Hydrodynamically induced crystallization of polymers from solution

Summary This paper primarily describes an investigation into the origin of persistent lamellar overgrowth of fibrillar crystals with a Shish-kebab morphology. Fibers grown from stirred p-xylene solutions of high molecular weight polyethylene at 108 °C exhibited an average spacing of 1 x 10³ Å between the lamellae when cooled down to room temperature in a normal way. Fibers quenched in liquid nitrogen displaved smaller lamellae with a periodicity of 0.4 x 10³ Å.It was found that the lamellae could be dissolved at 108 °C and recrystallized isothermally on the backbones at temperatures up to 94.6 °C. The spacing between the lamellae increased strongly with temperature in the range between 90.0 °C and 94.6 °C.The temperature dependence of this periodicity could be accounted for by the theory of secondary nucleation in polymer crystallization. The Poisson distribution of the numbers of lamellae along the backbones also suggests that lamellar growth is essentially controlled by the crystallization kinetics.The fibrillar polyethylene crystals grown at elevated temperatures from flowing solutions may be visualized as backbones constituted by partially crystallized polymer molecules and dangling ends extending from the backbones into the surrounding solution resembling centipedes.

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Zusammenfassung Dieser Bericht befaßt sich mit einer Untersuchung des Ursprungs von persistenter lamellarer Überwachsung von Faser-Kristallen mit Schaschlik-Morphologie.Faser-Kristalle, gewachsen aus geruhrten Xylol-Lösungen von hochmolekularem Polyäthylen bei 108 °C, zeigen ein mittleres Intervall zwischen den Lamellen von 1 x 10³ Å, wenn auf normae Weise abgekühlt zur Zimmertemperatur. Fasern, abgeschreckt im flüssigen Stickstoff, zeigen kleinere Lamellen mit einem Zwischenraum von etwa 0.4 x 10³ Å. Es wurde festgestellt, daß die Lamellen bei 108 °C gelöst werden können und von neuem auf den Faserkernen bis zu 94.7 °C kristallisieren. Die Temperaturabhangigkeit dieser Periodizität kann erklärt werden auf Grund der Theorie der sekundären Keimbildung bei polymerer Kristallisation. Wie sich zeigt, gruppiert sich die lamellare Überwachsung um den Kern entsprechend einer Poisson-Verteilung, was darauf hinweist, daß die lamellare Überwachsung wesentlich durch die Kristallisationskinetik bedingt wird.Die faserigen Polyathylenkristalle, bei erhöhter Temperatur hergestellt aus gerührten Lösungen, kann man sich als teilweise kristallisierte Moleküle, im Faserkerne aufgenommen, vorstellen mit langen Kettenenden im umgebenden Lösungsmittel, so ziemlich als einen Tausendfüßler.

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With 12 figures     

Hydrodynamically induced crystallization of polymers from solution

Summary This paper primarily describes the large diversity in morphology of fibrillar polyethylene crystals precipitated from stirred solutions at elevated temperatures. Shish-kebabs crystallized at 112°C from stirred xylene solutions of linear polyethylene exhibit lamellar overgrowths having thicknesses of the order of 150 Å from which it was concluded i. a. that at crystallization temperatures above 100°C only backbones are formed and that the surrounding polymer molecules deposit epitaxially on the substrate at lower temperatures. The variation in the appearance of the overgrowth and the inhomogeneity of the samples can be traced back to irregular stacking and to differences in washability of the fibrillar crystals, these differences being largely due to the non-uniform flow pattern along the various stirrer parts. Unusual lamellar shapes, interconnections, sheets between lamellae, the veil on the Shish-kebab noted byKeller andMachin, may well be explained byKeith, Padden andVadimskys mechanism of crystallization-induced orientation of macromolecules between the lamellae and the consequent formation of extendedchain crystals. The lamellar overgrowth appears to be arranged along the backbone according to a logarithmic normal distribution, and the average diameter of the lamellar crystals increases with the average spacing, both being approximately equal in size. The fibrillar crystals end in smooth tapered tails having lengths varying between 5 and 8 .These observed fibre ends agree best with the unroll model proposed for fibrillar crystal growth in flow fields if on the average each molecule introduces one chain fold and the chain ends are randomly assimilated by the crystal lattice. Linear polyethylene below molecular weight of 50×10³ and low density polyethylene did not crystallize in fibrillar habit. whereas copolymers of ethylene and slight amounts of propylene and butylene proved capable of forming fibrillar crystals and isotactic polypropylene yielded smooth and overgrown fibres.

