Monoclinic and Triclinic δ-Clathrates of Syndiotactic Polystyrene

Abstract

The paper compares X-ray diffraction patterns of δ-clathrates of s-PS with a large number of guests, having molecular volume in the range 0.058−0.195 nm3, mainly focusing on the distance between the ac layers of enantiomorphous polymer helices (d010), as precisely evaluated by measurements on films exhibiting the a∥c∥ uniplanar orientation. The reported data and the crystal structure determination of the δ-clathrates with dinitrobenzene and dibenzofuran allow concluding that beside the already known large number of δ-clathrates of s-PS with monoclinic unit cells there is also a large number δ-clathrates of s-PS with triclinic unit cells. In particular, monoclinic or triclinic δ-clathrates are generally obtained with guests whose molecular volume is lower or higher with respect to the cavity of the δ nanoporous crystalline form (≈ 0.125 nm3). Small guests are easily accommodated in the cavities of the δ phase, thus maintaining its monoclinic unit cell and P21/a symmetry. Bulky guests, on the contrary, can be accommodated by substantial changes of the structural organization of the starting δ 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 δ 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.