Influence of nucleating agents on crystallization of polypropylene

The effect of talc as an artificial nucleating agent in different concentrations on the crystallization of polypropylene (PP) has been studied. It is considered that the induction time should be taken into account in the Avrami evaluation of isothermal crystallization. From a study of nucleated PP samples prepared in different ways, it has been proved that the nucleating effect of talc slightly decreases with increasing time spent by the sample in the state of the polymer melt. It has been shown that crystallization of non-nucleated polypropylene strongly depends on the material of the sample pan. It has been established that dilatometry cannot be used to study the isothermal crystallization of nucleated polypropylene, since microcracks appear in the sample, partly compensating the volume decrease due to the crystallization process.

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Zusammenfassung Die Wirkung von in unterschiedlichen Konzentrationen als künstliches Keimbildungsmittel angewandtem Talk auf die Kristallisation von Polypropylen wurde untersucht. Bei der Auswertung der isothermen Kristallisation nach Avrami ist die Induktionszeit in Betracht zu ziehen. Bei der Untersuchung von auf verschiedene Weise hergestellten Polypropylenproben wurde festgestellt, dass die keimbildende Wirkung von Talk desto geringer ist, je länger sich die polymere Probe im geschmolzenen Zustand befand. Es wird gezeigt, dass die Kristallisation von keimbilden den Mitteln nicht ausgesetztem Polypropylen stark vom Material des Probengefässes abhängt. Es wurde festgestellt, dass die Dilatometrie nicht zur Untersuchung der isothermen Kristallisatior von mit keimbildenden Mitteln versetztem Polypropylen geeignet ist, da in der Probe Mikrokrack Produkte auftreten, die die durch den Kristallisationsprozess verursachte Abnahme des Volumens teilweise kompensieren.

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This work was presented in part at the 7th International Conference on Thermal Analysis, August 22–28, 1982, Kingston, Ontario.

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The authors are indebted to Messrs M. Pati and J. Volk for the preparation of the samples, and to Dr. A. Solti for her help during the dilatometric measurements.     

Gel-induced selective crystallization of polymorphs

Although nanoporous materials have been explored for controlling crystallization of polymorphs in recent years, polymorphism in confined environments is still poorly understood, particularly from a kinetic perspective, and the role of the local structure of the substrate has largely been neglected. Herein, we report the use of a novel material, polymer microgels with tunable microstructure, for controlling polymorph crystallization from solution and for investigating systematically the effects of nanoconfinement and interfacial interactions on polymorphic outcomes. We show that the polymer microgels can improve polymorph selectivity significantly. The polymorphic outcomes correlate strongly with the gel-induced nucleation kinetics and are very sensitive to both the polymer microstructure and the chemical composition. Further mechanistic investigations suggest that the nucleation-templating effect and the spatial confinement imposed by the polymer network may be central to achieving polymorph selectivity. We demonstrate polymer microgels as promising materials for controlling crystal polymorphism. Moreover, our results help advance the fundamental understanding of polymorph crystallization at complex interfaces, particularly in confined environments.     

Surface-esterified cellulose fiber in a polypropylene matrix: impact of esterification on crystallization kinetics and dispersion

Cellulose powders hydrophobized by surface esterification with carboxylic acids with different chain lengths (3, 10 and 18 carbons) were dispersed in a polypropylene matrix. Quality of the dispersion and nucleation activity of the filler were investigated by means of differential scanning calorimetry and optical microscopy. The results showed that the esterification decreases the crystallization rate in case of cellulose esterified with propionic or decanoic acid. On the other hand, the oleic acid ester demonstrated slightly higher crystallization rates than the unmodified cellulose, which was ascribed primarily to the newly arisen non-esterified surface after disintegration of the filler. Optical microscopy with hot stage showed the high nucleation ability of the natural cellulose fiber and its suppression in case of esterified surfaces. A complete inability to nucleate polypropylene crystallization was observed in case of decanoyl ester, while the other two retained some activity, but lower than that of the natural fiber. Finally, analysis of the filler dispersion and distribution revealed that the decanoyl and octadecanoyl esters disintegrate during melt mixing, while both dispersion and distribution of the fibers modified with propionic acid are poor.     

