Membrane Gas Separation

partial molar enthalpy of mixing,
Δ
H
m
, depends on the size of the sorbent and it was 
found that a plot of
Δ
H
m
against probe size shows a minimum for the sorbent size which 
best corresponds to the average size of a free volume element.
•
129
Xe - NMR spectroscopy [6] . This method evaluates the chemical shift of xenon - 129, 
dissolved in the polymer matrix. The chemical shift of the
129
Xe atoms depends on the 
interaction of the electron cloud with those of the atoms in the surrounding polymer. 
This interaction is stronger the smaller the size of the FVE in which the
129
Xe probe is 
located.
• Electrochromic probing [7] relies on the detection by UV spectroscopy of the reorienta-
tion of suitable electrochromic probes, usually azo or other dye molecules, dissolved 
in the polymer matrix and then oriented in an electric fi eld. The rate and kinetics of the 
reorientation after release of the electric fi eld is related to the probe molecule size and 
to the available free volume size.
• The spin probe method was one of the fi rst methods used to evaluate the free volume 
in polymers [1,7] . It is based on the principle that the rotational frequency of spin 
probes, usually stable nitroxyl radicals such as TEMPO (2,2,6,6 - tetramethylpiperidine -
1 - oxyl) is sensitive to the free volume. The relatively complex correlation between 
spectral data and FV makes this method more suitable for qualitative comparison of 
different polymers than for quantitative analysis of the FV [1,8] .
• The photochromic probe method uses different probe molecules, usually substituted 
stilbenes and azobenzenes, and in this case the evaluation is based on the capacity of 
the probe molecules to undergo a trans – cis isomerization upon irradiation (Figure 4.2 ) 
[9] . This depends on the probe size and on the size of the FVE in which it is located. 
The probe molecules are dissolved in the polymer matrix, isomerization is induced by 
a strong UV lamp and the degree of isomerization is determined from the absorbance 
spectra before and after the isomerization.

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Membrane Gas Separation