Study On Extensional Flow Induced Crystallization Of Poly(Lactic Acid) Melt

As a typical environmentally friendly biodegradable polymer,polylactic acid(PLA)has been widely used in many application fields like food,pharmaceutical and textile.PLA has many good physical properties including high modulus and good transparency.But at the same time,its inherent defects of slow crystallization speed,poor heat resistance and brittleness have impeded its further application.As for semicrystalline polymers,the macroscopic physical properties of PLA depend deeply on the microscopic crystal structure and morphology.It is expectable that tailoring the crystallization process through external flow field can overcome the material defects.In the past,researchers always tried to improve the mechanical performance of polymers by stimulating the formation of highly oriented shish-kebab superstructure through melt shearing process.However,it still remains a challenge to prepare PLA products filled with uniform and high content shish-kebabs based on melt processing methods.Aiming at the formation and evolution of flow induced shish-kebabs and the mechanism behind it,we have conducted a systematic investigation of PLA melt crystallization behavior under extensional flow by a combination of extensional rheology and in situ synchrotron wide and small-angle X-ray scattering.We have also proved a significant influence of shish-kebabs on the mechanical properties of PLA.The main results and conclusions are summarized as follows:(1)The formation process of shish-kebabs in PLA melt under fast extensional flow were studied.Lightly cross-linked PLA film was prepared by radiation method.According to in situ synchrotron wide and small-angle X-ray scattering,abundant shish-kebabs with high orientation were identified to form with Hencky strain above a critical value of 1.5?2.0,which led to not only tremendous acceleration of PLA crystallization kinetics but also superb heat tolerance of finally solidified sample.With an affine deformation assumption,the generation of shish-kebabs was recognized to require the entanglement network deformation in crosslinked component chains reaching 67%of its maximum elongation ratio,which is higher than 46%of polyethylene.This reveals the reason why the formation of shish-kebabs is difficult in PLA.In addition,we have confirmed a heterogeneous spatial distribution of crystalline polymorphs in shish-kebab body due to its morphological duality.The?-form contributed mainly to the central shish originating from strongly stretched crosslinked component chains,while the?/?'form to the lamellar kebab from moderately orientated uncrosslinked free chains.Dynamic mechanical analysis(DMA)further demonstrated a great contribution of shish-kebabs to the superb heat tolerance of material between T_g and T_m.A close relationship between chains deformation,crystalline polymorphs,crystal morphology and material property was well established.(2)The influences of flow-induced crystal structure and morphology on the mechanical property of PLA was investigated.PLA films with different crystalline states were prepared through quenching the melt after undergoing melt extension.According to WAXD and SAXS measurements,the crystalline states of PLA could be precisely tailored by combing melt extension and rapid quenching procedures.By quenching the melt immediately after extension with Hencky strain of 2.5,PLA film with only?-form crystal was prepared.The?-form crystal mainly contributed to the highly oriented shish entity.Tensile mearsurements showed that such a?-form PLA displayed an excellent elongation and toughness.By increasing the annealing time before rapid quenching,the mechanical properties dropped gradually,recomfirming the efficiency of?-form shish on enhancing the PLA property.Accorging to SEM observation of the fracture surface after tensile tests,the abundant shearing bands and crazing were identified at the surface of?-form PLA,proving that the?-form shish can effectively prevent the growth of tensile cracks and dissipate the deformation energy.This should be the basis of the enhanced mechanical properties.