Membrane Gas Separation

Tailoring Polymeric Membrane Based on Segmented Block Copolymers
229
based on poly(ethylene oxide) (PEO) exhibited excellent CO 
2
permeability, and many 
research groups demonstrated that they can effectively separate CO 
2
from other gases 
[35 – 42] . 
Poly(amide 
- 
b 
- 
ethylene oxide) copolymers were presented in 1990 as a promising 
membrane material [43] . These block copolymers were developed in 1972 but in 1981 
began to be used for commercial purpose under the trade name Pebax 
® 
, produced by 
ATOCHEM [44] (now ARKEMA). Another important group of segmented poly(ester)s 
used for membranes are block copolymers based on PEO and PBT (poly(butylene tereph-
thalate), known under commercial name of Polyactive 
® 
[45] . By changing the polyamide 
and polyether segment, molecular mass and the content of each block, the mechanical, 
chemical, and physical properties are nicely tuned as well [46] . 
There are many techniques to improve gas transport properties of the above - mentioned 
polymers, and the simplest way to do it is by physical incorporation of additives into the 
polymeric matrix (blending). Because the performance of the blend membranes is tightly 
related to the nature of additives (fi llers) and polymer characteristics, they must be 
carefully selected in order to obtain a novel membrane material with superior properties. 
Generally, the presence of additives can have three effects on membrane transport: they 
can act as molecular sieves (zeolites, carbon molecular sieves), they can disrupt the 
ordering of polymer chains, which is refl ected in a permeability increase (non - porous 
nanoparticles and plasticizer), and they can act as barrier reducing the permeability (e.g. 
clays) [47 – 50] . The incorporation of plasticizers in polymers appears to have a relatively 
complex infl uence on the permeability [51] . Plasticizers may operate as a spacer, separat-
ing the macromolecular chains (free volume increase), thus they lead to a decline of the 
intermolecular attractive forces increasing the mobility of them and facilitating the diffu-
sion of the adsorbed molecules by lowering the activation energy. By inserting plasticizer 
between macromolecular chains, the glass transition temperature of the polymer is 
decreased due to the local segmental motions [52] ; accordingly, the diffusion coeffi cients 
are changed. In the literature, there are many publications related to the effect of plasti-
cizer on morphology and gas transport properties of polymer blends [53 – 55] . 
Immiscible or partially miscible polymers (blends) often form separated phases, leading 
to a complex morphology. In many cases, the miscibility in polymer blends [56] is a direct 
result of enthalpy - of - mixing (  
Δ
 H
mix
) contributing to the overall free energy  
Δ
 G
mix
, which 
is the fundamental thermodynamic quantity that controls the miscibility. To obtain homo-
geneous blend (single phase), the free Gibbs energy  
Δ
 G
mix
must be
<
0. The free Gibbs 
energy for mixing per unit volume is given by the classical Flory – Huggins equation 
(Equation 12.1 ):
Δ
Φ
Φ
Φ
Φ
Φ Φ
G
V
R T
M
M
mix
A
A
A
B
B
B
B
A
B
= ⋅ ⋅
⋅
⋅
+
⋅
⋅
⎡
⎣⎢
⎤
⎦⎥
+ ⋅
⋅
ρ
ρ
χ
A
ln
ln
(12.1)
where  
χ
  is the interaction parameter, R is gas constant, T is the absolute temperature and 
 
ρ
  ,  
Φ
  and M are the density, volume fraction and molecular mass of the polymer, respec-
tively. This equation includes implicitly enthalpy of mixing and entropy of mixing 
given by the Flory – Huggins model [57,58] . Although polymer blend phase behaviour is 
determined predominately by the nature of the molecular interactions, it may be infl u-
enced by many other factors, including molecular mass, copolymer composition, blend 

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230
Membrane Gas Separation
ratio, temperature and pressure. Morphology study is one of the most important research 
topics in polymer blends, and it has been shown that the surface and bulk chemical com-
positions of polymer blends are not the same even when they are miscible in the bulk 
[59] . However, a number of other factors such as solvent, specifi c interaction, crystalliza-
tion, etc. can also affect the blend, and consequently its transport properties. 
This chapter covers multi - step work of tailoring CO 
2
- selective membrane material, 
from new copolymers designs to tailor - made copolymer/PEG blends, moreover thin fi lm 
composite membrane performance are also discussed. The relationships between gas 
transport properties, structure, morphology and physical properties are analyzed. The 
performance at different operating conditions and with mixed gases is monitored as 
well in order to have a guideline for scaling up the membranes. The benefi ts of these 
membranes are the simplicity of preparation, low cost and resistance toward acid gas 
treatment.