Membrane Gas Separation

8.3.2
Gas Transport Properties of HBPI – Silica Hybrid Membranes 
Gas permeability, diffusion and solubility coeffi cients of the HBPI – silica hybrid mem-
branes are summarized in Tables 8.2 – 8.4 , respectively. Gas permeability of pure 6FDA -
TAPOB HBPI is higher than that of pure PMDA - and ODPA - TAPOB HBPIs, owing to 
its higher diffusivity. It is well known that gas diffusivity of polymers strongly depends 
on the fractional free volume (FFV) [23,24] . Densities of pure 6FDA - , PMDA - and 
ODPA - TAPOB HBPIs are 1.433, 1.399 and 1.396 g/cm 
3
, respectively, and the FFVs of 
them calculated from Equation (8.4) are 0.165, 0.138 and 0.129, respectively. The highest 
diffusivity of the 6FDA - TAPOB HBPI is, therefore, attributed to the highest FFV. Figure 
8.5 shows CO 
2
permeability, diffusion and solubility coeffi cients of 6FDA 
- 
TAPOB 
HBPI – silica hybrid membranes plotted against silica content. Gas permeability coeffi -
cients of the hybrid membranes increase with increasing silica content. The increased 
permeability suggests additional formation of free volume holes effective for gas transport 
behaviour. Similar enhancement of free volume holes has been reported by Merkel et al.
for high - free - volume, glassy polymer – nanosilica composites, and they have concluded 
that the nanosilica particles provide disruption of polymer chain packing and affect gas 
transport parameters [25,26] . He et al. have also reported that nano - sized inorganic fi llers 
incorporated into high - free - volume, glassy polymers with rigid molecular chains can 
function as a spacer material which disrupts molecular chain packing, leading to increases 
in fractional free volume and gas permeability [27] . It is worth noting, for MTMS and 
TMOS/MTMS combined systems, signifi cant improvements of gas permeability are 
mainly caused by the increased diffusivity observed. The enormous improvements of gas 
diffusivity suggest large enhancements of mean size and maybe total amount of free 
volume elements attributed to the loose Si – O – Si networks by the incorporation of methyl 
groups for MTMS and TMOS/MTMS combined systems.