Etched Fibers of Syndiotactic Polystyrene with Nanoporous-Crystalline

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Etched Fibers of Syndiotactic Polystyrene with NanoporousCrystalline Phases Christophe Daniel,* Pasqualmorica Antico, 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

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S Supporting Information *

ABSTRACT: Nanoporous-crystalline (δ and ε form) fibers of syndiotactic polystyrene (s-PS) have been prepared from melt-spun fibers by different guest-induced cocrystallization procedures. Their crystal structure, morphology, and uptake of volatile organic compounds (VOC) have been investigated. The use of chloroform, i.e., of a molecule being a suitable guest for both δ and ε forms but also a strong solvent for s-PS, leads to nanoporous-crystalline fibers with VOC sorption kinetics much faster than for s-PS nanoporous-crystalline powders and films and comparable with those of nanoporous-crystalline s-PS aerogels. This unexpected phenomenon is due to etching of the melt-spun fibers by chloroform that exposes internal fibrils and markedly increases the surface area up to 165 m2/g. Etched δ form fibers are more effective for sorption of most pollutants while etched ε form fibers are much more effective in sorption of long organic molecules, like ndecane. Outstanding sorption properties, simple preparation processes (easily scalable at the industrial level), and safe morphology make etched nanoporous-crystalline s-PS fibers particularly suitable as sorption media for VOC removal from water and air.

1. INTRODUCTION Two commercial polymers (syndiotactic polystyrene, s-PS,1,2 and poly(2,6-dimethyl-1,4-phenylene) oxide, PPO)3 can be crystallized in nanoporous-crystalline forms. Processes leading to nanoporous-crystalline forms (ultramicroporous according to IUPAC classification) are based on cocrystallization with low-molecular-mass guest molecules followed by suitable guest extraction procedures.4−6 These nanoporous-crystalline polymer phases are able to efficiently absorb guest molecules in their molecular-size cavities and have been proposed for several applications in molecular separation,7−13 sensors,14−16 and catalysis.17−19 For s-PS, two nanoporous crystalline forms with similar density (0.98 g cm−3, being definitely smaller than for the amorphous phase, 1.05 g cm−3), corresponding to different packing of polymer chains in the helical s(2/1)2 conformation, have been thoroughly described: the monoclinic δ form,1 with a = 1.74 nm, b = 1.185 nm, c = 0.77 nm, and γ = 117°, and the orthorhombic ε form,2 with a = 1.61 nm, b = 2.18 nm, and c = 0.79 nm. 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. Recently, for the nanoporous δ form, a triclinic modification different from the usual monoclinic modification has been also reported.6 This triclinic δ-form is typically obtained when solvent removal from the cocrystalline phase is achieved by the use of a volatile solvent (like acetonitrile).6 For the ε form, the empty space is instead present as channels crossing the unit cells along the c-axis and delimited, along b-axis, by two enantiomorphous helical chains. In these channels, guest molecules are generally hosted with © XXXX American Chemical Society

their longer molecular axis roughly parallel to the polymer chain axis. Besides these ordered nanoporous crystalline forms, disordered nanoporous crystalline phases have been also described.20 It has been observed that sorption kinetics of s-PS materials with nanoporous crystalline phases strongly depend on sample morphology. 21 Nanoporous-crystalline polymeric aerogels,22−32 which present nanopores of the crystalline phase beside amorphous mesopores and micropores (typical of all aerogels), are particularly effective in molecular sorption. In fact, these aerogels exhibit a high sorption capacity typical of nanoporous crystalline phases (due to sorption of molecules as isolated guests of the host crystalline phase) associated with the high sorption kinetics typical of aerogels (due to the high and disordered porosity).22−32 However, aerogel preparation procedures are energetically and environmentally expensive and thus not easily applicable to a large-scale industrial production. Fast VOC sorption kinetics have been also achieved for amorphous s-PS microfibers (with diameter of 3 μm) as activated by eco-friendly solvents (methyl acetate or ethyl acetate).33 However, their sorption capacity is limited by the presence of disordered nanoporous-crystalline phases20 rather than of well-formed δ or ε nanoporous crystalline phases. In this contribution, it is shown that s-PS fibers with δ or ε nanoporous-crystalline phases, if properly etched, can present Received: May 17, 2018 Revised: July 24, 2018

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DOI: 10.1021/acs.macromol.8b01044 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. X-ray diffraction patterns as taken by an automatic powder diffractometer (Cu Kα) of s-PS fibers: (A) as-prepared (i) and after treatment for 5 min with liquid dichloromethane (ii), tetrahydrofuran (iii), and chloroform (iv); (B) guest-treated s-PS fibers of (A) after guest extraction by liquid acetonitrile. ij ρapp yzz j zz P = 100jjj1 − z jj ρpol zz k {

sorption kinetics of volatile organic compounds (VOC) much faster than for usual nanoporous-crystalline s-PS films and fibers and comparable with those of nanoporous-crystalline sPS aerogels. The required etching method comprises chloroform induced recrystallization of α-form fibers, leading s-PS/ CHCl3 cocrystalline forms. In fact, chloroform is not only a suitable guest for s-PS cocrystalline forms, which can lead to both δ and ε nanoporous-crystalline forms, but also a strong sPS solvent, being able to etch the semicrystalline fibers, thus exposing crystalline fibrils and increasing surface area up to 165 m2/g.