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Zusammenfassung Der Bericht befaßt sich zuerst mit der großen morphologischen Mannigfaltigkeit faseriger, bei erhöhter Temperatur aus gerührten Lösungen gefällter Polyäthylenkristalle. Bei 112°C aus gerührten Xylollösungen von linearem Polyäthylen herauskristallisierte Schaschlik-Strukturen zeigen eine lamellare Überwachsung mit einer Dicke von etwa 150 Å, woraus u. a. geschlossen wird, daß sich bei Kristallisations-temperaturen über 100°C nur Faserkerne bilden und daß die umringenden polymeren Moleküle sich bei niedrigeren Temperaturen epitaxial auf dem Substrat ablagern. Die Verschiedenheit in der Form der Überwachsung und die Inhomogenität der Proben lassen sich erklären durch eine unregelmäßige Ablagerung auf dem Rührer und durch Unterschiede in der Auswaschbarkeit der faserigen Kristalle, welche Unterschiede weitgehend auf das nicht einheitliche Strömungsbild längs der verschiedenen Rührerteile zurückzuführen sind. Ungewöhnliche lamellenartige Formen, interlamellare Verbindungen, Häutchen zwischen den Lamellen und die vonKeller undMachin festgestellte Verschleierung der Schaschlikgebilde lassen sich sehr gut durch denKeith-, Padden- undVadimsky-Mechanismus einer durch die Kristallisation verursachten Orientierung von Makromolekülen zwischen den Lamellen und durch die anschließende Bildung von extended-chain Kristallen erklären. Wie sich zeigt, gruppiert sich die lamellare Überwachsung um den Kern entsprechend einer logarithmischen Normalverteilung; der mittlere Durchmesser der lamellaren Kristalle nimmt zu mit der mittleren Größe der Zwischenabstände, wobei beide ungefähr gleich groß sind. Die faserigen Kristalle enden in allmählich spitz auslaufenden Gebilden mit Längen von etwa 5 bis 8 . Diese Faserenden stimmen gut überein mit dem für das Wachsen faseriger Kristalle in Strömungsgebieten vorgeschlagenen unroll-Modell, falls im Durchschnitt jedes Molekül eine Kettenfaltung einleitet und die Kettenenden willkürlich in das Kristallgitter aufgenommen werden. Lineares Polyäthylen mit einem Molekulargewicht unter 50×10³ und Hochdruckpolyäthylen kristallisieren nicht in der faserigen Form, während —wie sich zeigt—Copolymere von Äthylen und geringen Propylen- und Butylenmengen in der Lage sind, faserige Kristalle zu bilden und isotaktisches Propylen schöne und überwachsene Fibrillen ergibt.

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With 25 figures in 28 details     

Extensional flow induced crystallization from solution

Summary An experimental apparatus and technique were developed for studying the crystallization of polyethylene from solutions undergoing an oscillatory, constant stretch rate, extensional flow. Investigation of the resulting fiber morphology, obtained at various temperatures and stretch rates, shows that the fibers exhibit a bead and stringlike appearance. Study of the melting, acid etching, and selective dissolution behavior of the fibers leads to the conclusion that fiber growth occurs in a fashion which directly results in the bead and string morphology and that the morphology is not a result of secondary, epitaxial crystal growth as previously believed. A model for the growth process, as suggested by the data, is presented.Evidence is also presented to suggest that at certain temperatures and stretch rates, a triclinic phase transformation occurs during the flow process. The effect is to produce two separate melting entities which appear to be stretch rate sensitive, in line with previously published studies on the effects of stretching on the behavior of bulk polymer.

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Zusammenfassung Eine experimentelle Apparatur sowie ein Verfahren wurden entwickelt zur Untersuchung der Kristallisation von Polyäthylen aus Lösungen, welche oszillatorischen Dehnströmungen mit konstanten Dehngeschwindigkeiten unterworfen waren. Untersuchungen der resultierenden Fasermorphologie, die aus Versuchen mit unterschiedlichen Temperaturen und Dehngeschwindigkeiten resultierten, ergeben ein perlen- und kettenförmiges Erscheinungsbild der Faser. Prüfung des Schmelzverhaltens, der Säure-Ätzbarkeit und der selektiven Löslichkeit der Faser erlauben den Schluß, daß das Faserwachstum in einer Art und Weise vonstatten geht, die sofort zur perlen- und kettenförmigen Morphologie führt, und daß diese Morphologie nicht durch sekundäres, epitaxiales Kristallwachstum entsteht, wie man bisher annahm. Auf der Grundlage der experimentellen Ergebnisse wurde ein Modell für den Wachstumsprozeß vorgeschlagen.Fernerhin wird der Beweis erbracht, daß in gewissen Temperatur- und Deformationsgeschwindigkeits-Bereichen eine trikline Phasentransformation während des Fließprozesses vonstatten geht. Zwei unterschiedliche Schmelzeinheiten werden erzeugt, welche von der Dehngeschwindigkeit abhängig sind, ein Ergebnis, das in Einvernehmen mit früher veröffentlichten Untersuchungen über den Einfluß von Dehnbeanspruchungen auf das Verhalten von festen Polymeren ist.

