Epitaxial growth of isotactic polypropylene on oriented polyethylene from solution

Oriented polyethylene (PE) films with surfaces bounded by the (100) plane were prepared. On the film surfaces, isotactic polypropylene (iPP) was crystallized epitaxially from solution as quadrits with their sides parallel and perpendicular to the polyethylene chain axis. In the through wide-angle x-ray diffraction pattern (taken with incident x-rays normal to the polyethylene film surface), the 111 iPP reflections was observed on the meridian (Parallel to the polyethylene chain axis). In the edge patterns (taken with x-rays incident on the edge of the polyethylene film), 040 and 060 reflections were observed on the equator. From the diffraction patterns, the following lattice coincidence was observed between polyethylene and isotactic polypropylene: (010)iPP//(100) PE, [101]iPP//[001] PE. The Small-angle x-ray scattering patterns showed that edge-on isotactic polypropylene lamellae 9 nm thick were arranged with their long axes inclined at an angle of 40° from the polyethylene axis. Molecular chains were oriented within the lamellae normal to the surfaces.     

Deformation of oriented polyethylene

The deformation of polyethylene in terms of structural processes has been investigated by low-and wide-angle x-ray diffraction in the case of low-density and, to a lesser extent, high-density polyethylene. The samples possessed a range of simple textures which enabled the deformation processes to be identified. The results are interpreted in terms of a model of stacks of lamellae which have axes along the original draw direction and which deform by lamellar slip, chain slip, and lamellar separation. In most cases these processes accounted for the macroscopic strain but in some cases discrepancies were observed which could be accounted for by inhomogeneous deformation or by the effects of a distribution of lamellar thicknesses. Attempts were made to identify fibrillar slip, without success. The relative contributions of the various deformation processes are examined as a function of temperature and sample treatment by defining a compliance constant for each process. Below room temperature, the results are consistent with expectations based on the α and β mechanical relaxations, whereas the unusual effects at high temperatures are attributed to gradual melting. The compliance constants are also found to depend on the annealing temperature of the sample, and are used to predict the mechanical anisotropy. The volume changes accompanying lamellar separation are examined. They were less than expected in low-density polyethylene, but satisfactory agreement was obtained in high-density polyethylene. A general relation is suggested between volume changes and the lateral development of the lamellae. Hence in narrow lamellae the interlamellar layer can contract laterally whereas the greater constraints imposed by wide lamellae lead to void formation. Other effects examined include the reversibility of the processes which is most marked in the case of chain slip and which is explained by the presence of restoring forces in the amorphous regions including the fold surface. Finally, the differences between low- and highdensity polyethylene are highlighted, emphasizing the part played in the deformation by the amorphous component.     

Deformation band studies of oriented polyethylene and polyethylene terephthalate

Deformation bands formed at the yield point in tensile tests on oriented high-density polyethylene have been studied by optical microscopy and wide-angle x-ray (WAXS) diffraction. The observations of the rotation of the optical extinction direction are shown to obey a simple scheme proposed previously by us: the principal directions of the refractive index ellipsoid within the deformation bands are everywhere parallel to the principal axes of the plastic strain ellipsoid, zero strain referring to the isotropic state. This result is similar to that obtained previously for polyethylene terephthalate (PET) and polypropylene despite the much higher crystallinity obtained with polyethylene. Independent measurements of the molecular reorientation in the deformation bands made using wide-angle x-ray scattering broadly confirm the optical measurements. The results taken together suggest that the material within the band, whether crystalline or not, becomes realigned about the new direction of maximum elongation as if controlled by the deformation of an effective molecular network.     

Redrawing of oriented polyethylene film

Drawn and subsequently annealed polyethylene film was restretched along the original draw axis at various temperatures. The internal deformation was analyzed in terms of the structural parameters of a simplified model. The elementary deformations are the rotation of crystals around the b axis and shear at the crystal interface. The rigidity of the crystal plays an important role during extension; and as a result, disorientation of chains in the crystal occurs at high strain. At the same time, crystals deform in such a way that the crystalline chains tilt about the b axis along the (h00) plane. This deformation of the crystal is affected by temperature. The increase in long spacing with extension can be interpreted roughly by the changes in structural parameters. The strain in amorphous region in also discussed in relation to these parameters.     

