Structural evolution of hard-elastic polyethylene cast film in temperature-strain space: An in-situ SAXS and WAXS study
Graphical abstract
Introduction
Polyolefin microporous membranes made by uniaxial dry process are widely used as lithium-ion battery separators, which are placed between anode and cathode in the battery. Generally, the polypropylene (PP) or polyethylene (PE) microporous membranes with homogeneous nano-scale cavities supported by neighboring lamellar stacks and several fibrillar bridges are produced through the two-step tensile deformation process at low and high temperatures of special materials, namely, hard-elastic PP or PE films. With the quick development of high-powered electric vehicles, three-layer PP/PE/PP composite microporous membranes, as one of the key materials in electrical lithium battery (high-performance separator), are required to possess well-connected pores with high porosity and maintain strong physical strength even at high temperature [[1], [2], [3]]. From the security perspective, PE microporous membrane in the middle layer serves as the shutdown agent while PP microporous membranes are the main mechanical support based on the different temperature sensitivity. Probably due to the different molecular structure, PE membranes with homogeneous micropores are more difficult to produce compared with PP even for single layer microporous membranes no matter in industrial or academic fields. To meet the market demand for the cost-effective but high-performance microporous membranes, it would be a good choice to figure out the deformation characteristics of hard-elastic polyethylene (HEPE) films at a wide temperature range before the usage of massive trial-and-error procedure in industrial processing. In fact, hard-elastic materials have attracted much attention in academic and industrial fields due to their special structures, great mechanical properties since 1960s [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. Similar to elastomers, hard-elastic materials possess the high elastic recovery property, which is provided by the well-organized morphology of oriented lamellar stacks with the normal parallel to the machine direction (tensile direction) [5,9,[16], [17], [18], [19], [20], [21]]. During the specific tensile deformation process, the nonequilibrium structural transitions are determined by the complex effects of temperature, strain, strain rate and other external parameters during the multi-step industrial process (such as cold and hot stretching) [12,13,[22], [23], [24], [25]]. Understanding the relationship between structural evolutions and mechanical properties in HEPE films is of great importance for the manufacture of PE microporous membranes.
For decades, due to the hierarchic structures like spherulites and randomly packed lamellae in most semi-crystalline polymeric materials, several crystal plastic deformation models like amorphization [26,27], slipping [[28], [29], [30], [31], [32], [33], [34]], twinning [35], melting-recrystallization [[36], [37], [38]], cavitation [39,40] and phase transition [41,42] were proposed to explain the macroscopic nonlinear mechanical behaviors. In fact, which specific deformation model determines the nonlinear mechanical behaviors depends on the coupled effects of exact external fields (strain and temperature) and the mobility of molecular chains in crystal and amorphous phases [[43], [44], [45], [46], [47]]. Amorphization and crystal slipping intend to occur at lower temperatures, while stress-induced melting-recrystallization are mainly observed during deformation at higher temperatures near the melting point [26,[48], [49], [50]]. As one special semi-crystalline materials, HEPE films with high crystallinity own perfect elastic recovery property without obvious necking phenomenon if tensile deformation along the machine direction is employed, where the corresponding stress-strain curves also behave nonlinearly, including yielding, stress plateau and strain hardening and so on. Therefore, it seems improper to simply borrow these theories mentioned above to account for the temperature effects on the deformation of hard-elastic films. With the techniques of small- and wide-angle X-ray scattering (SAXS/WAXS), infrared radiation (IR) spectrum, thermal analysis, scanning electron microscopy (SEM), etc., many efforts have been made to figure out how the row-nucleated lamellar structures deform at room temperature for decades, while the deformation mechanisms at higher temperatures have only received less attention, which might to some extent help to study on the intrinsic deformation mechanisms of PE oriented lamellar stacks. The deformation of hard-elastic materials during uniaxial stretching along machine direction at room temperature was roughly explained by lamellar separation and the failure of interlamellar amorphous phases [4,5,9,[11], [12], [13],51,52]. Cannon et al. concluded that the deformation of the row-nucleated lamellar structure might choose lamellar tilting and bending or intralamellar chain tilting as the deformation modes while little lamellar destruction or fibrillation could occur [53]. In detail, to account for the perfect elastic deformation of lamellar crystals, Sprague and Clark came up with the leaf-spring model and paper-bell model with the consideration on the lamellar-amorphous laminated structures [11,16,[53], [54], [55]] Statton et al. also proposed that lamellar shear would increase the surface area, which resulted in a high energy state and contributed to the retractive force for the elasticity property from the point of view of internal energy [56]. As for HEPE samples, Hashimoto and coworkers agreed that both lamellar bending and interlamellar slip process occurred with the existence of numerous tie chains during the deformation and voids were generated from the ultra-stretched interlamellar spacing [57,58]. Due to the limitation of experimental techniques, the conjectures mentioned above mainly focused on the origin of hard elasticity for tensile deformation with large strain but did not directly figure out the relationship between the nonlinear mechanical behaviors (like yielding or hardening) and the structural evolutions for hard-elastic PE films or fibers. And although Elyashevich et al. have tried to analyze the deformation mechanisms of HEPE films at different stages of stress-strain curve at room temperature [58], while most conclusions were mainly made based on the mechanical properties and conjectures of the deformation of oriented lamellar stacks without enough experiment evidence.
