Structure and properties of MDO stretched polypropylene

Two polypropylene cast films of different crystalline structures (one with coexisting small rows of lamellae and spherulites and the other with only a spherulitic structure) were prepared by extrusion. The produced cast films were uniaxially hot drawn at T = 120 °C using a machine direction orientation (MDO) unit and the changes in structure and morphology were examined and related to barrier as well as tear and puncture properties. Structural changes in terms of the degree of crystallinity and crystal size distribution, orientation of the amorphous and crystalline phases, and the deformation behavior at the crystal lattice and lamellae scales were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), respectively. A significant effect of the original crystal morphology on the alignment of the amorphous and crystalline phases was observed from FTIR and WAXD. The results also revealed that the deformation behavior of the crystal structure was dependent on the draw ratio (DR). Our findings showed that by increasing DR the crystal lamellae first broke up and oriented along the drawing direction and then, at large DR, they were deformed and created a fibrillar structure. Morphological pictograms illustrating the effects of original morphology and draw ratio on the stretched film microstructure are proposed. The tear resistance along the machine direction (MD) decreased significantly with increasing DR whereas the puncture resistance increased drastically. Finally, the oxygen transmission rate (OTR) of the MDO stretched films could be correlated with the orientation parameters as well as the β-relaxation peak magnitude of the amorphous tie chains.     

Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation

Cast films of a high molecular weight linear polypropylene (L-PP) were prepared by extrusion followed by stretching using a chill roll. An air knife was employed to supply air to the film surface right at the exit of the die. The effects of air cooling conditions, chill roll temperature, and draw ratio on the crystalline orientation, morphology, mechanical and tear properties of the PP cast films were investigated. The crystallinity and crystal size distribution of the films were studied using differential scanning calorimetry (DSC). It was found that air blowing on the films contributed significantly to the uniformity of the lamellar structure. The orientation of crystalline and amorphous phases was measured using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The amount of lamellae formation and long period spacing were obtained via small angle X-ray scattering (SAXS). The results showed that air cooling and the cast roll temperature have a crucial role on the orientation and amount of lamellae formation of the cast films, which was also confirmed from scanning electron microscopy (SEM) images of the films. Tensile properties and tear resistance of the cast films in machine and transverse directions (MD and TD, respectively) were evaluated. Significant increases of the Young modulus, yield stress, tensile strength, and tensile toughness along MD and drastic decreases of elongation at break along TD were observed for films subjected to air blowing. Morphological pictograms are proposed to represent the molecular structure of the films obtained without and upon applying air cooling for different chill roll temperatures. Finally, microporous membranes were prepared from annealed and stretched films to illustrate the effect of the PP cast film microstructure on the morphology and permeability of membranes. The observations of SEM surface images and water vapor transmission rate of the membranes showed higher pore density, uniform pore size, and superior permeability for the ones obtained from the precursor films prepared under controlled air cooling.     

Processing-structure-property investigation of blown HDPE films containing both machine and transverse direction oriented lamellar stacks

A series of blown films were prepared using two high density polyethylene resins of differing molecular weight and molecular weight distribution, using a high stalk process. Both the resins were processed at three frost line heights (FLHs) and three draw down ratios to determine the effect of processing parameters and resin characteristics on final film morphology and mechanical properties. By changing the FLH and the time to initiate transverse direction (TD) expansion, the relative number of lamellae stacked both perpendicular and parallel to the machine direction (MD) could be controlled for a constant blow up ratio (BUR) of 4:1. It was determined that the proportion of lamellae stacked parallel to the MD increased with increasing FLH. This effect was found to be related to the relaxation behavior of the melt and bubble shape. Increasing the stress in the stalk region was observed to lead to a reduction in stalk diameter just prior to bubble expansion, resulting in a greater effective BUR. Film morphology was observed to strongly influence end mechanical properties. Elmendorf tear resistance was found to increase in the MD and decreased in the TD with increasing FLH. The dart impact strength of these films was characterized with the surprising result that for one of the two resins studied, the dart impact increased with decreasing film gauge.     

