J. Phys. Chem. B 2008, 112, 12191–12194
12191
Free Volume Expansion and Nanofoaming of Supercritical Carbon Dioxide Treated Polystyrene Toshitaka Oka,* Kenji Ito, Chunqing He, Cedric Dutriez, Hideaki Yokoyama,* and Yoshinori Kobayashi* National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ReceiVed: March 13, 2008; ReVised Manuscript ReceiVed: July 17, 2008
Free volume of polystyrene films treated with supercritical carbon dioxide (SCCO2) were examined by positron annihilation lifetime spectroscopy. Variation of the free volume sizes after the SCCO2 treatment due to the release of trapped CO2 and structural relaxation of the polymer was observed. After 500 h from the depressurization, the free volume of the treated films free from CO2 was ∼0.306 nm in radius and much larger than that of the untreated films of ∼0.295 nm in radius due to the freezing of the swollen structure caused by the large solubility of CO2. 1. Introduction Supercritical carbon dioxide (SCCO2) has attracted much interest recently as an environmentally friendly solvent. It has been widely used for food processing,1,2 synthesis of organosilicates and polymers,3-5 dyeing,6,7 metallisation,8,9 polymer blowing,10-15 etc. These applications exploit the unique properties of SCCO2 such as the moderate critical point (T ) 31 °C, P ) 7.38 MPa)16 and its larger solubility in a variety of substances. Treatment with SCCO2 often leads to swelling and foaming of polymers, which may be useful for developing novel functional materials such as highly gas permeable membranes,11 biological scaffolds,12 etc. In spite of a number of successful studies of the micrometer-scale structure formed by SCCO2,13-15 only a few studies, relying mostly on X-ray and neutron reflectivity measurements, have been performed on the nanofoaming of SCCO2 treated polymers.10,11 Positron annihilation lifetime spectroscopy (PALS) is a powerful probe that has been used in the study of nanometer-size free volume holes and pores in polymers.17,18 In this work, we applied PALS19-22 to the study of nanofoaming of SCCO2 treated polystyrene from the viewpoint of the free volume. Application of PALS relies on the fact that some of the positrons injected into a polymer combine with an electron to form the hydrogen-like bound state, positronium (Ps). The intrinsic lifetimes of spin-antiparallel para-positronium (p-Ps) and spin-parallel ortho-positronium (o-Ps) in vacuum are 0.125 and 142 ns, respectively. In a polymer, o-Ps annihilates with a much shorter lifetime but still survives far longer than the free positrons with lifetime 0.4-0.5 ns. Thus a typical positron lifetime spectrum of a polymer contains three exponentially decaying components, due to the intrinsic annihilation of p-Ps (τ1), free positron annihilation (τ2), and o-Ps annihilation (τ3). The longest lifetime τ3 is governed by pick-off annihilation of o-Ps in a free volume with one of the surrounding electrons and increases with increasing hole size.23,24 Relative intensity I3 is the fraction of positrons that annihilate as o-Ps with lifetime τ3 and corresponds to the probability of o-Ps formation. * Corresponding authors. E-mail:
[email protected] (T.O.); yokoyama@ molle.k.u-tokyo.ac.jp (H.Y.);
[email protected] (Y.K.).
