Monolithic nanoporous–crystalline aerogels based on PPO

Abstract

Monolithic and robust aerogels exhibiting nanoporous–crystalline modifications of an industrially relevant polymer (poly(2,6-dimethyl-1,4-phenylene)oxide, generally known as polyphenyleneoxide, PPO), have been obtained from thermoreversible gels, by sudden solvent extraction with supercritical carbon dioxide. The aerogel formation occurs only in the presence of semicrystalline nanofibrils of syndiotactic polystyrene, a polymer presenting molecular miscibility with PPO in the amorphous phase. These mixed monolithic aerogels can present nanoporous–crystalline phases of both polymers and hence are very promising for water and air purification. For instance, the carbon tetrachloride uptake from 10 ppm aqueous solutions can be as high as 19 wt%.

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Introduction

Aerogels are a unique class of materials characterized by a highly porous network, being attractive for many applications such as thermal and acoustic insulation, capacitors or catalysis.¹ Although most aerogels are inorganic,^1a–h many organic and polymeric aerogels^1i,j have also been developed. The relevance of polymeric aerogels is mainly related to low cost, robustness, durability and easy processing typical of polymers.

Polymeric aerogels are generally characterized, as the other aerogels, by chemically bonded three-dimensional networks. In fact, highly cross-linked aerogels are obtained by using thermosetting polymers, e.g. resorcinol–formaldehyde or melamine–formaldehyde.^1i,j In more recent years, polymeric aerogels based on thermoplastic uncrosslinked polymers, where the knots of the three-dimensional networks are physical, i.e. not formed by covalent chemical bonds but by thin crystallites, have also been obtained.^2–4 These physically cross-linked aerogels are generally prepared by supercritical CO₂ (scCO₂) extraction⁵ of thermoreversible hydrogels² and organogels.^3,4

Besides aerogels, another class of porous materials, which has received attention in the last decade by the scientific community, is constituted by semicrystalline thermoplastic polymers whose crystalline phase is nanoporous, i.e. presents a density lower than the corresponding amorphous phase.⁶ These nanoporous–crystalline polymers are able to absorb low molecular-mass molecules even when present in traces and have been proposed for molecular separation,⁷ sensor⁸ and catalysis⁹ applications.

The preparation of aerogels with thermoplastic materials exhibiting nanoporous–crystalline forms has resulted in a special class of physically cross-linked aerogels, where the crystallites that constitute the physical knots of the aerogel exhibit a nanoporous–crystalline form.⁴ These nanoporous–crystalline aerogels also exhibit, beside disordered amorphous micropores (typical of all aerogels), all identical nanopores of the crystalline phases. In particular, monolithic aerogels based on the nanoporous–crystalline δ^4a–e and ε^4f forms of syndiotactic polystyrene (s-PS), with their typical crystalline cavities and channels, have been obtained. These sPS nanoporous–crystalline aerogels, due to their fast kinetics and high capacity of sorption of volatile organic compounds (VOCs) as well as their good handling characteristics, are particularly suitable for removal of traces of pollutants from water and air.⁴

Very recently, the presence of nanoporous crystalline modifications has been disclosed for another commercial thermoplastic polymer: poly(2,6-dimethyl-1,4-phenylene)oxide (PPO).¹⁰ However, extraction from PPO gels of the solvent, independently of its chemical nature and of the extraction procedure, leads to powders rather than to aerogels.^10a

In this paper, the preparation of monolithic aerogels, including PPO nanoporous–crystalline phases, is reported. The described preparation method is based on the formation in the gels (and in the derived aerogels) of fibrillar crystalline morphology as a consequence of blending PPO with s-PS. The method, by using gels with suitable PPO/s-PS compositions, also allows the formation of monolithic aerogels that include, beside amorphous nanopores, nanoporous–crystalline phases of both PPO and s-PS.

Experimental section

Materials and samples preparation

The PPO used in this study was purchased by Sigma Aldrich and presents weight-averaged and number-averaged molecular masses M_w = 58 500 g mol⁻¹ and M_n = 17 000 g mol⁻¹, respectively. The syndiotactic polystyrene used in this study was manufactured by Dow Chemicals under the trademark Questra 101. ¹³C nuclear magnetic resonance characterization showed that the content of syndiotactic triads was over 98%. Weight-averaged and number-averaged molecular masses were found to be M_w = 320 000 g mol⁻¹ and M_n = 82 000 g mol⁻¹. Solvents were purchased from Aldrich and used without further purification.

