Elsevier

Polymer

Volume 178, 12 September 2019, 121579
Polymer

Deformation mechanism of hard elastic polyethylene film during uniaxial stretching: Effect of stretching speed

https://doi.org/10.1016/j.polymer.2019.121579Get rights and content

Highlights

  • Effect of stretching speed on structures and mechanical property was studied.

  • Stretching speed is of importance in how microphase separation occurs and develops.

  • The deformation is inhomogeneous at low stretching speed region I.

  • Plastic deformation occurs earlier at low stretching speed.

Abstract

The effects of stretching speed on the structural evolutions and mechanical behaviors of hard-elastic polyethylene films are studied with in-situ and ex-situ small-angle X-ray scattering (SAXS), scanning electronic microscope (SEM) and tensile tests in a wide stretching speed range (0.04–4 mm/s). Based on the evolutions of structural parameters extracted from SAXS results and the surface morphologies from SEM experiments, the stretching speed space can be divided into two regions with the boundary of 0.8 mm/s. Stress induced microphase separation of amorphous phase triggers the yielding behavior, which distributes more homogeneously with the increase of stretching speed. In region I, microphase separation tends to develop into cavities at smaller strain due to the thorough relaxation process of molecular chains in amorphous phases, which results in the inhomogeneous deformation during further stretching. In region II, the relaxation of molecular chains is not enough to response to the variation of external tensile field, thus inducing the uniform distribution of the occurrence of microphase separation.

Introduction

Polypropylene (PP) and polyethylene (PE) microporous membranes produced by dry process serve as one of the most key materials in Lithium battery production, namely battery separator. With the quick development of electrical vehicles, high-performance Lithium battery separator in the market is required to possess great mechanical property and high porosity at the same time, which brings large challenge for industry due to the multi-step and multi-parameter properties of film processing. Generally, the production of the microporous membranes is based on the cold and hot stretching of hard-elastic precursor films, during which the relationship between structural evolutions and non-linear mechanical properties is still controversial.

For decades, the deformation mechanisms of hard-elastic materials have attracted much attention from polymeric researchers for their perfect hyperelasticity and the importance in the post-stretching processing of microporous membranes. The microporous structures supported by fibrillar bridges are indeed originated from the coupled effects of deformation temperature, strain and strain rate. Note that most related studies were based on ex-situ experimental techniques, including tensile tests, small- and wide-angle scattering (SAXS/WAXS), scanning electronic microscope (SEM) and atomic force microscope (AFM) and so on [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. With the combination of SAXS and SEM results, the hard-elastic materials (like PE or PP) have been proved to be composed of row-nucleated lamellae, i.e., highly oriented lamellae [1,[12], [13], [14], [15]]. Sprague and Clark tried to use leaf spring model (energy model) combined with interlamellar separation to explain roughly the elastic deformation of crystals at room temperature [1,5,16,17], which was also partially verified by the theoretical calculation results based on a microbuckling model for idealized hard-soft (lamella-amorphous) laminated composite structures recently [13]. Besides, Goritz and coworkers proposed another hypothesis on entropic effects of interlamellar layers to account for the hard elasticity, based on the analysis of thermal effects in the amorphous part during tensile deformation [2,18]. And void or cavitation observed from SEM pictures of samples after going through further hot stretching was believed to form from interlamellar amorphous region [10,11,14,15,19,20]. Since the synergetic effects of temperature, strain and strain rate play important roles on the intrinsic deformation of oriented lamellar stacks, it remains as a large challenge to figure out what exactly happens during tensile deformation at a specific external field.

Generally, molecular chains in crystal or amorphous phases possess different chain relaxation dynamics at a typical temperature, while the variation of strain rate might accelerate or decelerate the dynamics processes, to some extent, resulting in different deformation mechanisms [[21], [22], [23]]. Benefiting from the development of ultrafast in-situ SAXS/WAXS techniques, it has been found that at temperatures below alpha relaxation temperature (Tα) stress-induced amorphization or microphase separation of interlamellar amorphous phase might dominate the initial nonlinear mechanical behavior [24,25], while crystals slipping and melting-recrystallization phenomenon can be observed at higher deformation temperatures [[25], [26], [27], [28], [29]]. However, few studies focused on the effects of strain rate or stretching speed on the structural evolutions during tensile deformation of hard-elastic materials, which require more efforts.

In fact, strain rate also serves as one of the important roles during processing of other semi-crystalline polymers, which will affect the structure and mechanical property of plastic production [[30], [31], [32], [33], [34]]. Based on the previous studies on compression-molded or injected samples, crystals intends to deform plastically before the occurrence of cavitation in amorphous phase at low strain rates, while yielding stress increases with the increase of strain rate [30,35,36]. Due to the complicated and hierarchic property from the existence of spherules, the intrinsic mechanical behavior of lamellar stacks is hard to be obtained. And although the oriented lamellar stacks cannot fully represent that of lamellar stacks in spherulites due to their different microscopic chain conformation, but they after all exclude the interference of orientation factor or larger-scale spherulite are excluded, to some extent. Thus, considering that hard-elastic materials are composed of highly oriented lamellar stacks, they can be regarded as good but probably not perfect sample for the study on the deformation mechanisms of semi-crystalline polymers.