Self-hardening of highly oriented polyethylene

Ultra-oriented polyethylene fibers obtained by drawing to approximately 30 times their original length have a Young's modulus of approximately 800 kbar. Such fibers, if unconstrained, contract on heating to a length near the original. We have studied the forces causing this contractile behavior by monitoring the stress in the fiber while maintaining it at constant length. In the course of this we observed a complex sequence of both reversible and irreversible behavior. In the reversible case we observed first energy and then entropy elastic behavior. The most significant feature observed is that at sufficiently high temperature the fiber stress relaxes to an unmeasurably low value. A fiber allowed to relax in this way possesses a much lower room temperature tensile modulus (ca. 80 kbar) immediately after relaxation but, remarkably, this modulus increases to approach the initial high value over a period of a few hours when the fiber is stored either clamped or unclamped at room temperature. High x-ray orientation is preserved throughout the storage period but the density which dropped during the stress decay rose again in the course of the spontaneous stiffening. None of the stress relaxed fibers displays large-scale contractile behavior on subsequent heating. A phenomenological composite model is proposed which involves stiff microfibrils of short length—surrounded by a matrix present as a minority component. The softening of this matrix on heating and its subsequent stiffening on storage, involving a certain amount of melting and recrystallization, respectively, could then be responsible for the observed variations in the macroscopic tensile properties using simple fiber composite theories. The fibers are likely to be of extended-chain type produced by the initial drawing while the matrix may consist of a combination of oriented amorphous material (tie chains), randomly oriented chains, and transverse lamellar overgrowth present in varying proportions in the different stages of sample treatment. The wider implications, fundamental and practical, of this remarkable self-hardening process are indicated.     

Double crystallization of biaxially oriented polypropylene

The melting conditions of biaxially oriented polypropylene (BOPP) have a very strong effect on the courses of both non-isothermal and isothermal crystallization from the melt. It has been shown that the crystallization proceeds by two mechanisms. Depending on the melting conditions, both crystallization mechanisms can proceed individually or simultaneously (double crystallization). The high rate mechanism, occurring in non-isothermal crystallization at low undercooling and observed in isothermal crystallization, can be attributed to the ordered structure of the melt preserved after melting, presumably a mesomorphic state. The second component corresponds to crystallization from the isotropic melt.Anomalous crystallization of BOPP samples can be eliminated by energetic heat treatment, by the destruction of ordered structures. As a result of this treatment, the characteristics of crystallization and its quantitative parameters will become similar to those of unoriented polypropylene.

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Zusammenfassung Die Schmelzbedingungen haben bei biaxial orientierten Polypropylenen (BOPP) eine sehr starke Wirkung auf den Verlauf der sowohl nichtisothermen als auch der isothermen Kristallisation aus der Schmelze. Es wurde gezeigt, daß die Kristallisation nach zwei Mechanismen verläuft. In Abhängigkeit von den Schmelzbedingungen können beide Kristallisationsmechamismen einzeln oder simultan verlaufen (doppelte Kristallisation). Der Hochgeschwindigkeitsmechanismus, der bei der nichtisothermen Kristallisation bei geringer Unterkühlung auftritt und bei der isothermen Kristallisation beobachtet wird, kann der geordneten Struktur der Schmelze zugeschrieben werden, welche nach dem Schmelzen bis zu einem mesomorphen Zustand erhalten bleibt. Die zweite Komponente entspricht der Kristallisation aus isotroper Schmelze.Die anomale Kristallisation der BOPP-Proben kann durch energische Hitzebehandlung unter Zerstörung der geordneten Strukturen eliminiert werden. Als Ergebnis dieser Behandlung werden die Charakteristika der Kristallisation und ihre quantitativen Parameter denen von nicht-orientierten Polypropylenen ähnlich.