(1)

where ρpol is the density of the polymer matrix and ρapp is the aerogel apparent density calculated from the mass/volume ratio of the monolithic aerogels. 2.2. Characterization Techniques. X-ray diffraction patterns were obtained on a Bruker D8 Advance automatic diffractometer operating with a nickel-filtered Cu Kα radiation. WAXD patterns were also obtained, in transmission, by using a Philips diffractometer with a cylindrical camera (radius = 57.3 mm). In the latter case the patterns were recorded on a BAS-MS imaging plate (FUJIFILM) and processed with a digital imaging reader (FUJIBAS 1800). The degree of axial orientation (fc) has been evaluated by using the Hermans’ orientation function fc = (3 cos2 x − 1)/2 , where cos2 x is the average cosine-squared value of the angle, x, between the c crystallographic axis and the fiber axis. fc is equal to +1 or −0.5 when the c-axes of all crystallites are perfectly parallel or perpendicular to the fiber axis, respectively, while for random orientation fc is equal to 0. The degree of orientation evaluation has been conducted by using azimuthal scans of 110 and 010 reflections for α and δ form samples, respectively. All these reflections are located at 2θ < 9°, and as a consequence the correction of the azimuthal profile due to its elliptical shape is unnecessary. 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. As collection of meaningful infrared spectra of fibers in the transmission mode is a difficult task, FTIR spectra were collected in the diffuse reflectance infrared Fourier transform mode (DRIFT) using an Easy Diff accessory benchmark from Pike Technologies. 32 scans were signal averaged to reduce the noise. The degree of crystallinity of the fibers was evaluated by FTIR spectra34,35 and expressed as weight fraction χc, according to k = l/l′(1 − χc), where k is the subtraction coefficient and l and l′ are the thickness of the sample and of an amorphous reference film. The l/l′ ratio is estimated from the absorbance ratio of a conformationally insensitive peak (at 1601 cm−1). The morphology of the fibers was characterized by means of a Leica Cambridge Stereoscan S440. Before imaging, all specimens were coated with gold depositing approximately 20 nm of gold to prevent surface charging during measurement.

2. EXPERIMENTAL SECTION 2.1. Materials and Samples Preparation. S-PS fibers with diameter in the range 10−15 μm were obtained by melt processing with a melt-spin-draw type process using an extruding temperature 290 °C and a head temperature 310 °C while s-PS microfibers with diameter of 3 μm were obtained by melt blowing by using the following conditions: extruding temperature 300 °C and die air temperature 310 °C. Both s-PS fiber samples were supplied by Idemitsu Kosan Co. Ltd. using neat s-PS homopolymer (trademark Xarec). 13C nuclear magnetic resonance characterization showed that the content of syndiotactic pentads is over 98%. The mass average molar mass obtained by gel permeation chromotography (GPC) in trichlorobenzene at 145 °C was found to be Mw = 140000 g mol−1 with a polydispersity index Mw/Mn = 2.0. Solvents used to prepare the nanoporous fibers and for sorption investigations were purchased from Aldrich and used without further purification. Fibers with nanoporous crystalline phases were obtained by fiber treatments with liquid guests, leading to temporary formation of cocrystalline phases, followed by guest removal by immersion in acetonitrile. The δ-form aerogel samples were obtained by treating s-PS/toluene gels, with a SFX 200 supercritical carbon dioxide extractor (ISCO Inc.) using the following conditions: T = 40 °C, p = 200 bar, extraction time t = 60 min. For monolithic aerogels with a regular shape (i.e., spherical or cylindrical) 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 B


