Showing papers by "Jun-Ting Xu published in 2007"

Micellar Morphologies of Poly(ε-caprolactone)-b-poly(ethylene oxide) Block Copolymers in Water with a Crystalline Core

Abstract: The self-assembly of poly(e-caprolactone)-b-poly(ethylene oxide) block copolymers (PCLnPEO44 and PCLnPEO113) with narrow polydispersity in aqueous medium was studied using transmission electron microscopy In this system, the formed micelles are composed of a crystalline PCL core and a soluble PEO corona We demonstrated that the PCL-b-PEO block copolymers can form micelles with abundant morphologies, depending on the lengths of the blocks and composition It is observed that for PCLnPEO44 the micellar morphology changes from spherical, rodlike, wormlike, to lamellar, as the length of the PCL block increases In contrast, most of PCLnPEO113 (n = 21−147) block copolymers form spherical micelles, and only PCL232PEO113 exhibits mixed spherical and lamellar micellar morphologies The effect of microstructure on micellar morphology was semiquantitatively interpreted in terms of reduced tethering density (σ) It is found that lamellar micelles are formed when σ is smaller than a critical value of between 30 an

Synthesis and characterization of poly(ε-caprolactone)-b-poly(ethylene glycol) block copolymers prepared by a salicylaldimine-aluminum complex

Abstract: A series of poly(ϵ-caprolactone)-b-poly(ethylene glycol) (PCL-b-PEG) block copolymers with different molecular weights were synthesized with a salicylaldimine-aluminum complex in the presence of monomethoxy poly(ethylene glycol). The block copolymers were characterized by 1H NMR, GPC, WAXD, and DSC. The 1H NMR and GPC results verify the block structure and narrow molecular weight distribution of the block copolymers. WAXD and DSC results show that crystallization behavior of the block copolymers varies with the composition. When the PCL block is extremely short, only the PEG block is crystallizable. With further increase in the length of the PCL block, both blocks can crystallize. The PCL crystallizes prior to the PEG block and has a stronger suppression effect on crystallization of the PEG block, while the PEG block only exerts a relatively weak adverse effect on crystallization of the PCL block. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Effect of molecular weight on spherulitic growth rate of poly(ε-caprolactone) and poly (ε-caprolactone )-b-poly(ethylene glycol)

Abstract: The spherulitic growth rates of a series poly (ϵ-caprolactone) homopolymers and poly(ϵ-caprolactone)-b- poly(ethylene glycol) (PCL-b-PEG) block copolymers with different molecular weights but narrow polydispersity were studied. The results show that for both PCL homopolymers and PCL-b-PEG block copolymers, the spherulitic growth rate first increases with molecular weight and reaches a maximum, then decreases as molecular weight increases. Crystallization temperature has greater influence on the spherulitic growth rate of polymers with higher molecular weight. Hoffman–Lauritzen theory was used to analyze spherulitic growth kinetics and the free energy of the folding surface (σe) was derived. It is found that the values of σe decrease with molecular weight at low molecular weight level and become constant for high molecular weight polymers. The chemically linked PEG block does not change the values of σe significantly. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Lamellar orientation in thin films of symmetric semicrytalline polystyrene -b -poly (ethylene -co -butene) block copolymers : Effects of molar mass, temperature of solvent evaporation, and annealing

