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ABSTRACT: One of the most important issues for spin-coated diblock copolymer thin films is to distinguish the blocks by common techniques, especially for the diblock copolymers containing one block could form active domain. Aiming different estimations on the composition of surface microdomains in polystyrene-block-poly (4-vinylpyridine) (PS-b-P4VP) atomic force microscope (AFM) phase images, AFM, transmission electron microscopy, and droplet shape analyzer were used to identify the constructional details of the polymer thin film morphology. It was confirmed that PS block is harder than P4VP block in symmetric PS-b-P4VP films and corresponds to brighter regions in AFM phase image. It is helpful to distinguish the nanodomains that originate from the different blocks in PS-b-P4VP thin films. The structure evolutions of PS-b-P4VP film annealed with different selective solvents were studied and discussed. The results indicated that the core-corona inversion process is due to the cores of micelles swell and coalesce together rather than local reorganization of the chains.
Journal of Colloid and Interface Science 03/2013; · 3.07 Impact Factor
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ABSTRACT: Monte Carlo simulations are employed to investigate the adsorption of a flexible linear cationic polyelectrolyte onto a fluid mixed membrane containing neutral (phosphatidyl-choline, PC), multivalent (phosphatidylinositol, PIP2) and monovalent (phosphatidylserine, PS) anionic lipids. We systematically study the effect of chain length and charge density of the polyelectrolyte, the solution ionic strength as well as the membrane compositions on the conformational and interfacial properties of the model system. In particular, we explore (i) the adsorption/desorption limit, (ii) the interfacial structure variations of the adsorbing polyelectrolyte and the lipid membrane, and (iii) the overcharging of the membrane. Polyelectrolyte adsorbs on the membrane when anionic lipids demixing entropy loss and polyelectrolyte flexibility loss due to adsorption are dominated over by electrostatic attraction between polyelectrolyte and anionic lipids on the membrane. Polyelectrolytes with longer chain length and higher charge density can adsorb on the membrane with increased anionic lipids density under a higher critical ionic concentration. Below the critical ionic concentration, the adsorption extent increases with the charge density and chain length of the polyelectrolyte and decrease with ionic strength of the solution. The diffusing anionic lipids prohibit the polyelectrolyte chain to form too long tails. The adsorbing polyelectrolyte with long chain length and high charge density can overcharge membrane with low charge density, and conversely, the membrane charge inversion forces the polyelectrolyte chain to form extended loops and tails in the solution.
The Journal of Physical Chemistry B 01/2013; · 3.70 Impact Factor
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ABSTRACT: Morphologies and structures determined by crystallization of the blocks, microphase separation of the copolymers, and vitrification
of PLLA block in poly(l-lactide-b-ethylene oxide) (PLLA-b-PEO) copolymers were investigated using microscopic techniques and synchrotron small angle X-ray scattering. The PLLA-b-PEO copolymer films were crystallized from two different annealing processes: melt crystallization (process A) or crystallized
from glass state of PLLA block after quenching from melt state (process B). The relationship between the crystalline morphology
and microstructure of the copolymers were explored using SAXS. The morphology and phase structure are predominated by crystallization
of PLLA block, and greatly influenced by microphase separation of the copolymers. In process B, lozenge-shape and truncated
lozenge-shaped PLLA crystals of nanometer scale can be observed. The crystalline morphology is markedly affected by the microstructure
formed during the annealing process. Star-shaped morphologies stacked with PLLA single crystals were observed.