In our previous works, the effects of temperature on the deformation mechanisms of HEPP films have been systematically studied with in-situ SAXS/WAXS experiments. As the self-propelled trend of long period and corresponding micro-strains were observed in the nonlinear mechanical strain zone, microphase separation of interlamellar amorphous phase into high- and low-density regions combined with elastic deformation of crystals was proposed to be responsible to the reversible elastic instability at temperatures lower than alpha relaxation temperature [[19], [20], [21]]. Furthermore, a microbuckling model of lamellar stacks was developed on the basis of the good agreement between theoretical calculation and experimental results, which further proves the conjecture of reversible lamellar bending behavior [18]. Besides, crystal slipping and the gradual formation of fibrillar bridges play the main role at higher temperature, where irreversible cavities are formed. And further increasing deformation temperatures induces the occurrence of stress-induced melting-recrystallization [20]. As for HEPE films, it was found that the deformation at room temperature would also go through lamellar separation, interlamellar amorphous microphase separation and cavitation, while the increase of strain rate could improve the uniformity of microphase separation [21]. But due to the different temperature sensibility and molecular structures compared with polypropylene films, the structural evolutions during tensile deformation of HEPE films along machine direction in a wide temperature-strain space remains unclear, which requires more efforts to figure out the exact structural-mechanical relationship.
In this study, in-situ synchrotron radiation (SR) SAXS and WAXS techniques were used to study the deformation mechanisms of HEPE cast films during uniaxial tensile deformation along machine direction in a wide temperature range (25–135 °C) to reveal the temperature effects on the structural variations and the corresponding nonlinear mechanical behaviors. With the analysis of the evolutions of several quantitative mechanical and structural parameters, a roadmap of microscopic structural evolutions is built in the wide 2D temperature-strain space. This study will be beneficial for deeply understanding the deformation of HEPE films and guiding the manufacture of high-performance PE microporous membranes in industry.
Section snippets
Material and characterization
The annealed hard-elastic high-density polyethylene precursor films used in this study were supplied by Wuhan Bosheng Technology Company, Ltd. These films were produced with extruding-casting method and they perform great hard-elasticity at room temperature due to the structures of oriented lamellar stacks. The initial long period is around 31.3 nm and the initial crystallinity is around 61.6% according to ex-situ small- and wide-angle X-ray scattering results. The initial length, width and
Mechanical behaviors
Fig. 2 presents the mechanical information of HEPE cast films at different temperatures. The engineering stress-strain curves at the five different deformation temperatures of 30, 50, 70, 90 and 120 °C are plotted in Fig. 2(a) as an example. The mechanical results of all experimental temperatures ranging from 25 to 135 °C together with the enlarged plot are provided in Fig. S1 as a reference. To further analyze the mechanical behavior quantitatively, we extracted the onset strain for non-linear
Discussion
With the combination of in-situ SR SAXS and WAXS techniques, some interesting information can be extracted to allow us to deeply investigate the deformation mechanisms of HEPE cast films during uniaxial deformation at different temperatures. (i) With the basis of the evolutions of several structural parameters (Lm, εm, Ie, d200, Xc and f200) and the corresponding mechanical behaviors during stretching as well as the thermal analysis results, the experimental temperature space is roughly divided
Conclusion
The deformation mechanism of hard-elastic polyethylene films plays an important role in the processing of PE microporous membranes, which would serve as the key material for PP/PE/PP composite separators in lithium battery of electrical vehicles. With the combination of in-situ SAXS/WAXS measurements and the homemade stretching apparatus, we investigated the structural evolutions and nonlinear mechanical properties of HEPE cast films during uniaxial deformation along the machine direction in a
Acknowledgements
This work is funded by the National Natural Science Foundation of China (51903091, 51633009 and 51790503), the China Postdoctoral Science Foundation (2018M643076) and the Fundamental Research Funds for the Central Universities (2019XX13). And the in-situ SAXS and WAXS experiments were carried out in beamline BL19U2 of Shanghai Synchrotron Radiation Facility (SSRF).
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