Influence of wood mercerization on the crystallization of polypropylene in wood/PP composites

Mercerization process is very significant because the alkali treatment facilitates reactivity of lignocellulosic fillers, thus allowing better response to chemical modification. In the present study, the effect of mercerization of pine wood on the nucleation ability of polypropylene was investigated by means of differential scanning calorimetry. We discovered that for the composites with wood containing cellulose II, the decrease in the crystal conversion of the polymer matrix and increase in the half-time of crystallization values are significant. It can be concluded that the amount of cellulose II formed upon alkalization of lignocellulosic fillers determines their nucleation ability. To evaluate the transcrystalline effects caused by various woods, which were untreated or treated with sodium hydroxide, the polarized optical microscopy was also performed. The nucleation of polypropylene on the surface of wood was investigated by induction time measurement. It was found that surfaces of the unmodified wood generate epitaxial nucleation, whereas the mercerized wood generates nonepitaxial nucleation. The differences in the type of nucleation suggest that the effectiveness of formation of transcrystalline structures depends on the contribution of cellulose I and cellulose II. Moreover, the presence of epitaxy is not necessary for the appearance of transcrystalline structures. The results showed that the transcrystalline structures appeared in each system, even with wood containing significant contribution of cellulose II. The only difference noted was the change in the nucleation abilities of the wood surface. Results of this study imply the necessity of quantitative determination of the contributions of cellulose I and cellulose II, whose presence determine the type of nucleation and nucleation ability of the filler surface.     

Recognition of the syndiotactic polypropylene polymorphs via dynamic-mechanical analysis

Three syndiotactic polypropylene samples were crystallized under different conditions in order to obtain different polymorphs. A first sample was crystallized at high temperature, obtaining the helical form I; a second was crystallized from the melt at 0°C for many days obtaining the trans-planar mesophase; a third sample was obtained by solvent induced crystallization followed by annaeling of the trans-planar mesophase, leading to a mixture of both the helical forms I and II. In the dynamic-mechanical analysis the helical form I showed only one peak of tan δ corresponding to the amorphous glass transition. The other polymorphs also showed this transition centered at about the same temperature. Beside the peak corresponding to the Tg, the trans-planar mesophase was characterized by a peak appearing at 70°C, and the helical form II by a peak at 100°C. These peaks, unambiguously associated to transitions of the different forms, can be considered a distinctive evidence for the polymorphs obtained in different processing conditions.     

Effect of montmorillonite on polypropylene crystallization

The effect of additions of pure and modified montmorrilonite on the polypropylene crystallization rate was studied within the framework of Avrami model. In the presence of these fillers in amounts of 2.5–5.0 wt %, both the crystallization rate and the degree of crystallinity increase, and at the content of these fillers increased to 10% and higher values these parameters decrease. Introduction of fillers enhances the mechanical properties of polypropylene, which allows the developed material to be recommended for the production of packaging film.     

Surface crystallization of polypropylene

Crystallization of compression-molded isotactic polypropylene and polyethylene is invariably spherulitic; generally, nucleation occurs randomly throughout the sample. In a special case where nucleation predominates at the surface, spherulitic growth centers become crowded and are forced to propagate unidirectionally into the bulk (transcrystallinity). Conditions for the formation of transcrystallinity have been investigated by optical and scanning electron microscopy. The occurrence of transcrystallinity is attributed to heterogeneous nucleation induced at the mold surface. To be effective, the mold surface must have a nucleating efficiency equal to or greater than that of adventitious nuclei present in the polymer. As the crystallization temperature approaches the melting point, the activity of mold surfaces is found to increase leading invariably to transcrystalline formation. The degree of activity of various mold surfaces correlates with the known activity of specific dispersed nucleating agents having similar chemical structures. Contrary to claims in the literature, the surface energy of the mold surface and temperature gradients across the melt surface do not play a primary role in transcrystalline formation of polypropylene.     

Elastic modulus of the crystalline regions of cellulose polymorphs

The elastic modulus E_l of the crystalline regions of cellulose polymorphs in the direction parallel to the chain axis was measured by x-ray diffraction. The E_l values of cellulose I, II, III_I, III_II, and IV_I were 138, 88, 87, 58, 75 GPa, respectively. This indicates that the skeletons of these polymorphs are completely different from each other in the mechanical point of view. The crystal transition induces a skeletal contraction accompanied by a change in intramolecular hydrogen bonds, which is considered to result in a drastic change in the E_l value of the cellulose polymorphs. © 1995 John Wiley & Sons, Inc.     

The far-infra-red spectra of crystalline isotactic polypropylene polymorphs

Far-i.r. spectra of crystalline isotactic polypropylene (IPP) in its various polymorphic forms have been recorded over the range 500-100 cm⁻¹. Differences in the spectra, in particular the splitting of certain A line-group modes, reflect the environments of isolated IPP chains within the particular unit cell configurations of the crystal forms.     

Cyclodextrin-based isolation of Ostwald's metastable polymorphs occurring during crystallization

We describe a novel approach for the selective isolation of Ostwald's intermediate metastable polymorphs occurring during an early stage of crystallization, by utilizing the inclusion complex formed with a cyclic oligosaccharide derivative, 2,6-di-O-methyl-beta-cyclodextrin.     