KeywordsCrystallization–Morphology–Structure–Microphase separation–Vitrification–Poly(l-lactide-b-ethylene oxide) copolymer
Polymer Bulletin 04/2012; 67(5):885-902. · 1.53 Impact Factor
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ABSTRACT: In protein-polysaccharide complex systems, how nonspecific interactions such as electrostatic and van der Waals interactions affect complex formation has not been clearly understood. On the basis of a coarse-grained model with the specificity of a target system, we have applied Monte Carlo (MC) simulation to illustrate the process of complex coacervate formation from the association of proteins and polysaccharides. The coarse-grained model is based on serum albumin and a polycation system, and the MC simulation of pH impact on complex coacervation has been carried out. We found that complex coacervates could form three ways, but the conventional association through electrostatic attraction between the protein and polysaccharide still dominated the complex coacervation in such systems. We also observed that the depletion potential always participated in protein crowding and was weakened in the presence of strong electrostatic interactions. Furthermore, we observed that the sizes of polysaccharide chains nonmonotonically increased with the number of bound proteins. Our approach provides a new way to understand the details during protein-polysaccharide complex coacervation at multiple length scales, from interaction and conformation to aggregation.
The Journal of Physical Chemistry B 02/2012; 116(10):3045-53. · 3.70 Impact Factor
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ABSTRACT: Polyelectrolyte hydrogel is a ternary mixture of water, polymer network and mobile ions. The present paper examined two popular models describing the swelling and deformation behaviors of polyelectrolyte hydrogels, i.e. the multiphasic model and the transport model. The water flow, the network deformation and the ionic diffusion are coupled in the multiphasic model, and the gradient of the fluid pressure is taken as the driving force for the network deformation. However, the water flow is neglected in the transport model with the ionic osmotic pressure taking the role of fluid pressure. Two simplified experiments, i.e. the free swelling of a hydrogel sphere in response to the concentration change of the external salt solution and the bending deformation of a hydrogel strip under an external electric field, are simulated by the two models. Simulation shows that the two models lead to the same predictions for the swelling equilibrium of the hydrogel sphere but different predictions for the swelling kinetics of the hydrogel sphere and the deformation of the hydrogel strip under the external electric field. These are due to the fact that the two models are equivalent in thermodynamic equilibrium situations, but in thermodynamic non-equilibrium situations, the transport model is no longer applicable as it neglects the water flow in the hydrogel and takes the ionic osmotic pressure as a mechanical parameter to play the role of swelling pressure. The present work will be helpful for understanding the hydrodynamics of polyelectrolyte hydrogels and the application of the two models.
Journal of the mechanical behavior of biomedical materials. 10/2011; 4(7):1328-35.
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ABSTRACT: Using in situ atomic force microscopy (AFM) and grazing incidence ultrasmall-angle X-ray scattering (GIUSAX), the composition dependence of phase separation, dewetting, and interplay between them in blend [poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN)] ultrathin films (1Rg, radius of gyration) on silicon oxide substrate was investigated. It was found that phase behaviors depended crucially on the composition in blend. First, dewetting morphologies in SAN50 and SAN70 were still under the control of Uq0/E, which was introduced in our previous work [ Polymer 2009, 50, 4456] to distinguish different dewetting pathways. Here, Uq0 and E described the initial amplitude of the surface undulation and original thickness of film, respectively; second, composition produced significant influences on its gradient in composition fluctuation, resulting in the accelerated (or suppressed) dewetting; last, this kind of acceleration or suppression affected the following interplay between phase separation and dewetting so much. Using two samples of “dewetting/wetting–phase separation” and “wetting–dewetting/phase separation” with different components, we discussed the interplay mechanism of them in detail. In conclusion, our results indicate that dewetting, phase separation, and the following interplay between them are sensitive to composition in blend. Furthermore, composition gradient plays an important role in them.
