Monoclinic and Triclinic δ-Clathrates of Syndiotactic Polystyrene

The paper compares X-ray diffraction patterns of delta-clathrates of s-PS with a large number of guests, having molecular volume in the range 0.058-0.195 nm(3), mainly focusing on the distance between the ac layers of enantiomorphous polymer helices (d(010)), as precisely evaluated by measurements on films exhibiting the a(parallel to)c(parallel to) uniplanar orientation. The reported data and the crystal structure determination of the delta-clathrates with dinitrobenzene and dibenzofuran allow concluding that beside the already known large number of delta-clathrates of s-PS with monoclinic unit cells there is also a large number delta-clathrates of s-PS with triclinic unit cells. In particular, monoclinic or triclinic delta-clathrates are generally obtained with guests whose molecular volume is lower or higher with respect to the cavity of the delta nanoporous crystalline form (approximate to 0.125 nm(3)). Small guests are easily accommodated in the cavities of the delta phase, thus maintaining its monoclinic unit cell and P2(1)/a symmetry. Bulky guests, on the contrary, can be accommodated by substantial changes of the structural organization of the starting delta nanoporous form, leading to the occupation of only half of the cavities initially present while the other half is lost. This new kind of structural organization of the triclinic delta clathrates allows the inclusion of guest molecules definitely bigger than until now expected, although it halves the maximum guest/styrenic-unit molar ratio, which is reduced from the usual 1/4 to 1/8.