Showing papers by "Liangbin Li published in 2009"

Unusual Tuning of Mechanical Properties of Isotactic Polypropylene Using Counteraction of Shear Flow and β-Nucleating Agent on β-form Nucleation

Abstract: Isotactic polypropylene (iPP) has good comprehensive properties, viz. easy processing, high heat resistance, and good stiffness, etc., and in turn is widely used as a commodity plastic. However, under conventional processing conditions, iPP crystallizes into sizable spherulites with few tie molecules between spherulites. With such a crystalline texture, iPP exhibits very low impact toughness, especially at lower temperature, which restricts its more extensive application. Therefore, toughening of iPP has always been an open research. Up to now, four routes have been taken to improve impact strength of iPP, including copolymerizing with other olefin monomers, blending with rubber or thermoplastic elastomer, compounding with organic or inorganic fillers (e.g., nanoparticles), and adding β-nucleating agent. Without doubt, the modified iPP, more or less, increases its toughness. The enhancement of toughness is, unfortunately, at sacrifice of other properties, e.g., strength, heat resistance, etc. There are extremely few successful examples for simultaneously efficiently reinforcing and toughening iPP. It has been well established that for iPP oriented crystals (i.e., shish-kebabs) can bring out notable reinforcement on iPP, while β-form crystals of iPP can greatly increase its toughness. Flow (shear, elongational, or mixed) would induce formation of shish-kebabs whose content is governed to shear rate, shear duration, and molecular species and weight, etc. On the other hand, β-form crystals can be high production generated by addition of β-nucleating agent under quiescent conditions. This type of iPP crystals causes high toughness due to the β-R polymorphous transition and the loose structure of β-form crystals compared with R-crystals in favor of absorbing impact energy. Naturally, an idea arises that combination of flow-induced molecular orientation crystallization and β-nucleant-induced β-form crystals produces efficient reinforcement and toughening on iPP. However, as a matter of fact, there is not any example of it in the open literature. Themajor reason is that the coexistence of a β-nucleating agent and a shear flow at above a certain but low level of shear rate depresses β-form nucleation. Huo et al. performed a delicate study on iPP crystallization with β-nucleating agent and observed that as the shear rate rises, the content of β crystals constantly reduces, more greatly for higher nucleating agent content, showing counteraction between shear and β-nucleating agent for β-form formation. Apparently, it is a great practical challenge to prepare iPP parts withhighmolecular orientation and highβ-crystal proportion for the purpose of both strength and toughness enhancement. In the present work, we attempt to fabricate iPP with considerably increased strength and toughness, for the first time, utilizing the counteraction of shear-induced orientation crystallization and β-nucleating agent on β-form nucleation. The crystallization process of iPP is manipulated by two stages: one is flow-induced crystallization for high molecular orientation, and the other is β-nucleating-agent-induced nucleation for highβ-crystal content. More specifically, the oscillation shear supplied by an oscillation shear injectionmolding (OSIM) (its detailed definition is included in the Experimental Section) is imposed on β-nucleated iPP melt in the mold during injection molding. Initially, due to occurrence of shear flow and β-nucleating agent, β-form formation is restrained; thereby, the oriented R-crystals first form in the skin and intermediate layers of the sample.When the gate of the mold freezes, the shear ceases, and hereby, β-nucleating agent revives. The remainder of iPP melt in the core layer undergoes β-nucleation-induced crystallization. The results demonstrate that a large amount of shish-kebab structure appears in the surface and intermediate layers of the sample; at the same time, numerous β-form crystals form in the core layer, thus leading to prominent reinforcement and toughening for iPP.

Shear-Induced Nucleation and Growth of Long Helices in Supercooled Isotactic Polypropylene

Abstract: Flow-induced conformational ordering in the supercooled isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) coupled to an extrusion slit die. At temperature around the normal melting point of iPP, helices with monomer numbers up to 12 and 14 can be induced by shear. A window of shear strength exists to induce helices with different lengths, which increases with temperature. After the cessation of shear, the intensity of the 841 cm−1 band in FTIR spectra, corresponding to helices with 12 monomers, increases sharply in the first stage, which is followed by a slow growth process, whereas a reduction of shorter helices is observed after the cessation of shear. The different trends of intensity evolutions of the long and short helical bands suggest that a coupling between coil−helix transition and intermolecular ordering occurs with helices with a length of 12 monomers, which eventually leads to an isotropic−liquid-crystal transition. Therefore, the flow-induced...

