ε Form Gels and Aerogels of Syndiotactic Polystyrene - ACS Publications

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ε Form Gels and Aerogels of Syndiotactic Polystyrene Concetta D’Aniello, Christophe Daniel,* and Gaetano Guerra Department of Chemistry and Biology and INSTM Research Unit, Università degli Studi di Salerno, Via Giovanni Paolo II, 84084 Fisciano (SA), Italy ABSTRACT: Mixtures of two solvents allow obtaining stable physical gels of syndiotactic polystyrene (s-PS), whose physical knots are for the first time constituted by ε cocrystals. These ε gels, by simple scCO2 extraction procedures, lead to aerogels only exhibiting the nanoporous-crystalline ε form, with a degree of crystallinity higher than 45%. This preparation procedure is the first one leading to pure ε form aerogels. Although the BET surface areas for ε aerogels (exhibiting crystalline nanochannels) are much lower than for δ form aerogels (exhibiting isolated crystalline nanocavities), the uptake of long organic molecules is definitely higher for ε aerogels, with differences becoming very high at low guest pressures. Moreover, for all guests, pure ε aerogels present uptakes significantly higher than the previously known ε + γ aerogels.

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INTRODUCTION

Monolithic organic aerogels can be easily obtained not only by using cross-linked polymers like e.g. resorcinol−formaldehyde,1 melamine−formaldehyde,2 polyurethane,3 polyimide,4 or polyamide5 but also by drying of physical gels (both organogels6−11 and hydrogels12−16), as formed by linear un-cross-linked polymers. In particular, the robustness of physically crosslinked organogels and of the corresponding aerogels is associated with the presence of a polymer crystalline phase, preferentially with fibrillar morphology.6−11 In the past two decades, nanoporous-crystalline forms, presenting a density lower than the corresponding amorphous phase, have been discovered for syndiotactic polystyrene (sPS) 17−22 and poly(2,6-dimethyl-1,4-phenylene) oxide (PPO).23−26 These nanoporous-crystalline polymers are able to absorb low-molecular-mass molecules also when present in traces and have been proposed for molecular separation,27−31 sensor,32,33 and catalysis34,35 applications. In particular, for s-PS, two nanoporous crystalline forms, corresponding to the packing of polymer chains in the helical s(2/1)2 conformation, whose density is of 0.98 g cm−3, i.e., definitely smaller than that one of the amorphous phase (1.05 g cm−3),18 have been thoroughly described: the monoclinic δ form,17,18 with axes a = 1.74 nm, b = 1.185 nm, c = 0.77 nm, and γ = 117° (Figure 1A,A′) and the orthorhombic ε form,21 with axes a = 1.61 nm, b = 2.18 nm, and c = 0.79 nm (Figure 1B,B′). The distribution of the empty space is rather different for the two nanoporous-crystalline forms. For the δ form, the empty space can be described as isolated cavities having a volume close to 0.125 nm3, confined by layers of closely packed alternated enantiomorphous helices, parallel to the ac-plane (Figure 1A,A′).17 For the ε form, the empty space is instead present as channel-shaped cavities crossing the unit cells along the c-axis and delimited, along the b-axis, by two enantiomorphous helical chains (Figure 1B,B′).21 © XXXX American Chemical Society

Figure 1. Top (A, B) and lateral (A′, B′) views of the crystalline structures of the two nanoporous crystalline forms of s-PS. For the δ (A, A′) and ε (B, B′) forms, the porosity is distributed as cavities and channels, respectively.

Received: December 11, 2014 Revised: January 28, 2015

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DOI: 10.1021/ma502500n Macromolecules XXXX, XXX, XXX−XXX

