R. Kolb

Stony Brook University, Stony Brook, NY, United States

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Publications (17)17.96 Total impact

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    ABSTRACT: Flow-induced crystallization in a bimodal polyethylene blend was investigated by means of in situ shear-WAXD (wide-angle x-ray diffraction) and shear-SAXS (small-angle x-ray scattering) techniques. The blend contained a low molecular weight (Mw = 50 000 g mol−1 and polydispersity = 2) polyethylene copolymer matrix (MB-50k) with 2 mol% of hexene, and a nearly monodisperse high molecular weight (Mw = 161 000 g mol−1 and polydispersity = 1.1) hydrogenated polybutadiene component (MD-161k), which has the microstructure of an ethylene–butene copolymer with 4 mol% butene. At the experimental temperatures of 112 and 115 °C, MB-50k exhibited faster crystallization kinetics and higher crystallinity due to higher chain mobility and higher ethylene content than those of the MB-50k/MD-161k blend. However, both WAXD and SAXS results indicated that the high molecular weight component (MD-161k) is responsible for the formation of more highly oriented crystals, which we relate to a shear-induced precursor scaffold. Values of the lamellar long period in all experimental runs were found to slightly decrease in the beginning of crystallization and then reached a plateau value. Vonk's method for single lamella scattering was employed to estimate the lamellar thickness in the MB-50k/MD-161k blend at high temperature (115 °C), where the lamellar thickness was also found to decrease in the beginning and remained about constant afterward. Twisted lamellar structures were observed in all formed kebabs.
    Journal of Physics Condensed Matter 08/2006; 18(36):S2421. · 2.22 Impact Factor
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    ABSTRACT: Synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) have been utilized to elucidate the nature of shear-induced shish-kebab structures at the early stages of crystallization in a binary blend, containing 5 wt% of high-density polyethylene (HDPE) and 95 wt% of linear low-density polyethylene (LLDPE. The HDPE component possessed a bimodal molecular weight distribution with weight-average molecular weights Mw of 99 kg/mol and 1,100 kg/mole, respectively. X-ray results indicated that the crystallization kinetics and molecular orientation of the blend were significantly enhanced under shear as compared to LLDPE, even though the blend contained only a low concentration of high molecular weight HDPE chains (ca. 1 wt% of = 1,100 kg/mol above the overlap concentration, c*=0.5 wt%). The Avrami exponent of the blend, n=1.9, derived from the WAXD crystallinity evolution under shear, suggested that the two-dimensional (2D) kebabs are developed under diffusion-controlled and spontaneous nucleation conditions. The Avrami exponents were found to decrease with the increase in shear duration time (strain). The corresponding SAXS patterns showed a meridional streak, which can be modeled by a shish-kebab structure containing cylindrical symmetry along the shish axis and polydispersities in diameter, thickness and long period of kebabs. The shish-kebab model was formulated in a closed analytical form under the assumption of independent statistics. Furthermore, Ruland’s method to separate the effects of size and orientation, usually used for the equatorial streaks, was applied to the meridional streak for the first time, under the assumption that the widths of the distributions add as in Lorentzian-type distributions. The fitted mean diameter of kebabs using our shish-kebab model was found to be consistent with the value obtained by using the Ruland’s streak method. The radial growth rate, G, calculated from the kebab evolution initially followed the relationship G ∝ t -1/2, which confirmed the diffusion-controlled growth with spontaneous nucleation in the polymer blend. KeywordsSAXS–WAXD–shear–crystallization–shish-kebab–polyethylene
    06/2005: pages 223-238;
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    ABSTRACT: Structural changes during constrained melting of blown polyethylene (PE) films were followed by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Results indicated that thermally stable flow-induced crystallization precursor structures (i.e., shish-kebabs) can be revealed above the nominal melting point determined by differential scanning calorimetry (DSC). Two samples were investigated:  (1) linear low-density polyethylene (LLDPE) with M̄w of 116 kg/mol and (2) a binary blend of 95 wt % LLDPE and 5 wt % high-density polyethylene (HDPE). HDPE had a bimodal molecular weight distribution, containing 80 wt % of low molecular weight (LMW) component (M̄w = 99 kg/mol) and high molecular weight (HMW) component (M̄w = 1100 kg/mol). Even though the blend contained only a low concentration of HMW−HDPE chains (c = 1 wt %), above its overlap concentration (c* = 0.5 wt %), the initial film showed significantly higher lamellar orientation, and the high-temperature film showed a much more intense shish-kebab scaffold than LLDPE. The Ruland method for meridional streak analysis was used to analyze the kebab diameter and orientation during melting; the Vonk procedure for single lamella scattering was used to analyze the corresponding kebab thickness change. The temperature dependence of kebab thickness for both LLDPE and HDPE/LLDPE blend was used to determine the equilibrium melting temperature for PE (Tm° = 142.6 °C), which showed a very reasonable agreement with the literature value.