All gel samples were prepared in hermetically sealed test tubes by heating the mixtures until complete dissolution of the polymer and the appearance of a transparent and homogeneous solution had occurred. Then the hot solution was cooled down to room temperature where gelation occurred.

Gels were extracted with an SFX 200 supercritical carbon dioxide extractor (ISCO Inc.), generally using the following conditions: T = 40 °C, P = 250 bar, extraction time t = 300 min.

For monolithic aerogels with a regular cylindrical shape, the total porosity, including macroporosity, mesoporosity and microporosity, can be estimated from the volume/mass ratio of the aerogel.

Then, the percentage of porosity P of the aerogel samples can be expressed as:¹¹

where ρ_pol is the density of the polymer matrix and ρ_app is the aerogel apparent density calculated from the mass/volume ratio of the monolithic aerogels.

Techniques

WAXD. X-ray diffraction patterns were obtained on a Bruker D8 automatic diffractometer operating at a step size of 0.03°, a rate of 164 s/step and with a nickel-filtered Cu-Kα radiation. Diffraction patterns of aerogels were obtained with cylinder-shaped samples, with a thickness of 2 mm.

The degree of crystallinity of the samples was evaluated from X-ray diffraction data applying the standard procedure of resolving the diffraction pattern into two areas corresponding to the contributions of the crystalline and amorphous fractions for the 2θ range 6–35°.

FTIR. Fourier Transform Infrared (FTIR) spectra were obtained at a resolution of 2.0 cm⁻¹ with a Vertex 70 Bruker spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector and a Ge/KBr beam splitter. The frequency scale was internally calibrated to 0.01 cm⁻¹ using a He–Ne laser. A total of 32 scans were signal averaged to reduce the noise.

TGA. The content of the guest molecules in the samples was determined by thermogravimetric measurements (TGA) performed with a TG 209 F1 equipment from Netzsch under nitrogen atmosphere at a heating rate of 10 °C min⁻¹.

SEM. The internal morphology of the aerogelic monoliths was characterized by means of a scanning electron microscope (SEM, Zeiss Evo50 equipped with an Oxford energy dispersive X-ray detector). Samples were prepared by fracturing small pieces of the monoliths in order to gain access to the internal part of the specimen. In fact, the external lateral surfaces of all samples were found to be flat and free of porosity. Low energy was used (5 keV) in order to obtain the highest possible surface resolution. Before imaging, all the specimens were coated with gold using a VCR high resolution indirect ion-beam sputtering system. The samples were coated depositing approximately 20 nm of gold. The coating procedure was necessary in order to prevent surface charging during the measurement and to increase the image resolution.

Porosimetry. Surface area and pore volume were obtained by N₂ adsorption measurements carried out at 77 K on a Micromeritics ASAP 2020 sorption analyzer. All the samples were outgassed for 24 h at 30 °C before the analysis. The specific surface area of the polymers was calculated using the Brunauer–Emmet–Teller method,¹² which has been shown to provide a better fit of the experimental data than the Langmuir model (correlation coefficient of the linear fit r² > 0.9999).¹³ The micropore volume has been determined with the t-plot method,¹³ adopting the equation of thickness reported in ref. 3e.

Results and discussion

Preparation and structural characterization of PPO/s-PS aerogels

X-ray diffraction patterns of physical PPO gels with a polymer content in the range 30–40 wt%, with four different solvents, are shown in Fig. 1 A–D. All these patterns show, beside broad amorphous halos associated with the solvent and the polymer amorphous phase, well-defined crystalline peaks associated with the three-dimensional physical networks holding up the gel. In particular, the gels including decalin and α-pinene (Fig. 1A and 1B, respectively) exhibit diffraction peaks of the corresponding co-crystalline phases with PPO. In particular, the 101, 110, 102, 200, 201, 211, 220 and 221 reflections of the PPO/α-pinene co-crystalline form, located at 2θ_Cu-Kα ≈ 8.9°, 10.5°, 13.1°, 15.0°, 15.8°, 17.5°, 21.3° and 21.8°,^9,14 are clearly apparent in Fig. 1B. As for the gel with decalin, peaks at 2θ_Cu-Kα ≈ 9.1°, 10.6°, 12.7°, 17.4° and 21.8 are observed (Fig. 1A), which, from analogy with the X-ray diffraction pattern of the co-crystalline phase with α-pinene, can be indexed as 101, 110, 102, 211 and 220–221, respectively. The gels including CCl₄ (Fig. 1C) and benzene (Fig. 1D) show diffraction patterns that are markedly different from those of the co-crystals with α-pinene and decalin gels but also definitely different from each other as well as from those of the known PPO crystalline modifications.^9,15

Fig. 1 X-ray diffraction patterns (Cu-Kα radiation) of PPO gels (PPO content 30–40 wt%), prepared with different solvents (A–D), and of the corresponding powders, as obtained by complete solvent extraction by scCO₂ (A′–D′): (A, A′) in decalin; (B, B′) in α-pinene; (C, C′) in benzene; (D, D′) in carbon tetrachloride.

