Junpeng Zhao

University of Science and Technology of China, Luchow, Anhui Sheng, China

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Publications (11)38.28 Total impact

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    ABSTRACT: The aggregate structure of a thermoresponsive polystyrene-b-poly(N-isopropylacrylamide)-b-polystyrene (PS-PNIPAM-PS) triblock copolymer was investigated as a function of temperature in aqueous solutions by the combination of small angle neutron scattering (SANS) and dynamic/static light scattering (LS). SANS shows that the system consists of spherical intermicellar aggregates which self-assemble into larger clusters either at temperatures below or above the lower critical solution temperature(LCST). Upon the solubility transition of PNIPAM the mass of the clusters increases significantly, whereas their size remains constant and the size of the intermicellar aggregates decreases. This picture is consistent with the LS data where the signature of the two entities in solution is observed. The combination of LS and SANS studies has presented a clearer scenario on the thermoresponsive aggregation behavior of the investigated PS-PNIPAM-PS triblock copolymer.
    European Polymer Journal 07/2014; 56. DOI:10.1016/j.eurpolymj.2014.04.013 · 3.24 Impact Factor
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    ABSTRACT: We analyze in detail the morphology transition of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) diblock copolymer in water with small angle neutron scattering. A three-level hierarchical structure is found below the critical solution temperature (LCST) of PNIPAM. The small-scale structure corresponds to the unperturbed PEO-b-PNIPAM chains. These coils assemble into intermediate-size aggregates which have a constant number density in solution, independently of the solution concentration, under the assumptions of a simple model based on the concentration dependence of the fitting parameters. Large clusters are also observed, which comprise the large-size structure. Above the LCST the system transforms into a well-defined geometry of monodisperse elliptical vesicles which does not depend on the solution concentration. The vesicles become less asymmetric with a higher aggregation number at the highest temperature.
    Polymer 11/2013; 54(23):6373-6380. DOI:10.1016/j.polymer.2013.09.016 · 3.77 Impact Factor
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    ABSTRACT: We have synthesized nylon 3 via ring opening polymerization of 2-azetidinone (β-lactam) with 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)- phosphoranylidenamino]-2Λ5,4Λ5-catenadi(phosphazene) (t-BuP4) as the catalyst in a mixture of dimethylacetamide (DMAc) and LiCl. The polymers have been characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), laser light scattering (LLS) and viscometry. The synthesized nylon 3 is a linear and crystalline polymer with a molecular weight as high as 105 g mol−1. The intrinsic viscosity ([η]) relates to the weight average molecular weight (Mw) as [η] = 1.02 × 10−4Mw0.91. The effects of solvent, temperature and catalyst concentration on the polymerization have been examined. The molecular weight and yield increases with the amount of LiCl in the polymerization mixture, but both of them decrease with temperature at a temperature above 50 °C. As the catalyst concentration increases, the yield and the molecular weight of nylon 3 decrease. The possible mechanism for the initiation of polymerization is discussed.
    11/2011; 2(12):2888-2892. DOI:10.1039/C1PY00334H
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    ABSTRACT: We develop an experimental approach to analyze the water distribution around a core-shell micelle formed by polystyrene-block-poly[styrene-g-poly(ethylene oxide (PEO)] block copolymers in aqueous media at a fixed polymeric concentration of 10 mg/ml through contrast variation small angle neutron scattering (SANS) study. Through varying the D(2)O/H(2)O ratio, the scattering contributions from the water molecules and the micellar constituent components can be determined. Based on the commonly used core-shell model, a theoretical coherent scattering cross section incorporating the effect of water penetration is developed and used to analyze the SANS I(Q). We have successfully quantified the intramicellar water distribution and found that the overall micellar hydration level increases with the increase in the molecular weight of hydrophilic PEO side chains. Our work presents a practical experimental means for evaluating the intramacromolecular solvent distributions of general soft matter systems.