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typical procedure of gel preparation is the following: 0.7 g of polymer was added to 5 mL of chloroform in hermetically sealed test tubes heating the mixture above the boiling point of the solvents until complete dissolution of the polymer and the appearance of a transparent and homogeneous solution had occurred. Then a solution of 5 mL of chloroform and 10 mL of acrylonitrile were slowly added to the hot solution. Finally, the hot solution was cooled to room temperature where gelation occurred. The ε form aerogels were obtained by treating these ε form s-PS gels with a SFX 200 supercritical carbon dioxide extractor (ISCO Inc.) by using the following conditions: T = 40 °C, P = 200 bar, extraction time t = 5 h. The δ form aerogels were obtained by extracting by the same method s-PS gels prepared in o-dichlorobenzene at Cpol = 10 wt %. The γ form s-PS aerogels were obtained, with the same apparatus, from s-PS gels in 1,2-dichloroethane at Cpol = 5 wt %, by using the following conditions: T = 130 °C, P = 200 bar, extraction time t = 120 min. Aerogels prevailingly exhibiting the ε form but also minor amounts of γ form and/or δ form were prepared by following the previously reported process,38 i.e., by immersion in chloroform of γ form aerogel followed by chloroform removal with supercritical CO2 at 40 °C. The porosity P of the aerogels can be expressed as a function of the aerogel apparent density ρ as

In these channels, guest molecules are generally hosted with their longer molecular axis roughly parallel to the polymer chain axis. In recent years, the use of these polymers in aerogel preparation has allowed achieving a special class of monolithic physically cross-linked aerogels, where the crystallites that constitute the physical knots of the aerogel exhibit a nanoporous-crystalline phase.36−38 These nanoporous-crystalline polymeric aerogels present, beside disordered amorphous mesopores and micropores (typical of all aerogels), also all identical nanopores of the crystalline phases. X-ray diffraction, neutron diffraction, and differential scanning calorimetry characterizations have allowed clarifying that the junction zones of s-PS gels,39−44 precursor of the aerogels, are constituted by cocrystalline phases between polymer helices and low-molecular-mass guest molecules (δ clathrates or δ intercalates),19,45 which exhibit some structural similarity with the nanoporous δ form (Figure 1A,A′). ScCO2 drying of these gels not only removes solvent molecules leading to stable monolithic aerogels but also removes the guest molecules from the cocrystalline phases,46−48 always leading to nanoporous-crystalline δ phases.37−39 Although many different ε clathrates have been obtained,45,49−51 gels including ε clathrates have not yet been described. Moreover, s-PS aerogels exhibiting the nanoporouscrystalline ε form have only been obtained together with some amount of other crystalline forms (γ or δ). In addition, these prevailingly ε aerogels are obtained by using a rather complex procedure,38 where a δ cocrystalline gel is extracted by hightemperature scCO2 leading to a γ form aerogel, which is then treated with chloroform and subsequently extracted by lowtemperature scCO2.38 s-PS aerogels have been widely studied, mainly for their molecular sorption properties,37,38,52−55 and their physical properties have been tailored by chemical functionalization (mainly sulfonation)56,57 as well as by blending with other polymers58−60 or by making composites with nanofillers.35,61−64 All the cited literature reports include results relative to the easily prepared δ form aerogels while only a few reports also focus on aerogels with the ε form.38,54 The limited number of literature reports on ε form aerogels is possibly due to the definitely more complex preparation procedures as well as to the presence of additional s-PS crystalline phases. In this article we show, for the first time, s-PS gels exhibiting the ε clathrate form and the preparation of corresponding aerogels only exhibiting the ε nanoporous-crystalline form, by direct scCO2 drying in mild conditions. We also show that pure ε form aerogels (presenting crystalline nanochannels, Figure 1B,B′) are much more suitable than δ form aerogels (presenting crystalline isolated nanocavities, Figure 1A,A′) to absorb long guest molecules.

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⎛ ρ⎞ P = 100⎜⎜1 − ⎟⎟ ρS ⎠ ⎝ where ρS, the density of the polymer, is equal to 1.02 g/cm3 for semicrystalline s-PS samples with a crystallinity of 40%, exhibiting the ε phase. Characterization Techniques. X-ray diffraction patterns were obtained on a Bruker D8 Advance automatic diffractometer operating with a nickel-filtered Cu Kα radiation. Evaluation of the correlation length D of the crystalline domains (where an ordered disposition of the atoms is maintained) was effected using the Scherrer formula:

