Properties and Applications of Nanoporous-Crystalline Polymers

Nanoporous-crystalline (NC) forms, i.e. crystalline structure presenting nanocavities or nanochannels inside the crystalline lattice, are possible only for two polymers, both commercially available: syndiotactic polystyrene (sPS) and poly (2,6-dimethyl-1,4-phenylene) oxide (PPO). In particular, the two NC phases of sPS, named delta (δ) and epsilon ( ), showing pores as cavities or channels, are well described and characterized in literature since 1994 and 2007, respectively; whereas, as for PPO, evidences of the formation of NC phases have been established only in 2011. NC phases present several features which make them interesting for many industrial fields, for instance they can absorb suitable guest molecules, even at low activity from water and air, making them useful for applications such as air and water purification and/or molecular sensors. Moreover, the capability to absorb suitable guest molecules inside the NC lattice, leading to the formation of co-crystaline (CC) phases, can open the possibility of applications of these CC polymeric materials, in many different fields depending on the chemical nature of the guest molecule, as for instance fluorescence, photoreactivity, magnetism, ferroelectricity and antimicrobial. The present Ph.D. thesis aimed to investigate on new properties as well as on possible applications of these peculiar thermoplastic polymers, and the goals achieved are reported as follows. Firstly, WAXD and FTIR measurements have shown the formation of two well-separated NC (and CC) forms of PPO, named alpha (α) and beta (β). The NC α and β forms not only exhibit different chain packing but also slightly different chain conformations (c = 5.28 and 5.47 Å, respectively). Additionally, the α-form is favored by hydrophobic (with solubility lower than 0.11 mmol per 100 ml of water) and bulky guest molecules (with molecular volumes higher than 230 Å3 ) while the β form (being characterized by a higher chain periodicity) is favored by hydrophilic (with solubility higher than 2 mmol per 100 ml of water) and small guest molecules (with molecular volumes lower than 149 Å3 ). Another important aspect which has been point out is the strong influence of the crystalline phase orientation of NC PPO α phase with respect to the film plane on guest molecules diffusivity. In particular, NC PPO films exhibit higher diffusivity of the pollutant perchloroethylene (PCE) from vapors (5.6 × 10–10 cm2 s -1 ) as well as from aqueous solutions (2.1 × 10–10 cm2 s -1 ) when the orientation of the NC α phase is preferentially perpendicular to the film plane (c⊥ orientation). Specifically, diffusivity values are higher than for films with c// orientation (1.5 × 10–11 cm2 s -1 ) and much higher than NC sPS films (6.7 × 10–12 cm2 s -1 ). Moreover, guest uptakes for NC c⊥ PPO films, when expressed as mass of guest per polymer volume, are also much higher than for NC PPO powders and aerogels. This advantage becomes very important considering that for most purification processes the limiting factor is the volume of the absorbent material. [edited by Author]