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Macromolecules Nitrogen adsorption at liquid nitrogen temperature (77 K) was conducted with a Nova Quantachrome 4200e instrument. The surface area values were calculated using the Brunauer−Emmett−Teller (BET) method in the range (0.05−0.2)p/p0. Vapor sorption measurements of benzene and n-decane have been performed at 35 °C with a VTI-SA symmetrical vapor sorption analyzer from TA Instruments. A reservoir filled with the liquid adsorbate set at a given temperature and nitrogen gas bubbling through the container entrains the organic vapor. This N2−organic vapor stream continuously flows over the sample positioned in a sample chamber connected to the microbalance, and the adsorbate relative pressure is set by controlling the flow ratio of nitrogen going through the organic solvent evaporator and the flow of dry nitrogen for relative pressures. The vapor-phase sorption measurements were done at 35 °C for relative pressure p/p0 (p0 being the saturated vapor pressure) of the organic adsorbates in the range 0.01−0.05. 1,2-Dichloroethane (DCE) sorption kinetics from aqueous solutions were obtained by measurements of FTIR absorbances of DCE conformationally sensitive peaks located 1234 cm−1 (trans conformer) and 1284 cm−1 (gauche conformer) and the absorbance of a typical s-PS IR band located at 1079 cm−1. By using a calibration curve obtained from samples having a known amount of DCE, it is possible form the IR absorbances of DCE and s-PS to determine the quantity of DCE being absorbed.

the same samples after successive liquid acetonitrile sorption/ desorption are reported in Figure 1B. After treatment with the three guests, the diffraction peaks of the α form (curve i) have vanished while two broad peaks located at 2θ ∼ 8.2°−8.5° and 2θ ∼ 10.1°−10.2° appear (curves ii, iii, and iv), corresponding to 010 and 2̅10 reflections of the monoclinic clathrate cocrystalline phases.1 It is also worth adding that some partial cocrystallization of the amorphous phase may also occur during the solvent treatment. Treatments of the cocrystalline fibers by sorption/ desorption of liquid acetonitrile lead to complete guest removal as verified by FTIR spectra (not shown) and correspondingly the powder diffraction patterns show a strong intensity decrease of the 2̅10 reflection (located for the cocrystalline fibers at 2θ ∼ 10.1°−10.2°) and an increase of the intensity of the 010 reflection, located at 2θ ∼ 8.6°−8.7° (Figure 1B), as typical of the nanoporous-crystalline δ forms.1,6 The position of the 010 reflection after solvent removal with acetonitrile indicates the formation of the triclinic nanoporous δ form.6 The broadness of 010 reflections corresponds to small values of the correlation length perpendicular to the 010 plane (in the range 30−40 nm for the samples of Figure 2B) and indicates the occurrence of small and imperfect crystallites. As for formation of δ form by guest removal from cocrystalline phases, it is worth adding that fiber samples behave like film samples rather than as powder samples.6 In fact, after guest removal by acetonitrile,6 the monoclinic δ clathrates are transformed in triclinic (rather than monoclinic) nanoporouscrystalline phases. As the determination of the degree of crystallinity of oriented samples from X-ray diffraction patterns may lead to erroneous values, possible differences in the degree of crystallinity of etched and unetched fibers have been achieved by FTIR spectroscopy. The IR spectra of the etched and unetched δ fibers for the wavenumber ranges 1420−1040 and 600−480 cm−1 are reported in Figures 3A and 3B, respectively. Figure 3A shows that the absorbance of the IR band located at 1278 cm−1, which is typical of the s(2/1)2 helical conformation,35 of the δ form is larger for the etched fibers than for the unetched fibers. Figure 3B shows that with respect to broad IR band located at 538 cm−1, which is tyipical of amorphous s-PS samples, the s(2/1)2 helical conformation bands35 located at 572 and 502 cm−1 are more intense for the etched δ fibers. All these differences observed in the considered wavenumber interval indicates that the etched δ fibers are characterized by a larger fraction of the polymer chains with the ordered s(2/1)2 helical conformation50 than the unetched δ fibers An estimation of the degree of crystallinity for etched and unetched fibers done by spectral subtraction analysis34,35 of the spectra reported in Figure 3 gives a degree of crystallinity of 30% for the unetched fibers and 35% for the etched fibers. Photographic WAXD patterns of Figures 2 clearly show the occurrence of polymer orientation loss as a consequence of transformation of the trans-planar chains of the α form (Figure 2A, fc = 0.9) in the s(2/1)2 helices of δ form (Figures 2B and 2C). The orientation loss is already observed for the δ form fiber obtained by sorption/desorption of dichloromethane (Figure 2B, fc = 0.7) but becomes dramatic for the δ form fiber obtained by sorption/desorption of chloroform (Figure 2C, fc = 0.3).