D = 0.9λ /(β cos θ ) where β is the full width at half-maximum expressed in radian units, λ is the wavelength, and θ is the diffraction angle. For each observed reflection with θ < 1°, the width at half-height was evaluated by subtracting the unavoidable instrumental broadening using the formula

β 2 hkl = B2 hkl − b2 The instrumental broadening, b, was determined by obtaining a WAXD pattern of a KBr powder sample having a full width at halfmaximum, under the same geometrical conditions, of 0.17°. Infrared spectra were obtained at a resolution of 2.0 cm−1 with Bruker spectrometers (Vertex70 and Tensor27) equipped with deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter. The frequency scale was internally calibrated to 0.01 cm−1 using a He−Ne laser. Thirty-two scans were signal averaged to reduce the noise. The internal morphology of the aerogels was characterized by means of a scanning electron microscope (Zeiss Evo50 equipped with an Oxford energy dispersive X-ray detector or Leica 440). Samples were prepared by fracturing small pieces of the monoliths in order to gain access to the internal part of the specimen, and before imaging, all specimens were coated with gold depositing approximately 20 nm of gold. The coating procedure was necessary to prevent surface charging during measurement. Nitrogen adsorption at liquid nitrogen temperature (77 K) was conducted with a Nova Quantachrome 4200e instrument. Before the adsorption measurement, aerogel samples were degassed at 40 °C under vacuum for 24 h. The surface area values were calculated using the Brunauer−Emmett−Teller (BET) method in the range 0.05−0.2 P/P0.

EXPERIMENTAL SECTION

Materials. The syndiotactic polystyrene used in this study was manufactured by Dow Chemicals under the trademark Questra 101. 13 C nuclear magnetic resonance characterization showed that the content of syndiotactic triads was over 98%. The mass average molar mass obtained by gel permeation chromotography (GPC) in trichlorobenzene at 135 °C was found to be Mw = 3.2 × 105 g mol−1 with a polydispersity index Mw/Mn = 3.9. Solvents used to prepare the gels were purchased from Aldrich and used without further purification. The ε form s-PS gels were obtained by using a 50/50 by volume mixture of two different solvents: chloroform and acrylonitrile. A B

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Macromolecules The vapor sorption measurements have been carried out at 35 °C with a VTI-SA symmetrical vapor sorption analyzer from TA Instruments. The temperature of the solvent evaporator container was 40 ± 1 °C.

obtained from a gel with 3 wt % of polymer, having a porosity of nearly 92%, is shown in Figure 3A. This pattern only

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RESULTS AND DISCUSSION ε Form Gels. As described in previous reports, s-PS gels with solvents being suitable as guest of s-PS generally exhibit δ cocrystalline phases. Moreover, for solvents whose molecular volume is bigger than the crystalline cavity of the δ form, gels exhibiting the dense β38 phase are usually obtained. Even with chloroform, being the only known guest suitable to produce ε form samples (mainly by sorption in γ or α form samples),20−22 only δ form gels and aerogels are obtained.36 s-PS gels are presently prepared by using, as solvent, mixtures of chloroform and acrylonitrile, by following the procedure described in detail in the Experimental Section. The X-ray diffraction patterns of a chloroform/acrylonitrile (50/50 by volume) gel with a polymer concentration of 3 wt % after progressive desiccation in air at room temperature (A−D) and of the powder as obtained by subsequent complete desiccation by scCO2 (E) are shown in Figure 2.

Figure 3. X-ray diffraction patterns of s-PS aerogels with a porosity close to 90%, exhibiting different crystalline forms: (A) ε, (B) δ, (C) ε + δ, (D) γ, and (E) ε + γ.