The solvent removal from all these physical gels by supercritical carbon dioxide does not lead to aerogels,^10a as observed instead for many other thermoplastic polymers,⁴ but only leads to powders. These powders can be amorphous (Fig. 1A′) or can exhibit a PPO crystalline modification (Fig. 1B′–D′). In particular, as a consequence of complete benzene and CCl₄ removal from their gels (Fig. 1C′ and 1D′), the limit nanoporous–crystalline modifications with highest and lowest diffraction angles (similar to those described in the first and last columns of Table 1 of ref. 10a) are obtained, respectively.

It is worth noting that the X-ray diffractions of the gels of Fig. 1C, D are similar to those of the corresponding nanoporous–crystalline phases of Fig. 1C′, D′. This similarity suggests the presence in these gels of co-crystalline phases, being structurally related to the corresponding nanoporous–crystalline phases. In particular, the shift to a higher 2θ value (from 3.7° up to 4.55° and from 4.9° up to 5.15°) observed for the lowest diffraction peak, as a consequence of CCl₄ and benzene removal from their PPO gels, suggests unit cell reductions as a consequence of removal of guest molecules from the co-crystalline phases. This shift of the first reflection toward higher diffraction angle, as a consequence of guest removal, is analogous to those observed for the monoclinic δ-clathrate^5a–c,16 and intercalate¹⁷ co-crystalline forms of s-PS.

Hence, the present results confirm that, independently of the nature of the solvent in the gels and of the nature of the crystalline phase holding up the gel, solvent removal procedures lead to powders or extremely brittle samples rather than to robust aerogels. This is shown for instance for the PPO gel in 1,2-dichloroethane with a polymer content of 20 wt%, in the lower part of Fig. 2.

Fig. 2 Photographs of pieces of gels prepared in 1,2-dichloroethane with a polymer concentration of 20 wt%, before and after complete solvent extraction via supercritical carbon dioxide (units on the ruler are cm). Lower part: PPO; upper part: PPO/s-PS, 90/10 w/w.

Monolithic physically crosslinked aerogels can be instead easily prepared from mixed PPO/s-PS gels, by the usual scCO₂ extraction procedure, provided that the s-PS fraction is higher than 0.05. This is shown, for instance, for a PPO/s-PS, 90/10 gel in 1,2-dichloroethane with a polymer content of 20 wt%, in the upper part of Fig. 2. The obtained aerogels exhibit a toughness comparable to those of s-PS aerogels and the monoliths do not break even after heavy handling.

The X-ray diffraction patterns of aerogels obtained from gels in 1,2-dichloroethane¹⁸ (DCE) with a polymer content of 20 wt% are shown, for instance, in Fig. 3. The s-PS aerogel (Fig. 3A) shows strong reflections located at 2θ (Cu-Kα) ≈ 8.3° (010), 20.7° ((3̄21) and (301)) and 23.5° (4̄11), indicating, in agreement with a previous report,^17d the presence of the nanoporous δ-form.^5a–c

Fig. 3 X-ray diffraction patterns (Cu-Kα radiation) of PPO/s-PS gels in DCE, prepared with a polymer content of 20 wt%, after complete solvent extraction by scCO₂ , for different PPO fractions: (A) s-PS aerogel; aerogels with (B) x_PPO = 0.25, (C) x_PPO = 0.50, (D) x_PPO = 0.75, (E) x_PPO = 0.9; (F) PPO powder. For some patterns (C and F), diffraction peaks of PPO and s-PS (δ form) are labeled by bold and italic numbers, respectively.