In this work, with the combination of in-situ synchrotron radiation SAXS techniques and ex-situ SEM measurement, the structural evolutions of hard-elastic PE precursor films with highly oriented lamellar stacks during uniaxial tensile deformation is systematically studied at room temperature with different stretching speeds ranging from 0.04 to 4 mm/s (over two orders of magnitude). The corresponding initial strain rate is ranging from 0.003 to 0.3 s−1. The effects of stretching speed (strain rate) on the deformation mechanisms and structural evolutions during stretching are discussed, which play an important role to understand deeply the relationship between structure and mechanical property of hard-elastic materials and might guide the processing of microporous membranes by dry process in industry.

Section snippets

Material and characterization

The annealed high-density polyethylene (HDPE) precursor films composed of oriented lamellar stacks were supplied by Wuhan Bosheng Technology Company, Ltd. Based on differential scanning calorimetry measurement, the melting temperature is 134.5 °C. And the initial crystallinity is around 64.8% according to wide-angle X-ray scattering result. The thickness and initial length of the films are 25 μm and 30 mm, respectively. The other detailed structural information can be found in our previous

Results

Fig. 1(a) shows the engineering stress (σ)-strain (ε) curves of HDPE films at room temperatures with different stretching speeds ranging from 0.04 to 4 mm/s. And the corresponding enlarged image in small strain range is provided in Fig. 1(b) to observe the characteristics more clearly. Fig. S1 in the Supporting Information presents the corresponding true stress-strain curves as a reference. With v ≤ 0.8 mm/s, the stress-strain curves behave with similar variation trend: stress σ increases

Discussion

With the combination of in-situ and ex-situ SAXS experiments and SEM measurements, some interesting findings can be extracted. (i) Based on the evolutions of structural parameters (Lm-1, Lm-2, Ie and Φ) and mechanical data (σ and ER), the stretching speed range can be divided into two regions with 0.8 mm/s as the approximate boundary: low stretching speed region I (0.04–0.8 mm/s) and high stretching speed region II (0.8–4 mm/s). (ii) In region I, the stress-strain curves only go through four

Conclusion

The effects of stretching speed on the structural evolutions of oriented polyethylene lamellar stacks are systematically studied with hard-elastic polyethylene films via SAXS, SEM and tensile tests with a wide stretching speed range (0.04–4 mm/s). Based on the evolutions of microscopic strain εm-2, equatorial intensity Ie, the ratio of the intensities of low-density and second-order scattering Φ, elastic recovery rate ER and other parameters, the stretching speed space can be divided into two

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51633009, 51790503), the China Postdoctoral Science Foundation (2018M643076) and the National Key R&D Program of China (2018YFB0704200). And the experiment is carried out in Shanghai Synchrotron Radiation Facility beamline BL19U2.

References (42)

  • S.S. Sarva et al.

    Stress-strain behavior of a polyurea and a polyurethane from low to high strain rates

    Polymer

    (2007)
  • L.E. Govaert et al.

    Influence of strain rate, temperature and humidity on the tensile yield behaviour of aliphatic polyketone

    Polymer

    (2000)
  • H.J.M. Caelers et al.

    The prediction of mechanical performance of isotactic polypropylene on the basis of processing conditions

    Polymer

    (2016)
  • X. Li et al.

    Strain and temperature dependence of deformation mechanism of lamellar stacks in HDPE and its guidance on microporous membrane preparation

    Polymer

    (2016)
  • X. Li et al.

    Mechanical energy and thermal effect controlled micropore nucleation and growth mechanism in oriented high density polyethylene

    Polymer

    (2017)
  • B. Sprague

    Relationship of structure and morphology to properties of “hard” elastic fibers and films

    J. Macromol. Sci., Part B: Physics

    (1973)
  • S. Cannon et al.

    Hard‐Elastic fibers.(A review of a novel state for crystalline polymers)

    J. Polym. Sci. Macromol. Rev.

    (1976)
  • R. Hosemann et al.

    Refraction effects and structural changes of hard elastic polypropylene (HEPP) during stretching

    Colloid Polym. Sci.

    (1987)
  • E. Clark

    A mechanism of energy-driven elasticity in crystalline polymers

  • H.D. Noether

    Factors affecting the formation of hard elastic fibres

    Polym. Eng. Sci.

    (1978)
  • H.D. Noether et al.

    Small-angle X-ray diffraction studies and morphology of microporous materials and their 'hard' elastic precursors

    J. Appl. Crystallogr.

    (1978)
  • Cited by (23)

    • Deformation mechanisms in PBT at elevated temperatures

      2023, Materials Today Communications
    • Structural evolution of UHMWPE gel fibers as high degree plasticized system during stretching: An in-situ wide and small angle X-ray scattering study

      2022, Polymer
      Citation Excerpt :

      Moreover, UPEp96 and UPEp88 both have higher stress at the corresponding strains at a higher stretching speed. This is also the conventional understanding that more molecular chains are pulled apart and less prone to relaxation, thus allowing greater stress to be transmitted at higher stretching speeds [36–38]. Notably, the UPEp88 shows a more significant stress variation at different speeds than the UPEp96.

    View all citing articles on Scopus
    View full text