Chun Wang

University of Freiburg, Freiburg, Baden-Württemberg, Germany

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Publications (7)26.16 Total impact

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    ABSTRACT: The formation of holes during the late stage of the isothermal crystallization in thin films of isotactic poly(propylene) between two cover glasses was observed by light microscopy and atomic force microscopy. This behavior can be described consistently by the well-known negative pressure effect. Light microscopy reveals the simultaneous and sudden occurrence of a large number of small holes at the liquid-solid interface after the liquid in front of the spherulites is completely confined by other spherulites for a certain time interval. In exceptional cases only a few holes appear and finally large cavities are formed. Atomic force microscopy measurements carried out in the height mode are able to prove the hole formation in front of the spherulites. Furthermore, a substantial thinning of the two-dimensional spherulites in thin films can be observed prior to the hole formation.
    Macromolecular Chemistry and Physics 03/2003; 197(3):1085 - 1091. · 2.39 Impact Factor
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    ABSTRACT: The occurrence of banded spherulites in blends of poly(ε-caprolactone) with poly(styrene-ran-acrylonitrile) is studied by means of optical-, scanning electron-, environmental scanning electron-, and atomic force-microscopy as well as smallangle X-ray scattering. The measurements reveal that lamellae are confined in fibrils that bend periodically from the center of spherulites in a radial direction. The fibrils show also a regular spacing in the lateral direction. Single bent lamellae can be observed by atomic force microscopy after permanganic etching. The enrichment of amorphous material near to the growth front of the spherulites can be directly observed by polarized light microscopy after temperature jump experiments. The excluded amorphous material on the surface of the growing spherulites has the shape of droplets. This might be the main reason for the initiation of the bending process because the result is a non-uniform stress distribution acting on the growing lamellae at the interface between spherulites and the surrounding melt. The amorphous material of the blends can be found in three areas as observed by small-angle X-ray scattering experiments and microscopical methods: (i) between the lamellae, (ii) excluded from the interlamellar region but within the spherulite, and (iii) for high contents of non-crystallizable material, it can be excluded from the spherulite. The banding periodicity of spherulites as a function of the crystallization temperature can be described in terms of two models, based either (i) on the inherent twisting of lamellae perpendicular to the axis of a screw dislocation or (ii) on the dependence of the diffusion of amorphous material away from the growth front of lamellae and the temperature dependence of the rate of crystallization. Both models fail at very low supercoolings near to temperatures where the banding disappears completely.
    Acta Polymerica 03/2003; 48(9):354 - 362.
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    ABSTRACT: The supramolecular assembling of small amounts of octyl- and benzyl-substituted complementary tectons based upon barbiturates (BA) and 2,4,6-triaminopyrimidines (TP) was studied in a polystyrene matrix by means of differential scanning calorimetry (DSC), dynamic-mechanical spectroscopy, and transmission electron microscopy (TEM). Self-organization of these tectons within the compounds drastically changes the melt rheology in comparison to the pure matrix. Depending on external conditions such as temperature and mechanical deformation, the formation of BA/TP assemblies in polystyrene via self-organization leads to strongly increased dynamic moduli and a networklike behavior. A minimum content of 3 wt % of BA/TP is required for these rheological effects. A similar self-organization and reinforcing effect could not be observed in a more polar poly(methyl methacrylate) matrix. The resulting BA/TP structures were also examined by TEM and polarized light microscopy in both polymer matrices as well as at the interface of sandwiches consisting of PMMA and polystyrene layers. The microscopic techniques confirm the rheological results with respect to the dynamic self-organization and provide information about the concerning morphology development.
    Macromolecules 11/1999; 32(25). · 5.93 Impact Factor
  • Jörg Kressler, Chun Wang
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    ABSTRACT: The film formation behavior of a semicrystalline polymer, poly(ε-caprolactone) (PCL), during spin-coating from solution in cyclohexanone and after thermal annealing, has been studied by light microscopy, light scattering, scanning electron microscopy, and atomic force microscopy. During the spin-coating process, film formation is ruled by competition of dewetting and crystallization. Depending on the concentration of the solutions, PCL films completely cover the glass substrate, open spherulites are formed, or for very dilute solutions, PCL forms dispersed droplets on the glass substrate. Films, covering the substrate, show during heating above the melting point of PCL dewetting phenomena that are influenced by both film thickness and initial crystalline morphologies.
    Langmuir 08/1997; 13(16). · 4.38 Impact Factor
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    ABSTRACT: The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) (Mw = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.
    Macromolecules 12/1996; 29(26). · 5.93 Impact Factor
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    ABSTRACT: Blends of isotactic and syndiotactic polypropylene were studied by wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), light microscopy, scanning electron microscopy and atomic force microscopy. WAXS measurements show that both polymers crystallize in different unit cells already during precipitation from a common solvent. Both polymers have a very similar long period and lamella thickness after isothermal crystallization at 135°C as revealed by SAXS. From the crystallization morphology, it can be concluded that the crystallization of isotactic and syndiotactic polypropylene after isothermal annealing in the melt occurs always in large, macroscopic domains. Isotactic polypropylene crystallizes preferably in different spherulitic forms which can usually not be detected for syndiotactic polypropylene crystallizing preferably as needle-like entities. The crystalline morphology of the blends is very complex and depends strongly on the thermal history in the melt, the crystallization temperature and blend composition. It can clearly be seen that the blends undergo liquid-liquid phase separation in the melt which yields isotactic polypropylene in a matrix of syndiotactic polypropylene; syndiotactic polypropylene in a matrix of isotactic polypropylene or a co-continuous morphology for nearly symmetric blends.
    Polymer 01/1996; · 3.77 Impact Factor
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    ABSTRACT: The morphology of syndiotactic polypropylene (s-PP), containing 91% racemic pentads, isothermally crystallized from the melt has been investigated by means of light microscopy, atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). Light microscopy shows a typical needle-like structure of crystalline entities at crystallization temperatures ranging from 115 to 150°C. Large bundles of lamellae and rectangular entities with single crystal character were the main morphological structures observed by AFM. These single crystal-like entities exhibit two typical fracture types; (i) transverse straight fractures with an average distance between the cracks of approximately 2 μm, and (ii) irregular fractures which yield a small mosaic-like structure. A lamellar thickness of 10 nm and a long period of 20 nm of s-PP crystallized isothermally at 135°C were obtained from the two-dimensional correlation function of SAXS measurements. This long period is in good agreement with that measured by AFM. Rare structures such as spherulites, hedrites and aggregates having a clear lateral periodicity perpendicular to the growth direction can be observed. Light microscopy of s-PP crystallized at 126°C and 3.5 kbar shows mainly spherulitic growth with an irregular banding structure.
    Polymer 01/1995; · 3.77 Impact Factor