Understanding of the tensile deformation in HDPE/LDPE blends based on their crystal structure and phase morphology

The mechanisms of tensile deformation in high density polyethylene/low density polyethylene (HDPE/LDPE) blends were studied by a video-controlled tensile set-up, combined with dynamic mechanical analysis and small angle X-ray scattering. When quenching from the melt to room temperature, HDPE forms well-organized spherulits with high crystallinity and rigid amorphous layers between lamellae, and LDPE forms irregular aggregates with low crystallinity and mobile amorphous layers between lamellae. A separate lamellar stack-like structure is formed in HDPE/LDPE blends during the quenching. The deformation is affected by both the crystal structure and the phase morphology. Because the semi-crystalline polymers are made up of two interpenetrating networks, one is built up by the entangled fluid part and the other by the crystallites, at low deformations the coupling and coarse slips of the crystalline blocks dominate the mechanical properties, which allows the system to maintain a homogeneous strain distribution in the sample. The assumption of a homogeneous strain distribution can now be further proved by the tensile deformation in HDPE/LDPE blends, which shows two-step processes, with HDPE crystallites being broken down first at imposed strain of 0.4 and then LDPE crystallites being broken later, at an imposed strain of 0.6.     

Real time synchrotron SAXS and WAXS investigations on temperature related deformation and transitions of β-iPP with uniaxial stretching

In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were carried out to investigate the uniaxial drawing-induced deformation and structure transitions of β form isotactic polypropylene (iPP) at varying temperatures (30 °C, 60 °C, 80 °C, 100 °C and 120 °C). The WAXS results indicated that the initial strain for the strain-induced β–α transformation decreases with the tensile temperature according to the engineering stress–strain curves. The SAXS data showed that the long period increased along the direction perpendicular to the tensile force and changed little along the tensile direction with increasing strain in the elastic deformation stage before the yield point. The analysis of the obtained scattering results indicated that the angle between parent and daughter lamella rotates from initial 40° or 140° to close to 90° accounts for the lateral expansion of the samples with tension, which matches the essential auxetical behavior. A structure deformation and transition mechanism was proposed for β form iPP with uniaxial drawing. The initialization of the crystalline structure transition is after the yield point, then the mechanical loading-induced β–α transition seems to be a gradual process with lamella slippage and breaking which triggers the β–α polymorphic transition.     

Direct Observations on Structure Evolutions in Polyamide 6 during Deformation at High Temperatures with WAXS and SAXS

The deformation induced structure evolutions of polyamide 6 (PA6) during uniaxial tension at high tensile temperatures (60 °C and 90°C) were investigated with in situ wide- and small-angle X-ray scattering (WAXS and SAXS) technologies. The obtained data on structure evolutions revealed that they were different from the results measured at low temperature (30 °C). The α-phase got oriented once upon the beginning of deformation. After yielding the γ-phase started to be oriented following the α-phase. While, the breakdown of PA6 crystals along a and c axis overcame partial crystalline orientation at the high tensile temperatures (60 and 90 °C). The competition between stretch of amorphous phase and slippage of lamellae after yielding affected the deformation behavior of PA6. The collapse of lamellae was also confirmed from SAXS analysis and such disrupted lamellar structure resulted in the decrease of long spacing of PA6. The results showed that PA6 materials may show higher ductility at high temperatures. Therefore the crystals could be broken more easily and the formed lamellar fragments of PA6 could be preserved at larger strain at 90 °C. In addition, the yielding of PA6 and γ-phase orientation depended on the lamellar slippage during the deformation. POLYM. ENG. SCI., 60:581–586, 2020. © 2019 Society of Plastics Engineers     

Study on structure and property relations of α‐iPP during uniaxial deformation via in situ synchrotron SAXS/WAXS and POM investigations

