual carbon increased,and the maximum thermal decomposition temperature increased and then decreased with the increase of sulfuric acid concentration.The sulfuric acid concentration had a large effect on the yield of SNC,which decreased sharply with the increase of sulfuric acid concentration.Transmission electron microscopy(TEM)showed that disk-shaped SNC with a diameter of about 100 nm could be obtained by 3.2 mol/L sulfuric acid hydrolysis.Combining the above results,SNC with a crystallinity of 48.99%,a yield of 10.8 wt%and a particle size of about 100 nm can be obtained by hydrolyzing waxy corn starch with 3.2 mol/L sulfuric acid.(2)The PLA/SNC composites were prepared by solution blending,and the effect of PEG addition on the properties of PLA/SNC composites was further investigated.The mechanical tests showed that the tensile strength of PLA/SNC4 composites increased from 45.39 Mpa to 47.22 Mpa and the elongation at break decreased from 8.70%to 7.83%compared with PLA,and the mechanical properties of the composites became worse with the increase of SNC content.After adding PEG,the tensile strength of PLA/SNC4/PEG composites slightly increased and then decreased with the increase of PEG content,and the elongation at break kept increasing.Among them,the PLA/SNC4/PEG2 composites had the best mechanical properties with the tensile strength and elongation at break of 47.86 Mpa and 10.20%,respectively.Differential scanning calorimetry(DSC)analysis showed that no crystallization peak was present for PLA,and the crystallization rate of PLA/SNC and PLA/PEG composites increased significantly with the increase of SNC and PEG contents.With the synergistic effect of both,the crystallization rate of PLA/SNC4/PEG4 composites reached the maximum compared with PLA/SNC4 and PLA/PEG4 composites with Tp and XC of 109.6℃ and 30.18%,respectively.XRD showed that the addition of PEG had no effect on the crystalline form of PLA/SNC composites.Polarized light microscopy(POM)showed that both SNC and PEG can improve the crystallization ability of PLA,with the greatest effect when the two were synergistic.Scanning electron microscopy(SEM)showed that PEG improved the interfacial compatibility between SNC and PLA.(3)The PLA/A-SNC composites were prepared by modifying SNC with silane coupling agent.XRD showed that the crystalline form of the modified composites did not change and the crystallinity decreased.Mechanical tests showed that the mechanical properties of PLA/A-SNC composites were improved compared with PLA/SNC,among which,PLA/ASNC4 composites had the best mechanical properties with tensile strength of 49.14 Mpa and elongation at break of 8.26%.POM shows that PLA/A-SNC composites have smaller spherical crystal size,higher density and higher crystallization rate compared to PLA/SNC.SEM shows that PLA/A-SNC composites have stronger interfacial bonding compared to PLA/SNC. |