The PPO/s-PS aerogels, for low PPO contents (x_PPO = 0.25, Fig. 3B), show only the crystallinity of the δ form of s-PS. For intermediate polymer composition (e.g. x_PPO = 0.50, Fig. 3C) both s-PS and PPO crystallinities are clearly present. As for the PPO crystallinity, well apparent for x_PPO > 0.5, diffraction peak positions change with x_PPO. In fact, for the pure PPO powder, as obtained by DCE removal from the corresponding gel, diffraction peaks at 4.95°, 7.65°, 12.3°, 16.1°, 21.5° are observed (Fig. 3F), which are not far from those observed for powders obtained by desiccation of benzene gels (Fig. 1C′, indicated as high limit diffraction angles in the first column of Table 1 in ref. 10a). As the PPO fraction decreases, a progressive shift of the PPO crystalline peaks is observed and for x_PPO = 0.50 (Fig. 3C) the diffraction peaks are observed at 4.4°, 7.15°, 11.2°, 15.0° and 18.1°, i.e. at 2θ values lower than those of powders obtained by desiccation of carbon tetrachloride gels (Fig. 1D′, indicated as low limit diffraction angles in the last column of Table 1 in ref. 10a).

The formation of PPO physical aerogels, already in the presence of small amount of s-PS, clearly indicates a strong influence of s-PS on PPO crystallization. This influence is confirmed by the remarkable shift between PPO crystalline modifications (approximately from the limit modification with lowest diffraction angles toward the limit modification with highest diffraction angles), which is induced by the increase of the s-PS content in the blend (Fig. 3).

The observed s-PS influence on PPO crystallization is not surprising, due to the well-known miscibility of the two polymers in their amorphous phases.¹⁹ In fact, it has been clearly established that PPO is able to alter the s-PS polymorphic behavior, favoring the densest and thermodynamically stable β crystalline form,²⁰ both for melt crystallization^19a and for solvent-induced crystallization from the amorphous state.^19b The patterns of Fig. 3 show that, in turn, s-PS is able to alter the PPO polymorphic behavior favoring, for solution crystallization, the crystalline modifications exhibiting highest diffraction angles.

Scanning Electron Microscopy (SEM) images of some of the samples of Fig. 3 are shown in Fig. 4. The differences between the morphologies of the s-PS and PPO semicrystalline samples prepared in the same conditions are remarkable. In fact, the δ form s-PS aerogel (Fig. 4A) presents a fibrillar morphology with fibril diameters of 60–150 nm, while the PPO nanoporous–crystalline powder exhibits granules of micrometric size, possibly constituted by aggregates of spherulites (Fig. 4D). The SEM images of the aerogels obtained from PPO/s-PS blends (Fig. 4B, C) clearly show the maintenance of the fibrils typical of s-PS aerogels, together with a size reduction of the PPO granules (whose diameter becomes roughly lower than 1 μm). Hence, the miscibility between the amorphous phases of PPO and s-PS leads to finer PPO morphologies. It is reasonable to hypothesize that the maintenance of the fibrillar morphology of the s-PS crystalline phases in mixed gels and aerogels (Fig. 4B, C) makes for feasible PPO-rich monolithic aerogels.

Fig. 4 SEM images of samples, as prepared from DCE gels with polymer content of 20 wt%, after complete solvent extraction by scCO₂: (A) s-PS δ form aerogel; (B) aerogel with x_PPO = 0.5; (C, C′) aerogel with x_PPO = 0.90; (D) PPO powder.

Monolithic aerogels can be obtained from PPO/s-PS gels for a large range of polymer concentrations. The X-ray diffraction patterns of aerogels obtained from gels with 5, 10, 20 and 30 wt% of polymer, with x_PPO = 0.50, are shown in Fig. 5. The obtained aerogels present a nearly constant s-PS crystallinity (roughly 50%) while showing a variable PPO crystallinity (in the range 15–48%), reaching a minimum for the aerogel prepared from the most diluted gel (for C_pol =5 wt%, χ_PPO = 15%).

Fig. 5 X-ray diffraction patterns (Cu-Kα radiation) of PPO/s-PS aerogels with x_PPO = 0.50 , prepared from DCE gels after complete solvent extraction by scCO₂, for different polymer contents: (A) 5 wt%; (B) 10 wt%; (C) 20 wt%; (D) 30 wt%.

