| Traditional polyolefin materials are mostly used in packaging,pipeline and agriculture.Nowadays,high-performance polyolefin products（such as functional products）are also widely applied in the high-technology fields such as automotive,aerospace,military and new energy.For several specific cases,high-performance polyolefin products are involved under high strain rate（or dynamic）loading conditions during the processing and real service.Therefore,the study on the relationship between microstructure and macroscopic performance of polyolefins under such harsh conditions is of great interest for guiding the processing and optimizing the properties of polymer products.However,the machnism of microstructure evolution involved in such non-equilibrium conditions is a major challenge in the academic and industry areas.This is mainly due to the lack of effective experimental methods.At present,the theoretical models and experimental evidence for the deformation mechanism of polymers are still limited to the quasi-static loading process,the research focusing on the physical mechanism on the non-equilibrium high strain rate states is very lacking.In this thesis,the mechanical properties and structural evolution of polyolefin films over a wide strain rates range,especially during high strain rate tensile process,are systematically investigated by combining the high flux synchrotron radiation X-ray scattering technique and a homemade high-speed tensile apparatus.Based on the quantitative data,the structure evolution and phase transition mechanism of polyolefin materials under non-equilibrium high strain rate conditions are further analyzed.The specific research contents and results are as follows:（1）The high-speed tensile apparatus of our group is upgraded and optimized.For the upgraded apparatus,any prescribed strain rate can be reached within 10 ms.A set of active pressure rollers with controllable pressure is added to avoid the sliding of films in the previous machine.High temperature is controlled by heat gun and air compressor,and low temperature is controlled by nitrogen gas and liquid nitrogen.The upgraded apparatus can perform the tests in a strain rate range of 10-3 to 102 s-1,spanning six orders of magnitude.The temperature can vary from-60 to 300℃.This apparatus can be well combined with synchrotron radiation X-ray,high time-resolution detector and high-speed CCD camera.With such experimental system,the stretching-induced structural evolutions of polymers under a wide range of strain rate conditions are successfully constructed with the time resolution up to 0.2 ms.（2）The structural evolutions of linear low-density polyethylene（LLDPE）are investigated by in-situ synchrotron radiation wide-angle X-ray diffraction（WAXD）during tensile deformation over a wide strain rates range from 0.005 to 250 s-1.The phase transition mechanism is discussed by combining the phase diagram in 2D true stress（σtrue）-strain rate（ε）space with the evolutions of several micro-structural parameters,such as the crystallinity（χc）,crystal sizes（L110,L200）and lattice parameters（ao,bo）.The true stress space can be roughly divided into three regions by two critical true stresses at the onset formation of the monoclinic（σM-onset）and the hexagonal（σHonset）crystals.In region Ⅰ where only orthorhombic crystals exist,lattice distortion under the effects of stress plays a dominant role.In region Ⅱ,the martensitic transformation from orthorhombic to monoclinic crystals occurs and monoclinic crystals load the main tensile stress.In region Ⅲ,stress-induced melt recrystallization occurs and hexagonal crystals appear.In addition,the effects of strain rate on the stress-induced melt crystallization and lattice deformation are also discussed.This study extends and deepens the existing understanding of structural evolutions of LLDPE film during high strain rate process and real service.（3）With the assistance of the ultrafast time resolved synchrotron radiation wideangle WAXD and a homemade hyphenated high-speed tensile apparatus,the structural evolutions in the crystalline domain of polybutene-1（PB-1）are elucidated even within several milliseconds.The stretching-induced phase transition mechanism of PB-1 consisting of form Ⅱ crystals is investigated in three strain-rate regions（A,B,and C）spanning six orders of magnitude(from 0.005 to 100 s-1).During the quasi-static loading process in region A(0.005 s-1<ε<0.5 s-1),metastable form Ⅱ crystals progressively transform into the stable form I ones.This classical transition is ascribed to the stress-induced change of the long-range chain position and conformation.Under dynamic loading conditions,in region B(1 s-1<ε<10 s-1),besides the form Ⅱ to Ⅰtransition,several form Ⅱ crystals are directly melted with increasing stress.And in region C(ε>50 s-1),not only the form Ⅱ crystals but also a large amount of the formⅠ crystals are melted when the imposed stress reaches the threshold value.This abnormal stretching-induced phase transition of PB-1 is related to both the high strain rate and accompanied heating effect.When the lattice is subjected to an ultra-high rate of energy input,the appearance and growth of conformational defects,which are related to the change of short-range contour shape of chains,can be involved.These defects lower the energy barrier of phase transition between the crystalline and amorphous structures significantly.For the crystals containing a large number of defects,they tend to be melted with increasing stress rather than undergo a phase transition in the crystalline domain as those in the quasi-static loading region.（4）The structure evolution of hard-elastic isotactic polypropylene（iPP）films composed with highly oriented lamellar stacks during uniaxial stretching in the strain rate regions（Ⅰ and Ⅱ）spanning five orders of magnitude(from 0.005 to 10 s-1)has been investigated by in-situ ultra small-angle X-ray scattering（USAXS）technique.In regionⅠ(0.005 s-1<ε<0.1 s-1),the interlamellar amorphous undergoes stress-induced microphase separation,forming the high-density region with high content of tie chain（strongly constrained）but low content of cilia chain,and low-density region with rich content of cilia chain.In region Ⅱ(ε>5 s-1),the above low-density microphase develops and transforms into cavitation at higher strain rates due to the rapid energy input. |
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