Morphology of polypropylene crystals. III. Lamellar crystals of thermally decomposed polypropylene

Morphology and crystal structure of solution-grown and melt-grown crystals of thermally decomposed polypropylene have been studied by electron microscopy and x-ray diffraction. By crystallization from dilute α-chloronaphthalene or carbitol solutions well-defined lamellar crystals about 100–150 A. in thickness are obtained from fractions (number-average molecular weights 1600–2100) of thermally decomposed crystalline polypropylene. The structure is monoclinic as crystallized from very dilute α-chloronaphthalene or carbitol solutions (0.02–0.005 wt.-%). However crystals of the triclinic as well as of the monoclinic forms are precipitated from carbitol solutions of higher concentrations (0.05-1 wt.-%). On the other hand, the separated triclinic form has been obtained from the melt. Crystals of both modifications have similar morphology. In addition, the chain molecules cannot be expected to fold, within the thickness of lamellae in crystals of either modification prepared from the low molecular weight fractions used in this study.     

Pyrolysis kinetics of polypropylene

The pyrolysis of oil shale and plastic wastes is being presently considered as an alternative means of partial substitution of fossil fuels to generate the necessary energy to supply the increasing energy demand and as well as new technology to reduce the negative environment of plastic wastes. However, Knowledge of pyrolysis kinetics is of great imponrtance for the design and simulation of the reactor and in order to establish the optimum process conditions. In this study, the thermal decomposition of polypropylene, oil shale and their mixture was studied by TG under a nitrogen atmosphere. Experiments were carried out for various heating rates (2, 10, 20, 50 K min⁻¹) in the temperature range 300–1273 K. The values of the obtained activation energies are 207 kJ mol⁻¹ for polyethylene, 57 kJ mol⁻¹ for the organic matter contained in the oil shale and 174 kJ mol⁻¹ for the mixture. The results indicate that the decomposition of these materials depends on the heating rate, and that polypropylene acts as catalyst in the degradation of the oil shale in the mixture.     

Crystallization kinetics of syndiotactic polypropylene

Melting-point and spherulite growth rate measurements for a sample of syndiotactic polypropylene (S = 0.716 and η = 0.356) were analyzed for the parameters characterizing crystal formation and growth: T_m = 159 ± 2°C, σ_e = 47 erg cm ^?2, σ = 4.4 erg cm^?2, and q = 5.6 kcal per mole of folds. The q and σ_e values place syndiotactic polypropylene in the group of “unhindered” polymers. Failure of the isotactic-polypropylene spherulite growth rate data to follow current theories of crystal growth precluded a comparison of crystal parameters of the two stereoisomers. At comparable degrees of supercooling, the absolute growth rates for the two forms are of the same order of magnitude and exhibit one or more crossover(s) in relative position.     

Autoxidation kinetics of atactic polypropylene in bulk

The kinetics of the reaction between atactic polypropylene (APP) and oxygen in bulk at temperatures ranging from 170 to 210°C and oxygen partial pressures from 160 to 760 torr have been studied by thermal differential analysis. The reaction takes place in two successive steps, both giving hydroperoxide groups as product. Partial reaction orders with respect to APP and oxygen for the first step, which corresponds to the uncatalyzed attack of a C? H tertiary bond to give a hydroperoxide, are one and two, respectively. In the second step, interpreted as another attack on a tertiary C? H by oxygen, catalyzed by a neighboring hydroperoxide group, reaction orders are one and one-half for APP and oxygen, respectively. Activation parameters have been determined and a reaction sequence is proposed. Hydroperoxidated APP subsequently decomposes via a zero-order process giving methylketone groups as its main product. An interpretation of this process is also given.     

Morphology of polypropylene crystals. II. Twinning of lamellar crystals

Twinned crystals were obtained from fractionated isotactic polypropylene of M?_w = 600,000 by isothermal crystallization at 130°C. for 20 hr. from dilute α-chloronaphthalene solution (0.005 wt.-%). Electron microscopic observations confirm that the molecular chains of polypropylene lamellar crystals extend along the [100] direction while the folding itself occurs within the (010) planes in the monoclinic crystal form. On this basis it is shown that polypropylene forms twinned crystals in which the composition plane is the (1k0) planes. It can be deduced that the formation of twin nucleus occurs before twinning, and then growth occurs from the neighboring region of the crystal boundary by chain folding along the [100] direction.     

