Biomedical Engineering Reference
In-Depth Information
was confirmed by DSC analysis which exhibited two distinct Tgs (- 128.1
◦
C
and - 122.3
◦
C).
Many polyacrylate/PDMS IPNs are synthesized via such a swelling
method. Some of these systems are silicone rubber/hydrogels. They are
extensively used when the application requires hydrophilicity of the rub-
ber. With this end, the most reported monomers are methacrylic acid and
2-hydroxyethylmethacrylate. Turner et al. [76, 77] developed a typical synthe-
sis to form a PDMS/polymethacrylic acid (PMAA) IPN via a two-step process.
First, a PDMS film was prepared by hydrosilylation of vinyl-terminated PDMS
by tetramethylcyclotetrasiloxane (a hydride crosslinker). Then, the mixture of
components plus a platinum catalyst were sprayed onto an appropriate sup-
port and afterwards cured at 55
◦
Ctoobtaina“pre-IPNfilm”.Theauthors
studied the influence of the second step (MAA polymerization) on the mate-
rial morphologies and properties. Co-continuous structures are known to be
obtained at high concentrations of the host monomer and the so-called “sea-
island morphologies” (nucleation and growth phase separation mechanism)
are obtained for low concentrations of the host monomer. Co-continuous
structures lead to permeable materials whereas “sea-island morphologies”
lead to impermeable materials. Depending on the targeted applications, one
of these two morphologies is needed. The authors [77] prepared different
rubber-hydrogel IPN membranes and tested their permeation to water sol-
uble components (vitamin B12). They concluded that when the crosslinked
PDMS film is swollen by MAA monomer and crosslinker (triethylene gly-
col dimethacrylate and UV sensitive free radical initiator) and disposed onto
a support to be UV-cured, a gradient of concentration of the monomer in
the PDMS film occurred. This gradient can be attributed on the one hand
to the nature of the interface with the support (glass in this study which
has specific interactions with the MAA) and on the other hand with the air.
Moreover, some evaporation of the monomer undoubtedly occurred during
handling thus increasing the gradient. The IPNs in these cases were im-
permeable to water soluble components such as vitamin B12 since the low
concentration of monomer on one side of the film leads to a “sea-island
morphology” forming locally. These low MAA concentrations were observed
as expected either opposite the glass-IPN interface or at the air-IPN inter-
face. The authors developed an alternative method: the crosslinked PDMS
film was submerged into the mixture of monomer, initiator and crosslinker
and once it was swollen, it was UV-cured. In this case, no gradient was ob-
served and hence the membrane was permeable to vitamin B12. The influence
of initiator and crosslinking agent concentrations, UV light intensity and the
IPN morphology on the permeation properties were also studied [78, 79].
A similar material was patented [80] for contact lens applications since the
final material is sufficiently hydrophilic thanks to the PMAA and is also oxy-
gen permeable. Poly(2-hydroxyethyl methacrylate) was also combined with
PDMS to obtain a hydrogel/silicone IPN (poly(HEMA)/PDMS) [81]. This
