Thermal Stability of Nanoporous Crystalline and Amorphous Phases

Article pubs.acs.org/Macromolecules

Thermal Stability of Nanoporous Crystalline and Amorphous Phases of Poly(2,6-dimethyl-1,4-phenylene) Oxide Christophe Daniel,* Daria Zhovner, and Gaetano Guerra Dipartimento di Chimica e Biologia e Unità INSTM, Università degli Studi di Salerno, via Ponte Don Melillo, 84084 Fisciano (SA), Italy ABSTRACT: A study of the thermal stability of the outstanding sorption properties of nanoporous-crystalline and amorphous phases of poly(2,6dimethyl-1,4-phenylene) oxide (PPO) is presented. In particular, the structural changes, as induced by thermal treatments in the temperature range 100−270 °C, of two limit nanoporous-crystalline modifications have been investigated by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Although both PPO nanoporous-crystalline modifications are metastable, their crystallinity reduction is small for thermal treatments up to 200 °C. For the entire thermal treatment range, the uptake of benzene from vapor phase for the PPO nanoporous-crystalline samples is higher than for PPO nanoporous-amorphous samples. After thermal treatments above the polymer glass transition temperature (Tg ≈ 220 °C), the guest solubility becomes negligible for the fully amorphous samples, while remains large for the nanoporous-crystalline phases, up to the polymer melting (Tg ≈ 250 °C). This high thermal stability of sorbent materials based on PPO nanoporous-crystalline phases is particularly relevant for their thermal regeneration processes.

1. INTRODUCTION Crystalline phases are extremely relevant for properties and applications of many polymeric materials. In fact, their amount, structure, and morphology constitute the main factors controlling physical properties of fibers, films, and thermoplastics1−3 and can be also relevant for properties of rubbers4,5 and gels.6−8 Only in the past two decades the relevance of crystalline phases in molecular transport phenomena has been recognized. In particular, nanoporous-crystalline forms9−16 exhibiting a density lower with respect to the density of the corresponding amorphous phases have been shown to be suitable for many applications in the fields of molecular separations,17−21 gas storage,22−24 sensors,25−28 and catalysis.29 Generally, the removal of the low-molecular-mass guest molecules from polymer cocrystalline forms generates host chain rearrangements, leading to crystalline forms that, as usual for polymers, exhibit a density higher than that one of the corresponding amorphous phase. However, in a few cases (to our knowledge, up to now only for syndiotactic polystyrene (sPS) 9−13 and poly(2,6-dimethyl-1,4-phenylene) oxide (PPO)14−16), by using suitable guest removal conditions,30−34 nanoporous-crystalline forms, which display solubility of gases and volatile organic compounds (VOC) significantly higher than for the corresponding amorphous phases, are obtained.9−16 The nanoporous-crystalline phases of s-PS (δ9−11 and ε12,13) are thermally unstable. In fact, as the temperature approaches the glass transition temperature, both crystalline phases are transformed into the dense γ crystalline phase.35−37 In © 2013 American Chemical Society

particular, their guest sorption ability vanishes after thermal treatments above 80 °C.35−37 As for PPO, the occurrence of nanoporous-crystalline modifications exhibiting high guest solubility already for low guest activity has been only recently established.14−16 Semicrystalline samples of PPO present particularly relevant transport properties because also its amorphous phase can exhibit high guest solubilities and diffusivities, although lower than for the nanoporous-crystalline phase.38−46 PPO also presents the advantage of high glass transition (Tg ≈ 220 °C) and melting (Tm ≈ 250 °C) temperatures.46,47 In previous reports it has been shown that by suitable selection of the crystallization solvent, it is possible to prepare crystalline PPO modifications whose X-ray diffraction patterns are comprised, in a continuum way, between two limit patterns whose reflections assume minimum or maximum diffraction angles.14 For possible applications of these nanoporouscrystalline modifications, it is of course relevant to explore their thermal stability as well as their guest sorption ability after thermal treatments for both amorphous and crystalline phases. In this paper the thermal stability of two nanoporouscrystalline modifications of PPO, with limit low and high 2θ angles, has been explored by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Moreover, changes in benzene uptake at low vapor activities, as a consequence of polymer thermal treatments up to 270 °C, have Received: October 26, 2012 Revised: December 21, 2012 Published: January 9, 2013 449

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Figure 1. X-ray diffraction patterns of nanoporous-crystalline PPO powders obtained from benzene (A) and CCl4 (B), before and after annealing at different temperatures. 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.48

been compared for semicrystalline and amorphous powders, as obtained by similar solution processes.