     

A molecular model for flow induced crystallization of polymers

We present in this paper a thermodynamic model for flow induced crystallization of a thermoplastic. The thermomechanical framework (generalized standard materials) allows us to couple in a very natural way the kinetics of crystallization with the mechanical history experienced by the thermoplastic^[1]. In describing the viscoelastic properties of the polymer with a molecular theory, we obtain a model for flow-induced crystallization that couples the chain conformation to the kinetics of crystallization. This model intends to be valid both for shearing and elongation. We present the equations for two cases: Maxwell and Pom-Pom constitutive equations. We finally illustrate our model with injection molding simulations achieved with a dedicated Finite Element code.     

HDPE solution crystallization induced by electrospun PA66 nanofiber

Nanohybrid shish?Ckebab (NHSK), induced by polyamide 66 (PA66) nanofiber, was successfully fabricated in high-density polyethylene (HDPE)/xylene solution via isothermal crystallization. The crystalline morphological features of NHSK were observed by scanning electron microscopy. In the structure of NHSK, PA66 nanofiber serves as shish and HDPE lamellae act as kebabs periodically surrounding the nanofiber. Additionally, it reveals that both HDPE solution concentration and crystallization time have significant effects on the size of HDPE kebab. That is, as the concentration and crystallization time increase, the diameter of the kebab increases. Moreover, when crystallization time further increases, the crystals decorated on PA66 nanofiber exhibit a three-dimensional growth (i.e., aggregate of crystallites) rather than a two-dimensional one (i.e., disk-like lamellae normal to the axis of nanofiber).     

The kinetics of flow-induced crystallization from solution

In situ birefringence measurements of the seeded growth in a tubular flow geometry of 0.01 wt% solution of a polyethylene fraction in xylene have been used to determine the flowinduced crystallization kinetics as a function of temperature and flow rate. In contrast to earlier reports on higher molecular weight polyethylene and polypropylene systems, orientational properties of the crystallized fibers do not show a clear correlation with growth conditions (i.e., temperature and flow rate). The combined kinetic data from these experiments and our earlier studies of higher molecular weight polyethylene—xylene and polypropylene—tetralin systems are analyzed in terms of a modified from of the Avrami equation which provides a basis for separately correlating temperature and flow rate effects. The observed temperature dependency of the crystallization process can be interpreted in terms of nucleation and growth models while the flow rate dependency can be interpreted on the basis of entanglement formation arguments. Results showing liquid phase precursor formation in an atactic polystyrene system are also presented to further document the liquidphase separation which can be induced in polymers under flow.     

Chiral crystallization of helical polymers

Crystallization of achiral or racemic helical polymers in chiral crystals is quite frequent and while in some cases it may relate to the thermodynamic stability of the chiral polymorph, in others it must be associated with kinetic factors. Analysis of the available literature data suggests that in the second instance a key role must be played by the nucleation step, i.e. specifically by the formation of precrystalline entities. Furthermore a survey of the chiral crystal structures for helical polymers evidences that they are frequently characterized by a quasi-hexagonal packing. The hexagonality index H, defined as the ratio of the largest to the smallest distance between the axis of the reference helix and its six nearest neighbors, appears to be a reliable indicator of the presence of helices of a single handedness or, respectively, of both in a given crystal structure. A detailed analysis of the general energetic and entropic factors favouring chiral crystallization of helical polymers is carried out. It is shown that a hexagonal or pseudo-hexagonal arrangement (i.e. the six-fold coordination) in either the crystalline or in a precrystalline state, promotes chiral crystallization and is in its own turn favoured by clustering of isochiral helices.     

Using multimodal blends to elucidate the mechanism of flow‐induced crystallization in polymers

The effect of polydispersity on the formation of flow‐induced, oriented morphology in polyolefins is investigated by polarized light imaging and small angle X‐ray scattering. A torsional shear flow was applied at different temperatures to model polyethylene blends (bimodal and trimodal hydrogenated polybutadienes) comprising of two kinds of long chains with different molecular weight (1080 and 1770 kDa) in a matrix of short chains (18 KDa), and the results were compared to those of polydisperse materials. While a single boundary associated with the threshold flow conditions for the onset of oriented morphology is observed in the bimodal blends, two boundaries corresponding to the orientation of the longest chains (1770 kDa) and next longest chains (1080 kDa) are detected in the trimodal blends. The results obtained, herein, are extended by inference to polydisperse polymers. It is demonstrated that the shear rate dependence of the critical specific work parameter for the onset of oriented morphology in polydisperse polymers is dictated by the molecular weight distribution and that the longest chains mainly control the process with some contribution from shorter chains involved in the formation of flow‐induced precursors at higher flow rates. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Folded chain crystallization of random terpolymer liquid crystal polymers from the nematic state

