5.3. Makrolon A Polymer for Time-resolved
5.3.1. General Properties of Makrolon
measurements of the SPR and RIfS setups are based on sensitive layers, which
were prepared using a polycarbonate polymer. This polymer is commercially available
as Makrolon M2400 from Bayer AG, Leverkusen, and will be
further referred to as Makrolon. The structure of the monomer unit is shown
in figure 17. The interesting property of Makrolon
is the microporous structure of the glassy polymer. According to ,
the size of these pores follows a distribution with a mean pore size of 0.1
nm3 determined by the use of the positron annihilation lifetime spectroscopy
(PALS). The PALS detects the decay of positrons and exploits the effect that
the positrons are more probably present in the pores than in the polymer matrix
due to the lower electron density in the pores ,.
figure 17: Structure of the monomer
unit of the polymer Makrolon M2400.
18, the shift of the SPR wavelength is shown while the device was exposed
to an alternating sequence of synthetic air and relative saturation pressures
of methanol of 0.31, 0.62 and 0.80. At these high concentration levels, the
sensor signal increases rapidly at the beginning of the analyte exposure meaning
that the refractive index of the sensitive layer increases. This increase of
the refractive index can only be explained by methanol (nD20=1.329) sorbing into the micropores
of the polymer and replacing air (nD20=1)
in these polymers as the sorption of methanol into the polycarbonate matrix
(nD20=1.58) would decrease the
refractive index of the sensitive layer. The molecules of methanol have less
volume (0.068 nm3) than the mean size of the pores of Makrolon and
therefore can easily sorb into the micropores.
figure 18: SPR sensor response
of the SPR device when exposed to different relative saturation pressures of
methanol of 0.31, 0.62 and 0.80 and alternating to synthetic air.
a long-term exposure of Makrolon to methanol (see figure
18), the refractive index decreases. This decrease reinforces with higher
concentrations of methanol and can be explained by two effects. The first effect
is based on an expansion of the micropores of the polycarbonate matrix when
these micropores are occupied by analyte molecules resulting in a decrease of
the refractive index of the sensitive layer. When the exposure to methanol stops
the refractive index decreases rapidly as the methanol molecules quickly desorb.
After the exposure to high concentrations of methanol, the refractive index
decreases below the initial value, as the pores and with it the Makrolon matrix
are still expanded. The initial refractive index is gradually reached while
the Makrolon layer is shrinking to the initial thickness. The expansion of the
Makrolon matrix can only be detected when exposed to analyte for a long time.
This effect allows the modification of the kinetics of sorption and desorption
for bigger molecules and is exploited in section 5.3.4.
The second effect is an unspecific sorption of the analyte into the polymer
matrix besides of the specific sorption into the pores resulting in a swelling
of the matrix. As the analyte has a lower refractive index than the polymer
matrix, the shift of the SPR wavelength decreases. This unspecific sorption
is a Henry type sorption (see section 3.2) whereas the
sorption into the micropores can be considered as a Langmuir sorption (see section
3.2). In figure 19 the isothermal calibration
curve for the sorption of methanol into Makrolon is shown. The curve shows the
typical shape of the Langmuir sorption and hardly any portion of Henry sorption
can be detected whereas the isothermal calibration curves of R22 and R134a show
a significant Henry sorption (see figure 7
in section 3.2 and additionally the discussion in section
3.2). At the moment, further research is done by combining measurements
of an ellispometric device, an SPR device and a RIfS device for distinguishing
the sorption into the pores from the sorption into the matrix and for distinguishing
the long-term expansion of the matrix from the swelling due to analyte sorption.
First experiments allow following preliminary conclusions:
The Henry sorption plays only a role if the analyte molecules are so big
that a specific Langmuir sorption into the pores is not possible or very slow,
whereby the total amount of the Henry sorption is always very low due to the
high glass temperature of Makrolon.
For exposure times less than 30 minutes, the expansion of the polymer matrix
can be neglected.
The sorption of analyte is reversible and the layer is highly stable. For
example, the measurements, which are partly shown in figure
18, had a mean drift of 5.3*10-4 nm/h, which corresponds to
only 0.01 % of the maximum sensor response.
figure 19: Isothermal calibration
curve after 30 minutes of exposure to different concentrations of methanol.
The standard deviations of 3 measurements are represented by the error bars.