Penetrant permeability nanocomposites

Figure 24.13 Ratio of penetrant permeability in the nanocomposite to that in the pure polymer as a function of filler volume fraction (Merkel et al., 2002).

Merkel et al. [2002, 2003] carried out studies of gas and vapor permeability and PALS free volume in a poly(4-methyl-2-pentyne) (PMP)/fumed silica (FS) nanocomposite. It was observed that gas and vapor uptake remained essentially unaltered in nanocomposites containing up to 40 wt% FS, whereas penetrant diffusivity increased systematically with the spherical nanofiller content. The increased diffusivity dictates a corresponding increase in permeability, and it was further established that the permeability of large penetrants was enhanced more than that of small penetrants. PALS analysis indicated two o-Ps annihilation components, interpreted as indicative of a bimodal distribution of free-volume nanoholes. The shorter o-Ps lifetime remained unchanged at a value T3 2.3 to 2.6 ns, with an increase in filler content. In contrast, the longer lifetime, T4, attributed to large, possibly interconnected nanoholes, increased substantially from 7.6 ns to 9.5 ns as FS content increased up to 40 wt%. 

Where the loading of nanomaterial is increased, the resistance of nanocomposites towards different chemical media increases. This may be due to the more compact and cross-linked structure of the nanocomposites, along with a reduction of permeability characteristics compared to the pristine system. The delaminated structure of the nanocomposites reduces the permeability and, as a result, the various ions or species present in the different media cannot easily penetrate the surface as they have to follow an indirect path. A significant increase in the thermal degradation temperature and a decrease in the thermal degradation rate have also been observed in most of these nanocomposite systems where there is an increase in the amount of dispersed nanomaterial. This improvement of thermal stability in the nanocomposites systems is related to the well-dispersed nanomaterials, which hinder the diffusion of volatiles and assist the formation of char after thermal decomposition. 

Solubility increase mechanism depends on the interaction between the penetrants and nanofillers. The functional groups of nanofillers such as hydroxyl when occur on the surface of the inorganic nanofiller phase in rubber composites may interact with polar gases such as SO2. This condition can increase the penetrant solubility in the nanocomposite rubbers and, in turn, increase the gas permeability. The solubility increase mechanism model due to permeation coefficient parameter of gas, P is described using the Arrhenius equation ... 

Barrier properties. At a high aspect ratio which can be achieved in nanocomposites (with exfohated clay) significant decreases in permeability are predicted and observed in practice (Hollaway and Hackman, 2004). The barrier properties of polymers can be significantly altered by inclusion of inorganic platelets with sufficient aspect ratio to alter the diffusion path of the penetrant molecules. 

Theoretical approaches on the barrier properties of nanocomposites beat fillers as impermeable nonoverlapping particles and assume no permeability changes in the polymer matrix. Effectively, this means that the permeability of the composite will be smaller than the permeability of the matrix (unfilled polymer) by a factor equal to path tortuosity in the composite (simply assuming that the penetrant path cannot cross any filler particles). This path tortuosity was calculated by Nielsen for completely aligned filler particles (aU fillers have then-larger surface parallel to the film surfaces, but there is no order in the filler center of mass), and its contribution to the composite permeability was derived to be... 

Fig. 8.6 Gas transport properties of CNT nanocomposite membrane. Gas transport properties of CNT/PS/PDMS membrane (triangle). CNTs/PS membranes (square), and Knudsen diffusion model (solid line), (a) Effeet of the pressure drop on the permeance of helium through CNTs/PS membrane, (b) Single-gas permeability as a funetion of the inverse square root of the molecular weight of the penetrant, (c) Single gas seleetivity with respect to He calculated from singe-gas permeability data, (d) Mixed-gas selectivity (CO /CH ) of CNTs/PS membrane. The composition of gas mixture was COjiCH =1 1. The feed pressure was 50 psi, and the pressure differential across the membrane was maintained by drawing a vaeuum on the permeate side. Operating temperature was maintained at 308 K. (From [8])...

As it is known [50-52], introduction of organoclay in a polymeric matrix results in essential reduction of permeability to gas of the nanocomposites obtained by such a mode in comparison with a matrix polymer. As a rule, such permeability to gas reduction is explained by an increase in the meandering trajectory of the gas-penetrant molecules through the nanocomposite by virtue of the availability of organoclay anisotropic particles within it [50, 52]. So, the relative permeability to gas characterising a reduction in this parameter for nanocomposites in comparison with a matrix polymer, is defined as follows [50] ... 

The decreased permeability of the nanocomposites arises from the longer diffusion pathway that the penetrants must travel in the presence of clay nanolayers. In general, there are two reasons for the enhancement of gas-barrier properties in nanostructured polymer blends with clay. First, gas-impermeable nanoclay layers dispersed in the polymer matrix form tortuous pathways, which retard the diffusion of the gas molecules through the composites. Second, exfoliated and intercalated clay-layer bundles strongly restrict the motions of the polymer chain probably reducing the coefficient of diffusion of the gas molecules. 

Nanocomposite mixed-matrix membranes have been investigated for close to a decade. Ti02-poly(amide-imide) membranes showed selectivity improvement but suffered loss of productivity when TiOa was added. Nonporous, nanoscale, fumed silica was embedded in a glassy, amorphous polymer, poly(4-methyl-2-pentyne), which resulted in enhancements in both permeability and selectivity for the mixed-matrix membrane. These membranes were discovered to be reverse selective, so the membrane is selective for the larger penetrant. This phenomenon is attributed to increased free volume in the bulk polymer from chain packing disruption, which occurs when the filler is added. ... 

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