Surfaces - The Polysiloxanes - page 124

An example of attachment by adsorption involves making PDMS sur-
faces more hydrophilic by addition of hydrophobins (a family of small,
cysteine-rich and amphipathic fungal proteins). 75 A more complicated ad-
sorption approach is illustrated by the adsorption of segments of a graft
copolymer onto a PDMS surface that had been treated with an oxygen or
ammonia plasma. 76, 77 The PDMS surface then contains hydroxyl-carboxyl
groups that can attract the lysine segments of a poly(L-lysine- graft -
poly(ethylene glycol)), leaving hydrophilic ethylene glycol segments cover-
ing the PDMS surface. This adsorption technique can be extended to an
embedding approach, as illustrated in figure 6.2. The PDMS sequences of a
PDMS-poly(ethylene oxide) block copolymer embed into a PDMS elasto-
mer. 78 The ethylene oxide segments are then relegated to the surface,
giving the PDMS a hydrophilic coating.
In a gas phase approach, undecenyl-trichlorosilane vapor was con-
densed onto a silicon wafer surface to form a self-assembled monolayer
film whose surface was composed of olefin groups. 79 hese groups react
with SiH-functionalized PDMS in a hydrosilylation reaction to give an un-
usually thin coating of PDMS.
A hybrid approach involves adsorption followed by chemical bonding.
Specifically, a vinyl-ended oligonucleotide was adsorbed onto a PDMS sur-
face and then a hydrosilylation reaction used to bond it to the surface
chemica l ly. 80 This modification can greatly improve separation assays or
the performance of biochips by enhancing target hybridization.
Whether bonded or physically adsorbed or embedded, surface poly-
mers dangle from the polymer surface, as do the chains in any PDMS
elastomer. Such chains are important with regard to mechanical prop-
erties such as adhesion, adhesion hysteresis, 81 and tack (“stickiness”). 82
Their rheological and dielectric behavior have been modeled using mo-
lecular dynamics. 83
Figure 6.2:
Modifying a surface using a diblock copolymer in which one block (thin lines) is the same
as the substrate polymer, and the other block (rejected to the surface) has the desired
amount of hydrophilicity or hydrophobicity.