Fabrication Of Thermoplastic Starch With "soft And Hard"characteristics And Their Effects On The Performance Of Biodegradable Polyester

To solve the“white pollution”evoked by traditional polyolefin polymers and reduce dependence on petrochemical resources,the best effective strategy is developing novel bio-based and biodegradable polymers to replace them.Due to its merits in cheap,renewable and biodegradable,starch is widely utilized in many fields.However,although the processability of thermoplastic starch（TPS）is promoted to improve after plasticizing modification of native starch,TPS still displays some defects,such as brittleness,mechanical properties varied by humidity sensitively,high hodrophilicity and low thermal stability,which restricts the application of TPS in biodegradable plastic.To improve the performance of TPS and extend its application,the reactive extrusion was used to“stiffen”（to improve T_g）and“soften”TPS（to obtain rubber like elastomer）via coordination reaction and acidification,respectively,and the“softened”TPS was melt extruded with PLA and PBS respectively to fabricate polyester based fully biodegradable blends.A new coordination supramolecular interactions“stiffened”thermoplastic starch（TPS-ZAs）was developed using reactive extrusion for the mixtures of starch,glycerol and anhydrous zinc acetate.Various characterizations were used to investigate the mechanism of coordination reaction and the effects of coordination supramolecular interactions on structure and performance of TPS-ZAs.The results revealed that Zn²⁺mostly coordinated with the oxygen atoms of 3O-H of starch via an exchange process during reactive extrusion and the molecular weight of starch decreased.TPS-ZAs showed interfacial and mechanical improvements compared with crude TPS,including low wettability,good shape memory,high heat resistance and high strength.The surface contact angle increased from 55.9°for pure TPS to 118.2°for TPS-ZAs,the shape recovery ratio increased from 50%and 27%for pure TPS to 60%and 50%for TPS-ZAs during the first and second test cycles,respectively,and T_g increased from 48.1℃for pure TPS to 89.1℃for TPS-ZAs.The high hydrophobicity of TPS-ZAs is surprising considering that ZA added in the TPS is high soluble in water.Morphology characterizations revealed the improvements were related to the micro/nano hierarchical architectures induced by coordination-driven self-assembly.The results of this chapter revealed that the introduction of Zn²⁺-starch coordination supramolecular interactions can“stiffen”TPS and significantly improve its performance.To investigate the effect of elastomer like TPS on the performance of PLA,tartaric acid（TA）,glycerol and starch were firstly extruded to fabricate"softened"TPS-TA.Then TPS-TA with PLA at different ratios were extruded to obtain PLA/TPS-TA（P/T-TA）blends.The effects of TA and TPS-TA contents on the compatibility,mechanical properties and folding resistance of P/T-TA were investigated in detail.The results revealed that TA acid hydrolyzed starch and improved the melt flow index of TPS.Consequently,TPS presented a“softening”virtue with low modulus and high ductility.Interestingly,the dispersion of TP-TA and the compatibility of phases were improved and the average particle diameter of TPS-TA in P/T-TA20 decreased from 2.87μm for TPS to 0.71μm.The unnotched impact strength of P/T-TA20 was 38.4 KJ/m² and the value was 2.6 times when PLA was used as the control.The results of folding endurance test showed that both P/T-TA20 and P/T-TA30 folded more than 1000 times without fracture,while the value of pure PLA was only 16 times.The mechanism was ascribed to that TPS-TA could induce the shear yield of PLA matrix and promote PLA to orient and fibrillate during folding process,which dissipated the shearing work in time.SEM results exhibited that an incomplete shish-kebab structure was formed in the folding subcenter region of P/T-TA20.The results demonstrated that the incorporation of TPS-TA could improve the toughness of PLA,especially for the folding endurance.Cellulose nanocrystal（CNC）was used as a reinforcement filler to further modify TPS-TA（TPS-TA-CNC）.The selected TPS-TA-CNC（0.5 wt%CNC）,which presented the best mechanical properties,was reactive extruded with PBS at different ratios to obtain PBS/TPS-TA-CNC blends（PBS/T-TA-C）.Effects of TA and TPS-TA-CNC contents on the morphology and properties of blends were evaluated by SEM,DMA,DSC and XRD.They revealed that TA improved the compatibility and the dispersion of TPS.The average dispersion size of TPS-TA-CNC decreased from 2.99μm for pure TPS to 0.50μm.Moreover,the hydrogen bonded cross-linking networks with soft coat and stiff core could be formed between TPS-TA and CNC.Therefore,the notched impact strength of PBS/T-TA-C30 significantly increased to 6.0 KJ/m² and the improvements were 76.5%and 114.3%compared with that of PBS/T-TA30 and PBS/T30,respectively.This value also even higher than that of pure PBS（5.3 KJ/m²）.However,the addition of dispersed phases restricted the crystallization of PBS matrix.These results demonstrated that the incorporation of TPS-TA-CNC（30 wt%）could improve the toughness of PBS with lower cost.