Micro‐ and meso‐scale structure changes of α‐form isotactic polypropylene (α‐iPP) during uniaxial stretching is studied by time‐resolved synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray scattering (WAXS). The structure/property relations are investigated at different temperatures, and the effects of isothermal crystallization are also studied with POM. The X‐ray scattering results show that the long period increased and the lamellar oriented along the stretching direction in the elastic deformation stage. The lamellar and crystals start destructing after yielding. And from it POM images it can be seen that with higher crystallization temperature the spherulites connected to form a crystalline network, on which the stress is mainly loaded. It turns out different environment temperatures affect mostly the amorphous domains. And samples exhibit different yielding mechanisms with different thermal histories. A hypothetical structural mechanism is proposed based to explain the observed relationship between the processing parameters, thermal history and the structure/property relations of α‐iPP. POLYM. ENG. SCI., 58:160–169, 2018. © 2017 Society of Plastics Engineers     

Structure evolution during uniaxial tensile deformation of high density polyethylene before and after irradiation by 1 MeV electrons

The structural evolution of high density polyethylene (HDPE) during uniaxial tensile deformation, before and after irradiation by 1 MeV electrons, is in situ studied by synchrotron small angle X‐ray scattering (SAXS) and wide angle X‐ray diffraction (WAXD). Both the pristine and the irradiated HDPE exhibit three regions of deformation behavior. It is shown that the deformation in the first region is in accord with the change in long period of the lamellar structure. In the following two regions, both the strain‐induced melting and strain‐induced crystallization could occur. The tensile stress decreases with strain in the second region due to the dominant melting effect. In the third region, the synergistic effect of the melting and crystallization results in stress leveling off first, and then the tensile stress increases again because the crystallization effect becomes dominant at higher strains. For the irradiated HDPE, the irradiation‐induced crosslinking network slows down the deformation process. Compared to the pristine one, all the tensile stress is rather higher at a given strain for the irradiated HDPE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40269.     

The mechanism of orientation in cold-drawn polyoxymethylene as revealed by crystallographic pole figures

The orientation mechanism of cold-drawn, partially crystalline polyoxymethylene (ULTRAFORM) samples was studied by performing wide angle X-ray scattering (WAXS) measurements. The anisotropic samples were prepared in uniaxial tensile tests around 130°C, in a temperature range between the glass transition and the melting point. The process of the plastic deformation is discussed for different degrees of anisotropy. The orientation distribution of the crystalline lamellae was qualitatively characterized by performing pole figure intensity calculations from the measured WAXS intensities. The degree of orientation was quantitatively described by calculating the orientation factors for the [100] normal vector of the unit cell. The texture of the necked samples with high orientation degrees was a mixture of axial and uniplanar-axial textures. An explanation for the formation of this kind of orientation is proposed assuming an influence of the sample geometry on the orientation process.     

Uniaxial compression and stretching deformation of an i-PP/EPDM/organoclay nanocomposite

In this study, the morphology and rheological properties of nanocomposites prepared by melt mixing of isotactic polypropylene (i-PP), ethylene-propylene-diene terpolymer rubber (EPDM), and Cloisite 15A organoclay, were investigated at rest and under of uniaxial compression and stretching deformations at room temperature. Transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD) experiments revealed that intercalation took place between the clay and the blend. The lamellar long period (L) of the polymeric structure determined by small-angle X-ray scattering (SAXS) was found to increase upon the addition of nanoclay, which also resulted in variations in the angle of rotation (?) between the polymer and clay lamellae. The intercalated nanoclay improved the storage and loss modulus of the resulting materials in the melt state significantly, as determined from oscillatory rheology analyses. Finally, we verified that the uniaxial plane deformation caused by compression of the nanocomposites contributed to the reduction of crystalline domains in the blend, while the crystallinity remained almost constant in the case of uniaxial stretching deformation.     

