Crystallization Of Linear Polyethylene Research Articles

Self-nucleation and heterogeneous nucleation in ethylene/1-octene random copolymer and its linear polyethylene blends

Self-nucleation normally denotes an increase in the crystallization temperature during cooling down from certain preheated (seeding) temperature due to the existence of unmolten crystalline nuclei or some heterogenous structures which can induce melt memory effect. It is one of the crucial parts in the study of polymer crystallization behavior which provides a meaningful guidance in both academia and industry to accelerate crystallization of polymers. In this work, self-nucleation effects of a commercial ethylene/1-octene random copolymer with 12 mol% 1-octene content and its blends with small amount of linear polyethylene (LPE) were investigated. The ethylene/1-octene random copolymer possesses strong self-nucleation effect whereas in the blends the crystallization temperature increases with the seeding temperature and eventually stabilized at certain constant temperature when the seeding temperature is high enough. This result is clearly due to the heterogenous nuclei generated by the crystallization of LPE during cooling. The strength of the heterogenous nucleation effect i.e. the crystallization temperature is positively related to the degree of dispersion of LPE in the mixture.

Epitaxial Crystallization of Linear Polyethylene in Blends with Isotactic Polypropylene

The structure and morphology of extrusion-oriented ribbons of polypropylene/polyethylene blends is described. The blends with 20%, 30%, and 40% of oriented isotactic polypropylene fibrils show homo- and heteroepitaxial structures. Partial mutual solubility of the blend components influenced the melting and crystallization behavior.

Solid-State Structure and Crystallization in Double-Crystalline Diblock Copolymers of Linear Polyethylene and Hydrogenated Polynorbornene

Double-crystalline diblock copolymers of linear polyethylene (LPE) and hydrogenated polynorbornene (hPN) are synthesized, and their crystallization behavior and morphology are examined using small-angle (SAXS) and wide-angle X-ray scattering (WAXS). In symmetric hPN/LPE diblocks with molecular weights above 50 kg/mol, the hPN block has previously been shown to crystallize first and set the solid-state microstructure. Two-dimensional WAXS on hand-drawn fiber specimens reveals that the LPE crystals formed in confinement stack orthogonally to the hPN crystals. By adjusting total molecular weight, the order of block crystallization may be reversed, even while holding the block length ratio fixed. At a diblock molecular weight of 20 kg/mol, simultaneous time-resolved SAXS/WAXS reveals that the LPE block crystallizes first, even when LPE is the minority component, and restricts hPN to crystallize between the LPE lamellae. The relative orientation of the LPE and hPN crystals in the lower molecular weight diblocks is examined by modeling changes in the SAXS primary peak intensity on cooling two diblocks through the hPN crystal–crystal transition, where hPN densifies as it adopts a rotationally ordered crystal structure. Only a perpendicular stacking of hPN and LPE crystals consistently yields the large reduction in primary SAXS peak intensity observed for both diblocks. Thus, even though the templating block switches from hPN to LPE as the diblock molecular weight is reduced, the orthogonal stacking motif is retained for both high- and low-molecular-weight copolymers.

A Study on the Chain−Particle Interaction and Aspect Ratio of Nanoparticles on Structure Development of a Linear Polymer

