Polymer Nanoporous and Co-crystalline Phases
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Polymer Nanoporous and Co-crystalline Phases Gaetano Guerra and Christophe Daniel Dipartimento di Chimica e Biologia, Nanomates and INSTM Research Unit Università degli Studi di Salerno Via Ponte don Melillo, 84084 Fisciano (SA), Italy ABSTRACT The first part of this presentation will be devoted to the description of the two nanoporous crystalline forms and of the various co-crystalline structures which can be obtained with syndiotactic polystyrene (s-PS). Then, the second part will deal with the co-crystals formed with active guest molecules (nonlinear active, fluorescent, photoreactive, chiral, ...) while the final part of the lecture will focus on the possible applications of materials characterized by nanoporous crystalline s-PS phases. INTRODUCTION In recent years, new nanoporous polymer materials based on syndiotactic polystyrene (sPS) have been discovered and extensively studied. Depending on the preparation conditions, two nanoporous crystalline phases can be obtained. In the so-called δ-form, the crystalline nanopores consist of cavities characterized by a well-defined shape and volume (≈120 Å3) and regularly located into the crystalline lattice [1] while in the ε-form the nanopores are channel-shaped cavities crossing the unit cells along the c axis [2].
(A)
(B)
Figure. 1. Representation of the nanoporous δ (A) and ε (B) crystalline forms of s-PS. These materials are able to absorb volatile organic compounds (VOC) from water and air, also when present at very low concentrations [3]. As these materials are hydrophobic they seem particularly suitable for applications in chemical separation and air/water purification [4]. Moreover, the use of these materials as sensing elements for volatile organic compounds in molecular sensors [5] and for active packaging of fruit and vegetable has been also proposed [6]. In addition to these nanoporous materials it is possible to obtain co-crystalline phases between host helical s-PS chains and low molecular-mass guest molecules. Depending on the chemical nature of the guest molecules and the preparations procedures, different type of cocrystalline phases can be achieved.
For most s-PS co-crystals, isolated molecules are imprisoned as guests into cavities formed between layers of alternated enantiomorphous s(2/1)2 polymer helices, which characterize the nanoporous δ phase. These s-PS co-crystals have been defined as δ-clathrates and in most cases by a guest/monomer-unit molar ratio of 1/4. The occurrence of a second class of s-PS co-crystals, defined as δ-intercalates (or simply intercalates), has been also established. For these co-crystals, the guest molecules are not isolated into host cavities but are contiguous inside layers intercalated with the above-cited layers of helices. These intercalate structures are characterized by a higher guest content than the clathrate structures, corresponding in most cases to a guest/monomer-unit molar ratio of 1/2. Very recently also a third class of s-PS co-crystals has been disclosed, defined as εclathrates, for which the gues
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