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. WAXD Analysis of Thermally Treated Nanoporous-Crystalline PPO Samples. The X-ray diffraction patterns of PPO nanoporous-crystalline powders, as obtained from benzene and carbon tetrachloride gels, are shown in the lowest parts of Figures 1A and 1B, respectively. As already described in a previous report, this crystallization procedure leads to two limit crystalline modifications corresponding to highest and lowest 2θ values, respectively. The room-temperature X-ray diffraction patterns of the same powders after annealing at T = 100, 150, 200, 220, 250, and 270 °C are also shown in Figure 1A,B. During annealing, no shift of the diffraction peaks is observed, clearly indicating that there is no thermal transition between different PPO modifications neither variation of the crystal unit cells and hence no densification of the nanoporouscrystalline modifications. However, for both nanoporous-crystalline forms, a reduction of the degree of crystallinity as a consequence of thermal annealing is observed (Figure 2). The decrease of crystallinity is more pronounced for the highly crystalline sample exhibiting the nanoporous-crystalline modification from CCl4. For instance, for thermal treatments up to 150 °C, the relative decrease of crystallinity is of nearly 14% and 8% for semicrystalline samples exhibiting nanoporous-crystalline modifications obtained from carbon tetrachloride (Figure 1B) and benzene (Figure 1A), respectively. It is worth adding that, as the annealing temperature approaches the PPO glass transition (≈220 °C), the decrease of crystallinity becomes sharp. This sharp decrease of degree of crystallinity well agrees with previous data from Wunderlich and co-workers,47 as obtained by calorimetric measurements on commercial PPO powders (also reported as triangles in Figure 2). The observed decrease of crystallinity by annealing clearly indicates a metastable nature for both PPO modifications,

2.1. Materials. The PPO used in this study was purchased by Sigma-Aldrich and presents weight-averaged and number-averaged molecular masses Mw = 59 000 and Mn = 17 000, respectively. PPO gel samples were prepared in hermetically sealed test tubes by heating the mixtures above the boiling point of the solvent 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. PPO powders were obtained by sudden solvent extraction from 20 wt % gels with a SFX 200 supercritical carbon dioxide extractor (ISCO Inc.), using the following conditions: T = 40 °C, P = 250 bar, extraction time t = 300 min. The PPO semicrystalline powders of Figure 1A,B have been prepared from benzene and carbon tetrachloride gels while the PPO amorphous powders have been obtained from decalin gels. PPO samples have been isothermally annealed using a KW-4AH hot plate from Chemat Technology Inc. having a 1 °C temperature resolution. For temperatures up to 250 °C, samples were annealed for 3 h while at 270 °C samples were annealed for 30 min. After annealing, samples were removed from the hot plate and simply left cooled down at room temperature. 2.2. Characterization Techniques. The thermal behavior of PPO samples was investigated by means of a DSC 2920 from TA Instruments under a dry nitrogen atmosphere. The temperature and the heat flow of the DSC were calibrated for every heating rate with indium. X-ray diffraction patterns were obtained on a Bruker D8 Advance automatic diffractometer operating with a nickel-filtered Cu Kα radiation. The degree of crystallinity of powders and films was obtained from the X-ray diffraction data, by applying the standard procedure of resolving the diffraction pattern into two areas corresponding to the contributions of the crystalline and amorphous fractions. The vapor sorption measurements have been carried out at 35 °C with a VTI-SA symmetrical vapor sorption analyzer from TA Instruments. Surface area was obtained by N2 adsorption measurements carried out at 77 K on a Micromeritics ASAP 2020 sorption analyzer. All the 450