3. RESULTS AND DISCUSSION 3.1. Preparation and Structural Characterization of Fibers with Nanoporous Crystalline Forms. 3.1.1. Characterization of the Starting α Form Fibers. The X-ray diffraction pattern, as taken by an automatic powder diffractometer, of the as-prepared fibers with diameter of 10−15 μm (curve i, Figure 1A) displays diffraction peaks located at 2θ = 6.7°, 11.6°, 13.5°, and 17.8° with Miller indexes 110, 300, 220, and 410, respectively, typical of the α′ modification36−39 of s-PS. The high intensity of hk0 reflections and the nearly negligible intensity of the usually very strong 211 reflection (at 2θ = 20.4°) clearly suggest the presence of axial orientation of the α crystalline phase as previously reported for s-PS fibers obtained by melt spinning process.40 The photographic 2D WAXD pattern of the as-prepared fibers (Figure 2A) confirms the occurrence of axial orientation which can be quantified with a good accuracy as fc = 0.9 by a noncorrected intensity azimuthal scan of the 110 reflection.

Figure 2. Photographic X-ray diffraction patterns of parallel bundles of s-PS fibers: (A) as-prepared α form; (B) δ form (by sorption/ desorption of dichloromethane); (C) etched δ form (by sorption/ desorption of chloroform).

3.1.2. δ Form Fibers. Nanoporous-crystalline δ form s-PS samples can be easily obtained by guest removal4−6 from cocrystalline samples,41−49 being in turn obtained by guestinduced recrystallization of trans-planar α form samples.41−43 The X-ray diffraction patterns of as-prepared fibers of curve i of Figure 1A after treatment with liquid dichloromethane, tetrahydrofuran, and chloroform for ca. 5 min are shown in Figure 1A, curves ii, iii, and iv, respectively. WAXD patterns of C


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Figure 3. FTIR spectra of etched δ fibers (red line) and unetched δ fibers (black line) in the wavenumber ranges 1420−1040 cm−1 (A) and 600− 480 cm−1 (B).

3.1.3. ε Form Fibers. The procedure leading to the nanoporous-crystalline ε form of s-PS from the α form, already reported in details for s-PS films,51 is rather complex and requires several steps: (i) solvent treatment of an α form sample, leading to a δ cocrystalline form; (ii) thermal annealing of the δ cocrystalline sample in the 130−150 °C temperature range, leading to γ form; (iii) treatment of the γ form sample with liquid chloroform, leading to a cocrystalline s-PS/CHCl3 form; (iv) chloroform removal from its cocrystalline phase, leading to the nanoporous-crystalline ε form.2 X-ray diffraction patterns of s-PS fibers, as obtained by successive treatments on the as prepared fiber of Figures 1 and 2, are shown in Figure 4. In particular, pattern i refers to the as-

Pattern iii of Figure 4 refers to a γ form sample after sorption/desorption of the chloroform guest and shows two peaks at 2θ = 6.9° and 8.1° which are typical 110 and 020 reflections of the ε form.2 The presence of the diffraction peak at 2θ = 9.2° in pattern iii indicates, as previously reported in Figure 3E of ref 31, that a residual amount of γ form is still present in the fibers. It is also worth noting that after heating at 135 °C and obtaining of the γ form (curve ii), the high intensity of the (hk0) reflections located at 2θ = 9.3° (200 planes) and 2θ = 10.3° (020 planes) indicates that the high degree of orientation fc = 0.7 obtained after treatment of the as-prepared fibers with dichloromethane is maintained. After treatment of the γ form fibers with liquid chloroform and obtaining of the ε form (curve iii) we can observe that the (hk0) reflections of the ε form located at 2θ = 6.9° (110 planes) and 2θ = 8.1° (020 planes) still present a high intensity with respect to (hk1) reflections such as for example the (211) reflection at 2θ = 16.1°, indicating that the axial orientation is still maintained in the ε fibers. This result suggests that the degree of orientation of the ε fibers is similar to the one of etched δ fibers. 3.2. Morphology of the Nanoporous-Crystalline Fibers. Photographs of the as-prepared α form s-PS fibers and of the δ nanoporous-crystalline fibers, as obtained by liquid dichloromethane and chloroform sorption/desorption, are reported in Figures 5A, 5B, and 5C, respectively. The

Figure 4. X-ray diffraction patterns of the melt-spun fibers after subsequent solvent and thermal treatments, leading to different crystalline forms: (i) by liquid dichloromethane for 5 min, leading to cocrystalline s-PS/dichloromethane form; (ii) by thermal annealing at 135 °C for 1 h, leading to γ form; (iii) by liquid chloroform for 10 min and successive chloroform removal by liquid acetonitrile, leading to ε form.