06/2011;
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Soft Matter 01/2011; 7:8056. · 4.39 Impact Factor
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ABSTRACT: Telechelic polyelectrolytes, bearing short hydrophobic blocks at both ends, will ionize into polyions and their counterions when dissolved in water. With the variation of concentration, the interplay between short range attraction and the long range electrostatic interaction as well as the counterion distribution exerts a major influence on the chain conformations (two basic conformations: loop and nonloop, the latter can be subdivided into three association types: free, dangling, and bridge), the cluster structure and the forming of a physical gel. For weak hydrophobic interaction, the relative strong electrostatic interaction dominates the gelation progress; sol-gel transition occurs at higher concentrations due to electrostatic screening and mainly involves the forming of stretched nonloop conformations such as dangling and bridge. While for strong hydrophobic interaction, the hydrophobic interaction dominates and the electrostatic interaction provides a contribution to the formation of gels by maintaining a spatial swelling structure, resulting in a much lower concentration region of sol-gel transition; besides, the sol-gel transition is characterized by the competition of the forming of loop and bridge chains.
The Journal of chemical physics 01/2011; 134(3):034903. · 3.09 Impact Factor
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ABSTRACT: Self-assembly of symmetric ABC three-arm star copolymers confined in cylindrical nanopores is investigated by means of a lattice Monte Carlo simulation method. The dependence of morphologies on the degree of confinement and preference of pore surface is studied systematically. For the symmetric ABC three-arm star copolymers which form polygonal cylinder structures with periodic spacing L(0) in bulk, various novel structures are observed inside the nanopores. In the nanopores with a neutral surface, we find a minimum diameter value (D(min) ≈ L(0)) under which helical arranged droplets are formed; otherwise, parallel polygonal cylinder structures are identified. By adjusting the preference between component A and the pore surfaces, a number of novel structures such as A cylinder + BC single-strand helix and complex multilayer double helices are identified. Additionally, the confinement-induced morphology transition is interpreted by the frustration parameter D/L(0).
The Journal of Physical Chemistry B 11/2010; 114(49):16318-28. · 3.70 Impact Factor
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ABSTRACT: Film thickness dependence of complex behaviors coupled by phase separation and dewetting in blend [poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN)] films on silicon oxide substrate at 175 °C was investigated by grazing incidence ultrasmall-angle X-ray scattering (GIUSAX) and in situ atomic force microscopy (AFM). It was found that the dewetting pathway was under the control of the parameter U(q0)/E, which described the initial amplitude of the surface undulation and original thickness of film, respectively. Furthermore, our results showed that interplay between phase separation and dewetting depended crucially on film thickness. Three mechanisms including dewetting-phase separation/wetting, dewetting/wetting-phase separation, and phase separation/wetting-pseudodewetting were discussed in detail. In conclusion, it is relative rates of phase separation and dewetting that dominate the interplay between them.
Langmuir 09/2010; 26(18):14530-4. · 4.19 Impact Factor
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ABSTRACT: A multiphasic model for the volume change of polyelectrolyte hydrogels that takes into account conservation of mass and momentum is derived. The gradient of chemical/electrochemical potentials of water and mobile ions is taken as the driving force for the volume change of the polyelectrolyte hydrogel, which is damped by the frictional forces between different phases and balanced by the elastic restoring force of the polymer network. Employing the model constructed here, the free swelling of a spherical polyelectrolyte hydrogel immersed in salt solution is simulated by the finite element method. The simulation shows that the polyelectrolyte hydrogel swells from the surface to the interior when the concentration of the external salt solution decreases. The swelling kinetics for ordinary hydrogels with high frictional coefficient between the polymer network and water is controlled by the collective diffusion of the polymer network, while for fast-response hydrogels it is controlled by the ionic diffusion in the hydrogel.
The Journal of chemical physics 09/2010; 133(11):114904. · 3.09 Impact Factor
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ABSTRACT: Transport of star polymers under pressure-driven flow in a pipe with pipe radius being at least twice the size of polymers has been examined with standard dissipative particle dynamics (DPD) simulations. Equilibrium dynamics of star polymers in bulk solution were found to obey the Zimm model very well, indicating that DPD simulation correctly incorporates the hydrodynamic interaction in the stars. Under pressure-driven flow, star polymers with more arms were found to migrate toward the center of pipe more, leading to a net faster velocity and hence a shorter retention time in the pipe. The stretching of star polymers along the flow was found to follow similar scaling behavior as the linear polymer chains, except that the Weissenberg number Wi for the stars should be reduced by arm number f. After rescaling of the Weissenberg number, the stretch ratio Sx, defined as the ratio of square of radius gyration of the chains along the flow, Rgx2, over its corresponding value in dilute bulk solution, was found to scale with Wi linearly when Wi 1. The compression of the chains in the dimension perpendicular to the flow Sy were found to scale with Wi−0.5 when Wi 1.0.
