PLLA With High Ductility And High Transparency:Fabrication And Mechanism During Reactive Blending

Poly(L-lactic acid)(PLLA),as a new biodegradable material,has been used in agricultural plastic film,food packaging,medical products and other fields.It exhibits high strength,good processing properties.However,its inherent brittleness does limit its wide applications.In order to improve its ductility,melt-blending with flexible or elastic polymers with PLLA has attracted much attention.Compared with chemical copolymerization,polymer blends have been widely recognized as a simple,efficient and economical method to develop new polymer materials.Due to the poor miscibility between different components,it is very difficult to prepare PLLA blends with high toughness by simple physical blending.Therefore,the interfacial compatibility between PLLA matrix and flexible or elastic polymer is the key factor to fabricate high performance PLLA blends.In this work,a reactive comb compatibilizer(RCC)was used to enhance the compatibilization of poly(vinylidene fluoride)(PVDF)and poly(L-lactic acid)(PLLA)blends.PLLA with high ductility and transparency were fabricated by blending with tiny amount of PVDF and compatibilizers.The compatibilization and toughening mechanism of the blends were investigated in details.(1)Fabrication of PLLA with high ductility and transparence by blending with tiny amount of PVDF and compatibilizers: PLLA with high ductility and transparence is fabricated by a blending tiny amount of PVDF and RCC.Upon blending,the reaction between terminal carboxyl groups in PLLA and expoxy groups in RCC produces graft copolymer(PLLA-g-poly(methyl methacrylate)(PMMA)),which locates at the PVDF/PLLA interface due to the balanced stress on two sides.On the one hand,the PVDF domain size decreases remarkably with the help of compatibilization,accounting for the excellent transparence.On the other hand,the interface is enhanced significantly,resulting in an activated PLLA layer surrounding PVDF domains.The thickness exhibits higher magnitude because of the high molecular weight of grafted PLLA(relative to premade compatibilizers).During uniaxial tension,the activated PLLA layers(in addition to PVDF)can cause effective energy absorption and dissipation in the case of percolation of them,which is the reason for the better ductility.These results provide an efficient strategy to improve the ductility and transparence simultaneously.(2)Investigation of migration behaviors of RCC in PVDF/PLLA blend: In this work,the localization of RCC(containing PMMA backbone with randomly distributed glycidyl methacrylate(GMA)on it)at the PVDF/PLLA interface has been manipulated by means of GMA contents.At the very beginning of mixing,RCC tends to stay in the PVDF phase due to the miscibility between PVDF and PMMA.Upon further shearing,more and more PLLA chains have been grafted on PMMA backbone,producing PLLAg-PMMA copolymer.The balanced stress on two sides accounts for the localization of compatibilizers at the PVDF/PLLA interface.Finally,the stress of the PLLA side has been enhanced remarkably due to the higher graft density of PLLA,resulting in the enrichment of the copolymer in the PLLA matrix.The migration of RC from the PVDF phase to the immiscible interface and PLLA matrix can be accelerated by employing RC with higher GMA content.Furthermore,the compatibilizer localization produces a significant influence on the morphology and ductility of the PVDF/PLLA blend.Only when the compatibilizers precisely localize at the interface,the blend exhibits the smallest domain and highest elongation at break.Our results are of great significance for not only the fabrication of PLLA with high ductility,but also the precise localization of compatibilizers at the interface of the immiscible blend.(3)Reaction extent: quantitative investigation and its relationship with torque spectrum: The bi-continuous structure of PVDF/PLLA blend was obtained by adjusting the feed ratio.Upon blending,a percentage of PLLA is grafted onto RCC because of the reaction between its terminal carboxyl group and epoxide group in RCC.In the resultant bi-continuous structures,extraction and acid hydrolysis have been employed to distinguish reacted and free(un-reacted)PLLA,in which only free PLLA and all(un-reacted and reacted)PLLA can be removed,producing extraction ratio(ER)and acid hydrolysis ratio(AR)respectively.The graft ratio(GR)of PLLA,calculated based on the difference between AR and ER,represents the reaction extent directly.The comparison of GR and torque indicates that they exhibit the same evolution with not only similar increase/equilibrium regimes but also comparable critical transition time.Therefore,our results confirm that torque spectrum can be used to describe the reaction extent as accurately as graft ratio,which is significant for the basic understanding and applications of reactive processing.