High Surface Area Nanoporous-Crystalline Polymer Films

Abstract

Based on a commercially available thermoplastic polymer (poly­(2,6-dimethyl-1,4-phenylene) oxide, PPO), nanoporous-crystalline (NC) polymer films with a surface area (SA) higher than 600 m2 g–1 and with exceptionally fast organic pollutant uptakes are presented. Preparation procedures require fast crystallizations of amorphous films, as induced by high uptakes of nonvolatile guest molecules (typically in the range 70–90 wt %), followed by guest exchange with a volatile guest. The high SA of these NC PPO films is associated with a nanofibrillar morphology, analogous to those observed in physically cross-linked PPO aerogels. Organic pollutant diffusivities for high-SA NC PPO films are much higher than for other high-diffusivity PPO films, like for example those exhibiting NC phases with chain axes (and hence crystalline channels) being preferentially perpendicular to the film plane. Much higher diffusivities are observed both for guest sorption from vapor phase and from diluted PPO aqueous solutions and for guest desorption. Surprisingly, kinetics of guest uptakes from high-SA NC PPO films are even faster than for NC PPO aerogels and then much faster when referred to volume of the sorbent material. Because of their extremely fast kinetics of pollutant uptakes, associated with high thermal and mechanical stability, these new high-SA NC PPO films are expected to find many applications, mainly in pollutant remediation and molecular sensors.

Poly(phenylene oxide) Films with Hydrophilic Sulfonated Amorphous Phase and Physically Cross-Linking Hydrophobic Crystalline Phase

“…For instance, the sorption kinetics of methyl benzoate (MB, a compound used in perfumery and as an eco-friendly solvent) in PPO films, from 500 ppm aqueous solutions, is shown in Figure a. As already well described for many organic compounds, − guest uptake is much higher for the NC film (green triangles) with respect to the corresponding amorphous film (black circles). For instance, after 2000 min of immersion, the amorphous PPO film (although well known as a high-free-volume amorphous phase) − shows a MB uptake lower than 3 wt %, while after 600 min the NC PPO film with c ⊥ orientation shows a MB uptake close to 18 wt %.…”

“…PPO does not crystallize on cooling from its melt phase or thermal treatments on amorphous samples and hence is generally used in its amorphous form. It was, however, known for many decades that PPO is able to crystallize in the presence of most organic solvents. − In recent years, it was established that, besides many cocrystalline (CC) phases between the polymer host and low-molecular-mass guest molecules, two crystalline phases can be obtained (α and β). − Both α and β forms are nanoporous crystalline (NC), i.e., their density is lower than for the corresponding amorphous phase and both are able to readily absorb low-molecular-mass organic guest molecules, even when present in traces. − Particularly interesting is the thermodynamically stable α form, , whose films can exhibit a melting temperature as high as T m = 260 °C and melting enthalpy up to Δ H m = 30 J/g …”

Transparent and flexible high-surface area nanoporous crystalline PPO films

“…All the high-SA films of Table 1, as those described in ref. 48 and 49, were completely opaque. Their density was always in the range of 0.80–0.85 g cm −3 , i.e.…”

“…Moreover, both guests when absorbed at 4 1C led to low SA NC films due to very slow guest sorption (and guest-induced crystallization) kinetics. 48,49 The three guests presented in Table 1 with the lowest diffusivities (limonene, eugenol, and carvacrol) lead to high-SA NC films only at the highest sorption temperature (65 1C). Guest-sorption procedures conducted at lower temperatures lead to low-SA NC films, with c > orientations.…”

“…47,48 Moreover, for PPO and sPS NC polymer films with SA higher than 600 m 2 g À1 and 120 m 2 g À1 , respectively, organic pollutant uptake is faster than for corresponding aerogels, even if evaluated per unit mass as was recently described. 49,50 Known preparation procedures for high-SA NC films require fast crystallization of the amorphous films, which is induced by high uptake (typically in the range 70-90 wt%) of nonvolatile liquid guests (carvone or dibenzyl ether), followed by a guest exchange with a volatile guest. However, due to the presence of mesopores beside the nanopores of their crystalline phases, the described high-SA NC films are opaque and exhibit poor toughness, and hence, are unsuitable for many applications.…”