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Résumé Les conditions de la fusion du polypropylène à orientation biaxiale (BOPP) ont un effet très important sur le déroulement de la cristallisation isotherme ou non, à partir du produit fondu. On montre que la cristallisation s'effectue suivant deux mécanismes. Suivant les conditions de fusion, les deux mécanismes peuvent intervenir seuls ou simultanément (cristallisation double). Le mécanisme à grande vitesse observé lors de la cristallisation isotherme, et lors de la cristallisation non isotherme avec une faible surfusion, peut être attribué à la structure ordonnée, préservée dans le produit fondu après la fusion, probablement sous un état mésomorphe. Le second mécanisme correspond à la cristallisation à partir d'un produit fondu isotrope.La cristallisation anormale des échantillons de BOPP peut être éliminée par un traitement thermique énergique, par destruction des structures ordonnées. Comme résultat de ce traitement, les paramètres quantitatifs et les caractéristiques de la cristallisation deviennent similaires à ceux du polypropylène non orienté.

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Deformation and structure of doubly oriented polyethylene

Doubly oriented low-density polyethylene with parallel lamellae was compressed along the initial draw direction (i.e., at right angles to the lamellar surfaces) at 20°C. Wide- and low-angle x-ray diffraction were used to determine the changes in the molecular orientation and in the texture. During the compression, specimens previously annealed at or near 102°C were found to undergo changes in length, in long spacing, and in molecular orientation which were consistent with an (001) chain slip mechanism. In specimens annealed at higher temperatures x-ray diffraction indicated that during compression some series component of the long spacing was compressed by a much smaller amount than the remainder of the long spacing, which deformed by chain slip; in these cases it was found that the macroscopic strain along the compression axis (ε_y) was greater than the strain in the long spacing along that axis (ε_d). It is suggested that the missing strain which makes ε_y greater than ε_d is due to partial melting and the consequent development of amorphous regions between the stacks of lamellae.     

Form birefringence in biaxially oriented polypropylene

The birefringence of several biaxially oriented polypropylene films swollen with a number of fluids has been measured and found to exhibit a minimum when plotted against the fluid refractive index, as predicted by the theory of Wiener. However, a discrepancy in the form birefringence behavior is observed when samples of different degrees of crystallinity but the same total birefringence are compared. These results are interpreted in terms of Bullough's theory and suggest that this discrepancy arises because of different morphologies. A refractometric technique was employed that makes possible the simultaneous determination of birefringence and the volume fraction of fluid.     

Recycled injection-molded and fibre-reinforced polyethylene/polypropylene

In this presentation, we are concerned with the fabrication and characterization of samples of compression-molded and injection-molded recycled polyethylene, with, in some cases, addition of cellulose fibre. We wish to know what is the performance of such plastics, as compared to the virgin materials and what is the effect of added fibre in both molding modes.     

Oxidation of polyethylene,polypropylene, and copolymers

This investigation of the autoxidation of ethylene–propylene copolymers and polyethylene–polypropylene mixtures was undertaken to determine whether reactivity is a linear function of composition. The copolymers and the mixtures were autoxidized in a trichlorobenzene solution at 100°C in the presence of 1,1′-azodicyclohexanecarbonitrile, and the rates of oxygen absorption were determined. The reactivity of the copolymers and the mixtures, after the underlying absorption of oxygen by initiator radicals is accounted for, is a nearly linear function of composition; however, the polymer mixtures and copolymers oxidized somewhat less readily than predicted by a straight line relationship. Several additional oxidations were performed on solutions of polypropylene so that the effects of initiation rate and substrate concentration could be evaluated. The oxidation kinetics of polypropylene even in dilute solution, are complex; titratable hydroperoxide yields are low. Further work will be required to specify the mechanism of oxidation.     

Sorption and diffusion of toluene in highly oriented polypropylene

The sorption and diffusion of toluene vapor at 30°C in polypropylene with draw ratios from 1 to 18 have been studied. Drawing leads to the transformation of the initially spherulitic material into the fibrous structure, with many taut tie molecules lying mainly on the outer boundary of the microfibrils. The free volume and hence the sorption sites are thereby reduced, and the microfibrils become less and permeable as the draw ratio increases. As a result, the equilibrium concentration and the zero-concentration diffusion coefficient drop by factors of 4 and 30, respectively. The diffusion coefficient increases exponentially with toluene concentration but the concentration dependence becomes weaker with increasing draw ratio, indicating that the severely constrained chain segments in the drawn samples have much less freedom to mix with penetrant molecules. Annealing relaxes the tie molecules and thus restores the sorption and diffusion properties to values corresponding to completely relaxed amorphous component, i.e., to values even higher than those of the undrawn but quenched material.     

