The Structure-Property Studies For Biodegradable Polymers Under Extension Field

With the deterioration of ecological environment,biodegradable polymers which are environmentally friendly and recyclable have attracted extensive attention.However,most of the biodegradable polymers have poor mechanical properties and cannot meet the practical application requirements.How to obtain biodegradable polymers with high performance is still a great challenge and a difficult problem to be solved.In this thesis,based on polylactic acid and other biodegradable polymers,the relationship between structure and properties under extension field was studied,aiming to obtain high-performance biodegradable polymer materials by adjusting the internal structure.Super-strong and super-toughened PLA-based blends with high heat resistant were prepared by pre-stretching above the glass transition temperature.The structural evolution under extension field and the strengthening and toughening mechanism were also explored.Secondly,the crystallization behavior of cross-linked polycaprolactone under tensile field was investigated by using a tensile machine coupled with a polarizing microscope,trying to reveal the relationship between the crystal morphology and the stress.Finally,high strength polyurethanes were prepared through cycle tensile process,and its structural evolution under extension field was studied by in-situ SAXS measurement.The specific content includes following aspects:1.Through pre-drawing process above the glass transition temperature,supertoughened polylactic acid(PLA)/ethylene-acrylic ester-glycidyl methacrylate terpolymer(EGMA)80/20 binary blend was prepared.The influence of tensile strain on tensile strength,impact toughness and micro-structure was systemically investigated.With the increase of pre-drawing strain,modulus,strength and notched impact strength of PLA/EGMA 80/20 blend gradually increased.Especially through drawing up to 200%at 70℃,the notched impact strength of PLA/EGMA 80/20 blend was improved up to 156.9±2.3 KJ/m2.In addition,the thermal deformation temperature of the material can be significantly increased by pre-stretching at 70℃.The results of scanning electron microscopy(SEM)showed that during pre-drawing above glass transition temperature process,the EGMA phase was highly oriented into nanofibers along the stretching direction,and the PLA lamella was induced perpendicular to the stretching direction,leading to the formation of hybrid shish-kebab structure.EGMA nanooriented fibers are interlinked by PLA lamella to form a unique continuous network structure.SAXS and ultra-small Angle X-ray scattering(USAXS)results proved the existence of oriented shish-kebab.Wide-angle X-ray diffraction(WAXD)test showed that predrawing process above Tg can induce the formation of α crystal of PLA and the crystallization of PE segments of EGMA component.This work provides an effective and simple method for the preparation of PLA materials with excellent comprehensive properties,especially the PLA materials with ultra-high notched Izod impact strength.2.Introducing a small amount of EGMA elastomer into PLA matrix,the PLA/EGMA 95/5 binary blend with super-strong and super tougheness was prepared through pre-drawing at 70℃.The effects of tensile strain on the mechanical strength,impact toughness and micro-structure of PLA/EGMA 95/5 binary blend were systematically studied.The results showed that when PLA/EGMA 95/5 blend stretched at 400%at 70℃,the break strength,modulus and Izod impact strength of the sample were significantly increased to 116.8±2.2 MPa、1.8±0.03 GPa,272.66±8.4 KJ/m2,which were increased by 109%,80%and 142%,respectively,compared with pure PLA.Meanwhile,the PLA/EGMA 95/5 binary blends also exhibited high thermal deformation temperature.SAXS,WAXD and SEM measurements were used to further proved that the improvement of strength and toughness was mainly related to the orientation hybrid shish-kebab and PLA orientation crystal network.Compared with PLA/EGMA 80/20 blends,PLA/EGMA95/5 blends have the higher notch impact strength.This is due to the reduction of EGMA dispersion size increases the specific surface area,leading to the improvement of interface interaction,which is conducive to the transfer of stress and dissipation capacity.3.In order to ensure the complete biodegradability of PLA-based blend material,natural rubber(NR)was selected as the toughening agent.Through melt blending and pre-drawing at 70℃ process,PLA4032D/NR 95/5 binary blend with high strength,super toughness and high thermal deformation temperature was prepared.The notched impact strength of PLA4032D/NR 95/5 blends reached the highest value of 177.8±13.3 KJ/m2 when the strain increased to 100%,then the notched impact strength has little change as the strain further increased.At the same time,the sample prepared through pre-drawing at 70℃ process still maintained excellent thermal deformation temperature.The influence of pre-drawing strain on the microstructure and morphology of the samples was studied by SEM,WAXD and S AXS measurements.It was found that PLA chains and NR phase gradually oriented along the tensile direction with the increase of strain.When reaching a certain strain,PLA molecular chains with higher orientation gradually accumulated into mesophase.Further increasing the strain induced PLA to produce α’ and α crystalline forms.4.Combining the Linkam tensile deformation instrument and an in-situ optical microscope simultaneously,strain-induced crystallization behaviors and changes in tensile stress during crystallization of crosslinked polycaprolactone(cPCL15)were investigated.Rheological method indicated the successfully introduction of crosslinking structure by through electron beam irradiation.cPCL15 samples were first subjected to be stretched at various strains at the molten state,and then subsequently cooled to isothermal crystallization under fixed strains.POM was used to observe the crystallization behavior of cPCL15 under different strains,and it was found that cPCL15 underwent a transformation from point-like nucleation to row-like nucleation and then to oriented network nucleation,revealing the relationship between different crystallization modes and applied strain.In addition,the stress changes during the isothermal crystallization of cPCL15 under different strains were recorded by the tensile deformation instrument,and it was found that the stress changing trend was related to the crystallization modes.The SAXS and rheological tests further proved that the cross-linking network delayed the stress relaxation behavior of cPCL15 samples under higher tensile strain.The mechanism of deformation and crystallization of cPCL 15 under extension was proposed.5.Choosing MDIPTMG、MDIPCL and TDIPTMG three kinds of polyurethane(PU)as the research object,high strength along machine direction and toughness perpendicular to the machine direction polyurethane materials were prepared through cycle tensile process.In situ SAXS measurement was used to research the microstructure evolution of PU during the loading and unloading process.The results showed that the hard phase distributed in the soft phase matrix become oriented along tensile direction,and the orientation structure can exist stably even after removal of external forces,which played an important role to enhancement of strength.In this work,by changing the internal microstructure of polyurethane through cyclic tensile process,a simple and effective method for improving strength of polyurethane was provided.