Morphology of polypropylene crystals. I. Dilute solution-grown lamellar crystals

The dependence of crystalline morphology of isotactic polypropylene crystallized from dilute solutions on its molecular weight and growing conditions and the mechanism of crystal growth were studied by electron microscopy and electron diffraction. Lathshaped lamellar crystals 150–300 A. in thickness are obtained from fractionated polypropylene powders of M _w (average molecular weight) = 600,000 and 240,000, but not from the samples of M _w = 82,000 and 44,000, by means of isothermal crystallization at 130°C. for 20 hr. in dilute α-chloronaphthalene solution (0.005 wt.-%). Precipitation of the fractionated polypropylene sample of M _w = 82,000 from a dilute solution of carbitol gives typical dendritic crystals under the same isothermal crystallizing conditions as mentioned above. The mode of chain folding in these crystals based on the orientation and the crystal structure of the lamellar crystals agrees with that proposed by Sauer, Morrow, and Richardson. From the morphological observations, the mechanism of growth pertinent to polypropylene lamellar crystals is presumed to be as follows: fibrils at first aggregate, then the molecular chains are folded to form small lamellae, and then these small lamellae accumulate compactly to grow to large, lath-shaped, lamellar crystals.     

Raman scattering in mixed defect chalcopyrite crystals

We have investigated with Raman spectroscopy pseudoternary Zn_xCd_1?xGa₂S₄ and CdGa₂(S_xSe_1?x)₄ phases over the whole composition range. In Zn_xCd_1?xCd_1?xGa₂S₄ a smooth variation of the spectra and a generalized one-mode behaviour has been observed, except for the lowest frequency modes. In CdGa₂(S_xSe_1?x)₄ the anion substitution has a larger influence on the spectra. Peculiar two-mode behaviour is displayed by the strong A-symmetry breathing modes.     

The effect of nucleating agents on the crystallization kinetics of polypropylene

Compositions of neat polyproylene (PP), PP–PE (polyethylene) blend and PP–PE–DBS (dibenzylidene sorbitol) were studied with respect to their crystallization kinetics by means of differential scanning calorimetry in isothermal mode. A modified Avrami equation was applied to obtain the crystallization parameters of PP, PP–PE and PP–PE–DBS. Optical and hot-stage microscopy and dynamical analysis were used for structure determination. Experimental results have indicated that PE addition inhibits PP crystallization rate and acts as a plasticizer, while addition of DBS changes the crystallization kinetics. A detailed study of the DBS nucleation effects indicates that owing to the high surface energy, nuclei are formed during primary crystallization, leading to a fine PP crystalline structure and improved mechanical properties.     

The mountain defect. A new kind of planar defect in surface stabilized smectic C liquid crystals

A new kind of planar chevron defect, which we call the 'mountain defect' due to its mountain-shaped appearance under the microscope, is observed in chevron surface stabilized smectic C liquid crystal cells for both chiral (ferroelectric) and achiral materials. Polarized optical microscopy investigations reveal that this kind of defect, which can either appear spontaneously and grow slowly over days, weeks and months or can be induced by applying an electric field or mechanical distortion, mediates change in the chevron interface position, separating chevron domains of differing chevron interface position. The full three dimensional layer structure of this defect and its relation with other commonly seen line defects in such cells, like zig-zag walls and field lines, will be presented. The formation of this kind of defect indicates that chevron structure is not necessarily a stable structure in these cells.     

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

The melting behavior of continuous fibrillar crystals of high-molecular-weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide-angle x-ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish-kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take-up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.     

Longitudinal growth of polymer crystals from flowing solutions V.: Structure and morphology of fibrillar polyethylene crystals

Summary Longitudinal fibrillar polyethylene crystals grown from xylene solutions subjected to simple shear flow have been investigated by various X-ray diffraction and electron microscopic techniques. The macrofibers with diameters in the micron range are composed of elementary fibrils of two basically different types. They may consist of either striated fibrils of the shishkebab type or smooth fibrils. The latter morphologies were obtained when the crystallization was carried out at temperatures above approximately 114 °C and the fibrillar crystals were allowed to grow longitudinally at the surface of the rotor of a Couette type instrument at a sufficiently high speed.The average spacings between the lamellae of the shish-kebabs as derived from the maximum in the small-angle X-ray scattering curve correspond remarkably well to the values obtained from transmission electron micrographs. The lateral dimensions of the elementary fibrils were acquired from equatorial line-broadening analysis of electron diffraction spots, wide-angle X-ray diffractograms, dark-field images and Guinier plots.They varied from 260 Å for fibrillar polyethylene crystals grown at 103 °C to approximately 150 Å for fibers formed at 118 °C The lateral crystallite size was also found to diminish at higher growth rates. Under the latter conditions the growth is accompanied by an extensive coil deformation as reflected by the presence of a considerable amount of the triclinic crystal modification.Dark-field electron microscopy indicates that the lengths of the crystallite blocks are of the order of 250–350 Å for fibers grown at 103 °C and 450 Å and longer for fibers formed at 118 °C. These values are in agreement with those obtained from the broadening of the (002) electron diffraction spots.