Ternary blends of high‐density polyethylene–polystyrene–poly(ethylene/butylene‐b‐styrene) copolymers: Properties and orientation behavior in plane–strain compression

Ternary blends of high‐density polyethylene (HDPE) with atactic polystyrene (PS) and styrene–ethylene/butylene–styrene block copolymer (SEBS) were deformed by plane–strain compression in a channel die. The samples were deformed up to the true strain of 1.8 (compression ratio of 6) at 100°C. Thermal and mechanical properties of the deformed blends were studied in addition to the study of the deformation process. The basic mechanism of plastic deformation is crystallographic slip, the same as that active in deformation of plain HDPE and binary blends of HDPE and PS. This slip is supplemented by the plastic deformation of an amorphous component. In blends of high SEBS content, the role of deformation of an amorphous component by shear and flow increases markedly due to reduced overall crystallinity of these blends. In such blends an amorphous component includes a semicontinuous embedding of crystallites, and therefore, the deformation process is dominated by deformation mechanisms active in a more compliant amorphous phase. Consequently, with increasing the content of SEBS in the blend, the texture of the oriented blends changes from a single‐component (100)[001] texture to a texture with a strong fiber component in addition to a (100)[001] component. In blends with high content of SEBS, the crystalline lamellae of polyethylene do not undergo fragmentation up to the compression ratio of 6, while in blends with low and moderate content of SEBS, such lamellar fragmentation was detected. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1746–1761, 2000     

Plastic deformation behavior of β-phase isotactic polypropylene in plane-strain compression at room temperature

Isotactic polypropylene (iPP) rich in β crystal modification (constituting 92% of crystalline phase) was deformed by the plane-strain compression with constant true strain rate, at room temperature. The evolution of phase structure, morphology and orientation was studied by DSC, X-ray and SEM.

The deformation sequence and the active deformation mechanisms were found out. The most important mechanisms were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands due to localization of deformation and the crystallographic slip systems, including the (110)[001] chain slip and (110)[10] transverse slip.

Shear within deformation bands leads to β → smectic and β →  solid state phase transformations. At room temperature the β → smectic transformation appeared to be the primary transformation, yielding the oriented smectic phase with high concentration of 19 wt.% at the true strain of e = 1.49. The β →  yields only about 4 wt.% of new -phase at the same strain. As a result of the deformation and phase transformation within numerous fine deformation bands β-lamellae are locally destroyed and fragmented into smaller crystals.

Another deformation mechanism is the cooperative kinking of lamellae, leading to their reorientation and formation of a chevron-like lamellar arrangement.

At high strains, above e = 1, an advanced crystallographic slip and high stretch of amorphous material due to interlamellar shear bring further heavy fragmentation of lamellar crystals, earlier fragmented partially by deformation bands. This fragmentation is followed by fast rotation of small unconstrained crystallites with chain axis towards the direction of flow, FD. This process leads to development of the final texture of the highly deformed β-iPP with molecular axis of both crystalline and smectic phases oriented along FD.     

Real-time in situ light scattering and X-ray scattering studies of polyethylene blown film deformation

A series of linear low-density polyethylene blown films were studied using the techniques of time-resolved, small-angle X-ray scattering (SAXS) using a synchrotron source and a time-resolved, small-angle light scattering. Scattering patterns and the load-extension curve were obtained simultaneously during deformation. It was found that the initial orientation of the film, with respect to the tensile axis, was important in determining the operative elastic deformation modes. Films drawn parallel to the machine direction (MD) showed evidence for lamellar separation, whereas interlamellar shear occurred in films drawn parallel to the transverse direction. In films drawn at 45° to MD, lamellar stack rotation was observed via SAXS. In all cases, the yield point corresponded to the activation of crystallographic deformation and the onset of the disruption of crystalline lamellae. In films drawn parallel to MD, the SAXS showed a distinct 4-point pattern upon macroscopic yield, indicating lamellar corrugation. Regardless of the initial orientation, a fibrillar morphology was achieved at some strain after yield that coexisted with the fragmenting lamellar morphology. Comparison of results from deformed spherulitic bulk samples showed that the study of oriented blown film containing a stacked lamellar morphology may be used, to a first approximation, as a model for the deformation of different regions of spherulites in unoriented spherulitic samples. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 321–339, 1998     

