Structural changes on hot-rolling of polyethylene. II. Crystallite size distribution and lattice distortions

The crystallite size distribution and lattice distortion in the [100] direction in hot-rolled high-density polyethylenes have been obtained by the analysis of the 200 x-ray line profile, using a Fourier transform technique. In the early stage of rolling (roll ratio <6) crystallite orientation is accompanied by breakup of crystallites due to intracrystallite slip, whereas in the heavily rolled stage (roll ratio ≧6) deformation proceeds through intercrystallite slip without degradation of crystallites. No marked change in lattice distortion can be found on hot-rolling, at least up to a roll ratio of 8. This shows that the lattice distortion in the rolled sample relaxes to the same extent as in the original lattice when the sample is processed in the crystalline relaxation (α_c) region.     

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     

Molecular and lamellar orientation in polyethylene thin films

Summary A combined wide (WA) and small angle X-ray diffraction (SAXS) study of melt compressed low density PE samples into the form of very thin films is reported. The WAXD patterns show an uniaxialb axis orientation normal to the film surface which can be interpreted in terms of a row structure in the plane of the film. The analysis of SAXS data indicates, in addition, a preferential orientation of bundles of stacked lamellae parallel to the film surface separated by longitudinal microvoids.With 3 figures     

Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low-density polyethylene,high-density polyethylene,and tetratetracontane heated in air

Samples of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and tetratetracontane (n-C₄₄H₉₀) free from additives were heated in air at temperatures between 120 and 180°C. As a comparison, “as received” HDPE containing unspecified additives has also been included. The structural changes have been studied with gel chromatography, viscometry, infrared spectroscopy, differential scanning calorimetry, and gravimetric measurements. LDPE, HDPE, and n-C₄₄H₉₀ follow the same course of thermooxidative degradation when they are free from additives and present in the molten state. Both molecular-diminishing and enlargement reactions occur. At temperatures below 150°C molecular enlargement is not observed until after rather long exposure times, whereas at higher temperatures enlargement occurs immediately. The difference is because “peroxide curing” becomes increasingly important above 150°C, whereas ester formation is operating at all temperature levels. Degradation below T_m is restricted to the amorphous phase that results in a different degradation pattern. In accelerated testing work extrapolations of the Arrhenius type in the prediction of structural change are thus not justified, even within the actual narrow temperature range. Neither are changes in commonly used standards like carbonyl content justified as a measure of the changes; for example, in mechanical properties. The stabilizer in the unpurified HDPE not only influences the induction period but also the course of the thermooxidative degradation.     

Crystallite orientation in stretched polychloroprene networks. II

The orientation of crystallites grown isothermally in several drawn trans-polychloroprene networks is studied as a function of crystallization temperature t_x, degree of crystallinity ω, and elongation ratio α. The orientation distribution is particularly simple for this polymer since the crystallographic c axis (chain axis) orients preferentially along the stretching direction, while a and b are randomly arranged about c. Hence the parameter cos² χ_c adequately characterizes the distribution, where χ_c is the angle between the c axis and the fiber axis, and the average is taken over all crystallites. A treatment due to Krigbaum and Roe is utilized to obtain values of v (the number of statistical segments comprising the crystallization nucleus of critical size) through comparison of the average orientation of crystallites and amorphous statistical segments. The behavior observed falls into two categories. First, if the initial amorphous network is well oriented, 〈cos² χ_c〉 is independent of crystallinity during both crystallization and melting, and v varies with t_z (or the degree of supercooling) as predicted by nucleation theory. If different networks are to have the same crystallite orientation distribution, they must not only be crystallized at the same supercooling, but must also have the same distribution of amorphous segment orientations. Both the relative elongation and the network crosslink density affect the latter distribution. Next, we consider the second category. If the initial amorphous orientation is poor, 〈cos² χ_c〉 decreases linearly during crystallization and increases along approximately the same path during melting. Further, 〈cos² χ_c〉 for a given t_z yields v values which are too large. These two behaviors can be explained if, in the former case, nucleation involves the best oriented statistical segments of all network chains, while in the latter there is a selection according to the chain displacement vector orientation. Thus, if the amorphous orientation is poor, both the orientation and thermodynamic stability of the crystallites decreases with further crystallization. If this decreased stability is reflected in shorter fold lengths, the reversible variation of long period spacing with temperature reported earlier for an oriented polychloroprene network can also be explained as a preferential melting process.     

Crystallite orientation effects in the 720/730 cm−1 doublet of transcrystalline polyethylene

Transcrystalline films of polyethylene were studied by polarized infrared spectroscopy with varying angles of incidence. The average orientation of the unit cells in the transcrystalline structure was shown to be such that the a axes were predominantly parallel to the surface of the films, and the b axes were predominantly perpendicular to the surface.     