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Zusammenfassung Dieser Bericht befaßt sich mit der Untersuchung von longitudinalen Polyäthylen-Faserkristallen, gewachsen aus Xylollösungen in einer Scherströmung, mit Hilfe verschiedener Röntgen-Diffraktions- und elektronenmikroskopischer Techniken. Die Makrofasern mit Durchmessern von einigen Mikron enthalten Elementar-Fibrillen von 2 grundsätzlilch verschiedenen Typen. Sie enthalten entweder gestreifte Fibrillen, die wie Schaschlik-Strukturen erscheinen, oder glatte Fibrillen.Die letztgenannten glatten Fibrillen wurden erhalten, wenn die Kristallisation bei Temperaturen oberhalb ungefahr 114 °C ausgeführt wurde und die Faserkristalle die Möglichkeit hatten, longitudinal an der Oberfläche des Rotors eines Apparates vom Couette-Typ zu wachsen, wobei der Rotor genügend rasch rotierte.Der mittlere Abstand zwischen den Lamellen der Schaschlik-Struktur, berechnet aus dem Maximum der Weitwinkel-Streuungskurve, stand in sehr guter Übereinstimmung mit den Werten der elektronenmikroskopischen Beobachtungen im Durchlicht. Die lateralen Dimensionen wurden aus der Verbreiterung der Reflexe erhalten, aus Weitwinkel-Aufnahmen Bowie aus Dunkelfeld- und Guinier-Aufnahmen.Die Abstände waren 260 Å für Polyäthylen-Faserkristalle, gewachsen bei einer Temperatur von 103 °C, und ungefähr 150 Å für Fasern entstanden bei 118 °C. Die laterale Kristallitgröße verringerte sich bei höheren Wachstumsgeschwindigkeiten. Unter den letztgenannten Verhältnissen ist das Wachstum verbunden mit größerer Knäuelverformung, was sich durch die Anwesenheit eines erheblichen Anteils von trikliner Kristallmodifikation bemerkbar macht.Dunkelfeld-elektronenmikroskopische Untersuchung hat gezeigt, daß die Länge der Kristallanteile von einer Größenordnung von 250–350 Å für Fasern, gewachsen bei 103 °C, and 450 Å und länger für Fasern entstanden bei 118 °C ist. Diese Werte stehen in guter Übereinstimmung mit den Werten, erhalten aus Verbreiterung der (002) Elektronen-Beugungsbilder.

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With 13 figures and 2 tables     

The kinetics and mechanism of solution of lead fluoride crystals in water

The kinetics of solution of lead fluoride crystals in water was studied under natural convection and rotating disk conditions at fixed temperatures from 0 to 80°C. The kinetic model of the process was suggested.     

The orientation of the dispersed phase and crystals in an injection-molded impact polypropylene copolymer

The orientation of the dispersed phase and crystals in the injection-molded bar of an impact polypropylene copolymer (IPC) containing isotactic polypropylene (iPP), ethylene-propylene rubber (EPR) and a β-nucleating agent (β-NA) were studied simultaneously. In the IPC, iPP and EPR act as the matrix and dispersed phase, respectively. The EPR is amorphous and the iPP is crystallizable in α- and β-crystalline forms in the presence of the β-NA. The orientation and orientation distribution for both of the EPR phase and the iPP crystals, as well as the crystallization behavior of iPP, were investigated by two-dimensional wide-angle X-ray diffraction (2D-WAXD), two-dimensional small-angle X-ray scattering (2D-SAXS), scanning electron microscope (SEM) and differential scanning calorimetry (DSC). The results of the experiment show that orientation exists for both the EPR phase and the iPP crystals. But their orientation distribution manifests an opposite tendency. The EPR phase was observed to be highly oriented in the core layer but the orientation of the iPP crystals was weakened gradually from skin to core. The difference in the orientation behavior between the EPR phase and the iPP crystals reflects the distinct response of the micrometer-scale EPR particles and nanometer-scale iPP chains upon the flow field and temperature gradient in the mold. The diffraction geometry of the β-crystals has also been discussed in detail. The observations in this study may shed light on the study in the structure and property relationship for the IPC injection-molded products.     

