Gexiao Sun

Equilibrium Interaction of Solid Surfaces across a Polymer Melt

Langmuir, Aug 7, 2004

Forces across polymer melts are poorly understood despite their importance for adhesion and fabri... more Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium.

Adhesion between Solid Surfaces in Polymer Melts: Bridging of Single Chains

Macromolecules, Jul 2, 2004

The adhesion of solid surfaces across polymer melts has been studied with an atomic force microsc... more The adhesion of solid surfaces across polymer melts has been studied with an atomic force microscope (AFM). As polymers we used poly(dimethylsiloxane) (PDMS, Mw) 18 000), poly(ethylmethylsiloxane) (PEMS, Mw) 16 800), and a diblock copolymer (PDMS-b-PEMS, Mw) 15 100). Upon retraction, adhesion peaks were observed which we interpret as bridging of single polymer chains. Bridging occurred seldom (in each 20th force curve) in PDMS and more often in PEMS (in each 8th force curve) and was most prominent in the diblock copolymer, where a typical force curve contained 5-10 adhesion peaks. The mean detachment force decreased with increasing retraction velocity, indicating that the bond to the surface or tip is not a direct contact with the solid surface.

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Exploring the functional attributes and in vitro starch and protein digestibility of pea flours having a wide range of amylose content

Food Chemistry, Mar 1, 2023

Interaction between solid surfaces in a polymer melt studied by atomic force microscopy

European Polymer Journal, Apr 1, 2005

ABSTRACT

Confined polymer melts studied by atomic force microscopy

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Dec 1, 2004

Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar so... more Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar solid surfaces have been measured across a long-chain poly(dimethyl siloxane) (PDMS, M W = 18,000 g/mol), a short-chain PDMS (M W = 4200 g/mol), a poly(ethylmethyl siloxane) (PEMS, M W = 16,800 g/mol), and a diblock copolymer consisting of one PDMS and one PEMS block (PDMS-b-PEMS, M W = 15,100 g/mol). The interaction changed significantly during the first 10 h after immersing the solids in the polymer melt. This demonstrates that the time scale of structural changes at a solid surface is much slower than in the bulk. On mica and silicon oxide both polymers formed an immobilized "pinned" layer beyond which a monotonically decaying repulsive force was observed. Attractive forces were observed with short-chain PDMS on silicon oxide and PEMS on mica and silicon oxide. On the basal plane of graphite PEMS caused a stable, exponentially decaying oscillatory force.

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Confined liquid: Simultaneous observation of a molecularly layered structure and hydrodynamic slip

Journal of Chemical Physics, Dec 8, 2002

Rupture of molecular thin films observed in atomic force microscopy. II. Experiment

Physical review, Sep 5, 2002

In atomic force microscope studies of thin films often a defined jump of the tip through the film... more In atomic force microscope studies of thin films often a defined jump of the tip through the film is observed once a certain threshold force has been exceeded. In particular, on lipid bilayers this is regularly observed. In a previous paper ͓H.-J. Butt and V. Franz, Phys. Rev. E 66, 031601 ͑2002͔͒ we presented two complementary models to describe film rupture. The aim of this study was to verify these models. Experiments were done with solid supported bilayers consisting of dioleoyloxypropyl-trimethylammonium chloride ͑DOTAP͒ and dioleoylphosphatidylserine ͑DOPS͒ in aqueous solutions and with propanol. Both models describe experimental results adequately. In particular, a narrow distribution of yield forces and an increase of the mean yield force with increasing loading rate is correctly predicted. For the lipid bilayers spreading pressures of roughly 20 mN/m ͑DOTAP͒ and 5 mN/m ͑DOPS͒ were measured. Line tensions for the edge of a lipid bilayer ranged between 3 ͑DOTAP͒ and 6 pN ͑DOPS͒.

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The combination of principal component analysis, genetic algorithm and tabu search in 3d molecular similarity

Journal of Molecular Structure-theochem, Apr 1, 2004

Recently, molecular similarity, as an important tool of computer-aided drug design has developed ... more Recently, molecular similarity, as an important tool of computer-aided drug design has developed rapidly. Its calculation has also developed from planar, rigid, 2D molecules to steric, flexible, 3D molecules. However, 3D molecular similarity calculation is easy to fall into local optima and the calculation is always time-consuming.In this paper, a method of flexible 3D molecular similarity calculation through the evaluation

Interaction between two solid surfaces across PDMS: influence of chain length and end group

Composite Interfaces, 2005

Forces between solid surfaces across polymer melts are poorly understood despite their importance... more Forces between solid surfaces across polymer melts are poorly understood despite their importance for adhesion and composite materials. Using an atomic force microscope (AFM) this force was measured for poly(dimethyl siloxane) (PDMS) on silicon oxide. The influence of molecular weight (4.0-40 kDa) was studied. Forces are attractive for low and repulsive for high molecular weight. In addition, changing the terminal methyl group for a hydroxyl group leads to an increased probability of bridging.