Plastic deformation of polyethylene. I. Change of morphology during drawing of polyethylene of high density

The transformation of microspherulitic quenched and annealed polyethylene film into highly oriented drawn material with the characteristic fiber structure was investigated by small-angle and wide-angle x-ray measurements and by a study of the thermograms after the fuming nitric acid treatment. With the details of deformation depending slightly on the crystallinity, one observes generally a preferential tilt of the platelets against draw direction at draw ratios below 2. At least in annealed material, an increasing tilt of the molecule within the lamella is also observed, which leads at higher draw ratios to slipping of blocks in the crystallites. With further drawing a new fiber structure appears, which is practically independent of the thermal history of the original film. This fact is established by investigation of the crystal thickness by three different methods; investigations of small-angle scattering, study of the width of the (002) reflection, and investigation of the debris after treatment with fuming nitric acid.     

Influence of the repeated extrusion on the degradation of polyethylene. Structural changes in low density polyethylene

The influence of the repeated extrusion on the molecular parameters of low density polyethylene (LDPE) Bralen NA 7-25 was studied. Virgin polyethylene was submitted up to 20 extrusion cycles and the processed samples were fractioned using precipitation fractionation. Non-fractionated samples and the individual polymer fractions were characterized by their weight average molar masses M_w (static light scattering), number average molar masses M_n (osmometry) and limiting viscosity numbers [η] (viscometry). Rheological properties in terms of shear viscosity curve, zero shear viscosity and flow activation energy were also determined by using high pressure capillary rheometer. The course of the changes in molecular parameters of LDPE is influenced both by the initial polymer structure and by the changes induced by the mechano-chemical degradation. The suggested degradation mechanisms during multiple extrusion of Bralen are chain scission predominating in the early stage of processing followed by recombination of macromolecules resulting in crosslinking and formation of microgel, which is clearly notable for the samples extruded 3-20 times.     

Structural changes in polymer networks under deformation

The mechanism of changes in the molecular-mass distribution of interjunction chains in crosslinked poly(ether urethane) elastomers under deformation has been studied by the method of NMR spectroscopy. In the course of tensile drawing, the molecular-mass distribution changes nonmonotonically. The character of stress-induced changes shows that the breakdown of some bonds is accompanied by formation of new bonds and the development of network fragments with short chains. The strength and thermal stability of poly(ether urethane) elastomers are characterized by extremal dependences on the width of the molecular-mass distribution of interjunction chains. In the case of densely crosslinked and unannealed polyepoxides, mechanical impact (pressure and shear; grinding) is accompanied by changes in their supramolecular structure; in turn, these changes entail an increase in the thermal stability of the samples.     

Polymer deformation. XI. Biaxial deformation of polyethylene single crystals

Uniaxial deformation of polyethylene single crystals has been reported in the previous papers of this series. This paper presents an extension of this study to the simultaneous biaxial deformation of polyethylene single crystals. Diamond-shaped crystals containing {110} fold domains and truncated crystals containing in addition {100} domains were used in these experiments. The results show that these crystals fail at deformations as low as 6%, giving rise to cracks predominantly in the a direction. Electron diffraction patterns suggest that {310} twinning is more favorable than {110} twinning at the lower degrees of deformation. No phase change from orthorhombic to monoclinic unit cell is observed.     

Epitaxy and lamellar twisting in transcrystalline polyethylene

The aim of this study was to investigate the micro structure of the transcrystalline interphase, obtained under isothermal conditions, in polyethylene-based single-polymer microcomposites. Analysis of the angular distribution of intensity in the X-ray diffraction patterns, obtained from transcrystalline layers of varying thickness, indicate that the transcrystalline growth most probably starts epitaxially with the c-axis of the orthorhombic unit cell aligned in the fiber axis direction. In the growth stage that follows, the lamellae twist as the crystals grow outward from the fiber surface, with the c-axis exhibiting variable angles with respect to the fiber axis for different transcrystalline layer thicknesses. The calculations based on the X-ray diffraction results, suggest that the pitch of the lamellar twist is 28.6 micrometers. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2429–2433, 1997     

Comparative study on the lamellar structure of polyethylene foams

A comparative study on the lamellar morphology of a collection of polyethylene foam (LDPE) has been performed in order to obtain a better understanding of the morphology of the crystalline phase of these materials. The lamellar structure was measured by small-angle X-ray scattering (SAXS), differential scanning calorimetric (DSC) and Raman spectroscopy. The results have shown that the lamellar structure of the foams is different to that of a LDPE solid sheet. Moreover, the different sensitivity of the three experimental techniques to the lamellar structure has also been analyzed.     

Polymer crystalline orientation resulting from melt deformation and solidification of talc and mica filled polyethylene compounds

An experimental study is presented of orientation of polymer crystallographic axes in talc and mica filled polyethylene which has been extruded from various dies, melt spun and compression molded. The data is interpreted in terms of uniaxial crystalline orientation factors. It is observed that in all cases theb crystallographic axis of the polyethylene seems to be oriented normal to the surfaces of the mica and talc flakes.     