The stability of metastable flow-induced precursor (FIPs) in the polymer melts in presence of nanoparticles, viz. single-walled carbon nanotube (SWCNT) and zirconia nanoparticles, is studied at, 142 °C, close to the equilibrium melting point of unconstrained extended chain crystals of linear polyethylene (PE). The results conclusively demonstrate the influence of chain−particle interactions, between PE and the nanoparticles, on the stretch of the long chains. With the applied flow, SWCNTs together with PE chains are observed to align along the flow direction, giving rise a strong streak like pattern along the equator. At the initial stages, intensity of the observed streak in the presence of SWCNTs is stronger than that for the neat polyethylene. The streak intensity stabilizes with time, where the time required for the stabilization depends on the amount of the dispersed nanotubes in the polymer matrix. On the contrary, in the presence of zirconia nanoparticles, where the chain−particle interactions between PE and the nanoparticles are weak the initially observed streak tends to disappear with time, where the time required is strongly dependent on the concentration of the nanoparticles in the polymer matrix. Thus, compared to the neat polymer, the presence of zirconia nanoparticles destabilizes the shish formation. The chain orientation along the flow direction is determined using Herman’s orientation function and the length of the oriented chains (shish) by Ruland’s streak analysis. On cooling, with the crystallization of the polymer, scattering develops along the meridian, indicating the development of folded chain crystals, where the oriented chains present along the flow direction provide the epitaxy matching thus suppressing the nucleation barrier. The meridional intensity (arising with the formation of crystals, called kebabs) at room temperature, shows strong dependence on the stable streak intensity (chain orientation along the flow direction, called shish) along the equator prior to cooling.

Crystalline Structures, Melting, and Crystallization of Linear Polyethylene in Cylindrical Nanopores

The crystalline structures associated with melting and crystallization behaviors of monodisperse linear polyethylene confined in cylindrical nanopores were investigated by X-ray diffraction and differential scanning calorimetry. The crystalline structures, melting, and crystallization behaviors of PE under the imposed cylindrical confinement were noticeably different from those of the bulk state. The isothermal crystallization experiments showed that the overall crystallization of polyethylene in cylindrical nanopores was dominated by the nucleation rather than the growth of crystallites. The c- and a-axes of orthorhombic PE crystals developed in nanoporous alumina were preferentially oriented perpendicular to the long axis of cylindrical nanopore while the b-axis was parallel to the pore axis. The melting temperature of polyethylene in the nanopores was substantially depressed, and it was analyzed with the Thomson−Gibbs equation. The crystallinity of linear polyethylene in cylindrical nanopores was less than 50%, whereas the bulk value was 71.6%.

Study of Crystallization Processes in Ethylene−Styrene Copolymers by Conventional DSC and Temperature-Modulated Calorimetry: Linear Polyethylene and Low Styrene Content Copolymers

Using a combination of differential scanning calorimetry and quasi-isothermal temperature-modulated calorimetry, we investigated the temporal evolutions of the melting temperature, degree of crystallinity, and excess heat capacity during crystallization of linear polyethylene and low styrene content ethylene−styrene copolymers. Describing isothermal crystallization as the succession of three stages (primary, mixed and secondary crystallization stages), we established the following correlations: (1) the evolution of the melting temperature with time parallels that of the degree of crystallinity, (2) the excess heat capacity increases linearly with degree of crystallinity during the primary stage, reaches a maximum during the mixed stage, and decays during the secondary stage, (3) the rate of decay of the excess heat capacity parallels the rate of secondary crystallization, and (4) the rates of shift of the melting temperature and decay of the excess heat capacity lead to apparent activation energies that are very similar to these reported for the crystal αc relaxation by solid-state NMR, dynamic mechanical, and dielectric spectroscopies. Strong correlations in the time domain for secondary crystallization by lamellar thickening and evolution of the excess heat capacity suggest that the reversible crystallization/melting phenomenon is associated with molecular events in the melt−crystal fold interfacial region. Specifically, we conclude that the excess heat capacity observed during the high-temperature crystallization of linear polyethylene and low styrene content copolymers is most likely to originate from the segmental processes in the crystal/melt fold region that have been discussed by Fischer, Mansfield, and Strobl. These studies also provide preliminary indications that the excess heat capacity observed during crystallization at lower temperatures in the case of ethylene copolymers of high comonomer content is consistent with the lateral surface model proposed by Wunderlich.