The relation between the o-Ps lifetime and free volume size smaller than 1 nm is given by the model proposed by Tao23 and Eldrup.24 In this model the free volume is approximated as a spherical potential well with radius R (e1 nm). The potential has infinitely high walls, and there is an electron layer with thickness ∆R on the wall surface. An additional assumption is that the o-Ps lifetime in a system with no free volume is 0.5 ns. The model provides the relationship between the lifetime and the free volume radius R in the form
[
τ3 ) 0.5 1 -
( )]
2πR R 1 + sin R0 2π R0
-1
(ns)
(1)
where R0 ) R + ∆R ) R + 0.166 nm.25 2. Experimental Section Commercially available polystyrene films (HF-77, PS Japan Corporation) with a thickness of 1 mm were annealed in a stainless steel high-pressure vessel at 60 °C for 1 h with CO2 at a pressure of 20 MPa. The vessel was connected to a highpressure liquid chromatography pump (PU-2086 plus, JASCO) with a cooling head and to a back-pressure regulator (SCFBpg, JASCO). After the annealing, the vessel containing the films was quenched in a water bath held at temperatures of 0, 30, and 60 °C while maintaining the pressure using the pump and regulator. After the vessel reached a desired temperature, the CO2 pressure was slowly reduced with a depressurization rate of 0.5 MPa/min.15 Because of the formation of microcells (∼30 µm), transparent films became opaque and colored white after depressurization at 30 and 60 °C. Of the two films, a far larger amount of microcells was formed in the film depressurized at 60 °C as is evidenced by much larger thickness increase. This is consistent with the scanning electron microscopy SEM observations by other authors.13 On the other hand, in the film depressurized at 0 °C, the color was unchanged and no microcells formed, which is due to the fact that no nucleation of CO2 in polystyrene for microcell formation occurs during rapid quenching to the lower temperature.14 Positron lifetime spectra of SCCO2 treated and untreated polystyrene films were recorded with a fast-fast coincidence system by determining the time interval between the detection
10.1021/jp802188p CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
12192 J. Phys. Chem. B, Vol. 112, No. 39, 2008
Oka et al.
Figure 1. Positron annihilation lifetime spectra for SCCO2 treated and untreated polystyrene recorded at room temperature. The SCCO2 treated polystyrene shows a longer o-Ps lifetime.
of a 1.27 MeV γ-ray from the β+ decay of a 22Na source and the detection of one of 0.511 MeV annihilation photons.26-29 Two identical samples sandwiched the 22Na source with an activity of 0.37 MBq sealed between two Kapton foils with a thickness of 7.5 µm.30 In order to avoid possible effects of positron irradiation,28 the source was moved after every measurement to a new position so that a fresh part of the films could be probed.30 The lifetime spectra were analyzed assuming three exponential components to deduce the lifetime τ3 and relative intensity I3 of the longest-lived o-Ps components by the POSITRONFIT program.31 The time resolution of the lifetime spectrometer was ∼300 ps full width at half-maximum. Correction was made for the positrons annihilating in the source material. All the measurements were carried out at room temperature in air. Weights of the SCCO2 treated films were measured at different times from the CO2 depressurization by an electric balance (BP221S, Sartorius) at room temperature.
Figure 2. (a) Variations of o-Ps lifetimes τ3 as a function of elapsed time after SCCO2 treatments at different temperatures for polystyrene. All of the measurements were carried out at room temperature in air. Dashed line represents τ3 of untreated films. The right-hand axis shows the free volume radius estimated from eq 1. Solid lines are drawn to guide the eyes. (b) Variations of o-Ps intensities I3 as a function of elapsed time after SCCO2 treatments at different temperatures. Dashed line represents I3 of untreated films.
3. Results and Discussion Typical positron lifetime spectra are shown for untreated and SCCO2 treated polystyrene in Figure 1. The o-Ps lifetime τ3 of the untreated film is 2.06 ns, which agrees well with the values for atactic polystyrene reported in the literature.32-35 Obviously the o-Ps lifetime is longer in the SCCO2 treated film, indicative of considerable free volume expansion after SCCO2 treatment. Figure 2 shows the variations of o-Ps lifetime τ3 and intensity I3 of the SCCO2 treated films as a function of elapsed time from the CO2 depressurization. The dashed lines represent the o-Ps lifetimes and relative intensities of the untreated film. The righthand axis of Figure 2a shows the free volume radius quantified from eq 1. Initially, the o-Ps lifetimes of SCCO2 treated films are appreciably longer than the untreated film. With increasing time after the depressurization, o-Ps lifetimes for all films gradually decrease, then approach ∼2.20 ns, which is still longer than the lifetime ∼2.06 ns of the untreated film. Therefore, relaxation of the SCCO2 treated films makes the free volume smaller, but the expansion remains even after 2000 h from the treatment. The o-Ps intensities I3 of the treated films are independent of elapsed time and close to I3 of the untreated film, indicating that the formation of Ps is not affected by the SCCO2 treatment and free volume relaxation. Figure 3 displays the variations of total weights of the films depressurized at different temperatures as a function of elapsed time. All the data are normalized relative to the films before the SCCO2 treatments. The increased weights are due to CO2 uptake by the SCCO2 treatments. In 20 MPa SCCO2, Tg of
Figure 3. Variations of the normalized weights of the films depressurized at different temperatures with elapsed time. Solid lines are drawn to guide the eyes. Continuous decrease of the weights is due to the gradual release of CO2. Inset emphasizes the variations within 10 h from the depressurization.