Deformation-mediated superstructures and cavitation of poly (l-lactide): In-situ small-angle X-ray scattering study

Cavitation and superstructure evolution of polymers during stretching play crucial roles to influence the mechanical properties of materials. In this study, we investigated deformation-mediated superstructures and cavitation of poly (l-lactide) (PLA) as well as their dependence on stretching temperatures by in-situ small-angle X-ray (SAXS) analysis coupled with mechanical testing. It is found that the cavitation and crystalline deformation are strongly influenced by stretching stress during deformation, which significantly depends on the stretching temperature. At lower stretching temperature (70 °C), the cavitation is initiated before the yielding and then stimulates the crystallite shearing. At higher stretching temperature (90 °C), however, the crystallites shear firstly and then crystalline deformation promotes the formation of cavities orientated along the stretching direction. High stretching temperature benefits the formation of relatively perfect crystals with high orientation. The results provide the basic knowledge of how to adjust the mechanical properties of polymer materials by controlling their superstructure in the deformation process.     

An in-situ X-ray scattering study during uniaxial stretching of ionic liquid/ultra-high molecular weight polyethylene blends

An ionic liquid (IL) 1-docosanyl-3-methylimidazolium bromide was incorporated into ultra-high molecular weight polyethylene (UHMWPE) and formed IL/UHMWPE blends by solution mixing. The structure evolution of these blends during uniaxial stretching was followed by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. During deformation at room temperature, deformation-induced phase transformation from orthorhombic to monoclinic phase was observed in both IL/UHMWPE blends and neat UHMWPE. The elongation-to-break ratios of IL/UHMWPE blends were found to increase by 2-3 times compared with that of pure UHMWPE, while the tensile strength remained about the same. In contrast, during deformation at high temperature (120 °C), no phase transformation was observed. However, the blend samples showed much better toughness, higher crystal orientation and higher tilting extent of lamellar structure at high strains.     

Effect of molecular structures on static and dynamic compression properties of clay and amphiphilic clay/carbon nanofibers used as fillers in UHMWPE/composites for high-energy-impact loading

Three different ultrahigh-molecular-weight polyethylene (UHMWPE)–clay nanocomposites (Muscovite, Cloisite 30B and amphiphilic clay/carbon nanofibers) were investigated with the nanocomposite nanomorphology studied before and after dynamic mechanical compressive tests at high strain rates. Their material structure and thermal properties were investigated using techniques such as step-scan differential scanning calorimetry, split Hopkinson pressure bar, synchrotron small angle X-ray scattering (SAXS), and dynamic mechanical analysis. Results were associated with morphological changes observed after deformation. chemical vapor deposition (CVD)-modified nanocomposite, due to the molecular bonding and the extra functional groups, is designed with crystalline structures with fewer defects and higher stability. The increase in particulate/polymer interactions observed for the CVD-modified material decreased the elongation in the quasi-static test. However, the dynamic mechanical behavior contradicted the quasi-static behavior because at very high strain rates there was not sufficient time for the interlamellar and intralamellar defect facilitated plastic flow and the material transitioned through the glassy state. The SAXS results show that deformation strongly induced changes in the UHMWPE and UHMWPE–clay nanocomposite morphology. SAXS indicates that CVD-modified samples became more compact and dense, thus corroborating the formation of additional secondary bonds between structures and/or the carbon nanofibers alignment. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47094.     

Real time development of structure in partially molten state stretching of PP as detected by spectral birefringence technique

Our main focus in this study is to investigate the deformation behavior of PP in temperature range where the PP is partially molten using a newly developed uniaxial stretching system. This system allows the real time study of the structural reorganization processes as reflected in birefringence coupled with true stress and true strain at temperature and deformation rates used in industrial film process. This instrument revealed that birefringence-stress relationship is linear beyond an initial yielding point until the onset of strain hardening beyond which negative deviation from this linearity is observed. At lower strains the films were found to continue to change significantly during holding stage leading to increase in birefringence. These changes observed in holding stage decreases with decrease of stretching speed and temperature and with increase of total strain. Increased strain rates result in destruction of crystallites that involves block rotations of these regions leading to observation of lower birefringence at faster rates. The latter process generates large amount of stretched amorphous chains that gradually convert to crystalline state during holding.     