Abstract: Orientation of the lamellar microdomains in thin films of three symmetric polystyrene-b-poly(ethylene-co-butylene) block copolymers (S 65 E 155 , S 156 E 358 , and S 199 E 452 ) on mica was investigated via atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The results show that lamellar orientation in the S x E y block copolymers greatly depends on the molar mass of the block copolymers, the temperature of solvent evaporation, and annealing. The nascent thin film of the low molar mass block copolymer, S 65 E 155 , shows a multilayered structure parallel to the mica surface with the PS block at both polymer/mica and polymer/air interfaces, but the high molar mass block copolymers, S 156 E 358 and S 199 E 452 , exhibit a structure with lamellar microdomains perpendicular to the mica surface. When the solvent is evaporated at a lower temperature, the crystallization rate is fast and a two-dimensional spherulite structure with the lamellar microdomains perpendicular to the mica surface is observed. Annealing of all the thin films with lamellar microdomains perpendicular to the mica surface leads to morphological transformation into a multilayered structure parallel to the mica surface. In all S x E y thin films on mica, the stems of PE crystals are always perpendicular to the interface between the lamellar PE and PS microdomains. A mechanism is proposed for the formation of different microdomain orientations in the thin films of semicrystalline block copolymers. When the thin film is prepared from a homogeneous solution, microdomains perpendicular to the substrate surface are formed rapidly for strongly segregated block copolymers or at a lower crystallization temperature and kinetically trapped by the strong segregation strength or solidification of crystallization, while for weakly segregated block copolymers or at slower crystallization rate, the orientation of the microdomains is dominated by surface selectivity.

Effect of substrate and molecular weight on the stability of thin films of semicrystalline block copolymers.

Abstract: The thermal stability of the thin film morphology of two symmetric oxyethylene/oxybutylene block copolymers (E 76 B 38 and E 114 B 56 ) on mica and silicon was investigated via atomic force microscopy (AFM). It is found that morphological transition of E m B n thin films during melting is strongly dependent on the molecular weight of the diblock copolymers and their interaction with the substrate. For E 76 B 38 on mica, a single-layered structure transforms into a double-layered structure upon melting, but the same polymer on silicon retains a single-layered structure after melting and spreads quickly to wet-out the silicon surface. Conversely a longer polymer, E 114 B 56 , has a thin film on mica that does not change much after melting of the crystalline E block. A mechanism was proposed to explain the relative stability of E 76 B 38 and E 114 B 56 thin films upon melting. Internal stress is produced during melting and can be released along two directions. The release along the vertical direction is restricted by the energy barrier related to the segregation strength, and the release along the horizontal direction is dependent on the mobility of block copolymer related to the interaction between the block copolymer and the substrate. Domain size affects the release rate of the internal stress along the horizontal direction and thus the thermal stability of E m B n thin films. Switching between horizontal and vertical releases can be realized by controlling the domain size of the thin films.

Control of the molecular weight distribution and tacticity in 1-hexylene polymerization catalyzed by TiCl4/MgCl2-NaCl/TEA catalysis system

Abstract: NaCl doped MgCl2 supported TiCl4 catalysts were used for 1-hexylene polymerizations in this paper. The catalysts’ activity, the molecular weight distribution and the tacticity of polyhexylene are influenced strongly by the doping of NaCl in MgCl2, based on the polymerization experiment, the deconvolution analysis of molecular weight distribution (MWD) profiles and the 13C NMR analysis. The results show that it is possible to control the active center distribution of supported TiCl4/MgCl2 catalysts by doping of support with NaCl.

Crystallization and morphology of cholesterol end-capped poly(ethylene glycol)

Abstract: Crystallization and morphology of polyethylene glycol with molecular weight Mn = 2000 (PEG2000) capped with cholesterol at one end (CS-PEG2000) and at both ends (CS-PEG2000-CS) were investigated. It is found that the bulky cholesteryl end group can retard crystallization rate and decrease crystallinity of PEG, especially for CS-PEG2000-CS. Isothermal crystallization kinetics shows that the Avrami exponent of CS-PEG2000 decreases as crystallization temperature (Tc). The Avrami exponent of CS-PEG2000-CS increases slightly with Tc, but it is lower than that of CS-PEG2000. Compared to the perfect spherulite morphology of PEG2000, CS-PEG2000 exhibits irregular and leaf-like spherulite morphology, while only needle-like crystals are observed in CS-PEG2000-CS. The linear growth rate of CS-PEG2000 shows a stronger dependence on Tc than PEG2000. The cholesterol end group alters not only the free energy of the folding surface, but also the temperature range of crystallization regime. The small angle X-ray scattering (SAXS) results show that lamellar structures are formed in all these three samples. By comparing the long periods obtained from SAXS with the theoretically calculated values, we find that the PEG chains are extended in PEG2000 and CS-PEG2000, but they are once-folded in CS-PEG2000-CS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2464–2471, 2007