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30 °C/min) are reported in Figure 4A while the variation of the melting temperature as a function of the heating rate is reported in Figure 4B. We can observe that, for all the investigated heating rates, only one endotherm peak is present on the DSC scans. Moreover, the peak melting temperature clearly increases with increasing heating rate. This trend which indicates that crystal thickening/perfectioning does not occur during heating is in agreement with the X-ray diffraction data showing the metastable nature of the PPO nanoporous-crystalline phases. In this respect, it is worth citing the metastable nature of the δ nanoporous-crystalline phase of s-PS. Also in this case a reduction of crystallinity is observed as a consequence of thermal annealing procedures in the temperature range 70−100 °C, i.e., close to the Tg of polystyrene.49−51 In that case, however, there is a transition toward the dense γ phase rather than melting.49−51 3.3. Guest Uptake from Thermally Treated PPO Samples. Benzene sorption isotherms at 35 °C and for pressures up to 0.05 P/P0, for an amorphous powder obtained from decalin and two semicrystalline powders, obtained from benzene and CCl4 (presenting BET surface areas of 320, 552, and 549 m2/g, respectively), before and after annealing at different temperatures in the range 100−270 °C, are reported in Figures 5A, 5B, and 5C, respectively. For the sake of comparison, the benzene uptakes at P/P0 = 0.01 for the three different samples are reported in Figure 6 as a function of the annealing temperature. The data of Figures 5 and 6 show that all the considered PPO samples, obtained by sudden solvent removal from gels, present large values of benzene uptake. Moreover, independently of the annealing temperature and of the guest activity, both semicrystalline samples present guest uptakes higher than for the fully amorphous PPO sample. Figures 5 and 6 also show that, for all PPO samples, as a consequence of annealing procedures at Ta < 200 °C, there is a gradual reduction of the solvent uptake, which is more pronounced for the nanoporous-crystalline PPO powder from CCl4, i.e., for the sample exhibiting a larger reduction of degree of crystallinity (Figure 2). It is worth noting that this behavior is consistent with the variation of the surface area determined from N2 adsorption measurements carried out at 77 K. After annealing at 150 °C, the SBET of the nanoporous-crystalline PPO powder from CCl4 decreases from 549 to 380 m2/g while the SBET of the nanoporous-crystalline PPO powder from benzene decreases from 552 to 467 m2/g. Sharper reductions of guest sorption capacity occur for the amorphous and semicrystalline samples after thermal treatments close to the glass-transition (Tg ≈ 220 °C) and melting (Tm ≈ 250 °C) temperatures, respectively (Figure 6). In fact, these high-temperature treatments lead to dense fully amorphous samples (e.g., upper curves in Figure 1), exhibiting BET surface areas lower than 3 m2/g. The above-described phenomena lead to markedly different sorption capacity of the three PPO samples after thermal treatment at 220 °C, being reduced for the nanoporouscrystalline sample from benzene (Figure 5B), for the nanoporous-crystalline phase from carbon tetrachloride (Figure 5C), and for the amorphous sample from decalin (Figure 5A) of nearly 35%, 55%, and 95%, respectively. The data of Figures 5 and 6, by making suitable assumptions, also allow an evaluation of the guest uptake from amorphous and crystalline phases of the semicrystalline samples. In

Figure 2. Variation of the degree of crystallinity as a function of the annealing temperature of nanoporous-crystalline PPO powders obtained from benzene (filled squares) and CCl4 (open circles). The degree of crystallinity evaluated from the heat of fusion of a commercial PPO powder, by Wunderlich et al.,47 is reported as filled triangles. Continuous lines are guide for eye.

which well agrees with their nanoporosity, i.e., their density lower than for the corresponding amorphous phase. 3.2. DSC Analysis of PPO Samples. In Figure 3 are reported the DSC scans of the amorphous PPO obtained from

Figure 3. DSC scans, at a heating rate of 10 °C/min, of PPO powders, as obtained by sudden solvent removal from gels, with supercritical CO2: (a) amorphous from decalin; semicrystalline from benzene (b) and semicrystalline from CCl4 (c).

decalin (curve a) and of the two semicrystalline PPO, obtained from benzene (curve b) and carbon tetrachloride (curve c), whose X-ray diffraction patterns are shown in Figures 1A and 1B, respectively. For the amorphous PPO sample, only the glass transition is observed (at ca. 217 °C). The DSC scans of nanoporouscrystalline PPO samples show endothermic peaks with similar melting enthalpies (c.a. 15 J/g) but different melting temperatures (247 °C for the sample obtained from benzene (curve b) and 255 °C for the sample obtained from CCl4 (curve c)). The higher melting temperature of the crystalline modification from CCl4 well agrees with the bigger crystallites, clearly shown by the narrower diffraction peaks of the patterns of Figure 1B with respect to those of Figure 1A. DSC scans of the nanoporous PPO sample obtained from benzene collected for different heating rates (5, 10, 15, 20, and 451

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Figure 4. (A) DSC scans at 5, 10, 15, 20, and 30 °C/min heating rate obtained with a nonannealed nanoporous semicrystalline from benzene. The heat flow is normalized by the heating rate. (B) Variation of the melting temperature as a function of the heating rate. Continuous line is a guide for the eye.