Reorganization and Melting of Polyethylene Single Crystals: Complementary TEM, DSC, and Real-Time AFM Studies

The reorganization and melting of single crystals of linear polyethylene with lozenge and truncated-lozenge habits has been studied in situ during slow continuous heating using atomic force microscopy. The morphological insights obtained are backed up by complementary experiments using transmission electron microscopy and differential scanning calorimetry and have enabled us to identify different stages of melting/recrystallization. Visible melting is preceded by local reorganization within the crystals, the extent of which depends crucially on the thermal history of the sample and can influence subsequent behavior such as the preferential melting of one crystal sector before another. This is followed by distortion of the crystal edges and limited thickening which does not progress into the bulk of the crystal. Next, larger scale melting and recrystallization occurs within the bulk of the crystal, which leads to a progressive increase in melting point. The observed behavior supports a model of local melting and recrystallization rather than solid-state thickening, and overlying layers can behave quite independently. Finally, the thickened crystals melt at a temperature dependent on the thermal history of the sample.

Melting of Isochronously Decorated Single Crystals of Linear Polyethylene, As Monitored by Atomic Force Microscopy

ADVERTISEMENT RETURN TO ISSUECommunication to the...Communication to the EditorNEXTMelting of Isochronously Decorated Single Crystals of Linear Polyethylene, As Monitored by Atomic Force MicroscopyNicolas Dubreuil, Stephane Hocquet, Marcel Dosière, and Dimitri A. IvanovView Author Information Laboratoire de Physique des Polymères, Université Libre de Bruxelles, CP 223 Boulevard du Triomphe, B-1050 Brussels, Belgium, and Laboratoire de Physicochimie des Polymères, Université de Mons-Hainaut, Place du Parc, 20, B-7000 Mons, Belgium Cite this: Macromolecules 2004, 37, 1, 1–5Publication Date (Web):December 11, 2003Publication History Received17 June 2003Revised19 November 2003Published online11 December 2003Published inissue 1 January 2004https://doi.org/10.1021/ma034823vCopyright © 2004 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views171Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (421 KB) Get e-AlertsSUBJECTS:Crystal structure,Crystallization,Crystals,Melting,Thickness Get e-Alerts

Morphology and Melting of Truncated Single Crystals of Linear Polyethylene

The thickness of (110) and (200) sectors in truncated single crystals of linear polyethylene grown from dilute n-octane solution at 95 °C was measured by atomic force microscopy (AFM) in tapping mode. The (110) sector was found to be 1.1 nm thicker than the (200) sector. This can be explained by a somewhat smaller tilt angle of the chains with respect to the basal plane of the crystal. The tilt of the chains in (110) and (200) sectors, obtained by electron diffraction, is 22° and 30°, respectively. This implies that the length of the stem, i.e., the length of the straight part of the chains between two consecutive folds, is identical in both sectors of the truncated single crystals. The melting of individual LPE truncated single crystals was revisited by AFM. The melting temperatures of the (200) and (110) sectors are 124.9 and 125.9 ± 0.3 °C, respectively. The lamellar thickness distribution calculated from AFM pictures clearly indicates that reorganization into thicker lamellae takes place during heatin...

The mechanism of crystallization of linear polyethylene, and its copolymers with octene, over a wide range of supercoolings