presents the reflections of the ε form and is similar to that one of the ε form powder of Figure 2E, with a similar degree of crystallinity (ca. 47%). The narrowness of the symmetric 110 reflection (at 2θ = 6.8°) indicates that the correlation lengths of the crystallites perpendicular to the 110 planes is close to 40 nm. This correlation length is about twice larger than the value obtained for the ε powder whose diffraction pattern is shown in Figure 2 (curve E). For the sake of comparison, the X-ray diffraction patterns of monolithic aerogels with porosity of 90% and exhibiting the pure δ and γ forms are shown in Figures 3B and 3D, respectively. The pattern of the δ aerogel exhibits the typical intense 010 reflection at 2θCuKα = 8.4°,16 while the pattern of the γ aerogel exhibits the typical 200 and 020 reflections at 2θCuKα = 9.3° and 10.5°,64 respectively. The monolithic s-PS aerogels as obtained by the previously known procedure, which involves chloroform sorption/ desorption in γ form aerogels, always present beside the ε form some amount of γ and/or δ form. This is shown, for instance by the X-ray diffraction patterns of two different monolithic aerogels with P ≈ 90%, which are shown in Figure 3C,E. The pattern of Figure 3C, similar to that one of Figure 6A of ref 38, presents all the peaks of the ε form, but with the 020 reflection shifted to 2θCuKα = 8.1° and with intensity much higher than for the 110 peak (at 2θCuKα = 6.9°). This indicates the presence of a significant amount of δ form, which can be evaluated as 30% of the overall crystallinity considering the calculated intensities of the δ form17 and ε form22 diffraction peaks. The pattern of Figure 3E, somewhat similar to that one of Figure 8B of ref 38, again presents all the peaks of the ε form but also shows broad reflections at 9.3° and 10.5°, indicating the presence of a significant amount of γ form. The intensity of the diffraction peak at 2θCuKα = 8.0° which is larger than the intensity of the diffraction peak at 2θCuKα = 6.9° also indicates that a minor amount of δ form is also present in the aerogel. The fraction of ε form, δ form, and γ form can be evaluated as 30%, 10%, and 60% of the overall crystallinity. The overall degree of crystallinity, which for the pure ε aerogel as obtained by the mixed solvents is 47%, can be

Figure 2. X-ray diffraction patterns of a s-PS gel in chloroform/ acrilonitrile (50/50% by volume) after progressive desiccation in air (A−D) and subsequent complete desiccation by scCO2 (E). The residual solvent content in the sample is indicated as percent by weight, close to the patterns.

All the diffraction patterns of Figure 2 present the peaks of the ε form at 2θCuKα = 6.8°, 8.0°, 13.6°, 16.2°, 20.3°, and 23.2°, which of course are better defined for the fully scCO2 desiccated powder (Figure 2E). The latter sample presents a degree of crystallinity of ca. 50%, and the intensity of the 110 reflection (at 2θ = 6.8°) higher than for the 020 reflection (at 2θ = 8.0°) clearly indicates the high purity of this crystalline phase.21 It is worth adding that acrylonitrile is not able to dissolve sPS, and as a consequence s-PS/acrylonitrile gels have not been prepared. Acrylonitrile is instead able to induce crystallization of amorphous s-PS, leading to the dense γ form. Hence, the determinant role of this solvent in obtaining ε form gels is not easy to rationalize. ε Form Aerogels. The extraction by scCO2 of gels of s-PS in chloroform/acrylonitrile (e.g., 50/50 by volume) leads to monolithic aerogels having a size similar to the original gels. The X-ray diffraction pattern of the monolithic aerogel as C

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The spectrum of the pure ε form aerogel of high crystallinity (Figure 4A) clearly shows the vibrational peaks typical of the ε form (at 965 and 914 cm−1)66 as well as an additional peak at 983 cm−1. All these peaks are possibly due to splitting of packing sensitive vibrational modes, which are dominated by out-of-plane bending of hydrogen atoms of the phenyl rings.67 An analysis of the whole FTIR spectra (not shown here) confirms that the overall degree of crystallinity is similar for ε + γ and for ε + δ aerogels, as obtained by the old method, and for pure ε aerogels, as obtained by the new method. In Figure 5, the SEM micrograph of pure ε-form aerogel A) is compared with the typical SEM obtained with a δ-form aerogel (B), a γ-form aerogel (C), and a mixed ε + δ aerogel prepared by the previously known procedure38 (D). The SEM analysis shows that morphology of the pure ε form aerogel obtained by the new preparation process is quite different from the morphologies observed with the other aerogels. Indeed, while the mixed ε + δ aerogels prepared by the previously known procedure is characterized by a sort of open pore morphology with pores with diameters in the range 0.5−2 μm (Figure 5D), the pure ε aerogels exhibits both fibers with a 30−60 nm diameter range (Figure 5A), similar to those observed for δ aerogels (Figure 5B) and for γ aerogels (Figure 5D) but also lamellar structures having diameters roughly in the range of 1−2 μm, which could explain the large correlation length of the crystallites perpendicular to the equatorial reflection (D110 ≈ 40 nm), as evaluated from the X-ray diffraction pattern of Figure 3A. Sorption N2 isotherms, where the sorption is expressed as cm3 of nitrogen in normal conditions (1 atm, 0 °C) per gram of polymer, are reported in Figure 6 for the aerogels exhibiting ε, δ, γ, ε + δ, and ε + γ forms, whose WAXD patterns are shown in Figures 3A to 3E, respectively.