    Macromolecules. 05/2005; 38(12).
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    ABSTRACT: Development of shear-induced crystallization precursor structure was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using binary polymer blends of high and low molecular weight polyethylenes near their nominal melting temperatures (120 °C). Two low molecular weight polyethylene copolymers, containing 2 mol % hexene, with weight-average molecular weights (Mw) of 50 000 (MB-50K) and 100 000 (MB-100K), and polydispersity of about 2, were used as the noncrystallizing matrices. A high molecular weight polyethylene homopolymer with Mw of 250 000 (MB-250K) and polydispersity of about 2 was used as the crystallizing minor component. Two series of model blends, MB-50K/MB-250K and MB-100K/MB-250K, each containing weight ratios of 100/0, 97/3, 95/5, and 90/10, were prepared by solution blending to ensure thorough mixing at the molecular level. At the chosen shear conditions (rate = 60 s-1, duration = 5 s, T = 120 °C), while no flow-induced structures were seen in pure MB-50K and MB-100K melts, the blends in both series showed distinct but different shear-induced structures. Results indicate that the high molecular weight component dominates the formation of crystallization precursor structures in the blend under shear, which can act as a template for further crystallization. A “shish-kebab” structure, detected by both SAXS and WAXD, was observed in the MB-100K/MB-250K (90/10) blend, while only a twisted lamellar structure (kebab) was seen in the rest of the blends under the same shear conditions. These findings suggest that the matrix viscosity plays an important role to influence the formation of crystallization precursor structure of the high molecular component under flow. In the MB-100K/MB-250K (90/10) blend, the length of the shish was estimated from the equatorial streak in SAXS, which showed a noticeable decrease with time, while the corresponding scattering intensity was found to increase. The evolution of the shish-kebab structure from SAXS is consistent with the appearance of the (110) peak in WAXD, which can be explained by the coil−stretch transition induced by flow.
    Macromolecules 06/2004; 37. · 5.93 Impact Factor
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    ABSTRACT: The formation of shish-kebab structure in polyethylene melt under intense shear flow was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) techniques. Model polyethylene blend was prepared by solution blending of low molecular weight polyethylene (Mw = 50,000, MWD = 2) matrix with 2 wt% of ultra high molecular weight polyethylene (Mw = 5 - 6 million). In this model blend, the low molecular weight PE matrix, which was non-crystallizing at the experimental condition (120^oC), behaved as a ¡°solvent¡+/-. A clear shish-kebab structure was observed in this blend after shear (shear rate = 60 1/s, shear duration = 5s). The total crystallinity of the blend after was around 2 %, indicating that the shish-kebab structure mainly came from the UHMWPE component. SAXS results indicated that shish formed first and subsequently kebab structure developed, indicating the existence of two populations of chain segments (stretched and coiled) in UHMWPE induced by shear. We argue that the stretched segments form the entity of shish while the coiled segments crystallize into kebabs. The evolution of shish length and the kebab spacing calculated from SAXS results will also be discussed.
    01/2004;
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    ABSTRACT: We report a method for producing stable low-density polymer films by using supercritical carbon dioxide ( scCO <sub>2</sub>). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO <sub>2</sub> along the density fluctuation ridge in P–T phase diagram. The swollen structures could be then frozen by flash evaporation of CO <sub>2</sub> without forming additional voids. X-ray reflectivity data clearly showed that exposure to scCO <sub>2</sub> could be used to produce uniform low-density films of about 2R<sub>g</sub> thick or less, where R<sub>g</sub> is radius of polymer gyration. © 2003 American Institute of Physics.