Plastic deformation of polyethylene

Summary The deformation behaviour during rolling is studied by small-angle X-ray scattering, density measurement, and investigation of the debris after fuming nitric acid treatment. Crystallinity and mechanical properties are compared with corresponding results on drawn material. Results obtained on quenched and annealed films of two polyethylene brands, Fortiflex and ACX, show many similarities in properties between rolled and drawn samples of similar draw ratio, i. e., a progressive lattice orientation, the appearance of a new long period different from that of the original film at =2 in Fortiflex and at =1.5 at ACX, decrease in crystallinity of annealed films, and constancy in quenched material during rolling. Therefore, it is concluded that, as in the case of drawing, the basic structure transformation during rolling is the destruction of lamellae with pulling out of microfibrils. Significant differences exist, however, between the two cases in ultimate tensile strength and in ultimate elongation.

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Zusammenfassung Das Deformationsverhalten während des Walzens wurde mit Kleinwinkelstreuung, Dichtemessung, Unter-suchung nach Behandlung mit rauchender Salpetersäure betrachtet. Kristallinität und mechanische Eigenschaften wurden mit entsprechenden Ergebnissen an verstrecktem Material verglichen.Die Ergebnisse, die man an abgeschreckten und getemperten Filmen von 2 Polyäthylenarten, Fortiflex und ACX erhielt, zeigen viele Ähnlichkeiten zu gewalzten und getemperten Proben vom gleichen Streckverhältnis, d. h. eine gleiche Gitterorientierung, das Auftreten neuer Langperioden verschieden von derjenigen des ursprünglichen Films, d. h. =2 bei Fortiflex und =1,5 bei ACX, Abnehmen der Kristallinität von getemperten Filmen und Konstanz des abgeschreckten Materials während des Walzens. Daraus wird geschlossen, daß wie im Fall des Verstreckens die Strukturänderung während des Walzens die Zerstörung der Lamellen unter Herausziehen von Mikrofibrillen verursacht. Entscheidender Unterschied zwischen beiden Deformationsarten existiert jedoch in der maximalen Zerreißfestigkeit und der maximalen Dehnung.

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With 13 figures in 18 details and 2 tables     

Large tensile deformation behavior of oriented high‐density polyethylene: A correlation between cavitation and lamellar fragmentation

Oriented high‐density polyethylene (HDPE), prepared by melt extrusion drawing, has been employed to address the correlation between cavitation and lamellar fragmentation at large strain. This has been done by investigating the volume strain, elastic recovery properties, and microscopic morphology. The results indicate that the reversible volume strain becomes saturation at a true strain of about 0.3, which is essentially consistent with the critical one related to lamellar fragmentation (point C). Morphological observations on the deformed samples provide structural insights into above deformation behaviors. Enlarged voids are hard to recover due to dominant plastic deformation of crystals once lamellar fragmentation sets in and thus a transition of reversible volume strain with strain is presented. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1202–1206, 2008     

Crystal orientation in a semicrystalline polymer in relation to deformation of polymer spherulites. II. Orientation distribution function of crystallites within crystal lamella as a function of lamellar orientation

A model relating crystal orientation in a semicrystalline polymer to the deformation of polymer spherulites is proposed. The distribution function for orientation of crystallites within crystal lamellae is assumed to be a function of lamellar orientation. In addition to the orientation of crystal lamellae in affine fashion, several parameters are introduced to characterize the untwisting of the crystal lamellae and the four different types of orientations of the crystallites within the crystal lamellae in the undeformed and deformed states of the spherulite. The model was tested by experiments in uniaxial stretching of a low-density polyethylene. The theoretical distributions of orientation of given reciprocal lattice vectors of the crystallites, such as the reciprocal lattice vectors of the (110) and (200) crystal planes, are compared with the results of x-ray diffraction experiments. It was found that the most important factors in fitting the model to experimental results are: (a) the fraction of crystallites having random orientation within lamella and, in turn, representing the degree of imperfection of the lamella in the undeformed state; (b) the ease of transition of crystal orientation within lamella from b-axis orientation parallel to the lamellar axis to two types of c-axis orientations (type C_a and type C_r) parallel to the stretching direction; and (c) the fraction of crystallites having orientation in type C_r (unfolding mechanism) rather than type C_a (rotation mechanism).     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.     

Structural changes and chain orientational behavior during tensile deformation of segmented polyurethanes

Structural changes and segmental orientation behavior of polyurethane have been studied during uniaxial deformation. The orientation function change of the two (free and hydrogen bonded C?O stretching peaks) peaks during a full cycle of deformation has been observed to be distinctively different. Even though the hydrogen-bonded C? O peaks showed hysteresis behavior, the free C? O peaks exhibited quite elastic behavior. It was thus concluded that the orientation behavior of free and hydrogen-bonded C? O stretching peak represents the deformation characteristics of soft and hard domains, respectively. The orientation behavior at different temperatures also has been studied. Temperature has a significant effect on the orientation behavior of the soft domain, whereas it has negligible effect on the hard domain orientation. It was also demonstrated that the structural change due to the deformation could be analyzed by infrared spectroscopy. Some of the hydrogen-bonded carbonyl groups have been observed to be transferred to the free carbonyl groups, indicating that a small amount of the hard segments in the hard domain have been pulled out into the soft matrix upon deformation. The orientational relaxation also has been studied as a function of time. The segmental relaxation of the hard segments appears to be quite characteristic depending on the nature of the domain in which they reside. © 1994 John Wiley & Sons, Inc.