calculated as ca. 45% for the ε + δ and ε + γ aerogels of Figures 3C and 3E, respectively. It is worth adding that, for more concentrated s-PS gels (Cpol > 5 wt %), the presently proposed procedure of aerogel preparation that uses mixtures of chloroform and acrylonitrile leads to aerogels with mixed ε and β phases. Highly informative is the comparison between FTIR spectra of s-PS pure ε, ε + γ, and ε + δ aerogels which are shown in Figure 4 for the chain packing sensitive66 spectral range 1000− 880 cm−1, by curves A (blue line), B (black line), and C (red line), respectively.

Figure 4. FTIR spectra, in the range 1000−880 cm−1, of s-PS monolithic aerogels exhibiting different crystalline forms: (A) pure ε (blue), (B) ε + γ (black), and (C) ε + δ (red).

Figure 5. Scanning electron micrographs of a pure ε form aerogel obtained by the new preparation process (A), a δ form aerogel obtained by supercritical CO2 extraction at 40 °C of a gel prepared in chloroform at Cpol = 0.1 g/g (B), a γ form aerogel obtained by supercritical CO2 extraction at 130 °C of a gel prepared in DCE at Cpol = 0.05 g/g, by supercritical CO2 extraction at 130 °C (C) and mixed ε + δ aerogels prepared by the previously known procedure (D). Size bar = 10 μm; (B) size bar = 3 μm. The porosity P of the aerogels is ≈90%. D

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For instance, sorption equilibrium uptakes of decane and tetradecane from vapor phase at 35 °C at pressures lower than 0.08 P/P0 for ε, δ, γ, and ε + γ aerogels are reported in Figure 7. As expected,37,52−55 the equilibrium uptakes of decane and tetradecane in aerogels with δ and ε nanoporous crystalline phase are much higher than for the γ aerogel. We can also clearly observe that the uptake in the ε aerogel is larger than for δ aerogel and the difference is particularly significant for low pressure activities. Thus, for instance, for a vapor pressure P/P0 = 0.01 the decane and tetradecane uptake in the δ aerogel is negligible while it is 1.5 and 0.8 wt %, respectively, in the pure ε aerogel. Moreover, at higher vapor pressures (P/P0 = 0.04− 0.08), the tetradecane sorption in the pure ε aerogel is in the range 4−8.5 wt %, and it is nearly double than for the δ aerogel as well as for the ε + γ as obtained by the old process. This higher sorption uptake observed for the pure ε aerogel with respect to the δ aerogel is of course due to the channel-shape cavities of the ε form.

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CONCLUSIONS The use of mixtures of acrylonitrile and chloroform allows the preparation of stable s-PS physical gels, where physical knots are for the first time constituted by crystallites exhibiting the ε cocrystalline form. These ε gels, by suitable scCO2 extraction procedures, can easily lead to powders and aerogels only exhibiting the nanoporous-crystalline ε form. This preparation procedure of ε form powders is the first one from solution, rather than by chloroform sorption in γ or α or mesomorphic trans-planar powders. Moreover, this preparation procedure of ε form aerogels is also the first one only leading to pure ε form (and with a degree of crystallinity higher than 45%) rather than ε form with significant amounts of the dense γ form or of the nanoporous δ form. The availability of s-PS samples only exhibiting the ε form and with high crystallinity allows to clearly point out some splittings of out-of-plane bending peaks of phenyl hydrogens (at 983, 965, and 914 cm−1), possibly due to chain packing effects specific of the nanoporous ε form. Although the nitrogen sorption (and hence the BET surface area) are much higher for δ aerogels exhibiting isolated nanocavities than for ε aerogels exhibiting nanochannels, the sorption of long organic molecules is higher for pure ε aerogels.