polystyrene is 30-40 °C36 and much lower than that of ∼100 °C in the CO2-free state. Therefore polystyrene in SCCO2 vitrifies during the depressurization or quenching to the lower temperatures, and CO2 is trapped inside. The continuous decrease of the weights after the SCCO2 treatments indicates that trapped CO2 is gradually released from vitrified polystyrene by diffusion. We note in Figure 3 that less CO2 is trapped in the sample depressurized at 60 °C than the other samples. During the depressurization at 60 °C well above Tg of SCCO2-swollen polystyrene, CO2 becomes less soluble in polystyrene and is supersaturated. Subsequent nucleation and growth of CO2 result
Supercritical Carbon Dioxide Treated Polystyrene
Figure 4. Variations of the free volume size as a function of logarithmic time from the depressurization. Solid lines are drawn to guide the eyes. I, II, and III show three stages of the free volume change for the films depressurized at 0 and 30 °C. I, free volume relaxation associated with CO2 release; II, free volume relaxation in the CO2-free state; III, metastable glassy state.
in the largest amount of microcells formed. The smaller CO2 uptake in the sample depressurized at 60 °C is supposed to be due to escaping of CO2 from the polystyrene matrix in the rubbery state via the microcells during depressurization. The weight decrease of the 1 mm thick films depressurized at 0 and 30 °C is consistent with a reported diffusion coefficient 7.5 × 10-8 cm2 s-1 for CO2 in bulk polystyrene.37 The more rapid weight decrease of the film depressurized at 60 °C suggests that CO2 trapped in this sample is more easily released through the microcells than the other samples depressurized at lower temperatures. Figure 4 shows the variations of the free volume size as a function of logarithmic time from the depressurization. It is clearly seen that the free volumes of SCCO2 treated films differ from each other. The smallest initial size ∼0.311 nm and fast relaxation of free volume up to 10 h from the depressurization are observed for the film treated at 60 °C. For the other two films depressurized at 0 and 30 °C, the free volume sizes are ∼0.324 and ∼0.323 nm, respectively, immediately after depressurization, slightly different from each other afterward, and the initial relaxation continues until ∼40 h from the depressurization (stage I). In light of the results in Figure 3, the smaller initial free volume of the film treated at 60 °C is attributed to the weaker plasticizing and nanofoaming effects of trapped CO2 in a smaller amount, whereas the faster initial relaxation of the free volume is attributed to the more rapid release of trapped CO2 through the large microcells formed during the depressurization at 60 °C. The slight difference of the free volume size between the samples depressurized at 0 and 30 °C is not related to the CO2 uptake in Figure 3. Further study will be required on this point. Note that even after the completion of the CO2 release, free volume relaxation continues for the films depressurized at 0 and 30 °C up to a few hundred hours from the depressurization (stage II). After ∼500 h from the depressurization, the free volume size of all the treated films becomes ∼0.306 nm in radius regardless of the depressurization temperature, indicating longterm relaxation results in a common metastable glassy state (stage III) with a free volume as large as that of untreated polystyrene at a temperature as high as 120 °C.34 This state is unchanged at room temperature at least for 5000 h (∼7 months) from the depressurization. We observed that it returns to the original glassy state with a free volume of ∼0.295 nm after annealing above Tg (∼100 °C) of polystyrene. Previously Hagiwara et al. reported that cooling of polystyrene pressurized
