Morphology of different types of isotactic polypropylene spherulites crystallized from melt

doi:10.1016/0032-3861(88)90071-7

Polymer

Volume 29, Issue 4, April 1988, Pages 591-596

https://doi.org/10.1016/0032-3861(88)90071-7 Get rights and content

Abstract

The morphology of melt-grown spherulites of isotactic polypropylene (iPP) has been investigated by use of a polarizing microscope. The low molecular weight iPP, isothermally crystallized at 150–160°C, was found to form new types of spherulites. The spherulites termed types I and II by Keith et al.² exhibit a simple Maltese cross on a polarizing microscope with crossed polars. However, the new spherulites do not exhibit such a Maltese cross. These spherulites have been named ‘pseudo-negative’, ‘pseudo-positive’, ‘neo-mixed’ and ‘flower-like’ according to their morphologies, respectively. The structure of iPP spherulites crystallized isothermally at 155°C, does not belong to the α₁-form ( $C 2 c$ ) group but to the α₂-form ( $P 2_{1} c$ ) group.

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The interference colors make specific Maltese-cross patterns in the spherulites. The different kinds of spherulites (like Maltese cross, ring-banded) and the explanation for this phenomenon can be found in the literature [24,45,46]. The spherulites grow uniformly in all directions and finally impinge upon each other forming polygonal units.
This work demonstrates the most widely used characterization methods and techniques of the supermolecular and lamellar structure of semicrystalline polymers. Polarized optical microscopy (POM) equipped with a hot-stage (thermo-optical microscopy, TOM), brightfield microscopy (BF), darkfield microscopy (DF), digital image processing techniques, optical profilometry (OP), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used as investigation techniques. The same iPP grade was used with different sample preparation techniques to compare these methods. The advantages and drawbacks of the sample preparation and investigation methods were discussed. The results show how the introduced techniques could reveal different kinds of information, and it is also shown how the experimental techniques should be matched to the goals of a structural study.
DSC/SAXS analysis of the thickness of lamellae of semicrystalline polymers-restrictions in the case of materials with swollen amorphous phase
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To emphasize changes occurring in the material as a result of the presence of nonadecane and after its extraction, the relationship of the value of long period in the function of the content of nonadecane has been presented in Fig. 5 (for polypropylene/nonadecane systems before and after the extraction process). Using the values of crystalline mass fraction (Tables 1 and 2) as well as the density of crystalline and amorphous component equal to 0.946 g/cm3 and 0.855 g/cm3 [30], the values of crystalline volume fraction were determined (Tables 5 and 6). This results together with the values of long period were then used to determine the thickness of lamellar crystals according to equation (2).
Due to the simplicity and speed of the analysis, calorimetric (DSC measurements and Gibbs-Thomson method) or x-ray methods (SAXS measurements and Bragg's law or correlation function method) are the most frequently used in determining the thickness of crystals. This paper revealed that the direct application of the above mentioned methods in order to analyze the lamellar structure of semicrystalline polymers, the amorphous phase of which was swollen, may lead to incorrect results. Swelling of the amorphous phase (without changing the thickness of crystals) leads to a shift of the observed melting temperature towards lower values, what in turn will result in underestimated values of thickness of crystals, in the case of calorimetric method. At the same time, an increase of the value of long period is observed, what in turn will result in overestimated values of thickness of crystals, in the case of classical x-ray method (with the use of value of long period estimated according to the Bragg's law and crystalline volume fraction). We have observed that only the analysis of x-ray data with the use of the correlation function method enables to obtain actual values of the thickness of lamellar crystals of the polymeric materials with swollen amorphous phase. The means for correct analysis are pointed out. The above effects have been presented on the example of model, miscible, non-cocrystallizing semicrystalline polymer/low molecular weight modifier systems.
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Observation of the Maltese cross pattern in polymer spherulite sections signifies only that the anisotropic crystals are arranged in such a way as to conform to Fig. 6(a) or Fig. 6(b). Structures grown at low undercooling are sometimes circular with a distorted cross pattern that changes appearance on sample rotation [28]; these are not spherulites. By methods that involve inserting an element of known birefringence into the optical path and observing systematic changes to the interference colors transmitted by the analyzer [29], one can determine if the largest refractive index is tangential (Fig. 6(a)) or radial (Fig. 6(b)).
Polymeric and non-polymeric materials often crystallize as spherulites when crystallized from viscous melts or solutions at large undercooling. The essential component of a spherulite is fibrillar crystals that grow in predominantly radial directions and branch irregularly. We review the growth, branching and twisting of crystals in the light of theoretical and experimental advances of the last decade, while maintaining an appreciation for historical context.
The crucial role of self-generated fields ahead of the crystal–melt interface is developed. Pressure gradients from volume contraction have been treated, as well as impurity gradients ahead of a growing crystal; fibril width W is predicted and found to be proportional to δ^1/2, where the diffusion length δ = D/G, the quotient of diffusivity and growth rate, conveys the extent of the field gradient. Ribbon-like spherulite radii grow at a constant rate under diffusion-coupled interface control.