Crystallization,structure, and enhanced mechanical property of ethylene-octene elastomer crosslinked with dicumyl peroxide

Crosslinking of polyolefin elastomer (POE, ENGAGE™ 8480) with Dicumyl Peroxide (DCP) can have effects on its crystallization dynamics, crystal structure, and properties. The POE crosslinked uniformly has significantly lower crystalline ability than the one with only amorphous phase crosslinked, which, in turn, has weaker crystalline ability than neat POE. The crystallinity and melting point depend on how the POE is crosslinked. The neat POE and POE crosslinked in amorphous phase only, are investigated with DSC and in-situ tensile/synchrotron radiation (WAXD/SAXS). In situ tensile/synchrotron X-ray during a uniaxial stretching process indicates that severe crystal fragmentation is observed at a strain around 45%, and with further increase in strain. The stress in the crosslinked POE is significantly larger than neat POE. For both samples, crystal orientation increases sharply within the strain range up to 88% where orientation-induced new crystals aligned in stretching direction are observed. The long period increases more in stretching direction for the crosslinked POE, consistent with larger stress in this sample, and the stress difference is more pronounced at large strains (27.3 vs. 10.9 MPa at a strain 435%). Permanent set of the crosslinked POE is smaller, consistent with less oriented crystals observed after the test for permanent set.     

Stretched PEO-LiCF3SO3 films: Polarized IR spectroscopy and X-ray diffraction

The effect of tensile stretching on pure PEO and P(EO)_xLiCF₃SO₃ (x = 10 and 20) films was studied using polarized FTIR spectroscopy, wide angle scattering X-ray scattering (WAXS) and small angle X-ray scattering (SAXS). The polarization effects observed in the spectra are readily explained by the selection rules governing IR transitions and symmetry-based vibrational mode assignments of PEO and the 3:1 compound, P(EO)₃LiTf. The degree of polymer chain orientation in the stretched samples as measured by the average of the second Legendre polynomial was calculated from the dichroic ratio for the PEO helices in both pure PEO and the 20:1 film. The degree of chain orientation was markedly higher in the 20:1 film than in the pure PEO film; however it was difficult to calculate the degree of orientation in the 10:1 film.The addition of LiTf causes a slight increase in the fractional crystallinity of the samples as determined from the WAXS data. Orientation of the films did not significantly affect the fractional crystallinity for the 10:1 and 20:1 samples, whereas a slight decrease in the fractional crystallinity was noted for pure PEO upon stretching. The SAXS data suggested that the average crystal size becomes smaller with the addition of lithium triflate to PEO. The SAXS data also indicated that substantial morphological changes occur upon stretching, since the Bragg peak due to the crystalline-amorphous phase separation disappeared upon stretching. These data, taken together, suggest that in the 10:1 film, the tensile stress tends to form small, unoriented domains of the 3:1 compound.     

Effect of initial take-up speed on properties and structure of as-spun and drawn/heat-set poly(ethylene terephthalate) filaments

The effect of initial take-up speed on the properties and structure of both as-spun and drawn/heat-set poly(ethylene terephthalate) filaments was characterized through measurements of birefringence, percent crystallinity, tensile properties, high temperature shrinkage, loss tangent temperature dependence, DSC melting behavior, and wide-angle (WAXS) and small-angle X-ray scattering (SAXS). While a steady trend toward improved as-spun filament orientation and tensile properties occurred with increasing initial take-up speed, the reduced drawability of these more structured precursor filaments resulted in corresponding drawn/heat-set filaments that were of relatively lower overall orientation and tensile strength. The observed trends in tenacity, initial modulus, and high temperature shrinkage of the drawn/heat-set filaments appeared to be well correlated with the extent and distribution of amorphous phase rigidity as perceived through inferences made from the loss tangent temperature dependence. The WAXS patterns of the drawn/heat-set samples indicated that these filaments all possess a well-developed and highly oriented crystalline structure. Application of a simple two phase model allowed the determination of an amorphous orientation factor, which for the drawn/heat-set filaments was generally found to decrease as the draw ratio imposed in order to achieve comparable levels of elongation to break decreased. The SAXS patterns of the drawn/heat-set filaments indicated that comparable long period spacings exist in all cases and that a transition from a four-point pattern to a two-point bar-shaped pattern occurred when the precursor filament possessed some significant amount of as-spun crystallinity. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2115–2131, 1998     