Zur struktur von schlauchfolien aus polyäthylen

The crystal orientation and the morphology of a tubular extruded LDPE film and two HDPE films, each of different molecular weight and different blow ratio, were investigated by means of small- and wide-angle X-ray techniques and, after staining, by electron microscopy. Only by combining these methods the structure of the films can be described in detail. In the LDPE film the orientation of the a-axis parallel to the machine direction is dominant. The morphology is more complicated than is usually supposed from X-ray measurements. The lamellae show a variety of orientations. The periodicity of the lamellae is more strongly marked in the machine direction than in all other directions. The morphology of one of the HDPE films consists of biaxially oriented stacked lamellae, their normals preponderantly lying in the plane of the film.In the other HDPE film the normals of the lamellae lie in the film plane, too, but they are oriented preponderantly in the machine direction. The distribution of the crystal orientations as determined from the pole figures corresponds to the electron microscopie results. An essential part of the morphology determining the mechanical properties of the film are the shish-kebabs. They form in the strained melt. The observed greater X-ray periods and radial intensity bridges in the small-angle region of the HDPE-films are due to greater periods along thc shish-kebabs.     

Plastic deformation of oriented lamellae: 1. Cold rolling of β-phase isotactic polypropylene

Isotactic polypropylene was crystallized by the oriented growth method and the oriented β-phase obtained. This has unidirectional lamellar orientation with the lamellar long axis parallel to the growth direction, the lamellae being twisted along this direction. The sample plates were cold-rolled in three orthogonal directions, and the deformation behaviour of each case was investigated chiefly by wide-angle and small-angle X-ray diffraction methods. It was revealed that deformation takes place by a different mechanism in each case, including rotation of lamellae, interlamellar slip, chain-directional and transversal chain slip. These results are discussed in connection with the anisotropic structure of these samples due to the lamellar orientation.When the β-phase samples are rolled, α-phase crystals appear with c-axis orientation and the proportion increases with draw ratio. For crystallographic reasons it is concluded in this case that by stretching the c-axis orientation is brought about not through block formation of the original β-phase lamellae and incorporation of these blocks into microfibrils, but by melting or unfolding of the original β-phase lamellae and recrystallization to the c-axis-oriented new α-phase.     

Morphology and texture of high-density polyethylene–polystyrene blends deformed by plane-strain compression

The blends of high-density polyethylene (HDPE) with atactic polystyrene (PS) were deformed plastically by plane-strain compression in a channel die. The samples were deformed up to the true strain 1.8 (compression ratio 6) in three temperature regimes: below, near, and above the glass transition temperature of polystyrene component. The morphology and the texture of crystalline component in the deformed blend samples were investigated by means of scanning electron microscopy and wide angle X-ray diffraction (pole figures technique). It was found that the deformation process in the blend of immiscible HDPE and PS does not differ markedly from the deformation of the one-component system from the point of view of the deformation mechanisms involved. The crystalline textures of the blend samples are qualitatively the same as in the plain HDPE deformed under similar conditions. The active deformation mechanisms are the same in deformation of both the plain HDPE and HDPE/PS blend. The mechanism identified are crystallographic slips: (100)[001], (100)[010], and (010[001] supported by the interlamellar slip. The presence of PS in blends modifies to some extent the deformation process and resulting orientation of the crystalline component of HDPE by modification of the stress distribution within HDPE matrix around PS inclusions. This influence is much stronger at low deformation temperatures, when PS is in a glassy state, than at temperatures above T_g of PS. © 1996 John Wiley & Sons, Inc.     

