| 1. |
- Chen, Tao, et al.
(författare)
-
Colloidal lithography for fabricating patterned polymer-brush microstructures
- 2012
-
Ingår i: Beilstein Journal of Nanotechnology. - : Beilstein Institut. - 2190-4286. ; 3, s. 397-403
-
Tidskriftsartikel (refereegranskat)abstract
- We exploit a series of robust, but simple and convenient colloidal lithography (CL) approaches, using a microsphere array as a mask or as a guiding template, and combine this with surface-initiated atom-transfer radical polymerization (SI-ATRP) to fabricate patterned polymer-brush microstructures. The advantages of the CL technique over other lithographic approaches for the fabrication of patterned polymer brushes are (i) that it can be carried out with commercially available colloidal particles at a relatively low cost, (ii) that no complex equipment is required to create the patterned templates with micro-and nanoscale features, and (iii) that polymer brush features are controlled simply by changing the size or chemical functionality of the microspheres or the substrate.
|
|
| 2. |
- Dunér, Gunnar
(författare)
-
Grafted Molecular Layers for Control of Surface Properties
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The goal of this thesis work was to develop responsive surface grafted brushlayers for control of surface properties and to gain insights in the molecular mechanisms that control these properties. Three types of grafted layers were investigated, as outlined below. In the first system studied, poly(AAc) was synthesized by a grafting from approach, utilizing a photopolymerization reaction from a macroinitiator cast onto QCM substrates. The responsiveness in terms of frequency change, Δf, of the resulting brushes to changes in bulk pH was studied with QCM. Further, the friction properties of poly(AAc) was elucidated with colloidal probe AFM as a function of pH and counterion valency. High friction (μ=0.27) was found in presence of CaCl2 at high pH (7.5), but not under any other condition explored. It was concluded that the high friction was due to intralayer COO--Ca2+--OOC bridges. QCM-D was utilized for studying viscoelastic properties of growing poly(AAc) films during in situ photopolymerization. By Voigt modeling, the thickness, shear elasticity and shear viscosity were extracted. These parameters were observed to undergo sudden transitions at a critical thickness, and from this thickness the grafting density of the growing poly(AAc) layers was determined. In addition, the sensitivity to changes in Δf and ΔD with respect to the thickness of the poly(AAc) films was investigated, and the results showed that high sensitivity in ΔD is retained at higher film thicknesses than for Δf, and that the sensitivity with respect to noise can significantly alter the thickness that is best suited forthe study of viscoelastic changes in sensor applications. The work with QCM-D also involved the detection of structural variations within a thick brush layer of poly(AAc). Since lower overtones have higher penetration depth, these frequencies sense polymer segments further out in the brush. It was found that the apparent pKa of the poly(AAc) was higher for lower overtones, indicating therelative ease of acid dissociation in segments further out in the polyelectrolytebrush. In the second system studied, phenylethylamine (PEA) was electrografted to flatsurfaces of glassy carbon (GC). The nanomechanical properties, such as topography, deformation, adhesion and dissipation, were investigated using PeakForce quantitative nanomechanical mapping (QNM). One main finding is that globular domains of 40 to 50 nm indiameter appear in the electrografted PEA layer. They are assigned to clusters of PEA formed due to less rapid reactions between radicals and the GC surface compared to reactions with already grafted PEA. The interactions between the PEA layer and a silica sphere were further investigated by surface force measurements. A main finding is that the PEA surfaces were heavily charge regulated due to the limited net charge of the PEA layer compared to that of silica. In the third system studied, the mechanical response of polyelectrolytes as a function of applied load, probing angle and pH was investigated with PeakForce QNM. The used polyelectrolyte was poly(2-dimethylaminoethyl methacrylate)(PDMAEMA) with pKa of 6.5 to 7.5 and grafted to silica nanoparticles. While most research on polyelectrolyte brushes is conducted by employing flat and smooth surfaces, with a roughness on the nanometer scale, real surfaces are rarely ever ideally flat but rather they possess topographic irregularities on nano- and micro-scales, which locally imparts high curvatures. The spherical geometry of the core-shell nanoparticles serves as a model for real surfaces with respect to topographical irregularities of real surfaces. The nanoscale brushes were probed with an ultrasharp AFM tip, providing nanoscale resolution of topography, deformation, adhesion and dissipation. It was found that the mechanical response of the polyelectrolyte corona is dependent on the applied load and the polar angle of the tip-brush interaction. All nanomechanical data show a non-monotonic variation with horizontal position, and the peak values are shifted in magnitude and position as a function of peak force. The effect of pH on deformation was further investigated. The results showed that the brush is more resistant against compression over the centre than it is to deflection at larger horizontal positions, and this effect is amplified by charging the brush. This work provided understanding of the direction dependence of the mechanical properties and is relevant for the design of brush boundary lubricating agents for rough surfaces, where the polymer chains are both being bent and compressed under the influence of load and shear.
