Membrane Gas Separation

8 
Physical and Gas Transport 
Properties of Hyperbranched 
Polyimide – Silica Hybrid Membranes
Tomoyuki Suzuki , Yasuharu Yamada , Jun Sakai and Kumi Itahashi
Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, 
Kyoto, Japan
8.1
Introduction 
Gas separation processes using polymeric membranes have greatly been developed during 
the last three decades. Especially, polyimides have been of great interest in gas separation 
membranes because of their high gas selectivity and excellent thermal and mechanical 
properties. In this regard, gas transport properties have been reported for a large number 
of polyimides [1 – 4] . In recent years, novel triamine - based hyperbranched polyimides 
(HBPIs) have been synthesized and characterized with the aim to assess their potential 
as high - performance gas separation materials. Fang et al. have synthesized HBPIs derived 
from a triamine, tris(4 
- 
aminophenyl)amine (TAPA), and commercially available dia-
nhydrides and studied their physical and gas transport properties [5,6] . These studies 
demonstrated that HBPIs have good gas separation performance compared to linear - type 
polyimides. In our previous work, gas transport properties of the HBPI prepared by poly-
condensation of a triamine, 1,3,5 - tris(4 - aminophenoxy)benzene (TAPOB), and a dianhy-
dride, 4,4 
′
- (hexafl uoroisopropylidene)diphthalic anhydride (6FDA), have been investigated, 
and it has been found that the 6FDA - TAPOB HBPI exhibits high gas permeability and 
selectivity arising from the characteristic hyperbranched structure [7] . 

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144
Membrane Gas Separation
Organic – inorganic hybrids (often called mixed matrix membranes) are attractive materi-
als since they generally possess desirable properties (combination of high permeability 
and permselectivity). Hybridization with inorganic compounds has been focused on the 
modifi cation of polyimides in order to improve their thermal, mechanical and gas transport 
properties [8 – 10] . We have also reported that gas transport properties of 6FDA - TAPOB 
HBPI – silica hybrid membranes prepared by sol – gel reaction using tetramethoxysilane 
(TMOS) have been investigated and compared with those of linear - type 6FDA - based 
polyimide – silica hybrid membranes synthesized with 1,4 - bis(4 - aminophenoxy)benzene 
(TPEQ) and 1,3 - bis(4 - aminophenoxy)benzene (TPER) as diamine monomers [11] . Figure 
8.1 shows ideal CO 
2
/CH 
4
selectivity (  
α
  (CO 
2
/CH 
4
)) of 6FDA - based polyimide – silica hybrid 
membranes. It is found that gas permeability and  
α
  (CO 
2
/CH 
4
) of the HBPI – silica hybrid 
membranes signifi cantly increase with increasing silica content, suggesting characteristic 
distribution and interconnectivity of free volume elements created by the incorporation 
of silica [12,13] .
In this paper, physical and gas transport properties of HBPIs synthesized with various 
dianhydride monomers and their silica hybrid membranes prepared by sol – gel reaction 
with two kinds of alkoxysilanes, TMOS and methyltrimethoxysilane (MTMS), individu-
ally or simultaneously, are discussed. The methyl group of MTMS will prevent the forma-
tion of robust three - dimensional Si – O – Si network in the hybrid membranes. It is expected 
that the loose Si – O – Si network formed from MTMS induces unique physical and gas 
transport properties of the hybrid membranes.
Figure 8.1 Ideal CO 
2
 /CH 
4
selectivity (  
α
 (CO 
2
 /CH 
4
 )) of 6FDA - based polyimide – silica 
hybrid membranes plotted against CO 
2
permeability coeffi cient; attached number 
represents silica content (wt%) in the membrane

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Physical and Gas Transport Properties
145
Figure 8.2 Chemical structures of monomers and alkoxysilanes