Studies on α to β phase transformations in mechanically deformed PVDF films

PVDF cast films were drawn at different temperatures to different draw ratios at constant draw rate to understand the mechanism of α to β phase transformation during mechanical deformation. WAXD and FTIR studies were carried out to determine the formation and content of β phase in the drawn films. Lower stretch temperatures gave higher fractions of β phase. The cast PVDF films were also drawn at suitable temperatures below the PVDF ambient melting point to the draw ratio of 6.4. The highest fraction of β phase obtained in these ultra drawn films was 0.98. SALS studies carried out for films at different stretch ratios show the change in spherulitic structure with the stretching parameters and give information for the understanding of phase transformation during stretching of PVDF films. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

On the plastic behavior of homogeneous ethylene copolymers compared with heterogeneous copolymers

The plastic behavior of homogeneous ethylene-octene copolymers and heterogeneous ethylene-butene copolymers has been compared on uniaxial tensile draw. True-stress-strain curves have been determined for various draw temperatures and strain-rates. Significant differences in yield behavior and strain-hardening have been revealed between the two types of materials, at equivalent crystallinity. In parallel, thermal analysis gave an indication that homogeneous copolymers have much narrow distributions of lamellae thicknesses together with lower values of the most probable thickness. Quantitative data from small-angle X-ray scattering support the latter point. An estimation of the surface free energy has revealed a more disordered chain-folding topology for the homogeneous copolymers. Interpretation of the mechanical behavior is based on the framework of a previous model for the plastic deformation of polyethylene involving competition of homogeneous and heterogeneous crystal slip processes. The former process governs the ability of the material to develop macroscopically homogeneous plastic deformation; the second process is a precursor to necking. It is proposed that the nucleation of dislocations, which is the basic mechanism of the homogeneous crystal slip, is favored for homogeneous copolymers, owing to the lower value of the most probable lamellae thickness. More tie molecules and chain entanglements in the disordered amorphous phase may also favor stable homogeneous plastic deformation in the case of homogeneous copolymers.     

The role of crystallinity in the crystallographic texture evolution of polyethylenes during tensile deformation

The crystallographic texture evolution of rapidly crystallized high-density polyethylene (HDPE) and ethylene-1-octene copolymer prepared with a Ziegler-Natta (LLDPE) catalyst, was studied during tensile deformation using wide-angle X-ray diffraction WAXD. The popLA software suite, a methodology based on spherical harmonics for texture analysis, was utilized to produce recalculated pole figures and orientation distribution function plots from the raw data. An important aspect of this work has been the in situ measurement of texture during tensile deformation and the subsequent measurement of texture in the relaxed samples. The difference in molecular structure of the polyethylenes had a strong effect in the initial texture as well as in the rate of texture evolution during deformation. Texture evolves slowly in the HDPE while a fast drastic change in texture is observed in LLDPE after yield. At higher strains the texture of LLDPE was basically unchanged revealing not only a strong (100)[001] ‘c-axis’ texture component, but also other weaker texture components such as (010)[001], (1×0)[010], (011)[100] and (201)[1?02]. Furthermore, while relaxation after unloading mitigated or eliminated two of the preferred texture components in HDPE strengthening the (001) component aligned along the extension axis, it was observed that the texture of LLDPE did not undergo any significant changes after relaxation. At large strains (ε>1.0), microfibers formed in both HDPE and LLDPE. A lamellar substructure that differs between the HDPE and LLDPE is observed by AFM images of drawn materials. The role of possible slip mechanisms and stress relaxation in the evolving crystallographic texture of these polyethylenes is discussed.     