|
|
| 3. |
- Horvath, A. Elisabet
(författare)
-
The effects of cellulosic fiber charges on polyelectrolyte adsorption and fiber-fiber interactions
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The surface charges of cellulosic fibers contribute to several papermaking operations that influence the manufacture and final properties of paper. This thesis investigates the effect of the surface charges on wet-end chemistry, e.g. through the interaction of cationic polyelectrolytes with the fiber surface charges, and on the network strength of pulp suspensions. The polyelectrolyte titration method was used to investigate the interaction of the fiber charges with cationic polyelectrolytes. Techniques were developed to fluorescent label the adsorbing cationic polyelectrolyte in order to visualize the adsorption behavior. Fluorescent confocal laser scanning microscopy (CLSM) was used to determine the extent to which the cationic polyelectrolyte adsorbs into the porous fiber wall. It was shown that the polyelectrolyte charge density limits the adsorption to the surface under electrolyte-free conditions. Adsorption into the fiber wall only occurs for two conditions: 1) if the molecular mass is sufficiently low or 2) the electrolyte concentration is high enough to screen the charges along the polyelectrolyte backbone but not the interactions between the polyelectrolyte and the fiber charges. Aside from the polyelectrolyte properties, the fiber charge density contributes to the adsorption behavior of cationic polyelectrolytes. The fiber charge profile was altered by bulk and surface carboxymethylation. The electrolyte concentration at which a deviation from 1:1 stoichiometry occurs was shown to be dependent on the amount of surface charges, such that the deviation in stoichiometry occurs at a higher electrolyte concentration for pulps having a higher surface charge. A hypothesis was developed to test the conditions at which the deviation in adsorption stoichiometry occurs, which was defined as the critical electrolyte concentration (CEC). It was found that the CEC corresponded to the electrolyte concentration at which the distance between the fiber charges was on the order of the Debye length. Electron spectroscopy for chemical analysis (ESCA) was used as an independent calibration procedure to validate for which a 1:1 stoichiometry occurs. The analysis with ESCA agreed well with the polyelectrolyte titration method for measurement of fiber surface charges. When measured under appropriate conditions, i.e. electrolyte concentration and molecular properties, the fiber surface charge can accurately be measured by the polyelectrolyte titration method. The charge profiles of various pulp types and treatments were also examined. Having been established as a valid technique, the polyelectrolyte titration method was again used to measure the surface charge while conductometric titration was used to measure the total charge content. The amount of bulk and surface charges vary depending on the pulping method and type of wood, although the ratio between the bulk and surface charge (i.e. the charge ratio) is similar for chemical pulps. The mechanical pulp has a higher charge ratio because it contains more fines material than chemical pulp. Bleaching of the chemical pulp decreases the amount of bulk and surface charges, although the charge ratio remains essentially constant. However, methods such as beating or carboxymethyl cellulose (CMC) grafting are available to increase the charge ratio. The effect of the charge profile on fiber-fiber interactions was studied on both a microscopic and macroscopic level. Colloidal probe microscopy (CPM) was used to investigate the microscopic interactions between two cellulose surfaces. Cellulose surfaces, prepared by spin-coating a dissolving pulp onto silica, were used to model the fiber surface, which is too rough for surface force measurements. The charge density of the model surface was increased by CMC grafting. Results showed that increasing the surface charge density created large electrosteric repulsions, due to CMC the chains protruding out from the surface. These interactions on the microscopic scale affect the fiber network strength, which was measured with a parallel plate rheometer. When the repulsion is increased between the fibers, caused by the increase in the surface charge, fiber flocs break apart more easily due to a reduced friction between the fiber surfaces. The forces acting on the fiber network can also be mechanical in origin. The fiber length and flexibility were altered in order to study the influence of mechanical surface linking and elastic fiber bending on the fiber network strength. Using the storage modulus (G’0) as a measure of fiber network strength, longer fibers were found to create a stronger network due to an increased amount of fiber contacts. Flexible fibers have a lower network strength than stiff fibers because the fibers come to rest in a less strained position such that the the influence of elastic fiber bending on the fiber network strength is predominant.
|
|
| 4. |
|
|
| 5. |
- Malmsten, Martin, et al.
(författare)
-
Colloids and surfaces in biology
- 2010
-
Ingår i: Current Opinion in Colloid & Interface Science. - : Elsevier BV. - 1359-0294 .- 1879-0399. ; 15:6, s. 393-394
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
|
| 6. |
- Malmsten, Martin, et al.
(författare)
-
Editorial overview : Colloids and surfaces in biology
- 2018
-
Ingår i: Current Opinion in Colloid & Interface Science. - : ELSEVIER SCIENCE LONDON. - 1359-0294 .- 1879-0399. ; 38, s. A1-A3
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
|
| 7. |
- Zhang, Jianming, et al.
(författare)
-
Novel evaluation method of neutron reflectivity data applied to stimulus-responsive polymer brushes
- 2008
-
Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 4:3, s. 500-509
-
Tidskriftsartikel (refereegranskat)abstract
- Neutron reflectivity (NR) measurements have been performed on stimulus-responsive polymer brushes containing N-isopropylacrylamide (NIPAAM) at different temperatures and contrasts using two different brush samples of roughly the same grafting density and layer thickness. The NR data were analyzed using a novel method employing polymer density profiles predicted from lattice mean-field theory augmented with a polymer model to describe polymer solubility that decreases with increasing temperature. The predicted density profiles at the different temperatures were self-consistent with the experimentally observed profiles; hence the experimental data lend credibility to the theory. We found that the brush thickness decreased from 220 to 160 nm and the polymer volume fraction increased from 55 to 75% when increasing temperature from 293 to 328 K. The new evaluation approach involved significantly fewer independent fitting parameters than methods involving layers of uniform densities. Furthermore, the approach can straightforwardly be extended to analyze neutron reflectivity data of grafted, weakly charged polymers that display pH-sensitive behaviour and also to block copolymers and to surfaces with adsorbed polymers. We propose that such accurate model calculations provide a tool to interpret results from NR experiments more effectively and design neutron reflectivity experiments for optimal outcome.
|
|