    The Journal of Chemical Physics 10/2010; 133(14):144912. DOI:10.1063/1.3493331 · 3.12 Impact Factor
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    ABSTRACT: Thermoresponsive brush copolymers with poly(propylene oxide-ran-ethylene oxide) side chains were synthesized via a “grafting from” technique. Poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by random copolymerization of mixtures of oxyalkylene monomers, using metal-free anionic ring-opening polymerization, with the phosphazene base (t-BuP4) being the polymerization promoter. By controlling the monomer feed ratios in the graft copolymerization, two samples with the same side-chain length and different compositions were prepared, both of which possessed high molecular weights and low molecular weight distributions. The results from light scattering and fluorescence spectroscopy indicated that the brush copolymers in their dilute aqueous solutions were near completely solvated at low temperature and underwent slight intramolecular chain contraction/association and much more profound intermolecular aggregation at different stages of the step-by-step heating process. Above 50 °C, very turbid solutions, followed by macrophase separation, were observed for both of the samples, which implied that it was difficult for the brush copolymers to form stable nanoscopic aggregates at high temperature. All these observations were attributed, at least partly, to the distribution of the oxyalkylene monomers along the side chains and the overall brush-like molecular architecture. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2320–2328, 2010
    Journal of Polymer Science Part A Polymer Chemistry 06/2010; 48(11):2320-2328. DOI:10.1002/pola.23997 · 3.54 Impact Factor
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    ABSTRACT: Thermoresponsive brush copolymers with poly(propylene oxide)-block-poly(ethylene oxide) side chains were synthesized via a “grafting from” technique. Near-monodisperse poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by sequential metal-free anionic ring-opening polymerization of the oxyalkylene monomers, using the phosphazene base (t-BuP4) and the phenolic hydroxyl groups in the backbone to generate the complex multifunctional initiating system. The length and composition of the side chains were varied by changing the feed ratios of the backbone and the side-chain monomers. By inverting the sequence of the monomer addition, two different molecular structures were achieved, with either poly(propylene oxide) or poly(ethylene oxide) linked to the backbone. In all cases, brush copolymers with high molecular weights and low molecular weight distributions were synthesized. The thermoresponsive behavior of the brush copolymers in dilute aqueous solutions was investigated by dynamic/static light scattering and fluorescence measurements. Temperature-induced intramolecular chain contraction/association and intermolecular aggregation could both be observed at different stages of the heating process. Intermolecular aggregation was more pronounced for the sample with the poly(propylene oxide) blocks located at the periphery. The results from fluorescence spectroscopy indicate the incompletely solvated state of the brush copolymer in aqueous solution at low temperature and the absence of compact hydrophobic domains in some of the aggregates due to the core−shell brushlike molecular structure of the copolymers.
    Macromolecules 01/2010; 43(4). DOI:10.1021/ma902590w · 5.93 Impact Factor
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    ABSTRACT: This work presents the synthesis of polystyrene-block-poly(p-hydroxystyrene-graft-ethylene oxide), PS-b-(PHOS-g-PEO), amphiphilic block−graft copolymers. The backbone diblock copolymers (PS-b-PHOS) were prepared by lithium-based anionic polymerization, followed by postpolymerization acid hydrolysis of the poly(p-tert-butoxystyrene), PtBOS, precursor block. The PEO side chains were synthesized by metal-free anionic ring-opening polymerization of ethylene oxide (EO), using the phosphazene base (t-BuP4) and the phenolic hydroxyl groups (PhOH) in the backbones as the complex multifunctional initiating system. In all cases, starlike block−graft copolymers with high molecular weights and low polydispersities were synthesized. Well-controlled polymerization was achieved even with the molar ratio of t-BuP4 to PhOH being equal to 0.2. Dynamic and static light scattering and fluorescence spectroscopy studies were carried out to investigate the solution behavior of the amphiphilic block−graft copolymers, including the critical micelle concentration and structural characteristics of the aggregates formed in aqueous solutions. Because of the high PEO content and the starlike macromolecular architecture, the PS-b-(PHOS-g-PEO) block−graft copolymers form highly swelled aggregates with low aggregation numbers, having nanostructures resembling hyperbranched clusters.