Plastic deformation and tearing in high density polyethylene blown films

Polymer films produced by tubular film blowing have a unique morphology that results from the large elongational flow in melt draw down and biaxial orientation due to bubble blow-up. Three high density polyethylene (HDPE) blown films were produced under similar processing conditions from resins which varied principally in molecular weight (MW) and molecular weight distribution (MWD). Scanning electron microscopy (SEM) showed that the lower MW and narrower MWD resin produced film which had a uniaxial orientation of stacked lamellar crystals. The higher MW (HMW) and broad MWD resins produced films consisting of a network of nearly orthotropically oriented lamellar stacks. Greater high molecular weight fraction (MW > 10⁶) in the resin resulted in more random orientation. The influence of these different structures on properties was studied by examining the plastic zone formation at crack tips and uniaxial tensile deformation with the SEM and comparing them to the macroscopic stress-strain behavior. A continuous deformation of the network structure was observed in the HMW films. Lamellar deformation occurred primarily in regions of stacks oriented parallel to the tensile axis. Macroscopic yield occurred at 6 to 10 percent strain via a shearing and opening the lamellar crystals. Irreversible deformation occurred from ?50 to 400 percent strain by transformation of the oriented lamellae to microfibrils. Eventually all the lamellar stacks in the network become aligned with the tensile axis. This process was found to improve the tear resistance in the crack propagation experiments. The lamellar stacks in the network orient perpendicular to the crack independent of crack propagation direction, insuring a more uniform transmission of stress and preventing local yielding. The tensile modulus, yield stress, and ultimate strength were highest in the film containing more high molecular weight polymer.     

Evolution of lamellar orientation and crystalline texture of various polyethylenes and ethylene-based copolymers in plane-strain compression

Deformation-induced orientation behavior of a series of polyethylenes and ethylene copolymers was studied by WAXS and SAXS. Plane-strain compression was employed as a deformation mode. Compression experiments were performed at room temperature and at 80 °C. Activity of crystallographic slip systems supported by interlamellar shear during deformation process resulted in formation of the (100)[001] primary texture component with chain axis aligned along the flow direction, FD, and (100) planes perpendicular to the loading direction, LD. A second texture component was produced on unloading by {310} twinning of primary component. Intensity of that twinning was controlled primarily by the molecular weight of polymer, through modification of the molecular network and its stress response on applied strain. Molecular weight influenced also the orientation behavior on the lamellar level. In all polymers studied the lamellar shear, intensive on the beginning of deformation and exhausting around the true strain of 0.75, induced cooperative kinks of lamellae. In samples of relative low molecular weight further advancement of deformation led to an additional intense fragmentation of lamellar structure and restructurization into new long period in the direction of flow (e>1.2). In polymers of higher molecular weight such an intense fragmentation did not occur due to topological constraints. Consequently, the lamellar structure of heavily deformed polyethylenes of low molecular weight consisted of relatively small blocks, oriented with their normals approximately along FD, while high molecular weight species, in absence of intense lamellae fragmentation, developed the structure of relatively long lamellar crystals, oriented in a chevron-like structure. It was found that the orientation produced by deformation process is controlled entirely by the strain applied to the material, while the differences induced by modification of the stress due to the change of the deformation temperature are of minor consequence.     

Preparation by chemical solution deposition and transparent conducting properties of LaRh1-xNixO3 thin films

Transparent conducting thin films have been widely used in lots of fields. The absence of high-performance hole-type transparent conducting thin films, however, seriously limits the wider applications. LaRhO₃ as a type of perovskite material shows hole-type conduction with semiconductor-like properties and no investigations have been carried out about transparent conducting properties on LaRhO₃ thin films. Here, LaRh_1-xNi_xO₃ (x = 0, 0.05, 0.1) thin films were firstly deposited by chemical solution deposition, showing epitaxial growth on single crystal SrTiO₃ (001) substrates with the epitaxial relationship of LaRhO₃(001)[110]||SrTiO₃(001)[110]. With the doping of Ni element, the surface morphology became denser. Hall measurements confirmed that the hole concentration was enhanced with Ni doping, resulting in the decreased resistivity. Low resistivity of 17.3 mΩ cm at 300K was obtained for the LaRh_0.9Ni_0.1O₃ thin films. The electrical transport mechanisms were investigated, showing thermal activation at high temperatures and variable range hopping model for the doped thin films at low temperatures. The transmittance within the visible range for all thin films was higher than 50%. The results will provide a feasible route to deposit hole-type transparent conducting LaRhO₃-based thin films.     