Deformation behaviour and mechanical properties of polypropylene processed by equal channel angular extrusion: Effects of back-pressure and extrusion velocity

Severe plastic deformation by equal channel angular extrusion (ECAE) is an ingenious deformation process used to modify texture and microstructure without reducing sample cross-section. The application of single ECAE pass to polypropylene (PP) was meticulously investigated at room temperature using a 90° die-angle tooling. The ECAE-induced deformation behaviour was examined in relation to the load versus ram-displacement curves. Depending on extrusion conditions, PP displayed various types of plastic flow. For ram velocities beyond 4.5 mm/min, severe shear bands consisting of successive translucent and opaque bands were observed, accompanied on the top surface by more or less pronounced periodic waves. Although the application of a back-pressure significantly reduced the wave and shear-banding phenomena, slightly inhomogeneous shear deformation was still observed. Shear bands were only suppressed by decreasing extrusion velocity. The strain-induced crystalline microstructure was investigated by X-ray scattering. Shear-banded samples exhibited a strong texturing of the (hk0) planes along the shear direction in the translucent bands whereas perfect crystalline isotropy appeared in the opaque bands. Application of back-pressure and/or reducing ram velocity resulted in uniform texturing along the extruded sample. Yet, texturing changed from single shear to twin-like shear orientation about the shear direction. Mechanical properties changes of the extruded samples due to back-pressure and extrusion velocity effects were analyzed via uniaxial tensile tests. The tensile samples displayed multiple strain localizations in shear-banded materials whereas quite homogeneous deformation appeared for non-banded ones. These effects were connected with the crystalline texturing. The results also revealed significant increase in the strain hardening after ECAE. Digital image correlation technique suitable for large deformation was used for determining the full-field strain of the tensile samples in relation to tensile strain and ECAE conditions.     

Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part I: Orientation mechanism and structural hierarchy

The structure which results from solid state extrusion using biaxial orientation is analyzed for oriented polypropylene. Structural changes on the spherulitic, lamellar, and macromolecular level during orientation are investigated using optical microscopy (OM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS). The results show that polypropylene spherulites undergo stepwise biaxial affine deformation and deform homogeneously into a disc-like morphology. During this spherulitic flattening process, lamellar rotation into the planar direction occurs prior to lamellar break-up at a baxial draw ratio of about 1.5. On the macromolecular level, the crystalline c-axis orients in the plane concurrently with the lamellar break-up, while the crystalline b*-axis gradually orients normal to the plane. Amorphous chains are also oriented preferentially in the plane of deformation. A hierarchical model is proposed to illustrate the nature of the orientation in the flattened spherulites.     

Structural evolution of β – iPP during uniaxial stretching studied by in–situ WAXS and SAXS

Polymorphism and crystal transition are of great significance for property mediation in polymer materials. Isotactic polypropylene (iPP) with β – crystal has been widely utilized for the preparation of high performance plastics or films. In the present work, the structural evolution of initially isotropic β – nucleated iPP (β – iPP) during uniaxial stretching at different temperatures was investigated by in–situ X – ray scattering using synchrotron radiation. The wide – angle X – ray scattering (WAXS) results confirmed that the β – crystal transformed either to the mesophase at lower temperature (30 °C) or to the α – crystal at higher temperature (60, 100 and 120 °C) during stretching. An interesting orientation of β – crystal with molecular chains perpendicular to the tensile direction was identified. As revealed by small – angle X – ray scattering (SAXS), cavitation took place in β – iPP stretched at temperatures lower than 120 °C. The size and shape of the cavities were observed by scanning electron microscope. A deformation mechanism of β – iPP combining the crystal transition, cavitation and orientation was proposed.     

