Toughening Polylactide With Plant Oil-based Elastomers

Biodegradable polymers,have attracted increasing interest due to the increased resource crisis and environmental concerns associated with traditional petroleum-based.Polylactide(PLA)is regarded as one of the most promising biobased and biodegradable polymers as it combines abundant advantages such as excellent biodegradability,biocompatibility,high mechanical strength,high melting temperature and easy processibility.However,PLA has some significant shortcomings,such as the inherent brittleness,which have largely restricted the wide applications of PLA.Therefore,aiming at that defect of PLA,this article provides a facial and cost-effective way to toughen PLA,i.e.using plant oil based elastomers produced by plant oils via dynamic vulcanization to toughen PLA.Plant oils as abundant natural products are thought as ideal feedstocks to various sustainable polymers,because they are composed of triglycerides with various active functional groups.Thereinto,dynamic vulcanization of castor oil with polyhydroxy groups in the presence of4,4'-diphenylmethane diisocyanate(MDI)in a polylactide matrix was performed to fabricate a tough blend composed of PLA and castor oil based polyurethane(PLA/COP).SEM analysis showed a phase-separated morphology with castor oil based polyurethane(COP)as a dispersed phase and PLA as a continuous phase.The content of COP in the blends played an important role not only in the size of dispersed COP particles but also in the mechanical properties,rheological and crystallization behaviors of the formed PLA/COP blends.The size of dispersed COP,melt viscosity,and storage modulus of the blends increased significantly with COP content.The crystallization rate of PLA was enhanced by incorporation of COP due to the increased nucleation effect arising from interfacial nucleation of the phase-separated blends.The thermal stability of the blends was slightly reduced compared to neat PLA according to TGA.Tensile test exhibited that the elongation at break of PLA was considerably increased by 45 times to 338%,compared to 7.5% of neat PLA,with theaddition of only 5 wt% COP;meanwhile,the mechanical strength and modulus were largely retained.Cavitation,arising from dispersed phase debonding from the matrix,induced matrix plastic deformation was the toughening mechanism for the present PLA/COP blends demonstrated by SEM of tensile fracture surfaces.Dynamic vulcanization of polylactide with castor oil(CO)and three different diisocyanates,namely 4,4'-diphenylmethane diisocyanate(MDI),hexamethylene diisocyanate(HDI)and isophorone diisocyanate(IPDI),was performed to study the effect of diisocyanate type on the vulcanization process and on the morphology as well as mechanical properties of the PLA/COP blends.The reactivity of the three diisocyanate followed the order of MDI > HDI > IPDI when reacting with castor oil during dynamic vulcanization.Interfacial compatibilization between PLA and the CO-based polyurethane occurred when the less reactive HDI and IPDI was used identified by 1H NMR.Among all the blends,PLA/CO-IPDI showed the finest morphology and the best toughening efficiency.Incorporation of 20 wt% CO-IPDI increased the elongation at break and notched impact strength of PLA by 47.3 and 6.6times,respectively.Cavitation induced matrix plastic deformation was observed as the toughening mechanism for the PLA blends with CO-based polyurethane.The effect of CO-IPDI content on the morphology and mechanical properties of PLA was studied in detail.The particle size of dispersed CO-IPDI and the elongation at break increased gradually,the tensile strength and Young's modulus decreased gradually,while the impact strength first increased and then decreased with increasing CO-IPDI content.The maximum impact strength of 222.9 ± 12.3 J/m appeared for the blends with 20wt% CO-IPDI.Furthermore,a series fully biobased PLA/VESO(W/W,80/20)blends were prepared by dynamic vulcanization of PLA with sebacic acid(SA)cured epoxidized soybean oil(ESO)precursors(SEP)in the torque rheometer.Interfacial compatibilization occurred through either chain-link reaction or transesterification between PLA and SEP and the chain-link reaction is proved much more efficient.The chemical structure of VESO,controlled by carboxyl/epoxy equivalent ratio(R),plays a critical role in the compatibility,morphology,and toughening mechanism and effect of the PLA/VESO blends.The compatibility decreases meanwhile the size of dispersed VESO particles increases with increasing R.High toughening effects are realized when the R is in the range of 0.3~0.5.The blend with R of 0.3 shows maximum tensile toughness of 150.6 MJ/m3,compared to 2.3 MJ/m3 of neat PLA andthe blend with R of 0.4 exhibits super toughness with impact strength of 542.3 J/m,compared to 34.1 J/m of neat PLA.The VESO chemical structure dependence of reaction mechanism,morphology,mechanical properties,and toughening mechanism and effect of the PLA/VESO blends was investigated in detail in this study.