The crystallization behavior of a linear polyethylene has been studied using conventional isothermal hot stage microscopy and with the Ding–Spruiell method of rapid cooling. When studied at rapid cooling rates the polymer generates its own pseudo-isothermal crystallization temperatures, in agreement with Ding–Spruiell's studies on other systems, permitting experiments to be carried out isothermally at a temperature as low as 90°C, thus extending the range of supercooling available from the 9°C (120–129°C) typical of conventional experimentation to 29°C (90–129°C). The points generated using both the isothermal and the rapid cooling techniques form a single consistent trend, as in polypropylene. In conventional crystallization experiments it was found, as expected, that the spherulite growth rates conformed to the regime I–regime II scheme, already well established for this polymer. When analyzed using a secondary nucleation approach all three regimes are found to exist and to adequately describe the data. The regime II–regime III transition temperature was found to occur at 120.6°C. The crystallization behavior of a series of ethylene-octene copolymers synthesized using metallocene catalysts has also been studied using the same experimental methodologies. In conventional crystallization experiments it was found, as expected, that the spherulite growth rates varied with octene content and molecular weight. When studied at rapid cooling rates, at the lowest temperatures of crystallization, the spherulite growth rates of all of the copolymers studied merge with the growth rate curve of the linear polyethylene and are virtually indistinguishable. The results indicate a major breakdown of all current theories of polymer crystallization, in that the overriding equation involving the relation between crystallization rate, lamellar thickness, surface free energy and supercooling appears to be superceded in the copolymers by some hitherto unrecognized process or law. The underlying physics behind this conclusion needs to be elucidated, but appears to be consistent with the formation of a partially ordered intermediate of 3–4 stems in size on the growth face under all conditions of growth in linear polyethylene and its copolymers. The degree of disorder in the cluster is believed to be strongly dependent on supercooling, permitting incorporation of hexyl groups into the intermediate. Subsequent ordering of the cluster produces the ultimate crystal packing and ejection of hexyl groups and other impurities. The rate of formation of the cluster is suggested to be the rate controlling step in secondary nucleus formation.

Yield and plastic resistance of α-crystals of isotactic polypropylene

Yield behaviour and plastic resistance of the (100)[001] chain slip system in α-crystals of isotactic polypropylene (iPP) were studied in samples of biaxially oriented films of plain iPP and blends of iPP with hydrogenated oligo(cyclopentadiene) (HOCP). Due to the orientation process the films investigated exhibit sharp and nearly one-component texture with the (010) plane parallel to the film surface and the chain direction parallel to the direction of final drawing (transverse direction, TD). The films were studied in tension at various angles with respect to the orientation direction. It was found that the yield stress obeys the Coulomb yield criterion, provided that the angle between the chain orientation axis and tensile axis is within the range of 30–50°. The results suggest that the (100)[001] chain slip is active as a single deformation mechanism in this range of sample orientation. The critical resolved shear stress necessary to activate this slip, τ c, was determined for iPP and iPP/HOCP blend samples. It was also found that the slip process is sensitive to the stress normal to the slip plane, similarly to the slip processes observed in linear polyethylene crystals. The value of τ c determined for plain iPP was 22.6 MPa, while for the 8:2 blend its value increased to 35.5 MPa. The increase is caused most probably by the presence of a small amount of HOCP molecules incorporated within iPP crystals, as well as by the layers of higher concentration of HOCP located at crystal–amorphous interfaces, which both cause immobilization of a part of dislocations and consequently an increase of the yield stress observed in the blend samples. The third probable cause of the increase of yield stress in blends may be an increase of the glass transition temperature of the amorphous phase of iPP in the blend, as compared with plain iPP samples.

The growth of single crystals of linear polyethylene

Journal of Polymer Science Part B: Polymer PhysicsVolume 34, Issue 7 p. 1169-1174 Letters to the Editors The growth of single crystals of linear polyethylene P. H. Till Jr., P. H. Till Jr. Research Division Polychemicals Department E. I. du Pont de Nemours & Company Wilmington, DelawareSearch for more papers by this author P. H. Till Jr., P. H. Till Jr. Research Division Polychemicals Department E. I. du Pont de Nemours & Company Wilmington, DelawareSearch for more papers by this author First published: May 1996 https://doi.org/10.1002/polb.1996.909AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume34, Issue7May 1996Pages 1169-1174 RelatedInformation

Commentary: Reflections on “The growth of single crystals of linear polyethylene,” by P. H. Till,J. Polym. Sci., XXIV, 301 (1957)

Commentary: Reflections on “The growth of single crystals of linear polyethylene,” by P. H. Till,J. Polym. Sci., XXIV, 301 (1957)

Perspective: Comments on “The growth of single crystals of linear polyethylene,” by P. H. Till, Jr.,J. Polym. Sci., XXIV, 301 (1957)

