Jun-Ting Xu

Structure and properties of polypropylene/poly(ethylene-co-propylene) in-situ blends synthesized by spherical Ziegler–Natta catalyst

Abstract A polypropylene/poly(ethylene- co -propylene) (iPP/EPR) in-situ blend synthesized by spherical Ziegler–Natta catalyst was fractionated by temperature-gradient extraction fractionation. The fractions were characterized using FTIR, 13 C NMR, DSC and WAXD. The in-situ blend was found to contain mainly three portions: an ethylene–propylene random copolymer, a series of segmented copolymer with PE and PP segments of different length, and propylene homopolymer. The impact strength of in-situ blends of different structural heterogeneity was measured, and the results show that increasing the amount of segmented copolymer has a positive effect on the impact strength. The segmented copolymer portion alone is found to increase the impact strength at room temperature greatly, while the low temperature impact strength can be markedly enhanced only when random copolymer coexists with the segmented copolymer.

Synthesis and Characterization of Organometallic Coordination Polymer Nanoshells of Prussian Blue Using Miniemulsion Periphery Polymerization (MEPP)

We report a conceptually new method for the synthesis of Prussian blue (PB) nanoshells with tunable size using miniemulsion periphery polymerization (MEPP). The process of MEPP involves the preparation of miniemulsion droplets using organometallic surfactants, followed by a coordination polymerization at the periphery of the droplets. The organometallic polymers with nanoshell structure were confirmed by using FT-IR, UV-vis, TEM, SEM, and AFM. WAXD measurements indicated that the shell structure suppressed the crystallization of the polymers. We also discovered that the nanoshell structure can significantly enhance the thermal stability of the materials. The hollow structures plus inherent magnetic properties of PB render the materials potential useful as multifunctional capsules.

Cooperative Effect of Electrospinning and Nanoclay on Formation of Polar Crystalline Phases in Poly(vinylidene fluoride)

Poly(vinylidene difluoride)/organically modified montmorillonite (PVDF/OMMT) composite nanofibers were prepared by electrospinning the solution of PVDF/OMMT precursor in DMF. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) show that in the bulk of the PVDF/OMMT precursor OMMT platelets are homogeneously dispersed in PVDF and can be both intercalated and exfoliated. It is found that the diameter of the PVDF/OMMT composite nanofibers is smaller than that of the neat PVDF fibers because the lower viscosity of PVDF/OMMT solution, which is attributed to the possible adsorption of PVDF chains on OMMT layers and thus reduction in number of entanglement. The crystal structure of the composite nanofibers was investigated using WAXD and Fourier transform infrared (FT-IR) and compared with that of thin film samples. The results show that the nonpolar alpha phase is completely absent in the electrospun PVDF/OMMT composite nanofibers, whereas it is still present in the neat PVDF electrospun fibers and in the thin films of PVDF/OMMT nanocomposites. The cooperative effect between electrospinning and nanoclay on formation of polar beta and gamma crystalline phases in PVDF is discussed. The IR result reveals that electrospinning induces formation of long trans conformation, whereas OMMT platelets can retard relaxation of PVDF chains and stabilize such conformation due to the possible interaction between the PVDF chains and OMMT layers. This cooperative effect leads to extinction of nonpolar alpha phase and enhances the polar beta and gamma phases in the electrospun PVDF/OMMT composite nanofibers.

Application of temperature rising elution fractionation in polyolefins

Abstract In this paper, the applications of temperature rising elution fractionation (TREF) in polyolefins, including conventional ethylene copolymers, polypropylene, propylene copolymers, polyolefin alloys and metallocene-based polyolefins were reviewed. TREF is helpful for the better understanding of the microstructure of polyolefins, polymerization mechanisms and the nature of catalysts. The microstructural characteristics of metallocene-based polyolefins and fractionation mechanisms were also discussed. In addition, preparative TREF can also be used to obtain polymer fractions with homogeneous structure to facilitate the study of polymer physics.

Separation and identification of ethylene-propylene block copolymer

Abstract In this paper, a commercial impact-resistant polypropylene alloy sample was fractionated with preparative temperature rising elution fractionation (t.r.e.f.) technique and a series of ethylene-propylene block copolymer fractions with different ethylene contents were obtained. Combined with fractionation results, the block structure of them was identified with 13 C nuclear magnetic resonance (n.m.r.) spectroscopy and differential scanning calorimetry (d.s.c.). There are some transition segments between polyethylene and polypropylene segment in the block copolymer. It was found that the number and kind of transition segments decreased with elution temperature, the length and melting temperature of polyethylene segment in block copolymer fractions showed a similar tendency.