Catalytic degradation of polyethylene and polypropylene to fuel oil

Thermal and catalytic degradation of plastic polymers, polyethylene and polypropylene to fuel oil were carried out in batch operations. The catalysts employed were acid silica-alumina (SA-1, SA-2) and zeolite ZSM-5, and non-acidic mesoporous silica FSM (folded sheet material). The yields of product gas, liquid and residues, recovery rate of liquid products, and boiling point ranges of liquid products due to degradation were compared with those of non-catalytic thermal degradation. Both the effect of catalytic contact mode and of catalyst type on the degradation were studied. In liquid-phase degradation of PP over SA-1, liquid hydrocarbon products were obtained in a yield of 69 wt.-% with a boiling point range 36-270°C, equivalent to the boiling point of normal paraffins C6 - C15. The liquid products from catalytic degradation have a carbon-number distribution very similar to commercial motor gasoline. In vapor-phase contact, the yield of liquid products was much lower (54%) and the rate of liquid recovery was much slower. With FSM, the initial rate of degradation of PP and PE to liquid products was as fast as that over acid catalyst SA-1, but the yield of liquid products was higher. The liquid products from catalytic degradation over FSM have a carbon-number distribution similar to kerosene and diesel oil. In repeated use, SA-1 deactivated very rapidly due to coke deposition on the catalyst, whereas FSM deactivated much more slowly. These findings suggest that mesopores surrounded by the silica sheet may act as reservoir for radical species, which accelerate the degradation of plastic melt.     

Vacuum-ultraviolet induced oxidation of the polymers polyethylene and polypropylene

The emission from low-pressure microwave plasmas in the vacuum-ultraviolet (VUV) region (λ < 200 nm) was investigated in order to use these plasmas as light sources for the study of the VUV photochemistry of polyethylene (PE) and polypropylene (PP) as part of the study of plasma-polymer interaction. These polymers, immersed in low-presure oxygen, were exposed to radiation with wavelengths down to 112 nm, the cut off of magnesium fluoride used as a window to separate the polymer specimen from the plasma light source. Total oxygen incorporation in the surface [O], and the formation of hydroxyl, carbonyl, and carboxyl groups were measured using XPS in combination with chemical derivatizations, particularly their dependence upon the radiation spectrum and the oxygen pressure around the sample. In most experiments the surface oxygen concentration [O] attained a constant value that appears to be related to the initial oxidation rate; this suggests a competition between oxygen incorporation and chain scission reactions, followed by the removal of volatile oxidation products. PE is usually oxidized to a higher level than PP, the latter appearing to be more susceptible to reaction with atomic oxygen than PE. A general initiation mechanism for the VUV experiments is proposed that allows us to explain the observed differences in behavior between PE and PP, and the results obtained under different irradiation conditions. The nature of oxidation products is in both cases very similar to what is observed after direct plasma treatment of the polymers. We conclude that short wavelength radiation contributes very appreciably to the observed surface modification effects during plasma treatment of PE and PP. © 1995 John Wiley & Sons, Inc.     

Epitaxial crystallization and oriented structure of linear low‐density polyethylene/isotactic polypropylene blends obtained via dynamic packing injection molding