Perspective: Comments on “The growth of single crystals of linear polyethylene,” by P. H. Till, Jr.,J. Polym. Sci., XXIV, 301 (1957)

Morphology and kinetics of crystallization of polyethylene from chain disentangled melt

AbstractShort description of crystallization processes in linear polyethylene with applied reptation concept and emphasized role of chain entanglements is presented. The influence of chain entanglements in Regimes of crystallization is discussed. The estimation of time of self‐diffusion and time required for complete pull‐out of a macromolecular chain from melt by crystallization forces on the basis of reptation friction leads to the conclusion that a crystallizing macromolecule is reeled out of the melt very quickly (order of microseconds) while other macromolecules are relatively motionless.Fast crystallization of polyethylene does not change end‐to‐end distance of macromolecules while slow isothermal crystallization causes partial disentaglement of chains as it could be concluded from an increased end‐to‐end distance.The route to complete disentanglement is a slow crystallization at elevated temperature (∼230°C) and under high pressure (∼6 kbars). High pressure crystallization delivers polyethylene samples with a high melting point (∼145°C) and completely extended chains (end‐to‐end distance − 1.0). Macromolecular chains in such samples after melting exhibit for a period of several minutes complete or partial disentanglement and can be used for studies of chain entanglements dynamics and also for studies of crystallization from chain disentangled melt.The studies of crystallization of linear polyethylene in disentangled melt showed almost 4 fold increase in the growth rate of spherulite at 126°C (end of Regime II of crystallization) and only 1.1 fold increase at 123°C (beginning of Regime II of crystallization). The results concerning the kinetics of crystallization and nucleation in chain disentagled PE melt as well as resulting morphology are discussed further in terms of Regimes and reptation concepts.

Absence of isothermal thickening for a blend of linear and branched polyethylene

In this paper we investigate the effect on melting stability of blending linear with branched polyethylene. Linear polyethylene (LPE) was blended in solution with branched polyethylene (BPE) at a concentration of 10% LPE. Both homopolymers and the blend were held at 126°C, for 80 h. At this temperature solid-liquid phase separation takes place in pure LPE and in the blend; pure BPE remains molten. The melting point of the isothermally crystallized blend lamellae was lower than the melting point of pure LPE lamellae crystallized under the same conditions. This depression in melting temperature has previously been explained in two ways: by the inclusion of BPE into the LPE crystals, and because of the different environment of the crystals. We assess these two factors and find them insufficient to explain the decrease in melting temperature observed. We discuss another important factor, the dimension of the crystals in the chain direction. By transmission electron microscopy we show that crystals isothermally grown in the blend are thinner than LPE crystals grown under the same conditions. This difference in crystal thickness is sufficient to explain the observed depression in melting temperature. We believe that the lamellae in the blend fail to thicken during the initial stages of crystallization. We envisage three mechanisms that explain the suppression of thickening present in the blend, and one of these is eliminated.

Crystallization of linear polyethylene from melt in isothermal compression

AbstractCrystallization and primary nucleation of linear polyethylene has been studied by means of a custom‐made miniature pressure apparatus. It has been shown that during isothermal compression of linear polyethylene melt at a constant rate crystallization occurs. In the range of fastest conversion rates the crystallization assumes isobaric character. The level of pressure at which the crystallization occurs increases with the increase of the crystal‐lization temperature and/or with the increase of the compression rate. The crystallization has a spherulitic character up to the highest pressure achieved in the apparatus (approx. 600 MPa). Surprisingly, there are no dependencies of average spherulite size, degree of crystallinity, and lamellae thickness on the pressure and the temperature of crystallization during melt compression, but there is a strong relation to the compression rate. Below 250 MPa and above 300 MPa the crystallization proceeds under pressure, ensuring a constant undercooling. The undercooling for the pressure above 300 MPa is approximately 10°C lower than that for the pressure below 250 MPa. For the pressure 250–300 MPa a change in a primary nucleation and spherulite crystallization has been observed that is connected with the transformation from orthorhombic to pseudohexagonal symmetry of crystals. No noticeable effect of molecular weight of linear polyethylene on crystallization during iso‐thermal melt compression has been observed.