Figure 5. Photographs of s-PS fibers: (A) as prepared, exhibiting the α form; (B, C) after sorption/desorption of dichloromethane (B) and chloroform (C), exhibiting the nanoporous-crystalline δ form.

induction in as-prepared fibers (Figure 5A) of the α→ δ transition by dichloromethane leaves fiber morphology substantially unchanged (Figure 5B) while by chloroform brings to fiber aggregation and embrittlement (Figure 5C). In Figure 6 are reported SEM micrographs of as-prepared fibers and of nanoporous-crystalline δ fibers obtained by treatments with dichloromethane and chloroform. SEM micrographs show that surfaces of δ form fibers obtained by dichloromethane remain smooth (Figures 6B,B′)

prepared fiber treated by dichloromethane, exhibiting the corresponding cocrystalline form (already shown as ii in Figure 1A). Pattern ii refers to the cocrystalline fiber after thermal treatment at 135 °C, which exhibits intense peaks at 2θ = 9.3°, 10.3°, 13.9°, and 15.9° corresponding to 200, 020, 220, and 030 reflections of the γ form.40,48 D


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Figure 6. SEM micrographs of as prepared fibers (A, A′) and of nanoporous-crystalline δ fibers, as prepared by sorption/desorption of dichloromethane (B, B′) and chloroform (C, C′).

while those obtained by chloroform present some cracks (Figure 6C). Moreover, higher magnifications of δ form fibers obtained by chloroform (Figure 6C′) clearly show that fibrils with diameter of ca. 500−800 nm appear within the cracks. Fiber etching with chloroform can be explained by the higher s-PS solubility with this solvent. In fact, liquid chloroform treatment for 5 min of the as-prepared α form fibers, after complete chloroform desorption leads to a weight loss (and hence to a polymer dissolution mainly involving the amorphous phase) close to 30 wt %. It is worth adding that a similar fiber etching also occurs during the preparation of the ε form fibers. As usually occurs for fiber etching, the revealed nanofibrils would be mainly crystalline and hence more resistant to solvents or to chemical attacks.52−54 In the particular case of sPS, crystalline fibrils are not totally dissolved but only undergo transition from the α form to a cocrystalline form and subsequently toward the δ form. The occurrence of a direct α → clathrate transition rather than of a crystal dissolution followed by cocrystallization is clearly demonstrated by the maintenance of a high degree of axial orientation (see Figure 2), even after complete guest removal. 3.3. Sorption Properties of Etched NanoporousCrystalline Fibers. 3.3.1. Adsorption of Nitrogen at 77 K. Adsorption isotherms of N2 at 77 K (with sorption expressed as cm3 of nitrogen in normal conditions per gram of polymer) collected within the relative pressure (P/P0) range of 0.05− 0.95 for the as-prepared fibers, standard δ nanoporous fibers (obtained by treatment with liquid dichloromethane for 24 h), etched δ nanoporous fibers (obtained by treatment with liquid chloroform for 5 min), and etched ε nanoporous fibers (obtained by the procedure described in section 3.1.2) are reported in Figure 7. Nitrogen uptake, which is negligible for the as-prepared fibers and low for the δ fibers from dichloromethane, becomes high for the etched δ and ε fibers. For instance, the nitrogen uptake at P/P0 = 0.1 is equal to 3, 26, and 34 cm3 g−1 for δ, etched ε, and etched δ fibers, respectively. The nitrogen uptake of the etched δ fiber is also much higher than for highly crystalline (typically, with 40% < Xc < 50%) δ form powders which is ca. 9 cm3 g−1 at P/P0 = 0.1.24 It is worth adding that as already observed for aerogels,25,31 also for etched fibers the nitrogen uptake is higher for the δ form than for the ε form. The specific surface area calculated using the Brunauer− Emmet−Teller method in the range (0.05−0.20)P/P0 is lower

Figure 7. Volumetric N2 sorption isotherms at 77 K collected in the relative pressure range P/P0 0.05−0.95 (P0 is the N2 saturation pressure) for s-PS fibers having a diameter of 10−15 μm: (i) asprepared; (ii) unetched δ (from dichloromethane treatment); (iii) etched ε; (iv) etched δ (from chloroform treatment).

than 1 m2/g for the as-prepared fibers while for unetched δ, etched ε, and etched δ fibers, the BET is ca. 10 ± 2, 120 ± 25, and 165 ± 25 m2/g, respectively. It is worth citing that for highly crystalline δ form powders a BET value of 43 m2/g has been reported24 while BET values in the range 200−350 m2/g were obtained for high porosity δ form aerogels with apparent porosity between 80% and 98.5%.24 The much higher nitrogen uptakes of etched δ and ε fibers with respect to highly crystalline δ form powders and the corresponding relevant increases of BET surface areas, which are observed despite the lower degree of crystallinity, are of course related to the etching procedures that expose crystalline fibrils. 3.3.2. VOC Sorption from Vapor Phase. For as-prepared, unetched δ, etched δ, and etched ε s-PS fibers, benzene sorption kinetics at T = 35 °C for relative pressure P/P0 = 0.01 and uptakes after 6 h for P/P0 < 0.05 are reported in Figures 8A and 8B, respectively. We can clearly observe in Figure 8A that benzene sorption kinetics at P/P0 = 0.01, for the etched δ and ε fibers, are much faster than for the as-prepared and unetched δ fibers. In E