Figure 6. Nitrogen sorption isotherms of s-PS aerogels, with porosity in range 88−92% and exhibiting different crystalline forms. The sorption is expressed as cm3 of nitrogen in normal conditions (1 atm, 273 K) per gram of polymer. The nitrogen uptake (and the derived BET surface areas) increases following the sequence γ, ε + γ, ε, ε + δ, and δ.

The minimum BET surface area occurs for γ aerogels (71 m2/g) and is similar to those of other s-PS aerogels exhibiting the dense s-PS crystalline β phase (60−70 m2/g).38,55 The maximum BET surface area occurs for the δ aerogels (290 m2/ g).38,55 For the pure ε aerogels the surface area (180 m2/g) is intermediate between these two values. As expected, the surface areas of the ε + γ and ε + δ aerogels, 150 and 230 m2/g, respectively, are intermediate between those of the corresponding pure crystalline forms. Although the nitrogen sorption (and hence the BET surface area) is much higher for δ aerogels than for pure ε aerogels, the sorption of long organic molecules which are more suitable for the channels of the ε form (Figure 1B,B′) than for the cavities of the δ form (Figure 1A,A′) is much higher for pure ε aerogels.

Figure 7. Decane (A) and tetradecane (B) sorption equilibrium uptakes at 35 °C and pressures lower than 0.08 P/P0 for ε, δ, γ, and ε + γ aerogels. E