On morphology and polymer blends: polystyrene and polyethylene

This paper compares the morphologies observed in two blend systems, namely blends of isotactic and atactic polystyrene, and blends of linear and branched polyethylene. Crystallization of isotactic polystyrene at high temperatures ( ≳ 200°C) produces immature sheaf-like aggregates of lamellae. Addition of atactic polymer results in modified morphologies that are based upon bundles of lamellae, separated from one another by non-crystalline regions. At high atactic contents sheaf-like objects are no longer observed, crystallization occurring to give structures that are better described as multilayered crystals. At temperatures below ∼ 200°C, mature spherulites develop. Addition of atactic polystyrene again results in morphologies in which lamellae are grouped into stacks separated from one another by non-crystalline regions. Again, at high atactic contents a less complex (sheaf-like) overall morphology is observed. In all the above microstructures, lamellae exhibit an inherently hexagonal habit. However, at lower crystallization temperatures observations suggest that lateral lamellar growth may be restricted. Isothermal crystallization of linear polyethylene at temperatures above ∼ 125°C results in morphologies in which a complex lamellar hierarchy exists, as a consequence of molecular fractionation during crystallization. Addition of branched polyethylene, which is unable to crystallize at these temperatures, suppresses fractionation such that the resulting morphologies may be described in terms of only one dominant and one subsidiary lamellar population. When viewed along the crystallographic b axis, dominant lamellae appear planar or mildly S-shaped in profile, whilst subsidiary lamellae are planar and inclined at an angle of ∼25° to the local dominant population. In blends with a high branched content, apparently non-crystalline regions can be seen between individual lamellae. Thus, in polyethylene blends, the non-crystallizing component is located between individual lamellae rather than between bundles of lamellae, as is the case in polystyrene. This difference arises from the parallel growth of dominant and subsidiary lamellae in polystyrene as opposed to the inclined development of subsidiaries with respect to the dominant lamellae in the polyethylene blends. In all the samples studied, spherulite growth occurs via a dominant/ subsidiary mechanism.

The influence of molecular weight on the regime crystallization of linear polyethylene

Overall crystallization rates of polyethylene fractions, ranging in molecular weight from 1.95 × 10 4 to 8.0 × 10 6, have been measured over the temperature range from 114 to 128°C. From kinetic analysis of these data, three distinct regimes have been detected. The assignment of the regimes has been made based on several mechanistic situations, which lead to the same temperature coefficient. The influence of molecular weight on the regime behaviour has been found to be very profound. It changes the type of regime that is observed, the slope of the temperature coefficient in regime II and the location of the transition temperature from one regime to another.

On isolated lamellae of melt-crystallized polyethylene

The growth, isolation and examination of individual melt-grown crystals of linear polyethylene is reported. Monolayers grown at 130°C have continuously curved elliptic prism faces (despite being grown under ‘regime I’ conditions) and fold surfaces close to {2 0 1}. Such monolayers, showing a sector boundary along their length, and distinct sectors differing very slightly in orientation across this boundary, have been revealed in dark-field electron microscopy. This is evidence for regularity in chain folding. A more frequent form is when a central screw dislocation is present near the centre of a lamella. The spiral terrace developing from this shows a remarkable habit in which adjacent layers splay apart, but growth soon stops, leaving the upper and lowermost layers plano-convex in outline. In every case, the planar outline is of a prism face making an acute angle with the previous layer. This is a consequence of growth having only occurred around screw dislocations of the appropriate hand: right-handed for a (201) surface. It is suggested that this habit, illustrating how spiral development selects screw dislocations of a consistent hand, may be the missing element in understanding the growth of banded spherulites in polyethylenes via systematic arrays of similar screw dislocations.