In this work, as a part of a long‐term project aimed at controlling of crystal structure and phase morphology for a injection molded product, we investigated the oriented structure and possible epitaxial growth of polyolefin blend (low‐density polyethylene (LLDPE)/isotatic polypropylene (iPP)), achieved by dynamic packing injection molding, which introduced strong oscillatory shear on the gradually‐cooled melt during the packing process. The crystalline and oriented structures of the prepared blends with different compositions were estimated in detail through 2D X‐ray diffraction, calorimetry, and optical microscopy. As iPP was the dominant phase (its content was more than 50 wt%), our results indicated that it could be highly oriented in the blends. In such case, it was interesting to find that LLDPE epitaxially crystallized on the oriented iPP through a crystallographic matching between (100)_LLDPE and (010)_iPP, resulting in an inclination of LLDPE chains, about 50° to the iPP chain axis. On the other hand, as iPP was the minor phase, iPP was less oriented and no epitaxial growth between iPP and LLDPE was observed; even LLDPE remained oriented. The composition‐dependent epitaxial growth of LLDPE on oriented iPP could be understood as due to: (1) the effect of crystallization sequence, it was found that iPP always crystallized before LLDPE for all compositions; (2) the dependence of oriented iPP structure on the blend composition; (3) the “mutual nucleation” between LLDPE and iPP due to their partial miscibility. Copyright © 2009 John Wiley & Sons, Ltd.     

Crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene

The changes in crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene (UHMW PE) were investigated by means of wide-angle X-ray scattering. The orientation distribution of crystallites in drawn UHMW PE is composed of two components differing in width. The narrow and broad components revealed in this study indicate the existence of two classes of crystallites with different orientability. Some of the crystallites are oriented almost perfectly even at low-draw ratios, while the others do not orient so effectively. The analysis of melting behaviour of such a texture composed of orthorhombic crystals indicates that highly oriented crystallites are formed by taut molecules and transform first to the hexagonal phase, while the molecules constituting low-oriented crystallites melt directly to the typical amorphous phase. The increase in orientation of highly oriented crystallites during their partial melting, observed in the samples kept at constant length and even those allowed to shrink under constant load, can be explained by the kinetic factor proposed by Ziabicki. Received: 11 September 1998 Accepted in revised form: 18 February 1999     

Scanning electron microscopy of oriented high density polyethylene

Summary The microfibrillar and lamellar morphologies in cold-drawn and cold-drawn/annealed high-density polyethylene sheets were observed by means of scanning electron microscopy. Differences in contrast on fracture surfaces for cold-drawn sheet are interpreted in terms of a preferential orientation of inter-microfibrillar tie molecules in the plane of the sheet brought about by the drawing mechanism. In annealed, cold-drawn sheet, stacks of lamellae were observed which showed twinned orientations of inclined lamellae. This roof-top structure is interpreted in terms of shear within the individual microfibrils during micronecking, and corresponds to the well-known 4-point small-angle X-ray pattern for this type of specimen. Light etching with fuming nitric acid was necessary in order to resolve the individual lamellar texture.With 9 figures     

Mechanical behavior of a highly oriented polyethylene rod

Dynamic mechanical and single step stress relaxation data are presented for a highly oriented linear polyethylene rod as produced by a high pressure extrusion technique. The results indicate that both major relaxation processes normally observed in unoriented polyethylene are virtually absent in torsion, whereas they are present in flexure. Stress relaxation in torsion data show a very highly nonlinear behavior when compared to similar data for an unoriented polyethylene rod.     

A fiber composite model of highly oriented polyethylene

A fiber composite model of highly drawn polyethylene is presented. Quantitative predictions and calculations are made using shear-lag theory. The drawing process is shown to occur in two stages, a neck and a postneck taper. It is shown that there is an empirical linear relationship, with a high correlation, between the parameter x in shear-lag theory (which involves the aspect ratio of the reinforcing elements and the square root of the ratio of matrix shear modulus to the Young's modulus of the reinforcing elements) and the 3/2 power of the taper draw ratio. It is concluded that crystalline fibrils (the reinforcing elements) deform homogeneously during the secondary, taper drawing process. The increase in aspect ratio resulting from this homogeneous deformation is held to be responsible for the increase in tensile modulus owing to the increased efficiency of the fibrils as reinforcing elements. The model is also used to explain the self-hardening process exhibited by these fibers and, using measurements of density of hardened fibers, to predict that immediately after the neck the aspect (length to diameter) ratio of the crystalline reinforcing elements is ca. 2 and that the shear modulus of the matrix material in as-drawn fibers is ～10³N/m² and does not change significantly during the taper-drawing process.