Pressure Effect of Various Inert Gases on the phase Behavior of

Jan 9, 2013 - †National Creative Research Center for Block Copolymer ... of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea...
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Pressure Effect of Various Inert Gases on the phase Behavior of Polystyrene-block-Poly(n-pentyl methacrylate) Copolymer Hye Jeong Kim,†,‡ Hong Chul Moon,†,‡ Hyunuk Kim,§ Kimoon Kim,§ and Jin Kon Kim†,* †

National Creative Research Center for Block Copolymer Self-Assembly, Department of Chemical Engineering and §Department of Chemistry, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea

Junhan Cho* Department of Polymer Science and Engineering and Center for Photofunctional Energy Materials, Dankook University, 126 Jukjeon-dong, Yongin-si, Gyeonggi-do 448-701, Korea ABSTRACT: We investigated the pressure effect of three inert gases (nitrogen, helium, and argon) on the phase behavior of polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing closed-loop phase behavior and baroplasticity. Helium gas pressure enhanced the miscibility between PS and PnPMA blocks similar to the hydrostatic pressure. Thus, the closed-loop size decreased with increasing helium gas pressure. Very interestingly, with increasing nitrogen and argon gas pressure, the miscibility between the two blocks decreased even though these two are also considered as inert gases. To explain these unexpected results, we measured the amount of gas absorption into each block. Helium gas showed almost no absorption in both PS and PnPMA; thus it simply acts as hydrostatic pressure. On the other hand, nitrogen and argon gases were more selectively absorbed into PnPMA compared with PS, which increased the free volume disparity between two blocks and enlarged closed-loop size with increasing gas pressure. The experimentally measured gas absorption results are consistent with the theoretical ones based on the Sanchez−Lacombe theory. The results in this study imply that well-known and widely employed inert gases such as nitrogen and argon could significantly affect the phase behavior of a weakly interacting block copolymer at high pressures.

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INTRODUCTION The phase behavior of block copolymers has been extensively studied due to the self-assembly to periodic nanostructures.1−4 Numerous experimental methods have been employed to investigate the phase behavior of block copolymers by using small-angle X-ray (or neutron) scattering (SAXS or SANS), rheometry, depolarized light scattering, and Fourier transform infrared (FTIR) spectroscopy.5−11 Previously, we reported closed-loop type phase behavior where lower disorder-to-order transition (LDOT) was found at lower temperature and upper order-to-disorder transition (UODT) was observed at higher temperature upon heating.12−14 We also found, via small-angle neutron scattering (SANS) or birefringence, the high pressure coefficient (and thus outstanding baroplasticity) for deuterated polystyreneblock-poly(n-pentyl methacrylate) copolymer (dPS-bPnPMA)15 or its derivatives13 when hydrostatic pressure (P) was used. With increasing P, the miscibility between PS and PnPMA was greatly enhanced, resulting in the shrinkage of the closed-loop size. Because of excellent baroplastic property of PS-b-PnPMA, it could be used to fabricate ultrahigh density array of nanoscopic indentations by pressure without heating.16 © 2013 American Chemical Society

Some research groups investigated supercritical carbon dioxide (sCO2) pressure effect on the phase behavior of a block copolymer. Vogt et al.17,18 measured the order-todisorder transition temperature (TODT) of polystyrene-blockpolyisoprene copolymer (PS-b-PI) and PS-block-poly(n-hexyl methacrylate) copolymer (PS-b-PnHMA) in the presence of sCO2. They found that the miscibility between PS and PI (and PS and PnHMA) blocks was enhanced (and thus TODT was decreased) with increasing sCO2 pressure.17,18 The depression of TODT was explained in terms of the enthalpic contribution. When gas molecules are absorbed into block copolymer chains, they can screen unfavorable interactions between dissimilar two blocks, which results in increased miscibility. This is similar to the case when a neutral solvent is added to a block copolymer.19,20 On the other hand, the opposite phenomenon was observed in PS-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) having lower disorder-to-order transition temperature (TLDOT) with the presence of sCO2, where the Received: October 15, 2012 Revised: December 3, 2012 Published: January 9, 2013 493

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(inner diameter) columns including particle size of 5 μm (PLgel 5 μm MIXED-C: Polymer Laboratories) were used with THF as an eluent and a flow rate of 1 mL/min at 30 °C. The volume fraction of polystyrene block ( f PS) was determined by 1H nuclear magnetic resonance spectra (1H NMR: Bruker Avance III 400) with a solvent of chloroform-d (CDCl3). The molecular characteristics for the samples are summarized in Table 1.