06/2010;
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ABSTRACT: We investigated the dependence of the dewetting velocity of a thin, low-viscosity polystyrene (PS) top film on a poly(methyl methacrylate) (PMMA) double layer consisting of a low-viscosity underlayer of thickness h(L) coated with a high-viscosity middle layer of thickness h(M). The addition of the liquid underlayer generated complex nonmonotonic behavior of the dewetting velocity as a function of increasing h(M). In particular, we observed an acceleration of dewetting for an intermediate range of h(M). This phenomenon has been interpreted by a combination deformation of the middle elastic layer and a concurrent change in the contact angle. On one hand, deformation led to the formation of a trench that dissipated energy during its movement through the liquid underlayer and thus caused a slowing down of dewetting. However, with an increase in the thickness of the elastic middle layer, the size of the trench decreased and its influence on the dewetting velocity also decreased. On the other hand, the deformation of the elastic layer also led to an increase in the contact angle. This increase in the driving capillary forces caused an increase in the dewetting velocity.
Langmuir 02/2010; 26(10):7270-6. · 4.19 Impact Factor
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ABSTRACT: A special kind of associating polyelectrolytes, telechelic polyelectrolytes, composed of a flexible linear polyelectrolyte and short hydrophobic blocks at both ends, is investigated by means of Monte Carlo simulations in aqueous media. It is found that the interplay between hydrophobic attraction and the long-range electrostatic interaction as well as the counterion distribution exerts a major influence on the chain conformations (two basic conformations: loop and nonloop), association behavior (loop association contributes to loop conformation; free, dangling, and bridge chains contribute to nonloop conformations), and the forming of clusters. At a concentration close to the overlapping concentration phi*, the clusters are found to be further connected by bridge chains under strong hydrophobic interaction, forming a 3D network. In addition, increasing Coulombic interaction strength is in favor of the formation of a gel network. With increasing Coulombic interaction strength, first chains tend to extend and then tend to collapse due to the screening originating from the counterion condensation layer.
The Journal of Physical Chemistry B 02/2010; 114(10):3449-56. · 3.70 Impact Factor
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ABSTRACT: Self-assembled behavior of T-shaped rod-coil block copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. Compared with rod-coil diblock copolymers with the anchor point positioned at one end, the copolymers with the anchor point at the middle of the rod exhibit significantly different phase behaviors. When the rod volume fraction is low, the steric hindrance of the lateral coils prevents the rods stacking into strip or micelle as that in rod-coil diblock copolymers. The competition between interfacial energy and entropy results in the formation of lamellar structures and the increasing thickness of the lamellar layer with increasing rod volume fraction. When the rod volume fraction is high, the graft density of the planar interface is decreased, which results in space-filling requirements and stretching penalty, thus leading to the stability of nonlamellar structures with curing interface. Furthermore, our results also suggest that the effect of the chain architecture on the self-assembled behavior is remarkable when the rod volume fraction is low, whereas the effect is weak when the rod volume fraction is high.
