Nanoscale pores introduced into paper via mesoporous silica coatings using sol-gel chemistry.

Mesopores, with diameters between 2 and 50 nm, not only increase the specific surface area, but also generate hierarchically porous materials with specific properties such as capillary fluid transport, ion specific pore accessibility, or size exclusion. Paper is a strongly hierarchical, porous material with specific properties, such as capillary force-driven fluid transport. However, paper fibers change their morphology during the initial step of wood disintegration. This results in changes of the porous fiber structure. In particular paper fibers loose their mesopores during the final drying step in the fabrication process. Here, we investigate silica mesopore formation in paper by sol-gel chemistry and evaporation induced self-assembly to specifically introduce and rationally design mesopore formation and distribution in cotton linter and eucalyptus sulfate paper sheets. We demonstrate the importance of synchronizing the solvent evaporation rate and capillary fluid velocity to ensure mesopore formation as well as the influence of the fiber type and sol-gel solution composition. The combination of argon and krypton sorption, SAXS, TEM and CLSM provides systematic analysis of the porous structure and the silica distribution along the cellulose paper fiber length and cross-section. These results provide a deeper understanding of mesopore formation in paper and how the latter is influenced by paper fluidic properties.

Surface-attached PDMAA-GRGDSP hybrid polymer monolayers that promote the adhesion of living cells.

Peptide-polymer hybrid molecules are being introduced, where one part of the molecule (i.e., the peptide) promotes the adhesion of living cells, whereas the other part of the molecule (i.e., the synthetic polymer) is known to prevent cell adhesion. The hybrid copolymer, poly(dimethylacrylamide) (PDMAA)-glycine-arginine-glycine-aspartic acid-serine-proline (GRGDSP) was synthesized by first preparing an initiator-modified peptide and in a second step growing the PDMAA block directly off the peptide through atom transfer radical polymerization (ATRP). The PDMAA block length can be varied by adjusting appropriate polymerization conditions, thereby changing progressively the amount of the cell-repelling part of the molecule. The hybrid copolymer was further used to prepare surface-attached peptide-polymer monolayers at planar solid glass substrates through a photochemical immobilization process. By blending of the hybrid copolymer with PDMAA homopolymer (i.e., without peptide), the apparent peptide film concentration can be varied in a very simple manner. The adhesion of human skin fibroblast cells in serum-free medium was investigated as a function of the amount of peptide-polymer in the solution used for film preparation. Cells do not adhere to a pure PDMAA monolayer; however, already 0.02 wt % of peptide in the film is enough to induce cell adhesion, and 0.1 wt % promotes stress-fiber formation within adherent cells. Using lithographical means, chemically micropatterned peptide-polymer films were prepared that allow for a spatial control of the adhesion of living cells and thus they constitute a simple platform for the design of live-cell biochips.

Electrolyte-induced collapse of a polyelectrolyte brush.

We have investigated the electrolyte-induced collapse of a polyelectrolyte brush covalently attached to a planar solid surface. Positively charged poly-4-vinyl [N-methyl-pyridinium] (MePVP) brushes were prepared in situ at the surface by free radical chain polymerization using a surface-immobilized initiator monolayer ("grafting from" technique) and 4-vinylpyridine as the monomer, followed by a polymer-analogous quaternization reaction. The height of the brushes was measured as a function of the external salt concentration via multiple-angle null ellipsometry. As predicted by mean-field theory, the height of the MePVP brushes remains unaffected by the addition of low amounts of external salt. At higher salt concentrations the brush height decreases. The extent to which the brush shrinks strongly depends on the nature of the salt present in the environment. MePVP brushes collapse to almost the dry layer thickness upon the addition of potassium iodide to a contacting aqueous medium. In contrast, the collapse of MePVP brushes having bromide or chloride counterions is much less pronounced. These brushes remain in a highly swollen state even after large amounts of salt have been added to the solution.