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Figure 8. Benzene sorption kinetics at 35 °C for as-prepared (black ■), unetched δ (red ●), etched ε (blue ◆), and etched δ (green ●): (A) kinetics for benzene relative pressure P/P0 = 0.01; (B) uptakes after 6 h for P/P0 < 0.05. P0 is the benzene saturation vapor pressure.

Figure 9. n-Decane sorption at 35 °C for as-prepared (black ■), unetched δ (red ●), etched δ (green ●), and etched ε (blue ◆) fibers: (A) kinetics for P/P0 = 0.02; (B) sorption after 20 h for P/P0 < 0.08. Lateral views of the crystal structures of δ and ε nanoporous crystalline forms of sPS, with empty space distributed as cavities and channels, respectively, are shown as inset in (A). P0 is the n-decane saturation vapor pressure.

Figure 10. Sorption kinetics of DCE from 100 ppm (A), 10 ppm (B), and 1 ppm (C) aqueous solutions in s-PS fibers: (black ■) as-prepared; (red ●) unetched δ form; (green ●) etched δ form.

particular, after 1 h, benzene uptake for etched δ form fibers is ca. 16 times higher than for as-prepared fibers and ca. 6 times higher than for unetched δ fibers. Moreover, for all considered benzene activities, uptakes after 6 h from the etched fibers remain definitely higher than for the unetched fibers (Figure 7B). The faster sorption kinetics observed for etched nanoporous-crystalline fibers is consistent with the much higher

surface areas as pointed out by N2 sorption data (Figure 6). Also interesting are sorption data relative to a long organic molecule, like n-decane. For as-prepared, unetched δ, etched δ, and etched ε fibers, n-decane sorption kinetics at T = 35 °C for P/P0 = 0.02 and uptakes after 20 h for P/P0 < 0.08 are reported in Figures 9A and 9B, respectively. n-Decane uptake is large in the nanoporous-crystalline fibers while it is negligible in the as-prepared fibers. Figure 9 also F


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Macromolecules clearly shows that decane uptake is much higher in the ε form fibers than in the δ form fibers, and this difference is particularly important for low activities. In particular, after 1 h at P/P0 = 0.02, the n-decane uptake for etched ε form fibers is ca. 7 and 40 times higher than for etched and unetched δ form fibers, respectively (Figure 9A). Moreover, for all the considered activities, n-decane uptake after 20 h from the etched ε fibers remains definitely higher than for etched and unetched fibers (Figure 9B). Because the surface area of the ε fibers is definitely smaller than for the etched δ fibers, the higher n-decane sorption is clearly due to the channel shape of the empty space of the nanoporous ε form, which allows a better fit with the long hydrocarbon molecule. 3.3.3. VOC Sorption from Diluted Aqueous Solutions. To assess the sorption capacity of VOCs from diluted aqueous solutions in nanoporous-crystalline fibers, the uptake of 1,2dichloroethane (DCE) has been investigated. The choice of DCE was motivated by its presence in contaminated aquifers and by its resistance to remediation techniques based on reactive barriers containing Fe0.55,56 The sorption kinetics of DCE from diluted aqueous solutions can be investigated by IR spectroscopy by collecting IR spectra of the fibers at different sorption times as shown in the Supporting Information, and the sorption kinetics of DCE from 100 ppm (A), 10 ppm (B), and 1 ppm (C) aqueous solutions in s-PS fibers are reported in Figure 10. The results of Figure 10 clearly show that as already observed for VOC sorption from vapor phase (Figures 7 and 8), also for VOC sorption from diluted aqueous solutions the etched nanoporous-crystalline fibers are much more efficient than other s-PS fibers. The differences become particularly large for low pollutant concentrations and low diffusion times. For instance, for 10 ppm concentration, the DCE sorption after 60 min is close to 1.4 wt %, 0.25 wt %, and negligible for δ etched, δ unetched, and as-prepared fibers, respectively. It is also worth noting that for the 1 ppm aqueous solution the DCE sorption capacity of etched nanoporous fibers is ca. 65% larger than activated carbon which shows a sorption capacity of ca. 0.8 wt %57 vs 1.35 wt % for s-PS fibers. 3.3.4. Comparison of Sorption Properties of NanoporousCrystalline S-PS Materials: Etched Fibers, Microfibers, and Aerogels. This section compares the sorption behavior of the etched nanoporous-crystalline s-PS fibers of this paper with those of the best s-PS based sorbent materials (activated 3 μm microfibers33 and δ form aerogels22,24−26) which have been shown to exhibit better sorption (both kinetics and capacity) of organic trace pollutants than Tenax a benchmark commercial sorbent material.33 The nanoporous samples considered here are prepared with different procedures leading to different nanonoporous modifications1,6,20 as can be seen from the X-ray diffraction patterns reported in Figure 11. As discussed in section 3.1, the diffraction pattern of the etched fibers exhibits broad peaks (Figure 11i) at 2θ ≈ 8.6°, 16.5°, 20.4°, and 23.5°, indicating the presence of small crystallites of the triclinic δ form6 which is typically obtained when solvent removal from the cocrystalline phase is achieved by the use of a volatile solvent (like acetonitrile). As reported above, the BET surface area of the etched δ fibers is 165 ± 25 m2/g. The 3 μm microfibers obtained by melt-blowing process are dissolved by most activating guests (dichloromethane and