Non-crystallographic branching is required to maintain the volume occupied by fibrillar crystals as the spherulite radius increases. Topological giant screw dislocations and induced nucleation at cilia tethered to crystals are observed mechanisms leading to branching normal to the wide dimension of lamellar crystals; but the relative importance of each of these is not yet established. Repetitive tip splitting by kinetic interface instability has been suggested as a branching mechanism in the wide dimension of lamellar crystals.
Larger molecular mass reduces the spherulite growth rate, more so at low undercoolings, for reasons that remain unresolved. Miscible diluents often profoundly reduce G by lowering both thermodynamic driving force and local transport dynamics that govern the secondary nucleation rate. Spherulite blend morphology is linked to the competition between radial growth rate G and diffusivity D of the diluent, expressed as the diffusion length δ.
Polymer crystals in which chain helices all have the same sense show banded spherulites, as do crystals in which the chain axes are not perpendicular to the basal surfaces. Recent analyses with optical birefringence and X-ray micro-diffraction support the presence of helicoidally twisted ribbons, although other structural arrangements have sometimes been revealed by microscopy. Assessments of twist directions in spherulites of chiral polymers point to unbalanced basal surface stress as the source of twisting, although a general mechanical analysis is lacking. Another twisting model employs regular arrays of isochiral giant screw dislocations; results are mixed for this model.
Shear-induced crystallisation of molten isotactic polypropylene within the intertube channels of aligned multi-wall carbon nanotube arrays towards structurally controlled composites
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A model of the microstructure of the MWCNT-iPP composite can be proposed (Fig. 3B), consistent with the reported effects of nanotube-induced iPP crystallisation [9] and a recent work by Haque et al. [22], demonstrating that iPP crystallised epitaxially in MWCNT composites as 15 nm-thick lammelar stacks. This model conforms tendency of the iPP lamellae to organise into characteristic lamellar branching [23–26], attributable to homo-epitaxial growth of a daughter T-lamella on the lateral (010) plane of the mother R-lamella [27,28]. A direct infiltration method, possibly scalable to laminated or wound structures, was applied in the fabrication of aligned MWCNT-iPP composites at high nanotube loading.
A melt processing route was successfully applied in the manufacturing of aligned multi-wall carbon nanotube/isotactic polypropylene (MWCNT-iPP) composites. As high as 25 wt% content of the catalytic Chemical Vapour Deposition (c-CVD) derived nanotube filler could be introduced into the iPP matrix. The composite was characterised by a practically retained nanotube alignment as well as an enhanced crystallisation of iPP. Analysis of the phase iPP composition in the composite using XRD and DSC revealed a dominating α-phase with polymer molecules uniaxially oriented in the same direction as the nanotube alignment. Furthermore, kinetic in situ Synchrotron XRD studies verified that the enhanced crystallisation was induced mainly by shear stress occurring in the initial stage of infiltration MWCNT arrays by flowing molten polymer and, to a lesser extent, by the template-based growth of iPP crystallites.
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It has four modifications: α, β, γ and smectic [1–4]. The lamella of monoclinic α-form presents “cross-hatch”structure [5–7], that is, daughter lamella grows accessorily on the parent lamella. Furthermore, α crystals are thermodynamically stable and possess relatively good mechanical performance.
It has been well established that periodical shear stress can improve the mechanical performance of isotactic polypropylene during injection molding. In the current study, Polarized Light Microscopy (PLM), two-dimensional Wide-Angle X-ray Diffraction (2D-WAXD), Scanning Electron Microscopy (SEM) were employed to investigate the morphology evolution of vibration sample. Compared with static sample, the morphology of vibration one, which is derived from periodical shear field, exhibits different hierarchy structure: there is an additional fiber layer containing cylindrulites between the shear layer and the core layer, and the orientation is enhanced obviously. Through etching technique, it is found that there exists non-crystalline component in the fibril (core) of cylindrulite, and it may induce the growth of β crystals directly regardless of its non-crystalline nature. Based on the investigated results, a model of formation of cylindrulite is proposed.
Polypropylene nanofiber sheets prepared by CO <inf>2</inf> laser supersonic multi-drawing
2012, European Polymer Journal
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The melting behavior of iPP crystallized by various methods has been studied [39–45]. The endotherm peaking at 161 °C is due to melting of the α1 form, and the higher endotherm peaking above 170 °C is due to melting of the α2 form [42]. Jacoby et al. [36] reported that the endotherm due to melting of the β form was observed at approximately 150 °C.
Isotactic polypropylene (iPP) nanofiber sheets were fabricated by winding nanofibers onto a spool. The nanofibers were prepared by irradiation of the iPP fibers with a carbon dioxide laser while drawing them at supersonic velocities. A supersonic jet was generated by blowing air into a vacuum chamber through the fiber injection orifice. The vacuum chamber used to produce nanosheets has seven fiber injection orifices and a spool to collect the nanofibers. A 17 × 18 cm², 17 μm-thick rectangular nanofiber sheet was obtained by collecting nanofibers for 10 min. The nanosheet is composed of nanofibers with an average diameter of 350 nm. This technique is a novel method for the production of nanosheets without using a solvent, and therefore, without the need to remove a second component.

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Polymer paperMorphology of different types of isotactic polypropylene spherulites crystallized from melt

Abstract

Eur. Polym. J.

Polymer

Polymer

Nuovo Cimento Supp.

Polymer paper
Morphology of different types of isotactic polypropylene spherulites crystallized from melt