Inducing Crystallization of Polymer through Stretched Network

Abstract: Introduction. Flow-induced crystallization of polymers is not only a fundamental nonequilibrium thermodynamic problem but also of importance for polymer processing and the final properties of polymer products. During polymer processing, such as extrusion, injection molding, and fiber spinning, polymer materials are subjected to various flow fields. Flow can enhance the crystallization rate up to several orders of magnitude and induce the formation of the so-called row nuclei or shish-kebab structure, which significantly increases the stiffness and thermo deformation temperature. Although the benefit from flow field is widely recognized in academia and polymer industry, no satisfactory molecular theory has been achieved yet. To explain flow-induced crystallization of polymer, entanglement-disentanglement (EDT) and coil-stretch (CS) transitions have been the basic ideas dominated in the community date back to Keller and de Gennes. They suggested that the flowinduced shish-kebab structure composes of extended long chains as row nuclei or shish and folded-chain crystals as kebab. Some recent elegant experiments and computer simulations further support the nucleation role of the oriented long chains. However, a different picture presented by Kimata et al. shows that the short chains or medium chains are more involved in the formation of shish. Indeed, more experimental evidence does not fit in the initial shish-kebab mechanism. Hsiao et al. observed that a single lamella can grow from multiple shish instead of a single shish. Zhang et al. showed that it is possible to produce row nuclei with a small shear rate and suggested EDT may not be a necessary condition for the formation of row nuclei. Without EDT, the stretched transient network is sufficient to accelerate crystallization and induce shish-kebab structure. With evidence from different facets, the two essential questions still remain as open debates, which prevent from further pursuing the molecular mechanism of the flow-induced crystallization of polymer. (i) What is the role of long and short chains in the formation of row nuclei or shish (or oriented nuclei in general)? (ii) What is the role of EDT in the formation of the oriented nuclei? We use “oriented nuclei” to emphasize on the enhancement of flow on nucleation in this work, as different structures such as point precursor, row nuclei, or shish-kebab can be produced by flow. In this Communication, two simplified mixtures containing short free chains and chemically end-linked network were designed to answer the above-mentioned two questions, which resemble the transient network of polymer blend with short and long chains. A schematic picture is illustrated in Figure 4A. These systems minimize the interferences of relaxation and macroscopic inhomogeneous of flow field, which may complicate the interpretation on the observed experimental phenomenon. Similar systems had been employed to verify the initial reptation theory and other mechanism related to polymer dynamics. Low molecular weight poly(ethylene oxide) (PEO) with a molecular weight of 2000 g/mol (PEO2F) which has two acetyl end groups was used as the free chains. To build the end-linked network, the mesh size with molecular weights M of 2000 and 6000 g/mol, denoted as PEO2N and PEO6N, were chemically cross-linked with 1,3,5-benzenetricarbonyl trichloride, respectively. As the critical entanglement molecular weight Me of PEO is about 2000 g/mol, 23 and the chemical cross-linking points in network were considered as the permanent entanglements points, PEO2N resembles the high molecular weight PEO with permanent “entanglements”, while PEO6N represents a polymer melt with about two possible disentanglement points.

Phase transition of [Cn-mim][PF6] under high pressure up to 1.0 GPa

Abstract: Behavior of the phase transition of an ionic liquid, [Cn-mim][PF6], has been investigated under pressures up to 1.0 GPa by using a high-pressure differential thermal analysis (DTA) apparatus. The T versus P phase diagrams of [BMIM][PF6] and [EMIM][PF6] are constructed. The DTA curve of [BMIM][PF6] shows one endothermal valley in heating course at each given pressure, which indicates that a simple phase transition from solid to liquid has taken place under high pressure and that the melting point is an increase function of pressure. However, the DTA curve of [EMIM][PF6] shows two endothermal valleys in the heating course within the tested pressure range, implying that there may exist another phase. After treatment of [EMIM][PF6] at different temperatures under high pressure, the structures of the recovered samples are also investigated by wide-angle x-ray scattering. By considering the results above, it indicates that another crystalline phase exists between the solid and liquid of [EMIM][PF6].

