Study Of The Intrinsic Deformation Mechanism Of IPP Oriented Lamellar Stacks

With the development of industry and improvement of people’s living level,polymer films do play a more and more significant role in energy,environmental protection and aerospace and other fields.Due to the multiple step,multi-processing parameter and multi-scale structure characteristics of semicrystalline polymer processing,how to produce the high-performance semicrystalline polymer films is still the unsolved technical and scientific problem in industry and academia.Almost all the processings of semicrystalline polymers involve the mechanical and structural responses of melt and crystal under different external fields(i.e.,shear and stretch field,temperature and humidity field).Due to the nonequilibrium nature of processing,it is hard to figure out the relationship between nonlinear mechanical behavior and crystal deformation and transition during post-stretching processing for precursor films or fibers.Although engineers and scientists have obtained a lot of experimental evidence and constructed some theoretical model after nearly one hundred years of efforts,it still remains controversial on the intrinsic deformation mechanism of specific scale structure in semicrystalline polymers.Therefore,in order to figure out the intrinsic deformation mechanism,the nonlinear mechanical behaviors and structural evolutions of specific-size structure in semicrystalline polymers,i.e.,oriented lamellar stacks,during uniaxial stretching are related together in this thesis.This job will provide theoretical foundation for studying the deformation behaviors under triaxial stress of the larger-scale structures like spherulites in the future.Based on the industrial background on dry process for producing isotactic polypropylene(iPP)microporous membrane and with the aids of ultrafast synchronous radiation small and wide angle X-ray scattering(SR SAXS/WAXD),differential scanning calorimetry(DSC),dynamic mechanical thermal analysis(DMTA)and scanning electron microscopy(SEM),the key processing parameters and structural parameters for making microporous membranes are analyzed in details.Besides,the multi-scale structural evolution in hard elastic precursor films,which are composed of oriented lamellae,are tracked with the coupled external fields’effects of stretching and temperature.This job will deepen the understanding on the related scientific problem on nonlinear mechanical behaviors in semicrystalline polymers.The main results and conclusions are summarized as follows:(1)Focusing on the preparation of iPP microporous membranes,the structure and mechanical behavior of isotactic polypropylene casting films prepared at different processing conditions are systematically studied with SAXS/WAXD measurements as well as tensile test.Microporous morphologies are also detected by SEM techniques to check the ability of precursor films for pore formation.The key structure and processing parameters of precursor films are defined after data processing and analysis.Among all the structural parameters obtained from X-ray scattering measurements,thickness of amorphous layer and the orientation of lamellar stack show critically positive correlations with the formation of micropores.Draw ratio and cast roll temperature are revealed as the key processing parameters,which determine orientation and thickness of lamellar as well as amorphous layer,respectively,i.e.,key structure parameters for pore formation.Considering the coupling effects of tight folds and lamellar-fibrillar transformation,a semi-quantitative deformation model for the formation of micropore is proposed to correlate surface porosity with key structural parameters of precursor film.(2)The mechanical instability of hard-elastic iPP films with highly parallel lamellar stacks is studied with in-situ ultrafast SR SAXS and WAXD techniques during cyclic tensile deformation.Unexpectedly,the micro-strain,the relative variation of lamellar periodicity,shows an accelerated increase at the onset of instability and reaches values larger than the corresponding macro-strain after yield.Meanwhile,no irreversible plastic destruction of crystal is observed.Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXD measurements,stress-induced spontaneous microphase separation of interlamellar amorphous phase is proposed to take responsibility for yield behavior and the hyperelasticity,which stems from the heterogeneous distribution of tie chain/trapped entanglement in interlamellar amorphous nano-layers.This reversible stress-induced non-equilibrium phase transition of interlamellar amorphous phase is different from current plastic deformation models with crystal destruction in semi-crystalline polymers but in line with the nearly 100%elastic recovery ratio of hard-elastic films.(3)The effects of temperature on the non-linear mechanical behaviors of hard-elastic iPP films are systematically studied with in-situ ultrafast SR SAXS/WAXD during uniaxial tensile deformation at temperatures from 30 to 160 ℃.Based on the mechanical behaviors and structural evolutions in strain-temperature space,three temperature regions(Ⅰ,ⅡandⅢ)are clearly defined with the a relaxation temperature(Tα≈80 ℃)and the onset of melting temperature(Tonset≈135 ℃)as boundaries,respectively,where different mechanisms dominate the nonlinear deformations after yield.In region I,micro-strain in lamellar stacks εm obtains an accelerated increase after yield and reaches a value significantly larger than corresponding macro-strain ε,during which neither slipping,melting nor cavitation occurs.Stress-induced microphase separation of interlamellar amorphous is able to account for the hyperelastic behavior in region I.Above Ta in region II,due to the reduced cohesive strength and enhanced chain mobility,the irreversible reduction of crystallinity and the formation of slender cavities suggest that crystal slipping overwhelms microphase separation and plays the major role in nonlinear deformation,during which chains in lamellar crystals are pulled out and recrystallize into nanofibrillar bridges.In region III,melting-recrystallization dictates the nonlinear deformation.A schematic roadmap for structural evolution is constructed in strain-temperature space,which may guide the processing of microporous membranes for Lithium battery separators as well as other high performance polymer fibers and films.(4)Considering that lamellar stack composed of alternatively arranging lamellar crystal and amorphous layers is one typical hard-soft laminated nano-composites,microbuckling instability of lamellar stacks is first proposed as a new deformation mechanism to trigger the nonlinear mechanical behaviors in semicrystalline polymers,due to Poisson contraction effects during uniaxial tensile deformation along the layer normal as reported in other composites.Based on the non-equilibrium process of crystallization and experimental observations,a three-phase theoretical model with lamellar stack(crystal and interlamellar amorphous)and the matrix amorphous layer is proposed to be the deformation unity in semicrystalline polymers.Based on the three-phase model and the proposition of buckling with shear mode,we deduce the theoretical critical strain for the sinusoidal microbuckling through linear stability analysis method.Taking hard-elastic isotactic polypropylene as an example,the theoretically calculated critical strain is in a good agreement with the experimental critical strain at temperatures below a relaxation temperature Ta.These results suggest that elastic microbuckling is indeed a possible mechanism to trigger nonlinear instability,which is different from current plastic deformation models with crystal destruction around yield in semicrystalline polymers.