Figure 11. X-ray diffraction patterns of the considered nanoporouscrystalline s-PS sorbent materials: (i) 15 μm etched fibers activated by chloroform; (ii), 3 μm melt-blown fibers, activated by MA; (iii) monolithic aerogel with porosity P = 80%, from toluene gels.

chloroform). The fiber cocrystallization can be achieved with eco-friendly solvents such methyl acetate and ethyl acetate without fiber dissolution. The typical nanoporous crytalline modification being obtained with eco-friendly solvents is a disordered nanoporous crystalline modification,58 and the Xray diffraction of the 3 μm microfibers activated with methyl acetate (MA) (Figure 11, curve ii) presents broad diffraction peaks at 2θ ≈ 9.5°, 16°, 20.4°, and 23.4°, typical of the disordered nanoporous crystalline phase.20−33 The BET surface area of these fibers is 41 m2/g. The aerogels have been obtained by supercritical CO2 extraction of s-PS gels in toluene, and as always being observed for δ aerogels,22,24−26 the diffraction patterns of the 80% porosity aerogel (Figure 11, curve iii) and of the 95% porosity aerogel (same diffraction of curve iii; not shown here) exhibits diffraction peaks located at 2θ = 8.3°, 13.5, 16.6°, 20.6°, and 23.3° typical of the monoclinic δ modification.1 The BET surface area of the aerogel with porosity P = 80% is 206 m2/g while the aerogel with porosity P = 95% has a BET of 300 m2/g. It is worth noting that with respect to the diffraction patterns of both type of fibers, the diffraction pattern of the aerogel shows sharper diffraction peaks, indicating the formation of larger and more perfect crystallites than for the fibers. The degree of crystallinity, as evaluated by these WAXD patterns is close to 50% for aerogels and 25% for the 3 μm fibers while the degree of crystallinity the etched fibers was evaluated to 35% (section 3.1.2). For the four considered nanoporous-crystalline s-PS samples, benzene sorption kinetics at 35 °C and P/P0 = 0.01 are reported in Figure 12A. For the same s-PS samples, benzene equilibrium uptake at T = 35 °C for P/P0 < 0.05, expressed as weight increase per polymer mass unit and per polymer (or sample) volume unit are reported in Figures 12B and 12C, respectively. It is apparent in Figure 12A that sorption kinetics increases with the increase of BET surface area which are 41 m2/g for the 3 μm nanoporous fibers (magenta ○), 165 m2/g for the etched δ fibers (green ●), 206 m2/g for the 80% porosity aerogel (blue ◆), and 300 m2/g for the 95% porosity aerogel (black ◆). As expected, the sorption equilibrium uptakes per polymer mass (Figure 12B) increase with the degree of crystallinity, G


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Macromolecules

Figure 12. Benzene sorption kinetics at 35 °C and benzene relative pressure P/P0 = 0.01 (A) and benzene sorption isotherms at 35 °C and P/P0 < 0.05 (B, C) for 3 μm disordered nanoporous fibers with BET = 41 m2/g activated by MA (magenta ○), etched δ fibers with BET = 165 m2/g obtained with chloroform (green ●), and δ form aerogels with P = 80% and BET = 206 m2/g (blue ◆) and P = 95% and BET = 300 m2/g (black ◆). In (A), the curves are reported in the normalized form, i.e., as gbenzene(t)/gbenzene(tinf) vs time. Benzene uptake is expressed per polymer mass unit and per sample volume unit in (B) and (C), respectively. P0 is the benzene saturation vapor pressure.

