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Lamellar stacks, arranged alternatively with hard crystal and soft amorphous nano-layers, are the basic building units to determine the intrinsic mechanical properties of semi-crystalline polymeric materials. The mechanical instability of hard-elastic isotactic polypropylene (iPP) films with highly parallel lamellar stacks is studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during cyclic tensile deformation. Unexpectedly, the micro-strain, deduced from 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, during which no irreversible plastic destruction of crystal is observed. Combining the unpredictable increase of long period and other structural information obtained with in-situ SAXS/WAXS measurements, we propose that stress-induced microphase separation of interlamellar amorphous phase is responsible for the yield behavior and the hyperelasticity of hard-elastic iPP films, 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. [ABSTRACT FROM AUTHOR]

Combining neutron and X-ray scattering (SANS, SAXS, and WAXD) as the contrast matching probes, the hierarchical structures of the multi-wall carbon nanotubes/high density polyethylene (MWCNTs/HDPE) composites (0.1–2.0 wt%) have been studied. SANS measurement avoided the correlation peak associated to the semicrystalline nature of HDPE from SAXS, and the passible scattering from the stretch induced voids. Benefitting from SANS data, the power law ( α ) taking the flexible cylinder suggested that whole MWCNTs in the unstretched composites follow a self-avoiding random walk ( α  ≈ 1.67). SANS data suggest a hollow flexible cylinder structure with outer and inner diameters (core-shell-like cross section) of the scatterers as ca. 12.6 nm and 4.1 nm, while the Kuhn length is about 43.0 nm. Tensile test displayed improved mechanical properties. Interestingly, the Hermans orientation parameter ( f ) of HDPE crystals ( f c ) in the stretched composites are around a plateau ca. 0.96 initially, while decreased remarkably to 0.776 when the fraction is further increased to 2.0 wt%. f obtained by SANS showed a similar tendency (decreased from 0.756 to 0.572). Combining all the results along with differential scanning calorimetry (DSC) measurement, a mechanism of adhered crystals on the surface of MWCNTs and a reinforcement model are proposed. [ABSTRACT FROM AUTHOR]

Aiming to unveil the effect of cold stretching conditions including strain and temperature on the morphology and performance of membrane product, high density polyethylene (HDPE) microporous membranes were prepared by uniaxial stretching. The evolutions of lamellar stacks and micropore formation were tracked by combining in-situ and ex-situ X-ray scattering and scanning electron microscopy methods. It was found that cold stretching strain mainly affected separation degree of lamellar stacks under strain lower than 60%, while slip of crystal at higher strain. As for cold stretching temperature, stretching below 60 °C can promote separation of lamellar stacks, while stretching over 80 °C is beneficial to pull out molecular chains from lamellae, resulting in fibrils. A linear relation between separation degree of lamellar stack and air permeability of final membrane was established, which can provide guidance for producing HDPE microporous membranes with high performance. [ABSTRACT FROM AUTHOR]

Aiming to establish the relationship between structures of precursor film and its capability of pore formation, high density polyethylene (HDPE) precursor films are prepared under varied draw ratios and die temperatures, which are further subjected to stretching to generate microporous membranes. Structures like long period, lateral size of lamellae, orientation parameter, and fibril content of the precursor films are obtained by small/wide angle X-ray scattering (SAXS/WAXS), and morphologies of the microporous membranes are examined with scanning electron microscope. It is found that the content of fibrils and orientation parameter of lamellar stacks in the precursor films play counter effects on the capability of micropore formation. A critical window to balance the conflict is revealed with an optimal fibril content of about 6.4%, at which maximal porosity and most homogeneous porous structures are obtained. A schematic model is proposed to account for the role of the fibrils and the oriented lamellar stacks during pore formation, which may guide the production of high-performance HDPE microporous membranes. [ABSTRACT FROM AUTHOR]

Hard-elastic polyethylene (HEPE) cast films are the key intermediate product for producing PE microporous membranes (dry process) used as separators in Lithium battery. The effects of temperature on the deformation mechanisms of HEPE cast films are systematically studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS/WAXS) techniques during stretching in a wide temperature range from 25 to 135 °C. The structural evolutions and mechanical behaviors show three distinct features, which contribute to the rough divisions of the temperature space into three regions (I/II/III) with α-I relaxation temperature (T αI) and the onset melting temperature (T onset) as the boundaries. On the basis of the evolutions of the structural parameters like long period (L m), micro-strain (ε m), orientation parameter (f 200), crystallinity (X c), and etc., lamellar separation is the main deformation mode in the linear elastic strain zone. And microphase separation of interlamellar amorphous, lamellar crystal slipping and melt-recrystallization are proposed to determinate the later non-linear mechanical behaviors in the three temperature regions, respectively. The full view of deformation mechanisms in the 2D temperature-strain space aids to deepen the understanding of the nonequilibrium structural evolutions in hard-elastic polyethylene films and guide the manufacture of high-performance microporous membranes with dry process. Image 1 • The structural roadmap of hard-elastic polyethylene films in temperature-strain space was constructed. • The deformation modes depend on the coupled effects of external field and chain mobility. • The microphase separation of interlamellar amorphous dominates in region I. • Plastic deformation like crystal slipping and melt-recrystallization is important in regions II and III. [ABSTRACT FROM AUTHOR]

Aiming to unveil the key structures and processing parameters of precursor film for the micropore formation, the structure and mechanical behavior of isotactic polypropylene (iPP) casting films prepared at different processing conditions are systematically studied with small and wide angle X-ray scattering measurements as well as tensile analysis. Draw ratio and cast roll temperature are found to be the key processing parameters, which determine orientation of lamellar stack and thickness of lamellar as well as amorphous layers, respectively. Furthermore, these two important structural parameters, the orientation of lamellar stack and the thickness of amorphous layer, show positive correlations with the formation of micropores. Considering coupling effects of tight folds and lamellar-fibrillar or lamellar-amorphous transformation, a semi-quantitative deformation model for the micropore formation is proposed to correlate surface porosity and elastic recovery rate with structural parameters of precursor film. [ABSTRACT FROM AUTHOR]

Combining the extension rheometry and simultaneous in-situ synchrotron radiation small/wide angle X-ray scattering (SAXS/WAXS) measurement, extension induced crystallization of cross-linked isotactic polypropylene (CL-iPP) is studied at different temperature from 130 to 160 °C. After extension at strain of 1.5, precursor is generated before crystallization and the relative stability increases with temperature. Precursor growth is confined due to limited chain diffusion, resulting in a periodic two-dimensional distribution. Crystallization takes place later with the existence of precursor, accompanied with the slowdown of precursor dynamics. Based on the results, a model of precursor assisted crystallization is proposed to illustrate this unique phase behavior. Image 1 • Flow induced precursor with long lifetime is observed in extended cross-linked iPP. • The model of precursor assisted crystallization is proposed based on the results. • Daughter lamellar with high self-similarity can be observed under SAXS. [ABSTRACT FROM AUTHOR]