Semicrystalline polymers finding wide applications in packaging,energy storage devices,etc.are mostly processed through stretching under various fields that modify their structures and final properties.Stretch-induced structural evolution especially ofpre-oriented.semicrystalline polymers has remained a challenge due to complex distribution of stresses among amorphous tie-chains and entanglements in interconnection with crystalline lamellae.On the other hand,though it becomes challenging to describe the structural evolution of semicrystalline polymers due to their multi-scale structure,such structure is beneficial for their wide applications since properties can be adjusted based on the size and arrangement of crystalline and amorphous phases.In this thesis,we used in-situ small-/wide-angle X-ray scattering（SAXS/WAXS）to study real-time uniaxial stretch-induced structural evolution of preoriented isotactic polypropylene（iPP）films.Later,the ultra-high molecular weight polyethylene and poly（4-methyl-1-pentene）（UHMWPE/PMP）blend films for Li-ion battery（LIB）separators were studied.These were processed by sequential biaxial stretching and have good thermal stability and excellent electrochemical performance.The major points marking the contribution of this work are underlined here.In-situ SAXS/WAXS is found a powerful technique to track the structural evolution induced by stretching.The stretch-induced cavitation in stretched iPP films was analyzed for sample processed at low（DR 10）and high（DR81）draw ratios.Combining structural parameters such as crystallinity,long period and orientation along with variation of relative mean intensities,the cavitation is initiated just after the yield point for DR 10.The initial cavities whose long axis is in equator,become extended in meridian direction in relation to the formation of fibrils and orientation of polymer chains after crystal destruction.However,the possibility of cavitation is limited at low strains for DR81.Instead,this may be formed at large strains when originating from low-density amorphous phase.In-situ SAXS/WAXS is also employed to study the stretch-induced structural evolutions of four different pre-oriented iPP cast films during uniaxial stretching at room temperature.A deformation model considering the effect of orientation is elaborated.The mechanical properties and structural evolutions show distinct features with the improvement in orientation of initial structure.For low-oriented films,the tensile stress induces the amorphization of lamellar crystals parallel to stretching direction,while the shear stress triggers the shearing of lamellar stacks at other angles through crystal slipping.For high-oriented films,the elastic deformation mode dominates as the normals of all lamellae are along stretching direction.Under tensile stress,micro-buckling or bending replaces the lamellar shearing to trigger the initial deformation instability of crystals.The stress-induced amorphization of lamellar crystals might also occur at large strains.This study uncovers the different structural evolution roads of low-and high-oriented films,which will aid to deepen the understanding of the orientation effects on the deformation mechanisms of other semicrystalline polymers.The advances of Li-ion batteries for energy storage continuously face major challenges like acquiring high performance,stability and safety.A separator plays major role in the safety and performance of batteries through its porous structure and interaction with electrolyte.In another work,the suitable microstructure of ultra-high molecular weight polyethylene/poly（4-methyl-l-pentene）（UHMWPE/PMP）blend film is achieved by tuning PMP content through sequential biaxial stretching.This results in significant increase of air permeability,porosity,wettability,electrolyte uptake and ionic conductivity with PMP content.A high thermal stability with low shrinkage of 0.7%in TD and 1.6%in MD is observed after exposing the film with 7.5 wt%of PMP（PM3）at 120℃ for 1h.Most importantly,a cell containing PM3 presents an excellent electrochemical performance with highest discharge capacity of 172.8 mAh g-1 at 0.1 C-rate and an efficient stable cyclic performance with coulombic efficiency of 99.89%over 100 cycles at 1 C-rate.Apart from existing blend separators,this work provides an alternative of using UHMWPE/PMP blend films and deepens an understanding of the role of porous structure of separator for designing future high performance Li-ion batteries.