Properties of aerogels based on nanoporous-crystalline PPO

Total surface areas S_BET and micropore areas S_micro, as obtained from N₂ adsorption data at 77 K are compared in Table 1 and Fig. 6 for the mixed PPO/s-PS aerogels. It is noteworthy that S_BET increases with the PPO content (Fig. 6A), but only for aerogels with crystalline PPO. In fact, the sample with x_PPO = 0.25 exhibiting only the s-PS crystallinity (Fig. 3B) shows a surface area close to that of the pure s-PS aerogel. Accordingly, an almost perfect linear correlation between aerogels’ S_BET values and their content of crystalline PPO (x_PPO,crys) is instead observed, as clearly shown in Fig. 6B.

Table 1 Total surface area (S_BET) and micropore area (S_micro) of polymeric aerogels as obtained for PPO/s-PS blends: (i) having the same polymer content (C_pol = 20%) and different PPO weight fractions (x_PPO) and (ii) as prepared from DCE gels presenting different polymer content (C_pol) and the same x_PPO value (x_PPO = 0.5)

x PPO	C pol (wt%)	S BET ^a(m^2 g^−1)	S micro ^b (m^2 g^−1)	x PPO,crys ^c	χ PPO ^d(%)
a Total area evaluated following the BET model in the standard 0.05 < P/P0 < 0.25 pressure range. b Micropore area obtained from the t-plot. c Aerogel fraction constituted by crystalline PPO, as evaluated from the patterns of Fig. 3 and 5. d Degree of crystallinity relative to the PPO fraction, as evaluated by the X-ray diffraction patterns of Fig. 3 and 5.
s-PS	20	206	27	0	0
0.25	20	217	49	0	0
0.5	20	337	118	0.24	48
0.75	20	341	141	0.27	36
0.9	20	483	172	0.53	58
PPO (powder)	25	535	195	0.59	59
0.5	5	112	19	0.08	15
0.5	10	273	92	0.20	39
0.5	20	337	118	0.24	48
0.5	30	343	108	0.18	35

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Fig. 6 Surface area (S_BET , m² g⁻¹) of PPO/s-PS aerogels (prepared from DCE gels and after complete solvent extraction by scCO₂), as evaluated by the BET model in the standard 0.05 < P/P₀ < 0.25 pressure range: (A, B) from gels with a polymer content of 20 wt% (aerogel apparent density of 0.2 g cm⁻³) versus the PPO fraction (x_PPO; A) or versus the crystalline PPO fraction (x_PPO,crys; B); (C) versus the aerogel apparent density, for x_PPO = 0.50.

The predominant contribution of PPO crystallinity to the aerogel surface area is also confirmed by comparing aerogels exhibiting equal PPO content. In fact, the high-porosity PPO/s-PS – 50/50 wt% aerogel prepared from diluted gels (C_pol = 5 wt%, 7th line in Table 1), which presents a low degree of crystallinity, exhibits a surface area nearly three times lower than for the denser highly crystalline aerogels, as obtained from more concentrated gels (C_pol = 10–30 wt%, last three lines in Table 1).

Particularly interesting is the comparison of the S_BET dependence on the apparent density for s-PS and PPO/s-PS aerogels, which is shown in Fig. 6C. In fact, while for s-PS aerogels S_BET increases as expected when the density decreases (black squares), the opposite unexpected behavior is observed for the PPO/s-PS aerogels (red circles). This behavior can be rationalized by the achievement of maximum content of nanoporous–crystalline phase of PPO only for denser PPO/s-PS aerogels (C_pol ≥ 10 wt%, Table 1).

For these monolithic PPO-based aerogels exhibiting high surface areas (up to 350–500 m² g⁻¹), also for densities higher than 0.2 g cm⁻³, the uptake of pollutant molecules as guests of the nanoporous-crystalline phases is expected to be large, mainly if expressed as content per volume rather than per weight.

The high potential of these nanoporous–crystalline PPO based aerogels for molecular separations is here shown, for instance, for the removal of traces of carbon tetrachloride from aqueous solutions. The reported results refer to CCl₄ uptake from 10 ppm aqueous solutions.

The FTIR spectra in the 800–770 cm⁻¹ range of the PPO/s-PS – 50/50 aerogel with crystalline PPO of Fig. 5C, before and after CCl₄ equilibrium sorption from a 10 ppm aqueous solution, are compared in Fig. 7. The appearance of the peaks at 788 and 785 cm⁻¹ clearly indicates the occurrence of a substantial sorption of CCl₄, which has been quantified by thermogravimetric analysis (Fig. 8). In particular, the aerogel of lower porosity (P = 80%) and high PPO crystallinity (χ_PPO = 48%, whose WAXD pattern is shown in Fig. 5C) and the aerogel of higher porosity (P = 95%) and poor PPO crystallinity (χ_PPO =15%, whose WAXD pattern is shown in Fig. 5A) exhibit a CCl₄ uptake of 13 wt% (Fig. 8c) and 7 wt% (Fig. 8b), respectively. This result clearly confirms that the uptake of carbon tetrachloride from diluted aqueous solutions is mainly due to the PPO nanoporous-crystalline phase of the aerogel.