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(20) Rizzo, P.; Daniel, C.; De Girolamo Del Mauro, A.; Guerra, G. Chem. Mater. 2007, 19, 3864−3866. (21) Petraccone, V.; Ruiz de Ballesteros, O.; Tarallo, O.; Rizzo, P.; Guerra, G. Chem. Mater. 2008, 20, 3663−3668. (22) Tarallo, O.; Schiavone, M. M.; Petraccone, V.; Daniel, C.; Rizzo, P.; Guerra, G. Macromolecules 2010, 43, 1455−1466. (23) Daniel, C.; Longo, S.; Vitillo, J. G.; Fasano, G.; Guerra, G. Chem. Mater. 2011, 23, 3195−3200. (24) Galizia, M.; Daniel, C.; Fasano, G.; Guerra, G.; Mensitieri, G. Macromolecules 2012, 45, 3604−3615. (25) Daniel, C.; Zhovner, D.; Guerra, G. Macromolecules 2013, 46, 449−454. (26) Galizia, M.; Daniel, C.; Guerra, G.; Mensitieri, G. J. Membr. Sci. 2013, 443, 100−106. (27) Manfredi, C.; Del Nobile, M. A.; Mensitieri, G.; Guerra, G.; Rapacciuolo, M. J. Polym. Sci., Polym. Phys. Ed. 1997, 36, 133−140. (28) Musto, P.; Mensitieri, G.; Cotugno, S.; Guerra, G.; Venditto, V. Macromolecules 2002, 35, 2296−2304. (29) Mahesh, K. P. O.; Sivakumar, M.; Yamamoto, Y.; Tsujita, Y.; Yoshimizu, H.; Okamoto, S. J. Membr. Sci. 2005, 262, 11−19. (30) Venditto, V.; De Girolamo Del Mauro, A.; Mensitieri, G.; Milano, G.; Musto, P.; Rizzo, P.; Guerra, G. Chem. Mater. 2006, 18, 2205−2210. (31) Albunia, A. R.; Rizzo, P.; Guerra, G. Polymer 2013, 54, 1671− 1678. (32) Pilla, P.; Cusano, A.; Cutolo, A.; Giordano, M.; Mensitieri, G.; Rizzo, P.; Sanguigno, L.; Venditto, V.; Guerra, G. Sensors 2009, 9, 9816−9857. (33) Erdogan, M.; Ozbek, Z.; Capan, R.; Yagci, Y. J. Appl. Polym. Sci. 2012, 123, 2414−2422. (34) Buonerba, A.; Cuomo, C.; Sanchez, S. O.; Canton, P.; Grassi, A. Chem.Eur. J. 2012, 18, 709−715. (35) Sannino, D.; Vaiano, V.; Sacco, O.; Ciambelli, P.; Longo, S.; Venditto, V.; Guerra, G. J. Chem. Technol. Biotechnol. 2014, 89, 1175− 1181. (36) Daniel, C.; Alfano, D.; Venditto, V.; Cardea, S.; Reverchon, E.; Larobina, D.; Mensitieri, G.; Guerra, G. Adv. Mater. 2005, 17, 1515− 1518. (37) Daniel, C.; Sannino, D.; Guerra, G. Chem. Mater. 2008, 20, 577−582. (38) Daniel, C.; Giudice, S.; Guerra, G. Chem. Mater. 2009, 21, 1028−1034. (39) Daniel, C.; Deluca, M. D.; Guenet, J. M.; Brulet, A.; Menelle, A. Polymer 1996, 37, 1273−1280. (40) Rastogi, S.; Goossens, J. G. P.; Lemstra, P. J. Macromolecules 1998, 31, 2983−2998. (41) van Hooy-Corstjens, C. S. J.; Magusin, P. C. M. M.; Rastogi, S.; Lemstra, P. J. Macromolecules 2002, 35, 6630−6637. (42) Daniel, C.; Musto, P.; Guerra, G. Macromolecules 2002, 35, 2243−2251. (43) Malik, S.; Rochas, C.; Guenet, J. M. Macromolecules 2006, 39, 1000−1007. (44) Itagaki, H.; Tokami, T.; Mochizuki, J. Polymer 2012, 53, 5304− 5312. (45) Jose, R. C.; Shaiju, P.; Nagendra, B.; Gowd, E. B. Polymer 2013, 54, 6617−6627. (46) Reverchon, E.; Guerra, G.; Venditto, V. J. Appl. Polym. Sci. 1999, 74, 2077−2082. (47) Ma, W.; Yu, J.; He, J. Macromolecules 2005, 38, 4755−4760. (48) Fang, J.; Kiran, E. Macromolecules 2008, 41, 7525−7535. (49) Tarallo, O.; Schiavone, M. M.; Petraccone, V.; Daniel, C.; Rizzo, P.; Guerra, G. Macromolecules 2010, 43, 1455−1466. (50) Albunia, A. R.; D’Aniello, C.; Guerra, G. CrystEngComm 2010, 12, 3942−3949. (51) Tarallo, O.; Schiavone, M. M. Soft Mater. 2011, 9, 124−140. (52) Malik, S.; Rochas, C.; Guenet, J.-M. Macromolecules 2005, 38, 4888−4893. (53) Malik, S.; Roizard, D.; Guenet, J.-M. Macromolecules 2006, 39, 5957−5959.

For instance, the uptake of decane and tetradecane from the vapor phase is much higher in ε aerogels than in δ aerogel, with differences becoming higher at low guest pressures. The higher uptake of these long organic molecules observed with ε aerogels is due to the channel shape cavities of the ε form. Large guest uptake improvements, for these pure ε aerogels with respect to the known ε + γ aerogel, are also reported. This improvement is due to the higher degree of ε form crystallinity (47% vs ca. 15%). The discovery of pure ε form aerogels further enlarges the applicability of nanoporous-crystalline polymeric aerogels in molecular separations as well as in water and air purification.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Vincenzo Venditto and Dr. Paola Rizzo of University of Salerno for useful discussions. The authors are indebted to Dr. Giuseppe Narciso of Istituto di Chimica e Tecnologia dei Polimeri (Pozzuoli) for making the SEM analysis. Financial support of the “Ministero dell’Istruzione, dell’Università e della Ricerca” (PRIN 2010 XLLNM3) and of Regione Campania is gratefully acknowledged.

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DOI: 10.1021/ma502500n Macromolecules XXXX, XXX, XXX−XXX