    Applied Physics Letters 12/2003; · 3.79 Impact Factor
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    ABSTRACT: The effect of chain length on shear-induced crystallization in model blend polyethylene melts was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) techniques. Model polyethylene blend was prepared by solution blending of low molecular weight polydisperse PE (Mw = 50,000, MWD = 2) with 10wt161,000, MWD = 1). While shear-induced oriented crystalline structures were not observed in SAXS and WAXD of the single component low molecular weight polydisperse PE (shear rate = 60 1/s, shear duration = 5 s at 115ºC ), the blend showed oriented crystalline structures after shear and these structures were stable at temperature near the nominal melting point. SAXS and WAXD results clearly show that the high molecular weight species (or long chains) in the blend directly affect the formation and stability of the orientation-induced crystalline structures in polymer melts under flow. This study verifies that upon cessation of flow, the longer chain molecules remain oriented and can form stable precursors for nucleation; on the other hand the shorter chains relax and loose their orientation rapidly due to short relaxation time.
    03/2003;
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    ABSTRACT: We have performed X-ray specula and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behaves as a simple liquid except that the shortest wavelength is determined by radius of gyration (Rg) rather than the monomer distance. When the molecular weight was higher than the entanglement length, the effects of the substrates propagate to the surface scaled as a Rg rather than film thickness. Furthermore films exactly 3Rg thick exhibit brush-like behavior where long wavelength cutoff vector, ql,c scaled as d-0.79.
    03/2003;
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    ABSTRACT: We have performed X-ray specular and off-specular measurements of free-standing polystyrene thin films as a function of molecular weight and thickness. The results show films thicker than a few radii of gyration (Rg) are well fit by a simple liquid model. This confirms the assumption that the anomalies previously reported in the scattering intensity from polymer films of comparable thickness were mostly due to confinement by long-ranged interactions with the substrate rather than a fundamental property of viscoelastic fluids. The simple liquid model was found to be insufficient to fit the transverse diffuse data for films thinner than a few Rg. Longitudinal diffuse scattering data demonstrated that the roughness at the two interfaces is highly correlated when the film thicknesses are approximately 2Rg. Hence, very thin films do not exhibit liquidlike behavior, and higher-order elastic terms may have to be included into models to describe their rheological behavior.
    07/2001;
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    ABSTRACT: X-ray reflectivity data of polymer bilayer systems have been analyzed using a Fourier method which takes into account different limits of integration in q-space. It is demonstrated that the interfacial parameters can be determined with high accuracy although the difference in the electron density (the contrast) of the two polymers is extremely small. This method is not restricted to soft-matter thin films. It can be applied to any reflectivity data from low-contrast layer systems. © 2000 American Institute of Physics.
    Applied Physics Letters 05/2000; 76(19):2713-2715. · 3.79 Impact Factor
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    ABSTRACT: We investigated the interface of PS/PMMA and PS/ PVP as a function of film thickness by X-ray scattering, SIMS and AFM techniques. Although the neutron scattering of two immiscible polymer/polymer interfaces has been investigated by Sferrazza et.al( PRL, 78, 1997), the disadvantages of small qz range( 0.15A-1) and lower resolution of neutron reflectivity brought difficulties to detect within reasonable error range. By the way X-ray reflectivity can overcome these limitation of neutron reflectivity because of its much larger qz ranges and high resolution, but the small contrast, which is that most of polymers have very similar refractive index of X-ray, was only problem. Recently, we found that X-ray reflectivity can be analyzed using Fourier method (Seeck et al, APL, in preparation) to detect the ultra-small density contrasts in thin film layer system. Therefore with great advantages of X-ray scattering compared to neutron scattering, this method can be applied specially to polymer-polymer interfaces without any further contrast alteration. The results are showing that interfacial energy as well as film thickness are the most important factors to modify the intrinsic interfacial width and additional interfacial broadening by capillary wave. The results were well matched to the theoretical calculation within very small error ranges. Experiments are currently in progress for bilayer systems with diblock copolymer. The existence of diblock copolymer can reduce tremendously the interfacial tension of immcible polymer/polymer interface and bring the interfacial length broadening.