Figure 5. Benzene sorption isotherms obtained at 35 °C before and after annealing at various temperatures of a nanoporous-amorphous PPO powder obtained from a gel in decalin (A) and of nanoporous-crystalline PPO powders obtained from gels in benzene (B) and in CCl4 (C).

annealing at 220 and 250 °C, being very low for the amorphous sample and still high for the nanoporous-crystalline samples (in the range 1.5−4.5 wt % as shown in Figure 5). In fact, also in the limit assumption that the nanoporous-crystalline phases maintain unaltered the guest solubility of the unannealed crystalline phases, the benzene content for P/P0 = 0.01 (Figure 6) in the amorphous phase can be still evaluated as ca. 2.5 wt % for semicrystalline samples annealed at 220 and 250 °C. This demonstrates that, after annealing procedures in the temper-

particular, for the unannealed powders of Figure 6, the benzene sorption is similar for the two semicrystalline samples (6.5 wt %) and lower for the amorphous sample (4.9 wt %). Assuming that the benzene solubility in the amorphous phase of the semicrystalline samples is equal to the solubility in the amorphous sample, the benzene solubility in the two nanoporous-crystalline phases of Figures 1A and 1B can be evaluated as 8.5 and 7.6 wt %, respectively. In this framework, particularly interesting are the guest sorption data after 452

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nanoporous amorphous phases and much higher than for s-PS nanoporous-crystalline phases. This high thermal stability of sorption capacity of nanoporous-crystalline PPO samples should allows to establish simple thermal regeneration procedures for the derived sorbent materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Giuseppe Mensitieri, Dr. Michele Galizia of University of Naples “Federico II”, and Dr. Gianluca Fasano and Dr. Simona Longo of University of Salerno for useful discussions. The authors are indebted to Dr. Jenny Vitillo of University of Turin for making the N2 adsorption measurements. Financial support of the “Ministero dell’Istruzione, dell’Universita′ e della Ricerca” and of “Regione Campania” (CdCR) is gratefully acknowledged.

Figure 6. Benzene uptake at P/P0 = 0.01 as a function of the annealing temperature for amorphous and two semicrystalline PPO powders. Continuous lines are guides for the eye.

ature range between Tg and Tm (210−250 °C), the amorphous phase of the crystalline samples maintains some sorption ability and is not fully densified as for the fully amorphous sample.

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4. CONCLUSIONS The crystallinity changes of PPO samples exhibiting the two limit nanoporous-crystalline phases (with high and low 2θ reflections), after thermal treatments from room temperature up to the polymer melting temperature, have been studied by X-ray diffraction and DSC measurements. For both samples, the annealing procedure reduces (rather than increases) the degree of crystallinity. The progressive reduction of degree of crystallinity, with increasing annealing temperature, indicates that both PPO nanoporous-crystalline modifications are metastable, i.e., only kinetically stable, and hence there is no driving force for crystal size increase. The metastable nature of the PPO crystalline modification is confirmed by the decrease of melting temperature with decreasing heating rate, as observed by DSC measurements. The lack of thermodynamic stability of the nanoporouscrystalline modifications of PPO is not surprising due to their low density as generated by guest removal from host−guest cocrystalline phases.14 A smaller crystallinity decrease is observed for the nanoporous-crystalline modification of PPO exhibiting reflections at higher diffraction angles (Figure 1A). In fact, the crystallinity only reduces from 45% to 42% after annealing up to 150 °C. The uptake of low-molecular-mass guest molecules for the PPO nanoporous-crystalline samples remains higher than the uptake for the corresponding nanoporous amorphous samples, for the entire thermal treatment range. After thermal treatments above the polymer glass transition temperature (220 °C), the guest solubility becomes negligible for the fully amorphous sample due to its densification, while it remains large for the nanoporous-crystalline samples. In particular, for the nanoporous-crystalline sample from benzene (of Figure 1A), the benzene uptake for activity range 0.01 < P/P0 < 0.05, after thermal treatments at 150 and 220 °C, is in the ranges 4.8−13.5 and 4.2−9.2 wt %, respectively (Figure 5B). In summary, for relevant VOCs, the PPO nanoporouscrystalline phases present sorption capacity higher than for PPO amorphous and s-PS nanoporous-crystalline phases. Moreover, the thermal stability of sorption capacity for the PPO nanoporous-crystalline phases is higher than for PPO

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