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Exploring the functional attributes and in vitro starch and protein digestibility of pea flours having a wide range of amylose content

Food Chemistry

The combination of principal component analysis, genetic algorithm and tabu search in 3d molecular similarity

Journal of Molecular Structure: THEOCHEM, 2004

Recently, molecular similarity, as an important tool of computer-aided drug design has developed ... more Recently, molecular similarity, as an important tool of computer-aided drug design has developed rapidly. Its calculation has also developed from planar, rigid, 2D molecules to steric, flexible, 3D molecules. However, 3D molecular similarity calculation is easy to fall into local optima and the calculation is always time-consuming.In this paper, a method of flexible 3D molecular similarity calculation through the evaluation

Rupture of molecular thin films observed in atomic force microscopy. I. Theory

Physical review. E, Statistical, nonlinear, and soft matter physics, 2002

In atomic force microscope studies of molecular thin films, a defined jump of the tip through the... more In atomic force microscope studies of molecular thin films, a defined jump of the tip through the film is often observed once a certain threshold force has been exceeded. Here, we present a theory to describe this film rupture and to relate microscopic parameters to measurable quantities. We assume that the tip has to overcome an activation energy before the film ruptures. A universal relation between the force dependence of the activation energy and the approaching velocity of the tip is derived. Two complementary models for calculating the activation energy are presented: a continuum nucleation model and a discrete molecular model. Both models predict a narrow distribution of yield forces in agreement with experimental results.

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Adhesion between Solid Surfaces in Polymer Melts: Bridging of Single Chains

Macromolecules, 2004

The adhesion of solid surfaces across polymer melts has been studied with an atomic force microsc... more The adhesion of solid surfaces across polymer melts has been studied with an atomic force microscope (AFM). As polymers we used poly(dimethylsiloxane) (PDMS, Mw) 18 000), poly(ethylmethylsiloxane) (PEMS, Mw) 16 800), and a diblock copolymer (PDMS-b-PEMS, Mw) 15 100). Upon retraction, adhesion peaks were observed which we interpret as bridging of single polymer chains. Bridging occurred seldom (in each 20th force curve) in PDMS and more often in PEMS (in each 8th force curve) and was most prominent in the diblock copolymer, where a typical force curve contained 5-10 adhesion peaks. The mean detachment force decreased with increasing retraction velocity, indicating that the bond to the surface or tip is not a direct contact with the solid surface.

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Equilibrium Interaction of Solid Surfaces across a Polymer Melt

Langmuir, 2004

Forces across polymer melts are poorly understood despite their importance for adhesion and fabri... more Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium.

Confined liquid: Simultaneous observation of a molecularly layered structure and hydrodynamic slip

The Journal of Chemical Physics, 2002

The force profile between a glass microsphere and mica in 1-propanol has been measured with the c... more The force profile between a glass microsphere and mica in 1-propanol has been measured with the colloidal probe technique. Oscillatory solvation forces indicate a layered structure of the confined propanol for at least three layers. In the same experiment, hydrodynamic forces were measured at high approaching velocity. Comparing measured force curves with calculations we found a significant effective slip, which could be described by a slip length of 10–14 nm.

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Interaction between solid surfaces in a polymer melt studied by atomic force microscopy

European Polymer Journal, 2005

ABSTRACT

Interaction between two solid surfaces across PDMS: influence of chain length and end group

Composite Interfaces, 2005

Forces between solid surfaces across polymer melts are poorly understood despite their importance... more Forces between solid surfaces across polymer melts are poorly understood despite their importance for adhesion and composite materials. Using an atomic force microscope (AFM) this force was measured for poly(dimethyl siloxane) (PDMS) on silicon oxide. The influence of molecular weight (4.0-40 kDa) was studied. Forces are attractive for low and repulsive for high molecular weight. In addition, changing the terminal methyl group for a hydroxyl group leads to an increased probability of bridging.

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Confined polymer melts studied by atomic force microscopy

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004

Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar so... more Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar solid surfaces have been measured across a long-chain poly(dimethyl siloxane) (PDMS, M W = 18,000 g/mol), a short-chain PDMS (M W = 4200 g/mol), a poly(ethylmethyl siloxane) (PEMS, M W = 16,800 g/mol), and a diblock copolymer consisting of one PDMS and one PEMS block (PDMS-b-PEMS, M W = 15,100 g/mol). The interaction changed significantly during the first 10 h after immersing the solids in the polymer melt. This demonstrates that the time scale of structural changes at a solid surface is much slower than in the bulk. On mica and silicon oxide both polymers formed an immobilized "pinned" layer beyond which a monotonically decaying repulsive force was observed. Attractive forces were observed with short-chain PDMS on silicon oxide and PEMS on mica and silicon oxide. On the basal plane of graphite PEMS caused a stable, exponentially decaying oscillatory force.

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