Clathrate crystalline phases

By solution crystallization or by sorption of suitable compounds (e.g., methylene chloride, toluene, chloroform) in amorphous SPS samples as well as in samples in the a form, clathrate crystalline phases, always containing s(2/l)2 helical chains, can be obtained [3,9,10,38,40-43],... 

Clathrate formation is very attractive for exploitation in solid-state chemistry. It allows one to modify in a simple way the environment of the guest molecule, to place this molecule in a crystalline phase with a structure different from its own (one structure may be chiral, and the other not), and even to achieve a stable crystalline structure at a temperature above the melting point of the pure guest. Some of the variety available for a single compound, acetic acid, is... 

It stands out that the photochemistry is also dependent of the crystalline phases where the topochemical relations between nearest-neighbor molecules are not the same in the pure guest crystals and in the TOT clathrates. When the TOT/cw-stilbene clathrate was irradiated, photoconversion to trans-stilbene occurred smoothly with the additional formation of small amounts of phenanthrene and of an unidentified product. On the contrary the TOT/tru j-stilbene clathrate remained unchanged when irradiated for long periods of time. In the majority of cases, crystalline cw-cinnamic acid derivatives are entirely converted to tran -isomers on irradiation and the trans-to-cis isomerization is not observed However both TOT clathrates with the trans- and cis-isomers of methyl cinnamate photoisomerized under similar conditions. It is noteworthy that these clathrates and those of the stilbenes (Sect. 2.1.4) are isomorphous. 

Clathrates. or inclusion compounds, are the typical representatives of supennolecular species. We may define them as the compounds formed by inclusion of one kind of molecules, called guest molecules, into the cavities of a crystalline framework composed of the molecules of another kind (or into a cavity of one large molecule), called host molecules, without forming any specific chemical bond between guest and host. Unlike the case of traditional chemical compounds, favorable spatial complementarity of the guest and host subsystems, not chemical reactivity, plays the important role in formation of these compounds from the components. This principle of formation allows molecules that are coordination saturated and do not interact chemically with each other to be brought together, so that they form supermolecules or supennolecular crystalline phases that are thermodynamically more stable than a mixture of initial components. 

Gels of SPS with different solvents have been compared to clathrates. WAXD results using toluene (a good solvent for SPS) and decalin (a relatively poor solvent for SPS) show that the structure of the crystalline junctions of the gels is similar to that of the clathrate a phase. A difference can be found in the width of the (010) reflection, which is relative to the width of the (210) reflection, much broader for the gel than for the clathrate. This is caused by the difference in the mechanism involved in crystal formation in gels and clathrates. Experiments performed on quenched samples of SPS with the monomer benzyl methacrylate show that also for this gel the structure of the crystalline part is similar to that of the clathrate phase. This means that solvent is present in both the crystalline and the amorphous parts of the gel. By solid-state nuclear magnetic resonance (NMR) studies, a clear difference in the mobility of solvent molecules in the crystalline and amorphous parts of the gel has been observed [58]. 

Figure 10.1 Presentations of along-the-chain-projections of SPS co-crystalline phases (a) 5 clathrate phase with DCE (b) intercalate phase with norbornadiene (c) schematic e clathrate phase with DCE.

Figure 10.2 Top (a) and lateral views (b) of the ac layer of s (2/1)2 parallel helices of SPS, that is, the high-density and low-energy structural feature that is common to the 5-nanoporous form and to the corresponding co-crystalhne (both clathrate and intercalate) forms. The minimum interchain distance (0.87 nm) is achieved by alternating enantiomorphous helices (R and L stand for right-handed and left-handed, respectively). (c-e) Molecular models showing the three simplest orientations of the ac layers with respect to the film surface. The plane of the figure is assumed as parallel to the film plane. Arrows indicate the absence of axial orientation, (c) Both a and c axes are parallel to the film plane (otu Cil) (d) a parallel and c perpendicular to the fihn plane a Ci) (e) a perpendicular and c parallel to the film plane (a C ).These three tmiplanar orientations can be achieved for y, 8, and most co-crystalline phases of SPS. (See color insert.)...

It has been recently suggested that the structural feature determining these three different kinds of uniplanar orientations is the layer of close-packed alternated enantiomorphous helices [75] that characterizes the 8 phase of SPS, as well as all related clathrate and intercalate co-crystalline phases with low-molecular-mass guest molecules. 

In particular, it has been clearly established that in most cases the crystalline phases of the SPS gels are clathrate phases [85-88], However, low 29 peaks corresponding to d 1.5 nm have been observed for SPS gels in benzyl-methacrylate and in cyclohexyl-methacrylate [83,84], which could be easily rationalized by considering the possibility of formation of intercalate structures, analogous to those observed for semicrystalline films (see subsection 10.2.1.2). 

Shifts of Vibrational Peaks As well known for other polymeric co-crystalline phases (e.g., the clathrate structures of poly [ethylene oxide] with para-disubstituted benzene) [136], guest peak shifting attributed to specific host-guest interactions has been observed for some SPS co-crystalline phases. 

In particular, a spectroscopic analysis of films presenting the SPS/chloro-form 8 clathrate phase [120] has shown that when chloroform is absorbed in the crystalline phase, a significant perturbation of its vibrational spectrum takes place. This effect mainly involves the 1219 cm ( -c-a) peak, which clearly shows a fine structure in the form of an unresolved component at a lower wavenumber (1210cm ), and is not present in the spectrum of the isolated molecule (vapor phase) and in the case of chloroform sorbed in the amorphous phase of SPS [120], Hence, it is likely to be related to host-guest molecular interactions. 

Figure 10,5 Emission spectra (excitation at 265 nm) of SPS films exhibiting clathrate (guest content 9wt%) and intercalate (guest content 13 wt%) co-crystalline phases with 1,3,5-tri-methyl-benzene.

Figure 10.7 X-ray diffraction patterns (CuKa) of SPS semicrystallme powder samples presenting different helical crystalline and co-crystalline phases (A) 5 form (B) 5 clathrate with DCE (C) e form (D) e clathrate with DCE (E) y form. Bragg distances (d in nm) of relevant reflections are indicated.

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. 

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. 

Phase equilibria of the isothiazole-water system have been investigated by differential thermal analysis (76BSF1043), and it has been established that a stable crystalline clathrate (isothiazole-34H20) forms below 0 °C. 

Another interesting case is the much higher solvent resistance of the P crystalline form of s-PS, with respect to the other ones. In fact, it has been found that the sorption of solvents (which are suitable to produce transformations from the a or the y form toward clathrate structures) occurs only in the amorphous phase, for the case of P form samples [122-124]. Sorption kinetic curves of liquid methylene chloride in s-PS samples in the a and p form are, for instance, compared in Fig. 21 [124]. 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

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