The Journal of chemical physics 10/2009; 131(14):144905. · 3.09 Impact Factor
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ABSTRACT: The evolution of morphologies of isothermally crystallized thin films with different thicknesses of poly(L-lactide-b-ethylene oxide) diblock copolymer was observed by optical microscopy (OM) and atomic force microscopy (AFM). Dendritic superstructures stacked with lamellae were investigated in thin films with approximately 200 nm to approximately 400 nm thickness. The lamellar structure was a lozenge- or truncated-lozenge-shaped single crystal of PLLA confirmed by AFM observations. The contour of the dendritic superstructures is hexagonal, and two types of sectors, [110] and [100], can be classified in terms of the chain-folding and crystal growth directions. These phenomena are due to the interplay of the crystallization of the PLLA block, the microphase separation of the block copolymer, and the effect of the film thickness. The growth process of the superstructure can be classified into three steps: the growth of the main branches, the growth of the secondary side branches along the main branch, and the tertiary side branches. PLLA growth rates decrease in copolymer films thinner than 1 microm. Layer-layer phase structure of the copolymer driven by the crystallization of PLLA and the microphase separation of the copolymer appears to be a key factor explaining the crystallization and morphological behavior of this system.
Langmuir 08/2009; 25(22):13125-32. · 4.19 Impact Factor
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ABSTRACT: We investigate the dewetting behavior of the bilayer of air/PS/PMMA/silanized Si wafer and find the two competing dewetting pathways in the dewetting process. The upper layer dewets on the lower layer (dewetting pathway 1, the liquid-liquid dewetting) and the two layers rupture on the solid substrate (dewetting pathway 2, the liquid-solid dewetting). To the two competing dewetting pathways, the process of forming holes and the process of hole growth, influence their competing relation. In the process of forming holes, the time of forming holes is a main factor that influences their competing relation. During the process of hole growth, the dewetting velocity is a main factor that influences their competing relation. The liquid-liquid interfacial tension, the film thickness of the polymer, and the viscosity of the polymer are important factors that influence the time of forming holes and the dewetting velocity. When the liquid-liquid dewetting pathway and the liquid-solid dewetting pathway compete in the dewetting process, the competing relation can be controlled by changing the molecular weight of the polymer, the film thickness, and the annealing temperature. In addition, it is also found that the rim growth on the solid substrate is by a rolling mechanism in the process of hole growth.
The Journal of chemical physics 06/2009; 130(18):184903. · 3.09 Impact Factor
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Macromolecular Theory and Simulations 05/2009; 18(4‐5):268 - 276. · 1.71 Impact Factor
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ABSTRACT: The final structure of molten syndiotactic polypropylene (sPP) sheared under different conditions was investigated by synchrotron small-angle x-ray scattering (SAXS) and wide-angle x-ray diffraction (WAXD) techniques to elucidate the shear effects on sPP crystalline structure. The results obtained from the WAXD show that there is no variation on crystalline form but a little difference on the orientation of the 200 reflection. The SAXS data indicate that the lamellar thickness and long period have not been affected by shear but the lamellar orientation is dependent on shear. The experimental data of sPP crystallization from sheared melt may indicate a mesophase structure that is crucial to the shear effects on the final polymer multiscale crystalline structures.
The Journal of chemical physics 05/2009; 130(16):164909. · 3.09 Impact Factor
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ABSTRACT: The novel phase field model with the "polymer characteristic" was established based on a nonconserved spatiotemporal Ginzburg-Landau equation (TDGL model A). Especially, we relate the diffusion equation with the crystal growth faces of polymer single crystals. Namely, the diffusion equations are discretized according to the diffusion coefficient of every lattice site in various crystal growth faces and the shape of lattice is selected based on the real proportion of the unit cell dimensions. Spatiotemporal growth of syndiotactic polypropylene single crystals during isothermal crystallization has been investigated theoretically based on this phase field model. Two dimensional numerical calculations are performed to elucidate the faceted single crystal growth including square, rectangular, lozenge-shaped, and hexagonal single crystals. Our simulated patterns are in good agreement with the experimental morphologies, and the physical origin of polymer single crystal growth is discussed.
The Journal of chemical physics 12/2008; 129(19):194903. · 3.09 Impact Factor
