Chemistry Reference
In-Depth Information
6.7 MIGRATION
6.7.1 Surface Segregation
Because of its low surface free energy, PDMS tends to migrate to sur-
faces and interfaces. For example, in blends of PDMS with low-density
polyethylene
109
or linear low-density polyethylene,
109,
110
the surfaces
are enriched with the hydrophobic methyl groups from the PDMS
chains. The same is true for blends of PDMS with polypropylene
109
and
poly(3-hexylthiophene).
111
Similar enrichment of siloxane units occurs at the surfaces of block copo-
lymers or even random copolymers containing PDMS sequences. Examples
are poly(dimethylsiloxane-
b
-ethylene oxide)
112
and poly(dimethylsiloxane-
b
-imide) multi-block copolymers,
113
poly(dimethylsiloxane)/imine copoly-
mers,
114
and poly(urethane-dimethylsiloxane) anionomers.
115
Other mate-
rials, such as long-chain alkenes
116
and highly branched polymer additives
117
exhibit surface segregation. Much of the information on these changes
in surface composition has been obtained by measurements of contact
angles.
6.7.2 Recovery and Restructuring
In some cases attempts to make PDMS surfaces more hydrophilic were
only temporarily successful. Recovery or restructuring can occur, with
methyl groups from the siloxane units migrating to the surface to lower
the free energy of the system. Restructuring is facilitated by the high flex-
ibility of the polysiloxane backbone.
Studies of this restructuring process have been carried out on PDMS
with surfaces made hydrophilic by UV/ozone treatment,
118
oxygen
plasma,
10
or corona discharge.
119
In these cases, changes in surface compo-
sition were determined using chemical force microscopy,
120
Fourier trans-
form infrared spectroscopy and x-ray photoelectron spectroscopy,
10
or
atomic force microscopy.
119
Similar results have been obtained on PDMS elastomers. Examples are
tetraethoxysilane-cured hydroxyl-terminated PDMS, methylhydrogensi-
loxane-cured vinyl-terminated PDMS, and methylhydrogensiloxane-cured
vinyl-terminated poly(dimethylsiloxane-
co
-diphenylsiloxane) chemical
copolymer.
121
Restructuring has also been observed in un-cross-linked co-
polymers, for example PDMS-
co
-polystyrene,
121
using sum frequency gen-
eration vibrational spectroscopy.
