Membrane Gas Separation

4.3
Free Volume Analysis 
4.3.1
Photoisomerization Procedures and UV - Visible Characterization 
The trans – cis isomerization reaction (Figure 4.2 ) of all chromophores was carried out by 
irradiation with a UV - visible lamp (Helios Italquartz, model GRE 500W) selecting a 
wavelength of 350 nm with a quartz fi lter (OptoSigma). The back reaction was induced 
by irradiation at 440 nm, using a blue glass fi lter (OptoSigma). The lamp to fi lm distance 
was 15 cm. The band pass fi lters were placed directly onto the sample surface to avoid 
irradiation of the fi lms by unfi ltered scattered light from the sides, which may compromise 
the effi ciency of the photoisomerization. The UV - visible absorption spectra of the mem-
branes before and after irradiation were recorded on Shimadzu UV 1601 spectrophotom-
eter, interfaced to a personal computer for the data recording and elaboration.
4.3.2
Probes and Spectrophotometric Analysis 
Preliminary studies of the trans – cis photisomerization kinetics recommended an irradia-
tion time of at least 15 minutes to reach the photostationary state and to have the maximum 
photoconversion for all dyes. This time was therefore used in all photoisomerization 
experiments. Molar absorption coeffi cients,  
ε

trans
  and  
ε

cis
  , of the photochromic molecules 
dispersed in the polymer matrix were calculated from absorbance measurements, using 
the Lambert – Beer law:
A
C
d
= ⋅ ⋅
ε
(4.1)
where A is the value of the absorbance peak at the maximum absorption wavelength of 
the dye,  
λ
  , while d is the path length of the UV beam in the sample, i.e. the membrane 
thickness, and C is the dye concentration, calculated from the mass ratio of the dye and 
the polymer in the original casting solution. All absorbance measurements were carried 
out at ambient temperature. The cis isomer fraction, Y , in the fi lms is a measure of the 
trans – cis isomerization effi ciency, and is given by:
Y
A A
dark
cis
trans
= −
−
1
1
ε ε
(4.2)

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Amorphous Glassy Perfl uoropolymer Membranes of Hyfl on AD®
69
where
A
dark
is the initial peak absorbance with only
trans 
isomer present,
A 
is the 
actual peak absorbance in the photostationary state. In the present work a total of four 
azobenzene and two stilbene probe molecules with different substituents were used. Their 
molecular structures in the form of the trans isomer and the van der Waals structures of 
both isomers are shown in Table 4.1 . 
An example of the absorption spectra of a membrane containing unsubstituted azoben-
zene before and after isomerization is given in Figure
4.5 
. It must be noted that 
the isomerization is completely reversible over various cycles of isomerization with a 
wavelength of 350 nm and 440 nm, respectively.
The details of the spectrophotometric analysis and the interpretation of the results have 
been published recently [27] . The quantitative results of the cumulative free volume 
distribution, i.e. the relative trans – cis isomerization of the fi lms compared to the solution 
are listed in Table 4.2 as a function of the probe size.