Fig. 7 FTIR spectra in the 800–770 cm⁻¹ range of the PPO/s-PS - 50/50 aerogel with crystalline PPO (WAXD of Fig. 4C), before (a) and after (b) CCl₄ equilibrium sorption from a 10 ppm aqueous solution.

Fig. 8 TGA scans at 10 °C min⁻¹ of PPO/s-PS 50/50 aerogels. (a) untreated; (b, c) after CCl₄ equilibrium sorption from a 10 ppm aqueous solution: (b) with P = 95% and χ_PPO = 15% ;(c) with P = 80% and χ_PPO = 48%. The TGA scan of a PPO amorphous film is shown, for comparison, as (d).

In the same conditions, the CCl₄ uptake from the PPO/s-PS – 90/10 wt% aerogel with crystalline PPO (of Fig. 3E), exhibiting surface area higher than for 50/50 aerogels (Table 1), is close to 19 wt%. For the sake of comparison, it is worth adding that after three days of sorption, nanoporous–crystalline aerogels based on the δ form of s-PS, show a CCl₄ sorption lower than 0.5 wt% while PPO films exhibiting a nanoporous–crystalline form of PPO show a guest uptake lower than 1 wt%. The much higher guest uptake from aerogels with respect to PPO films, also exhibiting nanoporous–crystalline phases, are due to their much faster sorption kinetics associated with the aerogel morphology.

It is worth adding that the occurrence of higher molecular diffusivities in aerogels can also be responsible for the observed decrease of the temperature of onset of thermal degradation in TGA scans. In this respect, curve d in Fig. 8 shows the TGA of an amorphous PPO film obtained by compression moulding (being similar to scans of semicrystalline PPO films as obtained by different procedures like casting or solvent-induced crystallization, not shown). It is apparent that the aerogels (curves a, b and c in Fig. 8) present degradation phenomena shifted toward lower temperatures of nearly 40–50 °C.

Conclusions

X-ray diffraction characterizations of PPO gels with different solvents have shown that the gel physical cross-linking is constituted by co-crystalline phases between the polymer and solvent molecules. In particular, PPO gels with carbon tetrachloride and benzene exhibit co-crystalline phases, whose X-ray diffraction patterns are rather similar to those of the two nanoporous–crystalline limit modifications, obtained after complete solvent extraction by scCO₂.

All attempt to obtain monolithic aerogels from PPO gels were unsuccessful. PPO based monolithic aerogels were instead easily obtained starting from gels containing, beside PPO, a polymer miscible with PPO at molecular level and able to form aerogels (s-PS). SEM experiments suggest that the stability of PPO/s-PS aerogels is due to the s-PS fibrillar morphology, which is still apparent also for PPO/s-PS – 90/10 wt% aerogels.

X-ray diffraction experiments show that these mixed aerogels can exhibit nanoporous–crystalline phases of both polymers. In particular, aerogels with high degrees of PPO crystallinity (in the range 35–50%) can be easily obtained, at least for high PPO fractions (x_PPO ≥ 0.5) and high densities (ρ_app ≥ 0.1 g cm⁻³).

These nanoporous–crystalline PPO rich aerogels present high surface areas (up to 480 m² g⁻¹) and high guest solubilities and diffusivities, due to the presence of the crystalline cavities of both polymers, and hence are particularly suitable for removal of traces of organic pollutants from water and air. For instance, the carbon tetrachloride uptake from 10 ppm aqueous solutions for these aerogels can be as high as 19 wt%, while the corresponding uptake for s-PS aerogels and PPO films is lower than 1 wt%, after 3 days of exposure.

Acknowledgements

We thank Prof. Ernesto Reverchon, Prof. Vincenzo Venditto and Dr. Gianluca Fasano of the University of Salerno, Prof. Vittorio Petraccone and Dr. Oreste Tarallo of the University of Naples “Federico II”, and Prof. Giuseppe Spoto of the University of Turin for useful discussions. Financial support of the “Ministero dell'Istruzione, dell'Università e della Ricerca” and of Regione Campania is gratefully acknowledged.

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