Figure 13. Sorption kinetics of DCE from a 100 ppm (A, B) and a 10 ppm (C, D) aqueous solutions in etched δ nanoporous fibers obtained with chloroform (green ●), 3 μm disordered nanoporous microfibers obtained with MA (magenta ○), and δ form aerogels with porosity P = 90% (blue ◆) and with porosity P = 98.5% (black ◆). DCE weight uptake is expressed per polymer mass (A, C) as well as per sample volume (B, D). Aerogels data in panels A and C from Daniel et al.24

unit in the etched fibers is 2.5 and 9 times larger than for aerogels with P = 80% and P = 95%, respectively. DCE sorption kinetics from 100 ppm (A, B) and 10 ppm (C, D) aqueous solutions, expressed per mass unit (A, C) and volume unit (B, D), for active s-PS fibers and aerogels, are

going from MA activated microfibers (Xc = 25%) to etched fibers (Xc = 35%) and then to aerogels (Xc = 50%). However, as a result of the low density of the aerogels, their benzene uptake per volume unit is much lower than for fibers (Figure 12C). In particular, at P/P0 = 0.01, benzene uptake per volume H


Article

Macromolecules shown in Figure 13. In particular, the etched δ form fibers obtained with chloroform and the 3 μm microfibers are compared to monolithic δ form aerogels with P = 90% (BET = 270 m2/g) and P = 98.5% (BET = 350 m2/g). As a result of the degree of crystallinity, the DCE uptakes per mass of polymer (Figures 13A,C) from dilute aqueous solutions are highest for aerogels, intermediate for the etched fibers, and minimum for the 3 μm microfibers with disordered nanoporous-crystalline form. However, DCE uptakes per volume of polymer (Figure 12B,D) are maximum for the etched fibers, intermediate for the microfibers, and minimum for the aerogels. In particular, after 1 h of soaking in the 10 ppm solution, the DCE uptake in the etched fibers is ca. 17 times larger than for the high porosity aerogels. Higher pollutant uptakes per polymer volume are very relevant for water or air purification processes, which are generally based on cartridge filtration units having a fixed volume. With regard to the DCE sorption kinetics it is apparent for the 10 ppm aqueous (Figure 13B) that the sorption is slower for the low porosity aerogel than for the etched fibers. This behavior can be attributed to the presence of a skin of lower diffusivity which affects guest sorption kinetics from aqueous solutions mainly for lower-porosity aerogels.24

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AUTHOR INFORMATION

Corresponding Author

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

Christophe Daniel: 0000-0003-1629-5491 Gaetano Guerra: 0000-0003-1576-9384 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the “Ministero dell’Istruzione, dell’Università e della Ricerca”, of Regione Campania (POR Campania FSE 2014-2020- Asse III Obiettivo Specifico 14) and of INSTM-Regione Lombardia (Project BIOGASMAT) is acknowledged. The authors are grateful to Idemistu Kosan Co. Ltd. for the supplying of s-PS fiber samples.

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REFERENCES

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4. CONCLUSIONS Suitable chloroform treatments of α form s-PS fibers not only lead to α → δ or α → ε transitions, i.e., transitions toward both nanoporous-crystalline forms of s-PS, but also can lead to fiber etching that exposes nanoporous-crystalline fibrils. Etched s-PS fibers with nanoporous-crystalline δ and ε forms exhibit BET surface areas of 165 ± 25 and 120 ± 25 m2/g, respectively. These values are definitely higher than those observed for nanoporous-crystalline unetched fibers (≈10 ± 25 m2/g) as well as for nanoporous-crystalline s-PS powders (always lower than 50 m2/g). Etched nanoporous-crystalline fibers present VOC sorption kinetics (both from vapor and aqueous phases) much faster than for nanoporous-crystalline unetched fibers. For most guest molecules, δ form etched fibers are more efficient than ε form etched fibers. However, ε form etched fibers are much more efficient for sorption of long organic molecules. This confirms that for sorption in nanoporous-crystalline polymers the fit between the guest molecular shape and the host crystalline cavities is more relevant than the surface area value. VOC uptakes per polymer volume for the prepared etched fibers are by far the highest observed for nanoporouscrystalline s-PS sorbent materials. This is relevant for industrial purification processes, which are generally based on cartridge filtration units having a fixed volume. In summary, nanoporous crystalline (δ and ε) etched s-PS fibers, due to their simple and scalable preparation procedures, their excellent sorption properties, and safe macroscopic morphology, are particularly suitable as filter sorption medium to remove traces of pollutants from water and moist air.

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FTIR spectra of unetched nanoporous δ form fibers collected at different sorption times in a 100 ppm aqueous solution of DCE (PDF)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01044. I


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Etched Fibers of Syndiotactic Polystyrene with Nanoporous-Crystalline

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