    Macromolecules 11/2009; 42(22). DOI:10.1021/ma9016604 · 5.93 Impact Factor
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    ABSTRACT: Double hydrophilic poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, using a PEO-based chain transfer agent (PEO-CTA). The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. Temperature-induced aggregation behavior of the block copolymers in dilute aqueous solutions was systematically investigated by a combination of static and dynamic light scattering. The effects of copolymer composition, concentration (Cp), and heating rate on the size, aggregation number, and morphology of the aggregates formed at temperatures above the LCST were studied. In slow heating processes, the aggregates formed by the copolymer having the longest PNIPAM block, were found to have the same morphology (spherical “crew-cut” micelles) within the full range of Cp. Nevertheless, for the copolymer having the shortest PNIPAM block, the morphology of the aggregates showed a great dependence on Cp. Elongation of the aggregates from spherical to ellipsoidal or even cylindrical was observed. Moreover, vesicles were observed at the highest Cp investigated. Fast heating leads to different characteristics of the aggregates, including lower sizes and aggregation numbers, higher densities, and different morphologies. Thermodynamic and kinetic mechanisms were proposed to interpret these observations, including the competition between PNIPAM intrachain collapse and interchain aggregation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4099–4110, 2009
    Journal of Polymer Science Part A Polymer Chemistry 08/2009; 47(16):4099 - 4110. DOI:10.1002/pola.23470 · 3.54 Impact Factor
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    ABSTRACT: Low-molecular-weight cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB), were introduced to dilute aqueous solutions of thermosensitive poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers at concentrations (C(s)) either lower or higher than the critical micelle concentrations (cmc) of the surfactants. The copolymer/surfactant mixtures were investigated by dynamic and static light scattering at different temperatures. At temperature lower than the aggregation temperature (T(agg)), the disaggregation of the copolymers from the loose associations was observed upon addition of the surfactants(.) The thermo-induced aggregation behavior was found to be profoundly influenced with the cooperation of cationic surfactants in terms of T(agg) and the structural characteristics of the aggregates formed at high temperature. In general, T(agg) was increased together with the decrease in the size and molecular weight of the aggregates. These were attributed to the copolymer/surfactant interactions and the electrostatic repulsion coming from the ionic head groups of the surfactants within the mixed aggregates. These changes were much more pronounced at higher C(s). CTAB, which has a longer hydrophobic tail, displayed higher influences compared to DTAB. The formation of vesicles, by one of the copolymers, was suppressed in the presence of CTAB. At the higher CTAB concentration, only small mixed aggregates with very low mass were observed even at the highest temperature investigated.
    The Journal of Physical Chemistry B 08/2009; 113(31):10600-6. DOI:10.1021/jp9038896 · 3.38 Impact Factor
  • Macromolecular Chemistry and Physics 06/2009; 210(12):1026-1032. DOI:10.1002/macp.200900161 · 2.45 Impact Factor
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    ABSTRACT: The effects of temperature, pH, and salt concentration on the layer-by-layer (LBL) deposition of sodium poly(styrene sulfonate) (PSS)/poly[2-(dimethylamino)ethyl methacrylate] (PDEM) were investigated by use of a quartz crystal microbalance with dissipation (QCM-D). At pH 4, the frequency change (Deltaf) gradually decreased to a constant, indicating that the polyelectrolyte complexes of the layer were not dissolved. As the layer number increased, the -Deltaf oscillatedly increased, indicating that the thickness of the multilayer increased. At the same time, the dissipation change (DeltaD) oscillatedly increased with the layer number, indicating the chain interpenetration or complexation that led to the alternative swelling-and-shrinking of the outermost layer. For the same layer number, as the temperature increased, the amplitude of DeltaD increased, indicating that the chain interpenetration increased. The thickness also increased with temperature. Further increasing the pH to 7 led to a thicker layer, reflected in the larger amplitude of DeltaD. At pH 10, the polyelectrolytes no longer formed multilayers on the surface because of the lack of electrostatic interactions. On the other hand, the addition of NaCl also led to a thickness increase. The amplitude in DeltaD increased with NaCl concentration, indicating that the chain interpenetration increased. Our experiments indicated that the LBL deposition of polyelectrolytes was dominated by the chain interpenetration. Also, the polyelectrolyte complexes in the layer can redissolve into solution from the surface at a high temperature or a high salt concentration.
    The Journal of Physical Chemistry B 04/2008; 112(11):3333-8. DOI:10.1021/jp710600f · 3.38 Impact Factor