Nanoporous crystalline phases

The term polymeric nanoporous phase (and sometimes the term polymeric framework ) is used to indicate crystalline polymorphic phases, whose density are lower than the density of the corresponding amorphous phase and are suitable to absorb suitable guest molecules at low activities (e.g., from diluted solutions). 

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Preparation, Structure, Properties, and Applications of Co-Crystals and Nanoporous Crystalline Phases of Syndiotactic Polystyrene... 

Structure, preparation, properties, and applications of the nanoporous crystalline phases of syndiotactic polystyrene are described in section 3 of this chapter. 

Figure 10.8 Top and lateral views of the crystalline structures of the two nanoporous crystalline phases of SPS. For the 6-(upper figures) and e-(lower figures) phases, the porosity is distributed as cavities and channels, respectively. (See color insert.)...

Preparation of the Nanoporous Crystalline Phases By suitable procedures of guest-removal from 8 clathrate and intercalate SPS cocrystalline phases, the nanoporous crystalline 8 phases can be e Jlily obtained. 

The crystalline-phase orientation (axial, uniplanar, and uniplanar axial) that can be achieved for SPS co-crystalline phases can be maintained in the corresponding nanoporous crystalline phases [69-75,95],... 

There are few cases reported in the Uterature where the crystalline domain displays a nonnegligible solubility [171]. As for SPS, the two nanoporous crystalline phases (5 and e) have a density (0.98 g/cm ) significantly lower than the amorphous phase of SPS (l.OSg/cm ), which is close to that of atactic polystyrene. 

Chloroform sorption experiments at different temperatures have shown that if the relative pressure of chloroform is high enough, a, y, 5, and e phases are transformed into SPS/CHCI3 co-crystalline phases while the p phase remains unaltered [172]. Moreover, at low chloroform activities, SPS/CHCI3 co-crystalline phases are achieved only starting from the nanoporous 5- and e-forms. Experimental analyses of chloroform sorption, performed using in situ FTIR spectroscopy, have also allowed a quantification of the amount sorbed into the amorphous and nanoporous crystalline phases [120]. 

The reported sorption and desorption data not only conhrm that at low guest activities the sorption occurs nearly only by the nanoporous crystalline phase but also show that the guest transport behavior is dependent on the kind of uniplanar orientation of the host crystalline phase. In particular, in agreement with predictions based on molecular simulations [110], the lowest diffusivity has been measured for films with a Cu uniplanar orientation while the highest diffusivity has been measured for films with ai C// uniplanar orientation [97,98,111]. 

Moiecuiar Separations Particularly relevant molecular separations are those implying the removal of VOC from water and air. To this purpose, the nanoporous crystalline phases of SPS are extremely suitable because they are able to include apolar molecules, as guest of their crystalline cavities, and to exclude the highly polar water molecules. 

As already observed for other guest molecules of SPS, the ethylene diffusiv-ity in the host nanoporous crystalline phase is markedly reduced with respect to the diffusivity in the corresponding glassy amorphous phases. In addition, ethylene diffusivity can be further reduced by suitable selection of the unipla-nar orientation ((a// Cn or a Cj ) of the host crystalline phase. 

In summary, the 5 nanoporous crystalline phase of SPS presents high ethylene solubility and low ethylene diffusivity, which can also be controlled by the orientation of the crystalline phase, associated with negligible water uptake. These features make polymeric materials presenting the 5-nanoporous crystalline phase of SPS suitable for ethylene removal and storage and, due to its chemical and mechanical properties, it can be considered suitable candidates as (also repeated use) produce packaging. 

Several studies have shown that SPS films, presenting the nanoporous crystalline phases, are suitable sensing elements for detection of organic pollutants, being effective with most VOCs (mainly chlorinated and aromatic), which are present in industrial wastes like benzene, toluene, chloroform, methylene chloride, tetrachloroethylene, and trichloroethylene. 

Several exciting new material based on co-crystalline and nanoporous crystalline phases of syndiotactic polystyrene have been achieved. In particular, several kinds of polymer co-crystalline phases have been prepared, belonging to three different classes 8- and e-clathrates and intercalates. Polymer cocrystals with active guest molecules show unusual physical properties, hence are promising for several kinds of advanced materials. Moreover, the unprecedented achievement of polymeric nanoporous crystalline phases (8 and e) has given very interesting results in the fields of molecular separations, water/ air purification and sensorics. 

As for possible perspectives of the nanoporous crystalline phases, applications are expected in the field of controlled release of drugs and pesticides. 

As for nanoporous SPS films and aerogels, a relevant objective will also be the modification, with different kinds of functional groups, of the amorphous phase. The functionalization of the sole amorphous phase [193, 194] could bring several advantages, like increase of rates of guest sorption from the nanoporous crystalline phases. 

Larobina, D., Sanguigno, L., Venditto, V., Gnerra, G., Mensitieri, G. Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase. Polymer, 45,429-436 (2004). 

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