| 1. |
- Andersson, Martin, et al.
(författare)
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Single microgel particle studies demonstrate the influence of hydrophobic interactions between charged micelles and oppositely charged polyions.
- 2005
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 21:9, s. 3773-3781
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Tidskriftsartikel (refereegranskat)abstract
- The binding of two cationic surfactants, dodecyltrimethylammonium bromide (DoTAB) and N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium bromide (HFDePB), to covalently cross-linked sodium poly(styrenesulfonate) (PSS) microgels has been investigated by means of micromanipulator-assisted time-resolved light microscopy on single gels. It is demonstrated that repeated measurements on the same microgel under conditions of controlled liquid flow give highly reproducible results. The two surfactants are found to behave very differently with respect to degree of swelling, surfactant distribution in the gels, both during shrinking and at equilibrium, and kinetics of volume changes induced by them. The main difference is attributed to the presence of a hydrophobic interaction between PSS and the DoTAB micelles, absent in the case of HFDePB. Kinetic shrinking curves are recorded and analyzed using a model for steady-state transport of surfactant between the solution and the gels. Aggregation numbers for DoTAB in PSS solutions obtained from fluorescence quenching measurements are presented. A strong dependence on the surfactant-to-polyion concentration ratio is observed. Relations between surfactant binding isotherms, phase diagrams for linear polyelectrolyte/surfactant/water systems, and the binding to gels are discussed.
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| 2. |
- Bergman, Kristoffer, et al.
(författare)
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Hyaluronic acid derivatives prepared in aqueous media by triazine-activated amidation
- 2007
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 8:7, s. 2190-2195
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Tidskriftsartikel (refereegranskat)abstract
- A method is presented for the preparation of hyaluronic acid derivatives obtained through triazine-activated amidation. A number of amines were successfully reacted with hyaluronic acid carboxyl groups using 2-chloro-4,6-dimethoxy-1,3,5-triazine as an activating species in a mixture of water and acetonitrile under neutral conditions. By varying the amount of triazine reagent, it was possible to control the degree of modification. Depending on the amine chosen, degrees of modification ranging from 3 to 20% were obtained when using 0.5 equiv of the triazine to hyaluronic acid carboxyl groups. The possibility to perform the reaction in a mixture of water and acetonitrile facilitates the introduction of a wide range of both hydrophilic and hydrophobic amines. Triazine-activated amidation appears to be a highly versatile, controllable, and relatively mild technique for modification of hyaluronic acid, and we predict that it will be useful in the design of novel hyaluronic acid based biomaterials.
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| 3. |
- Carlsson, Tobias, et al.
(författare)
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Algorithm for generating a Brownian motion on a sphere
- 2010
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Ingår i: Journal of physics A: Mathematical and theoretical. - : IOP Publishing. - 1751-8113 .- 1751-8121. ; 43:50, s. 505001-
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Tidskriftsartikel (refereegranskat)abstract
- We present a new algorithm for generation of a random walk on a two-dimensional sphere. The algorithm is obtained by viewing the 2-sphere as the equator in the 3-sphere surrounded by an infinitesimally thin band with boundary which reflects Brownian particles and then applying known effective methods for generating Brownian motion on the 3-sphere. To test the method, the diffusion coefficient was calculated in computer simulations using the new algorithm and, for comparison, also using a commonly used method in which the particle takes a Brownian step in the tangent plane to the 2-sphere and is then projected back to the spherical surface. The two methods are in good agreement for short time steps, while the method presented in this paper continues to give good results also for larger time steps, when the alternative method becomes unstable.
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| 4. |
- Carlsson, Tobias, et al.