Identifying the phase behavior of biodegradable poly(hexamethylene succinate-co-hexamethylene adipate) copolymers with FTIR.

Abstract: Whether a phase separation or a cocrystallization occurs in poly(hexamethylene succinate-co-hexamethylene adipate) (P(HS-co-HA)) copolymers was studied with a combination of wide-angle X-ray diffra

Partial wave analysis of J/psi ---> p anti-p pi0

Abstract: Using a sample of 58 million J/{psi} events collected with the BESII detector at the BEPC, more than 100 000 J/{psi}{yields}pp{pi}{sup 0} events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/{psi}{yields}pp{pi}{sup 0})=(1.33{+-}0.02{+-}0.11)x10{sup -3}. A long-sought missing N*, first observed in J/{psi}{yields}pn{pi}{sup -}, is observed in this decay too, with mass and width of 2040{sub -4}{sup +3}{+-}25 MeV/c{sup 2} and 230{sub -8}{sup +8}{+-}52 MeV/c{sup 2}, respectively. Its spin-parity favors (3/2){sup +}. The masses, widths, and spin parities of other N* states are obtained as well.

Material stretching and compressing device used for multi-light source in-situ structure detection

Abstract: The utility model relates to a material tensile and compressing device for detecting structure of multi light source original position. A soleplate of the device is provided with an actuating motor provided with a retarder, a reverse-flighted screw and a slide guide; a pair of mated gears are arranged at the output end of the actuating motor and a corresponded end of the reverse-flighted screw; two ends of the reverse-flighted screw are respectively connected with fixed brackets through nuts; the other ends of the fixed brackets are respectively coordinated with the slide guide through slide rods; collets are respectively arranged on the two fixed brackets; a force sensor is arranged on the fixed bracket on one side; a pass-ray hole, a high frequency camera and a computer are arranged on the soleplate between a pair of chucks. The material tensile and compressing device for detecting structure of multi light source original position has the advantages of a small volume, high displacement precision, adjustable constant speed, capability of assorting a variety of light sources under different experimental temperatures, and high speed of real time detection mechanical property of detected material in the process of stretching (compressing) and corresponding relationship of micro structures of different dimensions.

Partial wave analysis of J/Ψ → PPπ0

Abstract: Using a sample of 58 million J/Ψ events collected with the BESII detector at the BEPC, more than 100000 J/Ψ → ppπ 0 events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/Ψ → ppπ 0 ) = (1.33 ± 0.02 ± 0.11) X 10 -3 . A long-sought missing N*, first observed in J/Ψ → pnπ - , is observed in this decay too, with mass and width of 2040 +3 -4 ± 25 MeV/C 2 and 230 +8 -8 ± 52MeV/c 2 , respectively. Its spin-parity favors 3/2 + . The masses, widths, and spin parities of other N* states are obtained as well.

Experimental studies of e + e −→ some charmless processes containing K S 0 at \sqrt{s}=3.773 and 3.65 GeV

Abstract: We measure the observed cross sections for the charmless processes e(+)e(-) -> (KSK-K-K+)-K-0 pi(+) + c.c., (KSK-)-K-0 pi(+)eta + c.c., (KSK-)-K-0 pi(+)pi(+)pi(-)eta + c.c., (KSK-K-K+)-K-0 pi(+)eta + c.c., (KSK-K-K+)-K-0 pi(+)pi(0) + c.c., (KSK-)-K-0 rho(+) + c.c. and (KSK-)-K-0 pi(+)rho(0) + c.c. We also extract upper limits on the branching fractions for psi(3770) decays into these final states at 90% C.L. Analyzed data samples correspond to 17.3 pb(-1) and 6.5 pb(-1) integrated luminosities registered, respectively, at root s = 3.773 and 3.65 GeV, with the BES-II detector at the BEPC collider.