Near-surface morphology effect on tack behavior of poly(styrene-b-butadiene-b-styrene) triblock copolymer/rosin films

The correlation between near-surface morphology and tack behavior of poly(styrene-b-butadiene-b-styrene) triblock copolymer (SBS)/rosin ester films was investigated using probe tack tests, transmission electron microscopy and small-angle X-ray scattering. The SBS/rosin films with rosin composition between 10 and 20 wt% rosin, prepared by slow evaporation of toluene during solvent casting, exhibited uniform near-surface morphology of lamellae oriented parallel to the surface. However, due to the limited solubility of rosin in the PS domains, the rosin started to phase-separate from the PS domains at 15 wt%, and formed fully separated micron-sized domains above 20 wt% rosin. The probe tack force of the SBS/rosin films increased steadily when the near-surface domain orientation changed from perpendicular cylinder to parallel lamellae on addition of rosin. Specifically, for a given lamellar morphology and surface orientation, macrophase separation of rosin plays a critical role in determining the tack properties of SBS/rosin films.     

Lamellar deformation and its variation in drawn isolated polyethylene spherulites

Observations of individual lamellae within spherulites of linear polyethylene, drawn under affine conditions between room temperature and ∼100 °C, show lamellae surviving to sample failure, thereby providing a strong memory of the initial morphology in the final product. Lamellae rotate and deform according to the angle their plane makes with the draw direction. Those parallel to the draw direction extend, to the full draw ratio, by shear in the basal plane, probably in {110} planes and at constant lamellar thickness. The same mechanism appears to occur for lamellae at higher angles with chain slip expected increasingly to operate. This latter mechanism is responsible for lamellar thinning, which becomes universal in elongated lamellae at higher draw ratios. Lamellae whose planes are transverse to the draw direction contract, with kinking—also by chain slip—to produce bands of sheared lamellae in spherulite centres. The temperature of drawing has little pronounced effect on the drawn morphology unlike draw rate whose influence is evident. Faster draw produces more severe local damage and less-well-organized co-operative kinking. The amelioration of these effects at a slower rate is attributed to molecular mobility and the influence of the surrounding molecular network.     

Modification of polylactide upon physical properties by solution-cast blends from polylactide and polylactide-grafted dextran

Polylactide (PLA)-grafted polysaccharides with various lengths and numbers of graft chains were synthesized using a trimethylsilyl protection method. The properties of the cast films prepared from graft-copolymers were investigated through thermal and dynamic mechanical analyses. The graft-copolymer films exhibited a lower glass transition temperature (T_g), melting temperature, and crystallinity, and higher viscosity properties compared to PLA films. Moreover, the usefulness of graft-copolymer as a plasticizer was investigated with 1:4 blend films prepared from the graft-copolymers and PLA. The blend films showed lower T_g and crystallinity, and higher viscosity properties compared to PLA films.     

Conformational development of polylactide films induced by uniaxial drawing

Uniaxially drawn polylactide (PLA) films were prepared using various draw ratios (2, 3, 4 and 4.5) at a constant draw rate and temperature. It was confirmed that the conformational structure of the PLA films prepared using the uniaxial drawing process was composed of the α′‐phase form. The conformational structure deformation of α′‐phase PLA films, according to the various draw ratios used, was investigated in terms of strain‐induced crystalline behavior and molecular orientation analysis. It is of utmost importance to confirm the α′‐phase structural deformation caused by the uniaxial drawing process because it directly relates to the characteristics of PLA films. The conformational structure deformations of the α′‐phase, created by uniaxial drawing, led to improved mechanical properties, as evident from mechanical testing and dynamic mechanical analyses results. © 2013 Society of Chemical Industry     

Cavitation in Irradiated and Oriented High-Density Polyethylene Cast Films with Stacked Lamellar Morphology

Cavitation (void formation) in high-density polyethylene cast films, having a stacked lamellar morphology, subsequent to tensile stretching of both nonirradiated and irradiated films was investigated. With nonirradiated films, the cavitation was not observed in the nonannealed films with lower draw ratios, while a limited number of voids was detected in the films of higher draw ratios. Annealing caused an intensive cavitation, and uniform distribution of cavities was noticed in highly oriented films. It was shown that cross-linking exerted a suppressive influence on cavitation in the annealed and oriented films; and with increasing the irradiation dose, the cavitation was completely eliminated.