Chemistry Reference
In-Depth Information
PDMS, which is often used because of its very high permeability to oxygen,
which is required for metabolic processes within the eye.
13-15
Such lenses
do not feel comfortable however because they do not float properly on the
aqueous tears that coat the eye. There are a number of ways to modify the
surfaces.
16-18
There is even a way to make “unreactive” silicones react with
inorganic surfaces.
19
In some applications it is useful to have hydrophilicity in the bulk of
the polymer instead of just at the surface. One way of doing this is by si-
multaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains
and hydrophobic PDMS chains.
20
Another way is to make a PDMS network
with a trifunctional organosilane R'Si(OR)
3
end linker that contains a hy-
drophilic R' side chain, such as a polyoxide.
21
Treating only the surfaces is
another possibility, for example, by adding hydrophilic brushes by vapor
deposition/hydrolysis cycles.
22
Such hydrophilic polysiloxanes can also
serve as surfactants.
23
It has also been possible to make radially layered
copolymeric dendrimers with hydrophilic polyamidoamine interiors and
hydrophobic organosilicon exteriors.
24
These ideas are being extended to materials that have high repellencies
to a variety of liquids, including oils, solvents, and other low surface-
energy liquids.
25
Such “superomniphobic” surfaces have been generated
on PDMS substrates.
26
6.2.2 Superhydrophilicity and Superhydrophobicity
There are also techniques for preparing surfaces of greater hydrophilicity
(“superhydrophilicity”)
27, 28
or increased hydrophobicity (“superhydropho-
bicity”).
29-43
More specific terminology calls low-contact-angle materials
hygrophilic and high-contact-angle materials hygrophobic.
32
Some of
these materials display multiresponsive surfaces that can change revers-
ibly between hydrophilicity or superhydrophilicity and superhydropho-
bicit y.
27,
44,
45
Polymers showing transitions of this type are examples of
stimuli-responsive materials, which are of increasing interest in a number
of emerging applications.
46
6.3 CHARACTERIZATION
6.3.1 Contact Angles
Measurement of advancing and receding contact angles of a water droplet
on a polymer surfaces the standard way to characterize interactions