miscibility between PS and PnBMA blocks became poorer (thus, the TLDOT of PS-b-PnBMA was decreased) with increasing sCO2 pressure. This is because the degree of the sCO2 absorption into PnBMA block is larger than that into PS block due to the favorable interaction between sCO2 and acrylate moiety in PnBMA block.18 As a consequence, the free volume disparity between PS and PnBMA blocks increased with increasing the sCO2 absorption, resulting in decreased miscibility between two blocks. Lavery et al.21 investigated the sCO2 swelling effect on the closed-loop phase behavior of PS-bPnPMA having both upper order-to-disorder transition temperature (TUODT) and TLDOT. Since the degree of sCO2 absorption in PnPMA was slightly larger than that in PS, the miscibility between PS and PnPMA blocks decreased with increasing sCO2 pressure. When one measures the phase transition temperatures such as TODT or TLDOT of block copolymers, the sample should be under a vacuum or in inert gas atmosphere to prevent thermal degradation by oxygen in air at high temperatures. The most widely used inert gas is nitrogen. Then, one interesting question could be raised regarding whether nitrogen indeed acts as an inert gas for phase behavior of a block copolymer. To answer this question, one should choose a special block copolymer having very large pressure coefficient of the transition temperature, for instance, PS-b-PnPMA showing a closed-loop phase behavior with excellent baroplasticity, since even a subtle variation of the transition temperatures by gas pressure could be detected. In this study, we investigated the effect of nitrogen gas pressure on the transition temperature of PS-b-PnPMA by using a birefringence experiment. Very surprisingly, the miscibility between PS and PnPMA blocks decreased (not increased) with increasing nitrogen gas pressure. If nitrogen had been acting as a true inert gas, the miscibility should have increased similar to the case of hydrostatic pressure generated by pressurizing a sample with silicone oil. This unexpected result arose from the larger amount of the absorption into PnPMA block compared with that into PS block. After we used two other inert gases (helium and argon), we found that only helium behaves as a true inert gas, namely, the miscibility between PS and PnPMA increases with increasing helium gas pressure. On the other hand, argon gas also decreases the miscibility, similar to nitrogen gas. To explain why these opposite phenomena occurred, we performed gas absorption experiments for nitrogen, helium and argon on each block of PS and PnPMA. The experimentally measured gas absorption results are consistent with the theoretical ones based on the Sanchez−Lacombe theory. The results indicate that nitrogen (or argon) gas at high pressure should be used very carefully in investigating the phase behavior of a block copolymer having close-loop type transitions (or very high pressure coefficient of the transition temperature).

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Table 1. Molecular Characteristics of Polymers Employed in this Study sample code

Mna

f PSb

Mw/Mna

PS-b-PnPMA-L PS-b-PnPMA-BL PS homopolymer PnPMA homopolymer

46 900 48 700 52 000 41 200

0.51 0.50 1.0

1.06 1.06 1.04 1.04

a Determined by SEC based on PS standards. bEstimated from 1H NMR spectra with known densities of 1.05 g/cm3 (PS) and 1.03 g/ cm3 (PnPMA).

Equilibrium gas absorption measurement into homopolymers PS and PnPMA was performed using a pressure decay method at room temperature by varying pressure from 1 to 70 bar. The amount of the sample was ∼1g. The transition temperatures (TUODT and TLDOT) as a function of pressure of three gases were determined by using birefringence with a specially designed pressure cell, as shown in Figure 1. A stainless steel pressure cell equipped with Pyrex windows was used working at pressures ranging from 1 to 120 bar. The gases were supplied from gas cylinders into the pressure cell, and the pressure of each gas was adjusted by the total amount of the gas inlet into the pressure cell in which two O-rings were used to avoid gas leakage. The sample size was a thickness of 3 mm and a diameter of 5 mm. A vertically polarized light from an HeNe laser with a wavelength of 632.8 nm passed through the sample and a horizontally oriented polarizer onto a photo detector to obtain the transmitted intensity. All measurements were performed at a constant gas pressure upon heating at a rate of 0.1 °C/min. We found that the temperature of the sample was the same as that of a gas in the cell.