Isothermal crystallization kinetics of polypropylene/POSS composites

Polypropylene (PP)/octavinyl polyhedral oligomeric silsesquioxane (POSS) composites were prepared by two different processing methods: reactive blending and physical blending, and the crystallization behavior of PP and PP/POSS composites was studied by means of differential scanning calorimetry and polarized optical microscope. The results showed that the crystallization of PP in PP/POSS composites was strongly influenced by the different processing methods. POSS particles can act as effective nucleating agent, accelerating the crystallization of PP. The crystallization rate increased more dramatically for the reactive blending composite due to the stronger nucleating effect of PP grafted POSS. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1762–1772, 2008     

Nonisothermal crystallization kinetics of polypropylene/montmorillonite nanocomposites

The nonisothermal crystallization kinetics of poly(propylene) (PP) and poly(propylene)/organic‐montmorillonite (PP/Mont) nanocomposite were investigated by differential scanning calorimetry (DSC) with various cooling rates. The Avrami analysis modified by previous research was used to describe the nonisothermal crystallization process of PP and PP/Mont nanocomposite very well. The values of half‐time and Z_c showed that the crystallization rate increased with increasing cooling rates for both PP and PP/Mont nanocomposite, but the crystallization rate of PP/Mont nanocomposite was faster than that of PP at a given cooling rate. The activation energies were estimated by the Kissinger method, and the values were 189.4 and 155.7 kJ/mol for PP and PP/Mont nanocomposite, respectively. PP/Mont nanocomposite could be easily fabricated as original PP, although the addition of organomontmorillonite might accelerate the overall nonisothermal crystallization process. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 408–414, 2002; DOI 10.1002/polb.10101     

Interaction kinetics of peroxides and organic sulphides in atactic polypropylene

Kinetics of the reactions between benzoyl peroxide and sulphides (dilaurylthyodipropionate and dibenzyl sulphide), of the reaction between diphenylethyl hydroperoxide and dibenzylsulphide in atactic polypropylene and of the reaction of benzoyl peroxide with dibenzyl sulphide in polyisobutylene have been studied. The stoichiometry of the reactions was determined and the dependence of the rate constants on the initial concentration of reactant was established. At low reactant concentrations, the rate constants decrease and at high concentration increase with increasing concentration. In model low-molecular-weight hydrocarbon, the reaction rate constants do not change with reactant concentrations. The reaction mechanism is discussed. It is based on specific features of the dissolution of low-molecular additives in polymer.     

Rotational dynamics and conformational kinetics in liquid crystals

Abstract

Two particular aspects of solute dynamics in ordered media are analysed on the basis of the solution of multivariate diffusion equations: the effects of the solvation dynamics on the rotational motions of dipolar probes in liquid crystal solvents, and the alteration of reaction pathways in isomerization kinetics caused by the solvent order. The introduction of a suitable solvent coordinate allows the interpretation of high frequency contributions in the rotational correlation functions observed by spectroscopic techniques, namely dielectric dispersion, IR and Raman spectroscopy, ESR lineshapes and optical Kerr effect. For molecular systems undergoing conformational changes, a method is offered to evaluate the modification of the torsional barriers resulting from the anisotropic torques modulated by the molecular shape changes along the reaction coordinate.     

Rotational dynamics and conformational kinetics in liquid crystals

Two particular aspects of solute dynamics in ordered media are analysed on the basis of the solution of multivariate diffusion equations: the effects of the solvation dynamics on the rotational motions of dipolar probes in liquid crystal solvents, and the alteration of reaction pathways in isomerization kinetics caused by the solvent order. The introduction of a suitable solvent coordinate allows the interpretation of high frequency contributions in the rotational correlation functions observed by spectroscopic techniques, namely dielectric dispersion, IR and Raman spectroscopy, ESR lineshapes and optical Kerr effect. For molecular systems undergoing conformational changes, a method is offered to evaluate the modification of the torsional barriers resulting from the anisotropic torques modulated by the molecular shape changes along the reaction coordinate.     

The thermal degradation kinetics of polypropylene: Part I. Molecular weight distribution

A kinetic model for the thermal degradation of polypropylene was developed and fit to molecular weight distribution data obtained by high-temperature size-exclusion chromatography. In a series of ampoule experiments, reaction temperatures of 275 to 315 °C were examined with reaction times of up to 48 h. A single-parameter version of the model, containing an apparent rate constant, was found to provide excellent fits of all molecular weight distributions. Values of the parameter varied with both temperature and reaction time. The variations with temperature provided Arrhenius plots at each time. A lower-than-expected overall activation energy of 123.8 kJ/mol was attributed to the temperature range examined and the presence of ‘weak links’ due to oxidized moieties in the polymer. The ‘weak links’ were below the detectability limit of Fourier transform infra-red spectroscopy applied to the reacted samples. However, other data on heavily oxidized polypropylene and a recent study using thermal gravimetric analysis¹ where an activation energy of 98.3 kJ/mol was determined for similar temperatures, did provide further support for the hypothesis.