Biomedical Engineering Reference
In-Depth Information
SAXS patterns were collected for the acid-catalyzed samples as
shown in Fig. 3.7. The intensity
I
(
q
) was measured over the
q
range
-1
-1
p
q
l
0.11 nm
<
q
< 3.27 nm
, where
q
= 4
sin
/
is the scattering
vector, 2
θ
is the scattering angle, and
λ
is the wavelength of incident
radiation.
The SAXS patterns for all samples contain a Guinier region at
high
(given by the plateau/hump in the data), which is expressed
by equation
q
2
2
I q
( ) exp(
R q
/3)
(3.7)
G
This equation is valid for
qR
< 1.2, yielding the radius of gyration
G
R
of the pores within the aerogel structure determined from the
slope of a plot of ln
G
2
of the pores
is shown in Table 3.2. The minimum micropore size is obtained
in CA4.8; there is a small reduction in the micropore size with
decreasing acetic acid content in the precursors with the exception
of CA5.2.
I
(
q
) vs.
q
. The radius of gyration
R
G
Table 3.2
The radius of gyration
of the pores within CAs derived from
different pH value precursors as calculated from SAXS
R
G
CA4.1
CA4.5
CA4.8
CA5.2
R
G
(nm�
0.68 ± 0.02 0.66 ± 0.03 0.59 ± 0.03 0.75 ± 0.02
It was suggested that acetic acid cannot fully activate the
aromatic rings since there is a lower acetic acid concentration in the
precursors, which leads to lower cross-linking, hence larger pores
formed, i.e., CA5.2. The micropores in the acid-catalyzed CAs are
quite similar to those in CAs synthesized without any catalysts [26]
as they contain both spherical pores and slit pores. The micropores
in CAs have been previously described as slit shaped [39]; however,
previous SAXS analysis on the CA used a spherical model to explain
the microporosity [40]. The difference in pore size given either
a spherical or slit/cylindrical pore shape model is related to the
assumed aspect ratio of the cylindrical pore [26]. Using a model for
pore shape requires information about the average micropore shape
within the entire CA structure.
The BET surface area was measured on aerogel powders that
were first out gassed at 110°C. The BET surface areas of the acid
catalyzed CAs were determined as 1420 m
2
2/
/g for CA4.1, 1798 m
g