J. Phys. Chem. B, Vol. 112, No. 39, 2008 12193 to 200 MPa with mercury in the liquid state to room temperature, followed by depressurization down to 10 MPa, results in a metastable glass with a reduced free volume of ∼0.284 nm.35 This densified glass returns to ordinary glassy polystyrene after annealing around 70 °C. It is interesting that the much less dense polystyrene glass formed by the SCCO2 treatments is unchanged and stable up to the glass transition temperature. Nanofoaming of SCCO2 treated polymer has been studied by different analytical techniques and discussed on the basis of various physicochemical properties, but little understood. For example, gas permeability of polystyrene film was reported to become higher after a SCCO2 treatment and the change of free volume was speculated.11 In this work, by applying PALS to SCCO2-treated polystyrene, we obtained direct experimental evidence of significant free volume expansion that is preserved up to the glass transition. 4. Conclusion Free volume in polystyrene treated with SCCO2 at 60 °C and depressurized at different temperatures was investigated by PALS. The initial relaxation of expanded free volume after the depressurization is associated with the release of trapped CO2 in the polystyrene matrix. For the films quenched to the low temperatures, this is followed by structural relaxation of decreasing free volume in CO2-free polystyrene. After ∼500 h from the depressurization, the free volume size of all of the treated films becomes ∼0.306 nm in radius regardless of the depressurization temperature, indicating long-term relaxation results in a common metastable CO2-free state with a free volume as large as that of untreated polystyrene well above Tg. We demonstrated that the nanostructure of a polymer can be altered by SCCO2 via nanofoaming, which may be useful for developing novel functional materials. Acknowledgment. The authors thank the New Energy and Industrial Technology Development Organization (NEDO) of Japan for the financial supports of this study. References and Notes (1) Brunner, G. J. Food Eng. 2005, 67, 21–33. (2) Gokulakrishnan, S.; Chandraraj, K; Gummadi, S. N. Enzyme Microb. Technol. 2005, 37, 225–232. (3) Pai, R. A.; Watkins, J. J. AdV. Mater. 2005, 18, 241–245. (4) Cooper, A. I.; Hems, W. P.; Holmes, A. B. Macromol. Rapid Commun. 1998, 19, 353–357. (5) Yates, M. Z.; Li, G.; Shim, J. J.; Maniar, S.; Johnston, K. P.; Lim, K. T.; Webber, S. Macromolecules 1999, 32, 1018–1026. (6) Schmidt, A.; Bach, E.; Schollmeyer, E. Dyes Pigment. 2003, 56, 27–35. (7) Gordillo, M. D.; Pereyra, C.; de la Ossa, E. J. M. Dyes Pigment. 2005, 67, 167–173. (8) Cabanas, A.; Long, D. P.; Watkins, J. J. Chem. Mater. 2004, 16, 2028–2033. (9) Kondoh, E.; Shigama, K. Thin Solid Films 2005, 491, 228–234. (10) Koga, T.; Seo, Y.-S.; Jerome, J. L.; Ge, S.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Seeck, O. H.; Tolan, M.; Kolb, R. Appl. Phys. Lett. 2003, 83, 4309–4311. (11) Koga, T.; Akashige, E.; Reinstein, A.; Bronner, M.; Seo, Y.-S.; Shin, K.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Satija, S. K. Physica B 2005, 357, 73–79. (12) Jacobs, M. A.; Kemmere, M. F.; Keurentjes, J. T. F. Polymer 2004, 45, 7539–7547. (13) Arora, K. A.; Lesser, A. J.; McCarthy, T. J. Macromolecules 1998, 31, 4614–4620. (14) Yokoyama, H.; Sugiyama, K. Macromolecules 2005, 38, 10516– 10522. (15) Yokoyama, H.; Li, L.; Nemoto, T.; Sugiyama, K. AdV. Mater. 2004, 16, 1542–1546. (16) Goel, S. K.; Beckman, E. J. Polymer 1993, 34, 1410–1417. (17) Ito, K.; Saito, Y.; Yamamoto, T.; Ujihira, Y.; Nomura, K. Macromolecules 2001, 34, 6153–6155.
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