(författare)
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Brownian dynamics of a compressed polymer brush model : Off-equilibrium response as a function of surface coverage and compression rate
- 2011
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:35, s. 16084-16094
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Tidskriftsartikel (refereegranskat)abstract
- We study the compressive behaviour of a polymer-covered surface (i.e., a "polymer brush'') using Brownian dynamics simulations. The model consists of grafted chains with variable flexibility, variable intra-and inter-chain interactions, as well as different surface coverage. We discuss the polymer brush response to confinement by considering variable rates of compression under a hard plane. Our results show a small degree of inter-chain entanglement, regardless of whether the interaction is attractive or merely excluded volume. We observe that the molecular shape depends strongly on the surface coverage. Dense brushes exhibit a limited degree of lateral deformation under compression; instead, chains undergo a transition that produces a local patch with near-solid packing. This effect due to surface density can be undone partially by increasing the attractive nature of the chain interaction, by modulating the rate of compression, or by allowing "soft anchoring'', i.e., the possible Brownian drift of the grafting bead on the surface. We have also studied the polymer brush relaxation while maintaining the compressing plane, as well as after its sudden removal. We find evidence that also the relaxation depends on surface density; dense brushes appear to be configurationally frustrated at high compression and are unable to undergo swelling, regardless of the pressure applied.
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| 5. |
- Carlsson, Tobias
(författare)
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Brownian Dynamics Simulations of Macromolecules : Algorithm Development and Polymers under Confinement
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I have used computer simulations to study the structure and dynamics of grafted polymers during confinement. These systems are of importance for understanding e.g. colloidal stability and surface coatings. We have used Brownian dynamics simulations with the polymers modeled as discrete wormlike chains allowing for a variable persistence length as well as different non-bonded interactions. The size and shape of the chains are characterized by the radius of gyration and the degree of oblateness/prolateness, and the entanglement is followed by calculating the mean overcrossing number. Starting in the dilute regime with a single polymer mushroom we have investigated how the rate of compression and solvent quality effects the behaviour of a compressed chain. In the brush regime, we investigated how the surface coverage effects the behaviour during compression. For low coverages the chains have the possibilty to increase their lateral extension during confinement but in general, the chains have a low inter-entanglement, as they strive to keep their integrity during the confinement process.To go from a polymer brush to the construction of a connected network, we have developed a method to construct a closed network without using periodic boundary conditions by building the network on a sphere in R4. In this way we avoid the restrictions of periodicity at the cell boundaries. We finally also show how to develop the idea of using spherical boundary conditions, by presenting a novel algorithm for simulating diffusion on a spherical surface. The method is more stable and allows for larger time steps, compared to commonly used methods in computer simulations.
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| 6. |
- Carlsson, Tobias, et al.
(författare)
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Off-equilibrium response of grafted polymer chains subject to a variable rate of compression
- 2011
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:24, s. 11757-11765
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Tidskriftsartikel (refereegranskat)abstract
- We present Brownian dynamics simulations of single grafted semiflexible chains (i.e., "polymer mushrooms'') with varying persistence lengths, intra-chain interactions, and subject to confinement. The results from different rates of compression are presented in the cases of an approaching infinite plane and a paraboloid tip. We discuss the different behaviour observed for grafted chains with strong and weak self-attraction (i.e., "hard'' and "soft'' polymer mushrooms). In both cases the effect on the size and shape is more pronounced for a slow compression rate, especially for "hard mushrooms''. We have also studied the relaxation of the chain while the compressing plane is maintained, and when it is removed suddenly. We find that the response depends strongly on the time allowed for relaxation in the compressed state. When using instead a paraboloid tip, the overall effects are similar yet less pronounced because the chain can dodge the confining object via an "escape transition.''
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| 7. |
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| 8. |
- Edvinsson, Tomas, et al.
(författare)
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Cluster identification and percolation analysis using a recursive algorithm
- 1999
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Ingår i: Molecular Simulation. - : Informa UK Limited. - 0892-7022 .- 1029-0435. ; 23:3, s. 169-190
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Tidskriftsartikel (refereegranskat)abstract
- A recursive algorithm for sampling properties of physical clusters such as size distribution andpercolation is presented. The approach can be applied to any system with periodic boundaryconditions, given a spatial definition of a cluster. We also introduce some modifications in thealgorithm that increases the efficiency considerably if one is only interested in percolationanalysis. The algorithm is implemented in Fortran 90 and is compared with a number ofiterative algorithms. The recursive cluster identification algorithm is somewhat slower than theiterative methods at low volume fraction but is at least as fast at high densities. The percolationanalysis, however, is considerably faster using recursion, for all systems studied. We also notethat the CPU time using recursion is independent on the static allocation of arrays, whereas theiterative method strongly depends on the size of the initially allocated arrays.
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| 9. |
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| 10. |
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