    03/2000;
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    ABSTRACT: Polystyrene freestanding thin films were investigated by X-ray specular and off-specular scattering techniques. From the specular scattering, we found that the films, which have the thickness(d) > 3 times of radius of gyration(Rg), have the identical rms roughness on both outer surfaces. The diffuse scattering of those films could be explained by a power law of a simple liquid due to the capillary wave fluctuation. The vertical correlation of both surfaces of the films has also been investigated successfully. In the range of 3Rg < d < 5Rg, an anticonformal state was found. However for very thick films, a mixed mode of conformal and anticonformal structure was found. For the very thin films(d
    03/2000;
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    ABSTRACT: The melt spinning of nylon 6 has been studied with on-line wide-angle synchrotron X-ray scattering techniques. The apparatus consisted of a single screw extruder and a metering pump mounted on a horizontal platform that could be translated in the vertical direction allowing a range of distances to be sampled with the X-ray beam. The structure development, equatorial crystallinity index, and crystalline orientation were studied as a function of take-up speed and position along the spinline. For low-speed (50 mpm) situations, the nylon chains crystallize into independent hydrogen bonded sheets that start to interact with each other as their concentration starts to increase. For higher speed situations, the nylon chains crystallize directly into the interacting hydrogen-bonded sheet structure. Upon conditioning at room temperature for 24 h, this interacting hydrogen-bonded sheet structure transforms into the well-known three-dimensional alpha and gamma phases of nylon 6, probably existing in a shish-kabob structure. The equatorial crystallinity index increases as distance from the spinneret increases and as take-up speed decreases. The crystalline orientation function is constant along the spinline for a constant take-up speed, and increases as take-up speed is increased. Conditioning further increases both the crystallinity and crystalline orientation of the fibers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1277–1287, 1999
    Journal of Polymer Science Part B Polymer Physics 04/1999; 37(12):1277 - 1287. · 2.22 Impact Factor
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    ABSTRACT: On-line simultaneous small- and wide-angle x-ray scattering (SWAXS) measurements were made using synchrotron radiation to investigate the structural and morphological development during the melt spinning of nylon-6 at low speeds (50-400 mpm). The position along the spinline where crystallization begins was determined, as well as the crystallinity development from that point. Analysis of the SAXS patterns reveals further morphological detail. A model of the possible crystallization mechanism during the melt-spinning process is proposed. Also, off-line measurements were conducted on these spun fibers after being conditioned at room temperature for 24 hours. Our results show a substantial increase in crystallinity in these conditioned fibers as compared to the as-spun fibers.
    03/1998;
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    ABSTRACT: This version is available at the following Publisher URL: http://apl.aip.org
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    ABSTRACT: In this study, the effect of chain lengths on shear-induced crystallization in polyethylene melts was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques. Two model polyethylene blends MB-50k/MB-250k(90/10) and MB-100k/MB-250k(90/10) were prepared by solution blending of 10 wt% of high molecular weight homopolymer PE (Mw = 250,000, MWD = 2) and 90% of low molecular weight PE matrix of low degree of crystallinity: MB-50k (Mw = 50,000, MWD = 2, hexane mol% = 2%) or MB-100k (Mw = 50,000, MWD = 2, hexane mol% = 2%). While shear-induced oriented crystallization was not seen in the pure matrix (shear rate = 60 1/s, shear duration = 5 s at 120ºC), both blends showed stable oriented crystalline structures after shear near the nominal melting point of PE. The "shish-kebab" structure was observed in the MB-100k/MB-250k(90/10) blend, while only the kebab structure was seen in the MB-50k/MB-250k(90/10) blend under the same shear condition. This indicates that the viscosity of the matrix also plays a very important role to influence the stability of flow-induced oriented structures.
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    ABSTRACT: This version is available at the following URL: http://apl.aip.org