One-Pot Preparation of Hollow Silica Spheres by Using Thermosensitive Poly(N-isopropylacrylamide) as a Reversible Template

Hollow silica nanospheres with mesoporous shells were successfully fabricated with a new one-pot strategy by using a thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAm), as a reversible template without the need of further calcination or chemical etching. By simply regulating the solution temperature with respect to the lower critical solution temperature (LCST) of PNIPAm, PNIPAm chains can reversibly form aggregates or dissolve in aqueous solution. The thermosensitive character makes PNIPAm chains behave as soft templates for the formation of core-shell silica nanospheres at elevated temperature (>LCST), and they will then diffuse out of the cores at lower temperature (<LCST), leading to the formation of hollow silica nanospheres. The TEM, SEM, XRD, and N(2) adsorption-desorption results indicate that the shells of such hollow silica nanospheres also contain large quantities of irregular mesopores. This new strategy was also tested with another thermosensitive polymer, poly(vinyl methyl ether) (PVME). However, only solid silica nanospheres with a broad size distribution were obtained when PVME was used. We speculated on the possible formation mechanism of hollow silica nanospheres with PNIPAm templates. The effects of the initial concentration of PNIPAm, the molecular weight of PNIPAm, and the pretreatment of silica precursor on the morphology and size of the resultant hollow silica nanospheres were also investigated. The PNIPAm soft templates were confirmed to be recyclable.

Fabrication and Properties of Thermosensitive Organic/Inorganic Hybrid Hydrogel Thin Films

We report on a facile method for fabricating thermosensitive organic/inorganic hybrid hydrogel thin films from a cross-linkable organic/inorganic hydrid copolymer, poly[ N-isopropylacrylamide- co-3-(trimethoxysilyl)propylmethacrylate] [P(NIPAm- co-TMSPMA)]. Fourier transform infrared (FT-IR) spectra confirmed the formation of hybrid hydrogel thin films after hydrolysis of the methoxysilyl groups (Si-O-CH 3) and subsequent condensation of the silanol groups (Si-OH). Atomic force microscopy (AFM) images revealed that the surface morphology of the hydrogel thin films depended on the supporting substrates. Microdomains were observed for the hydrogel thin films on a gold surface, which can be attributed to inhomogeneous network structures. The thermoresponsive swelling-deswelling behavior and the viscoelastic properties of the hydrogel thin films were investigated as a function of temperature (25-45 degrees C) by using a quartz crystal microbalance (QCM) operated in water. The high frequency shear modulus of the P(NIPAm- co-TMPSMA) hydrogel thin films was several hundred kilopascals.

SAXS/WAXS/DSC studies on crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer with lamellar morphology and low glass transition temperature

Abstract Crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene (S–EO–S) triblock copolymer, S40EO136S40, with lamellar morphology in the melt and low glass transition temperature (Tg=47 °C) of the S block was studied. The triblock copolymer was cooled from ordered melt and isothermal crystallization was conducted at crystallization temperatures (Tc) near the Tg of the S block. It is found that crystallization behavior of S40EO136S40 strongly depends on Tc. When Tc is far below Tg, an Avrami exponent n=0.5 is observed, which is attributed to diffusion-controlled confined crystallization. As Tc slightly increases, the Avrami exponent is 1.0, indicating that crystallization is confined and crystallization rate is determined by the rate of homogeneous nucleation. When Tc is just below the Tg of the S block, crystallization tends to become breakout and accordingly Avrami exponent changes from 1.0 to 3.2. Crystallinity and melting temperature of the EO block in breakout crystallization are slightly higher than those in confined crystallization. Time-resolved small and wide angle X-ray scattering (SAXS/WAXS) were used to monitor isothermal crystallization of S40EO136S40. It shows that the long period is constant in confined crystallization, but it gradually increases during breakout crystallization. WAXS result reveals that confined or breakout crystallization has no effect on the crystal structure of the EO block.

Effect of composition distribution on miscibility and co-crystallization phenomena in the blends of low density polyethylene with conventional and metallocene-based ethylene-butene copolymers

Abstract Three ethylene–butene copolymers (two metallocene-based and one conventional) having different composition distributions were blended with low density polyethylene (LDPE) in a wide proportion range. The miscibility of the blend melts was studied with the rheological method. It is found that zero-shear viscosity of all the blends does not obey the logarithmic additivity rule, indicating the immiscible state of the blend melts. The blends were rapidly quenched from the melts and co-crystallization phenomena were investigated by differential scanning calorimetry (DSC). The obtained results show that composition distribution has a great influence on co-crystallization. In the blends of the conventional ethylene–butene copolymer, which has a broad composition distribution, part of the co-crystals may segregate from the crystals of both LDPE and ethylene copolymer and exist as a separated population. This suggests that there exist three phases in the melts. In contrast, the blends of the metallocene-based ethylene–butene copolymers with a narrow composition distribution are only composed of two phases together with some exchange between the two components. It is also observed that broader composition distribution leads to more ethylene–butene copolymer transferred into LDPE.

Short chain branching distributions of metallocene-based ethylene copolymers

The short chain branching distribution (SCBD) of six metallocene-based ethylene copolymers was evaluated by preparative temperature rising eluting fractionation and by differential scanning calorimetry (DSC) after stepwise isothermal treatment of the samples at different temperatures. It is found that the SCBD of ethylene copolymers varies with the density of the copolymers and the type of the comonomer. Both samples with narrow SCBD and samples with broad SCBD can be prepared by the same metallocene catalyst. The lamellar thickness distributions of the samples were also obtained by fitting the DSC melting traces. The reasons that lead to broad SCBD of metallocene-based LLDPE were also discussed.