Exclusion effect of carbon dioxide on the crystallization of polypropylene

We investigated the crystallization growth of isotactic polypropylene under carbon dioxide (CO₂) at various CO₂ pressures and temperatures by in situ observation with a digital high‐fidelity microscope and a specially designed high‐pressure visualized cell. The fibrils within the spherulite were distorted and branched by crystallization under CO₂ at pressures higher than 2 MPa, and this suggested the exclusion of CO₂ from the growth front of the fibrils. The spherulite growth rate (G) at 140 °C increased with the CO₂ pressure, attained a maximum value around 0.3 MPa, and then decreased. Above 6 MPa, it became slower than that under air at the ambient pressure. An analysis of the crystallization kinetics by the Hoffman–Lauritzen theory revealed that the pressure dependence of G could be ascribed to the change in the transportation rate of crystallizable molecules (β_g) with pressure; that is, β_g increased and then decreased with pressure. The increase in β_g at a low pressure was caused by the plasticizing effect of CO₂, whereas the decrease in β_g at a high pressure was due to the exclusion of CO₂ from the crystal growth front. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1565–1572, 2004     

Modeling of the crystallization of isotactic polypropylene chains

The mechanical properties of polypropylene depend critically on the crystallizability of the chains, which depends in turn on their stereochemical structures. These dependences were investigated using Monte Carlo methods to generate chains having various stereochemical sequences and then scrutinizing parallel arrays of these chains to look for matches in isotactic stereochemistry that could lead to the formation of crystallites. The fraction of such units in matches, for example, gives a direct measure of the degree of crystallinity expected for the specified degree of isotacticity. Other quantities of interest obtainable in this way were the natures of the sequence distributions themselves, melting points, free energies of fusion, interfacial free energies, and moduli (calculated on the basis of the crystallites acting as crosslinks within an elastomeric network structure). © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2757–2764, 1997     

Double crystallization of biaxially oriented polypropylene

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

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

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

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High-pressure crystallization of isotactic polypropylene droplets

Dispersions of isotactic polypropylene (PP) particles in polystyrene (PS) were produced by interfacially driven breakup of nanolayers in multilayered systems that were fabricated by means of layer-multiplying coextrusion. The droplet size was controlled by the individual PP layer thickness ranging from 12 to 200?nm. In addition, PP was melt blended with PS to produce PP droplets larger than those formed by breakup of nanolayers. The dispersions of PP particles in the PS matrix were melted and annealed under high pressure of 200?MPa. Only the largest PP droplets, with average sizes of 170?μm, crystallized predominantly in the γ form. In the 42-μm droplets obtained by breakup of 200?nm layers, a minor content of the γ form was found whereas the smaller droplets obtained by breakup of the thinner nanolayers contained the α form and/or the mesophase. The results showed that the γ phase formed only in the droplets sufficiently large to contain the most active heterogeneities nucleating PP crystallization under atmospheric pressure. It is concluded that the presence of nucleating heterogeneities is necessary for crystallization of PP in the γ form under high pressure.     

Fast DSC applied to the crystallization of polypropylene

In an effort to find the limit of crystallization of polypropylene, a series of quantitative and semiquantitative DSC experiments at rates up to 10,000 deg/min are described. Even at these fast rates polypropylene crystallized on cooling between 350±15 K and 280±6 K. No fully amorphous polypropylene was produced. No initial stage crystallization to the condis state could be proven by quenching after partial crystallization.

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Zusammenfassung Eine Reihe von quantitativen und semiquentitativen DSC-Versuchen mit Aufheizgeschwindigkeiten bis 10000 K/min wurden ausgeführt, um die Grenze der Kristallisation von Polypropylen festzustellen. Sogar bei diesen schnellen Aufheizgeschwindigkeiten kristallisiert Polypropylen beim Abkühlen zwischen 350±15 K und 280±6 K. Kein völlig amorphes Polypropylen wurde erhalten.

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, 10 000 /. 350±15 280±6 . « » . .

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This work was supported in part by a grant of Hercules, Inc. to the ATHAS effort and in part by the Polymers Program of the National Science Foundation (Grant number DMR 8317097). The experimentation was helped by an instrument gift by the Celanese Company.     

On the crystallization behavior of cold-drawn syndiotactic polypropylene

The influence of the chain conformation on the crystallization behavior of cold-drawn syndiotactic polypropylene (sPP) has been investigated. The conformational and structural changes depending on drawing conditions and thermal treatments has been observed by x-ray diffraction, infra-red spectroscopy and modulated differential scanning calorimetry. A nucleation and crystal growth model is introduced, which explains the low crystallinity of cold-drawn sPP.     

Influence of the order of polymer melt on the crystallization behavior: II. Crystallization kinetics of isotactic polypropylene

The crystallization behavior of isotactic polypropylene (iPP) melts with a high order has been carefully examined by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The experimental results show that the helically ordered iPP melt crystallizes by heterogeneous nucleation with two-dimensional diffusion controlled growth and the Avrami exponent is about 2. The data available both from our DSC and PLM experiments and from the literature indicate that the order of a polymer melt can speed up the crystallization process. When some unmelted materials exist in the ordered melt, the crystallization will become more rapid. Received: 16 June 2000 Accepted: 16 October 2000