Study On Morphology Development And Mechanism Of Polylactic Acid (PLA)-based Multiphase System Via Elongational Flow

Polymer processing flow field plays a significant role for microstructure and propertiesof polymer materials. Screw is regarded as the core part for traditional polymer processingdevices, and processing methods are based on the shear flow field. Unlike screw processingdevices, vane plasticizing conveying equipment invented by Prof. Qu, which generateselongational deformation, inaugurates a new type of polymer processing device. It has owncharacteristics in polymer processing technology. Compared with the traditional screwprocessing equipment, the unique advantages of vane plasticizing conveying equipment aresmall in size, and low energy consumption and the wide adaptability for materials processing.Therefore, combined with the structure of vane plasticizing conveying equipment and theintrinsic characteristics of polymer materials, to deep study effects of the volume elongationalflow field on the crystallization and orientation of polymer materials, and dispersion mixingof polymer composite system, to systematically analyze polymer processing principle ofvolume elongational flow field for promoted the development of polymer materials, advancedprocessing technology and equipment have the vital significance.Vane extruder (VE) dominated by volume elongational flow field and/or twin-screwextruder (TSE) dominated by shear flow field plasticized and conveyed polylactic acid (PLA)material, respectively. The shock cooling, disassembling and sampling method was used forobservation of the crystalline form and orientation of PLA of extruder each section. Wideangle X-ray diffraction (WAXD) and polarizing microscope (POM), differential scanningcalorimeter (DSC) were adopted to test crystalline form, crystallization properties and crystalmorphology of PLA, small angle X-ray scattering (SAXS) was used to observe orientationbehaviors of PLA of the different set of vane plasticizing and conveying unit (VPCU). Theresults showed that: compared to those of TSE dominated by shear flow field, the PLAproduced by VE based on volume elongational flow field generated obvious β crystal andincreased crystallinity. Moreover, in view of crystal morphology based on VE, spherocrystalof PLA of rich solid state appeared as large size, and the spherical crystal of high integrity,that of rich melting state presented a number of smaller size, and that of the melting statepresented amount of small and uniform spherical crystal. In addition, SAXS patternspresented that orientation degree of PLA sampled from melting state of the VE was obviouslyhigher than those of rich solid state and rich melt state. These results indicate that VEdominated by the volume elongational deformation has strong induced crystallization capacity,compared with TSE dominated by shear deformation; more longer time elongational deformation, more strong orientation behavior. The elongational orientation behavior couldurge ordering arrangement of molecular chains materials and make insitu fibrillation.VE based on volume elongational flow field and/or TSE based on shear flow field wereused to prepare polylactic acid/thermoplastic polyurethane (PLA/TPU) blend for fixedcomponent ratio, respectively. The shock cooling, disassembling and sampling method wasused for in situ observation of dispersed phase morphologies and theirs evolution ofPLA/TPU blend derived from different segment of VE and/or TSE. Meanwhile, PLA/TPUblends for different component ratio were prepared by VE and/or TSE, and theirs dispersedphase morphology, crystalline properties and mechanical properties were observed andmeasured, respectively. High resolution scanning electron microscope (SEM) was adopted toobserve the dispersed phase morphology of PLA/TPU blends, WAXD, POM and DSC wereused to test crystalline form, crystallization properties and crystal morphology, respectively.Dynamic thermal mechanical analysis (DMA) was used to measure compatibility ofPLA/TPU blends. The results showed that: compared to that of TSE dominated by shear flowfield, dispersed phase morphologies of PLA/TPU blends prepared by VE had more apparentfibrous structure, and more uniform particles size. And PLA/TPU blends prepared by VE hada higher degree of crystallinity and better compatibility than that of TSE. These resultssuggest that insitu fibrillation capacity of polymer compound via elongational flow field hassuperior to that of TSE via shear flow field, its strong the mass and heat transfer ability canpromote the compatibility and improve the mechanical properties of the immiscible polymermaterials, which provide a novel and feasible polymer processing method.VE dominated by volume elongational flow field and/or TSE dominated by shear flowfield were used to prepare polylactic acid/mica (PLA/Mica) composites, respectively.Analysis of dispersed phase morphology, crystalline properties and mechanical propertiesfound that: mica particles in the composites prepared by VE had more uniform dispersionthan that of TSE. Compared with that of TSE, the degree of crystallinity showed a slightincrease. Eventually, mechanical properties of PLA/Mica composites prepared by VE weresuperior those of TSE. These results infer that VE via the volume elongational flow field hasthe better dispersive mixing effect, shorter mechanical thermal process, more strong theorientation effect and lower structure damage than TSE via shear flow field. The uniqueprocessing features will advance the processing device and technology of polymer material.Using silane coupling agent and titanate coupling agent modified Mica surface, thepreparation of the unmodified and/or modification Mica of PLA/TPU/Mica multiphasesystem were prepared by VE based on volume elongational flow field, respectively. Fourier transform infrared spectrometer (FTIR) was used to measure the Mica surface graftingcharacteristic, transmission electron microscopy (TEM) was adopted to carefully observedispersed phase morphology of the PLA/TPU/Mica multiphase system and distribution ofMica particles, DMA was used to characterize compatibility of the multiphase system, WAXD,POM and DSC were used to test crystal form, crystallization properties and crystalmorphology of the multiphase system, respectively. In addition, the relationship of betweenphase morphologies of the multiphase system and stress-strain curves was established. Theresults showed that: Mica particles were more likely to tend to distribution in thethermoplastic polyurethane phase, compatibility of the multiphase system for Mica modifiedby coupling agent were successfully improved. Moreover, crystallinity of the multiphasesystem were also a certain amount of increase, and the mechanical properties were slightincrease. In addition, a large number of long fiber structure of the multiphase system couldsignificantly increase the tensile stress and strain. These results indicate that: PLA/TPU/Micamultiphase materials based on volume elongational deformation with chemical modificationhave more strong adsorption than that of the multiphase system without surface modification.Finally, stability and security of the multiphase materials application are further promoted.Combination of the structure of VPCU dominated by volume elongational deformationand the corresponding experimental results, the special physical models for effects of thevolume elongational deformation on induced crystallization and orientation behavior and theevolution of dispersed phase morphology were established. These models explained that thevolume elongational deformation had more easier to accelerate ordering of molecular chainand insitu fibrillation of dispersed phase; and strong capacity of heat transfer and masstransfer could remarkably improve the disperse efficiency of polymer materials. The resultsfor the volume elongational deformation processing technology could prove an experimentand mechanism basis for development and promotion of advanced polymer processing.