The morphology of ca. equimolar random terpolymer liquid crystal polymers of an aliphatic segment of 4–7 carbon atoms, oxybenzoate, and dioxyphenyl crystallized from the nematic state in the form of thin films on glycerine by slow cooling and quenching has been characterized by electron microscopy (TEM) and diffraction (ED). In all cases a folded chain, lamellar structure is found. The ED studies suggest adjacent reentry, and despite a large ΔH, indicate no change in lateral molecular packing at the crystal-“liquid crystal” transition with a transformation to the nematic state at a higher temperature “liquid crystal”-liquid crystal transition. The results are interpreted as suggesting similar folded chain, lamellar morphology in the nematic state. © 1992 John Wiley & Sons, Inc.     

Temperature dependence of the bulk crystallization rate of polymers

Data already existing in the literature for the overall crystallization kinetics of a variety of polymers have been analyzed according to different possible nucleation mechanisms. The conclusions reached are similar to those previously deduced from an examination of ata for spherulite growth rates. It is demonstrated that, if allowed a reasonable choice for the equilibrium melting temperature, no unbiased selection of a unique nucleation process can be made. Moreover, a set of universal parameters exists for each of the nucleation and growth processes considered which allows the data for all polymers to be represented by a single straight line. The only quantities that are unique to a given polymer are the equilibriun melting temperature and the activation energy for transport.     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002     

Time-resolved small-angle neutron scattering study of polyethylene crystallization from solution

With time-resolved small-angle neutron scattering (TR-SANS), the crystallization kinetics of polyethylene from deuterated o-xylene solutions upon a temperature jump have been investigated. On the basis of a morphological model of coexisting lamellar stacks and coil chains in solution, experimental data have been quantitatively analyzed to provide structural information, such as the lamellar long period, the lamellar crystal thickness, the thickness of the amorphous layers between lamellae, the degree of crystallinity, and the crystal growth rate at various degrees of undercooling. The viability of TR-SANS for studying polymer crystallization is demonstrated through the consistency of these measurements and well-established knowledge of polyethylene crystallization from xylene solutions. One unique feature of this experimentation is that both the growth of lamellar crystals and the condensation of coil chains from solution are monitored simultaneously. The ratio of the crystal growth to the chain consumption rate decreases rapidly with a decreasing degree of undercooling. The Avrami analysis suggests that the growth mechanism approaches two-dimensional behavior at higher temperatures, and this is consistent with the observation of an increasing ratio of the sharp-surface area to the bulk crystal growth rate with temperature. The limitations, possible remedies, and potentials of TR-SANS for studying polymer crystallization are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3133–3147, 2004     

Thermal analysis of secondary crystallization in polymers

With a particular experimental technique of differential scanning calorimetry, the melting of the secondary crystals in polymers is observed separately from the melting of the primary crystals, and methods for the detection and quantitative analysis of the secondary crystals have been developed. By this means, a number of particular features of secondary crystals could be found which are not mentioned in the literature. Two types of crystal formation are found for linear polymers, with no regard as to kinetics, growth and other characteristics of crystallization. Secondary crystallization is interpreted as the formation of crystals of fringed micelle type. The equation describing secondary crystallization is deduced. It is shown that some specific features of the DTA and DSC curves of polymers are caused by the behaviour of secondary crystals in the polythermal regime.

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Zusammenfassung Es wird gezeigt, daß durch DSC bei einer besonderen experimentellen Technik zwischen dem Schmelzen von primären und sekundären Kristallen in Polymeren unterschieden werden kann. Methoden zur Detektierung und quantitativen Analyse sekundärer Kristalle wurden entwickelt. Mit diesen Methoden wurde eine Anzahl von in der Literatur noch nicht beschriebenen Besonderheiten der sekundären Kristalle festgestellt. In linearen Polymeren wurden zwei Typen der Kristallbildung festgestellt, die sich nicht auf die Kinetik, das Wachstum und andere Charakteristika der Kristallisation beziehen. Die sekundäre Kristallisation wird als die Bildung von Kristallen des fransigen Mizell-typs interpretiert. Die die sekundäre Kristallisation beschreibende Gleichung wird abgeleitet. Es wird gezeigt, daß einige spezifische Züge der DTA- und DSC-Kurven von Polymeren durch das Verhalten von sekundären Kristallen verursacht werden.

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