Biomedical subjects

B M Steels

Publications and source records attributed to B M Steels.

3 recordsLinked to original sources

PubMed2000

Analysis of compression of polymer mushrooms using self-consistent field theory.

A number of new technologies, including new-generation biomaterials and chromatography resins, are based on passivation and modification of surfaces by terminally attaching polymer chains to the surface. However, little is known about these systems at the molecular level. In this work the compression of a single end-grafted polymer chain (or mushroom) by a disc of finite radius was investigated using a self-consistent field (SCF) lattice model. In accordance with results predicted using scaling theory [Subramanian et al., Europhys. Lett. 29 (1995) 285 and Macromolecules 29 (1996) 4045], the compressed chain undergoes a smooth escape transition. However, under the assumption of angular symmetry, a first-order escape transition of the end-grafted chain is not observed, suggesting that the formation of a tether is required for the predicted phase transition. Segment density distributions and compression energies are calculated in a cylindrical lattice. The energy required to compress a chain increases monotonically as the disc is moved closer to the surface and becomes independent of chain length at strong compressions where the work of compression involves only confinement of the tether joining the escaped chain fraction to the grafting point.

Polymers↗

PubMed2000

Analysis of brush-particle interactions using self-consistent-field theory.

Non-specific protein adsorption can be reduced by attaching polymer chains by one end to a sorbent surface. End-grafted polymer modified surfaces have also found application in size-based chromatographic bioseparations. To better understand how to tailor surfaces for these applications, a numerical SCF model has been used to calculate theoretical results for the polymer density distribution of interacting polymer chains around a solute particle positioned at a fixed distance from a surface. In addition, the excess energy required to move the particle into the polymer chains (interaction energy) is calculated using a statistical mechanical treatment of the lattice model. The effect of system variables such as particle size, chain length, surface density and Flory interaction parameters on density distributions and interaction energies is also studied. Calculations for the interaction of a solute particle with a surface covered by many polymer chains (a brush) show that the polymer segments will fill in behind the particle quite rapidly as it moves toward the surface. When there is no strong energetic attraction between the polymer and solute we predict that the interaction energy will be purely repulsive upon compression due to losses in conformational entropy of the polymer chains. Above a critical chain length, which depends upon particle size, a maximum in the force required to move the particle toward the surface is observed due to an engulfment of the particle as chains attempt to access the free volume behind the particle. If an attraction exists between the polymer and solute, such that a minimum in the interaction energy is seen, the optimum conditions for solute repulsion occur at the highest surface density attainable. Long chain length can lead to increased solute concentration within the polymer layer due to the fact that an increased number of favourable polymer-solute contacts are able to occur than with short chains at a similar entropic penalty.

Entropy↗

PubMed2000

Size exclusion chromatography does not require pores.

Separation of macromolecules on the basis of their molecular weight by size exclusion chromatography has long been considered to be caused by the geometry-dependent partition of macromolecules between a continuous phase and the porous interior of a gel or cross-linked bead. The volume of a pore accessible to a solute is limited by its relative dimensions, so larger molecules will have access to a smaller volume and will remain in a bead for a shorter time than smaller solutes. Our recent alternate picture proposes that the partition coefficient can be calculated from a thermodynamic model for the free energy of mixing of the solute with the gel phase. Size-dependent exclusion caused by the unfavorable entropy of mixing associated with the partition is predicted; the magnitude of the effect is modified by enthalpic interactions between the solute and the gel phase. This concept is extended here to describe the partition of macromolecules into a layer of terminally attached polymer chains grafted onto a solid bead. Both simple mean field and self-consistent field theory calculations predict size-dependent entropic exclusion. Experimental results obtained with neutral polymer chains grafted onto solid polystyrene latex beads confirm the predictions.

Chromatography↗