Micro-phase separation and crystallization behavior of amorphous oriented PLLA/PVAc blends during heat treatment under strain

Amorphous oriented poly(l-lactide) (PLLA)/poly(vinyl acetate) (PVAc) 50/50 films were prepared by uniaxial drawing of melt-mixed blends at 65 °C. The morphology development and crystal organization of the blends during heat treatment under strain were investigated using small angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Equatorial scattering maxima in the SAXS patterns for samples annealed at 75 °C were observed before the appearance of crystal reflections. Further annealing of the samples at higher temperature induced two further discrete meridian scattering maxima. The observations indicated that homogenous oriented PLLA/PVAc film undergoes micro-phase separation first, followed by crystallization of PLLA in the PLLA-rich phase. The micro-phase separated PVAc nanodomains are aligned parallel to the stretching direction, whereas the crystallized PLLA lamellae are oriented perpendicular to the stretching direction (crystal c-axis along the stretching direction). Micro-phase separation was not observed when films were annealed at 120 °C, at which temperature the high crystallization rate of PLLA overwhelmed the micro-phase separation process.     

Effects of zone drawing on the structure of metallocene polyethylene

The influence of zone drawing on bulk properties and structure of metallocene polyethylene (m‐PE) is reported. Two different m‐PE materials were subjected to tensile stresses above the yield point by zone drawing in the temperature range from 50 to 100°C. Drawn materials were characterized by using small‐ and wide‐angle X‐ray scattering (SAXS, WAXS), molecular retraction, and small‐angle light scattering (SALS). Structural changes were studied as a function of drawing temperature, engineering stress, and draw ratio. WAXS showed strong crystalline orientation in drawn samples, and only the orthorhombic crystal modification was observed. SAXS showed lamellar orientation in drawn samples. At low drawing temperatures of 50 or 60°C, draw ratio increased as a step function of stress. There is a stress barrier, which must be exceeded before high‐draw ratios can be achieved at these temperatures. At drawing temperatures of 70°C or above, the barrier stress is low enough that draw ratio increases nearly linearly as a function of stress. Below the stress barrier, spherulitic structure is observed by small‐angle light scattering (SALS). Elongation occurs via deformation of the interspherulitic amorphous phase. Molecular retraction was low for these samples, indicating mostly plastic deformation of the amorphous material. Above the stress barrier, SALS showed that spherulites are destroyed. Elongation occurs via deformation of the intraspherulitic amorphous phase. Molecular retraction for these samples was high, indicating elastic deformation of the amorphous material. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3492–3504, 2001     

Stretching induced phase separation in poly(vinylidene fluoride)/poly(butylene succinate) blends studied by in-situ X-ray scattering

The structure evolution of poly(vinylidene fluoride)/poly(butylene succinate) (PVDF/PBS) blends during stretching above the melting point of PBS is investigated by synchrotron-based simultaneous wide angle and small angle X-ray scattering (WAXS/SAXS). Before stretching, PVDF crystallizes into the α-form, whereas the chains of molten PBS locate at the inter-lamellar amorphous phase of PVDF. Crystal transition from α to β of PVDF is observed in all samples during stretching. The morphological transformation from a lamellar structure into a fibrillar structure occurs at low and intermediate strains. With further deformation, a “stretching induced phase separation” phenomenon is observed. The final microstructure of PVDF/PBS blends contains PVDF microfibrils with PBS chains preferentially distributed in the inter-fibrillar region. The PBS molecular weight influences the onset and end strain for the transition. A new “two-step model” is proposed to describe the deformation process.