Structure and morphology of polyethylene/polypropylene in‐reactor alloys synthesized by spherical high‐yield Ziegler–Natta catalyst

Abstract: A series of spherical polyethylene/polypropylene (PE/PP) in-reactor alloys were synthesized with spherical high-yield Ziegler–Natta catalyst by sequential multistage polymerization in slurry. The morphology of PE/PP alloy granule was evaluated by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show PE/PP in-reactor alloy with excellent morphology, high porosity, and narrow distribution of the particle size. The PE/PP in-reactor alloys show excellent mechanical properties with good balance between toughness and rigidity. It was fractionated into five fractions by temperature-gradient extraction fractionation, and every fractionation was analyzed by FTIR, 13C-NMR, DSC, and WAXD. The PE/PP in-reactor alloy was found to contain mainly five portions: PP, PE, segmented copolymer with PP and PE segment of different length, ethylene-b-propylene copolymer, and an ethylene–propylene random copolymer. The characteristic chain structure leads to good compatibility between the fractions of the alloy that shows a multiphase structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2075–2085, 2007

Improvement of Structure and Properties of Polypropylene/Poly(ethylene-co-propylene) In-reactor Alloy by Modifying the Cocatalyst

Abstract: A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloy were synthesized by a TiCl 4 /MgCl 2 /SiO 2 /diester type Ziegler-Natta catalyst, using triethylaluminium (TEA), triisobutylaluminium (TIBA) or TEA/TIBA mixtures of different molar ratio as cocatalyst. Mechanical properties of the alloy are strongly influenced by the cocatalyst. Toughness-stiffness balance of the alloy synthesized using a 50/50 TEA/TIBA mixture as cocatalyst is much better than that of the alloy based on pure TEA cocatalyst. Changes in copolymer chain structure and composition distribution are thought to be the main reason for this improvement of properties.

Microstructure of Ethylene/Propylene Random Copolymers Prepared by a Fluorinated Bis(phenoxy-imine)Ti Catalyst

Abstract: Living copolymerization of ethylene and propylene was catalyzed by a fluorine-containing bis(phenoxy-imine) titanium catalyst. A series of ethylene-propylene copolymers with different propylene contents were prepared and the copolymers were characterized by 13C-NMR, GPC and DSC. The copolymers were found to have the following characteristics: (1) Molecular weight distribution of the copolymer is rather narrow; (2) There exist only isolated propylene units distributed along the polymer chains even at propylene content as high as 14.9 mol%; (3) The distribution of ethylene sequence are not homogeneous. Insertion of propylene changes from 2,1-insertion in homopolymerization into 1,2-insertion when the preceding unit is ethylene.

Modified polypropylene resin and its prepn process

Abstract: The present invention discloses one kind of modified polypropylene resin and its preparation process. The modified polypropylene resin contains propylene polymer in 70-98 wt% and styrene polymer in 2-30 wt%, and has complex melt viscosity at 190 deg.c temperature and 0.1 rad/s shearing frequency greater than 2.0x10 Pa .s, complex melt viscosity at 10 rad/s shearing frequency greater than 700-2000 Pa .s, gel content less than 2 % and weight average molecular weight not smaller than 90 % that of the linear polypropylene material. Its preparation process includes the following steps: 1. heating linear polypropylene powder at 120-150 deg.c for 2-6 hr before being cooled to room temperature in 0.5-1 hr; and 2. adding styrene with dissolved organic peroxide initiator to the polypropylene powder to reaction at 110-130 deg.c via stirring for 1-3 hr, cooling to room temperature, adding oxidant and pelletizing. The present invention has excellent machining rheological property.