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RESULTS AND DISCUSSION Parts a and b of Figures 2 show the change of birefringence intensity upon heating at a rate of 0.1 °C/min as a function of gas pressure of nitrogen and helium. The neat PS-b-PnPMA-BL showed closed-loop phase behavior with TLDOT of 160 °C and TUODT of 198 °C at atmospheric pressure (1 bar). For nitrogen gas, the closed-loop size was enlarged with increasing gas pressure. Namely, TLDOT was decreased but TUODT was increased with increasing gas pressure, in which dTUODT/dP = +305 °C/kbar and dTLDOT/dP = −291 °C/kbar. This result is quite unexpected because nitrogen has been considered as an inert gas. If nitrogen acts indeed as an inert gas for PS-bPnPMA, the miscibility between PS and PnPMA should have been enhanced similar to for the case of the hydrostatic pressure.13,15 On the other hand, Figure 2b clearly shows the shrinkage of closed-loop for PS-b-PnPMA-BL with increase of helium gas pressure. At a high pressure (100 bar), PS-b-PnPMA-BL became fully disordered state, which indicates that helium behaves as a true inert gas for PS and PnPMA. Based on Figure 2b, we obtained extracted pressure coefficients of dTUODT/dP = −179 °C/kbar and dTLDOT/dP = +286 °C/kbar for helium gas. Interestingly, these two values are smaller than those (− 725 and +725 °C/kbar) obtained from the hydrostatic pressure driven by silicone oil.15 The effect of silicone oil and He gas on the phase behavior (and thus the pressure coefficients) would

EXPERIMENTAL SECTION

We used three inert gases (nitrogen, helium, and argon) with high purity (>99.9995%). The PS-b-PnPMA used in this study was prepared by sequential anionic polymerization in tetrahydrofuran (THF) at −78 °C in the presence of lithium chloride salt under extremely pure argon atmosphere, and sec-BuLi was used as an initiator.12−14 The total number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn, in which Mw is the total weight-average molecular weight) were determined using size exclusion chromatography (SEC: Waters 2414 refractive index detector) based on PS standards. Two 300 mm (length) × 7.5 mm 494

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Figure 1. Experimental setup (left) for birefringence at high gas pressure and the cross sectional view of the pressure cell (right).

Figure 2. Temperature dependence of birefringence intensity for PS-b-PnPMA-BL with gas pressure of (a) nitrogen and (b) helium. (c and d) Plots of transition temperatures (TLDOT (●) and TUODT (○)) with gas pressures of nitrogen and helium, respectively.

PS and 0.024 wt % for PnPMA at ∼60 bar from the theoretical calculation (see the inset in Figure 5)). This means that He is practically considered as a neutral gas. However, a very tiny difference in the absorption amount of gases into PS and PnPMA chains can counteract to reduce the pressure coefficients of the ordering temperatures compared with silicone oil. The exact explanation on the different pressure

be same once they behave as a neutral pressurizing agent. A plausible explanation on the difference is as follows. The unfavorable interaction between polymers and silicone oil should be large so that the silicone oil makes a sharp interface against polymers without any penetration. On the other hand, even though He is not compatible with polymers energetically, the configurational entropy of He allows a very weak absorption of He into polymer chains (up to 0.020 wt % for 495

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amount of nitrogen gas into PnPMA is greater than that into PS. Moreover, the difference of the nitrogen absorption amount into PnPMA and PS increased with increasing gas pressure. This implies that nitrogen gas has more favorable interaction with PnPMA block. The different degree of the absorption amount could increase free volume (compressibility) disparity between the two blocks, resulting in a reduced miscibility of PS and PnPMA similar to sCO2.21 On the other hand, helium gas showed almost no absorption into both PS and PnPMA. Therefore, helium gas can be regarded as a true inert gas for PSb-PnPMA. We also investigated the effect of argon gas on the phase transition of PS-b-PnPMA. Argon is a well-known and widely used inert gas. Figure 4a shows the temperature dependence of birefringence intensity for PS-b-PnPMA-BL with argon gas pressure. The effect of argon gas on the phase behavior of PS-bPnPMA is similar to that of nitrogen gas: the miscibility between PS and PnPMA block decreased with increasing gas pressure. The pressure coefficients of the transition temperatures for argon gas are comparable with the values for nitrogen (see Figure 2c) as shown in Figure 4b. This is also explained by different amounts of gas absorption into PnPMA block compared with PS block (Figure 4c). The difference of the amount of argon gas absorption became larger with increasing argon gas pressure, which caused the greater free volume disparity. To explain such a different gas absorption tendency, we estimated, by using Henry’s law constant from the widely used Sanchez−Lacombe (SL) theory,22−24 the amount of the gas absorption into each block. To briefly mention the essence of

coefficients between silicone oil and He gas might deserve a future investigation. The decreasing of miscibility between two blocks with increasing sCO2 gas pressure has been reported for PS-bPnBMA18 and PS-b-PnPMA21 because sCO2 is more selective to PnBMA (or PnPMA) compared with PS block. Thus, to clarify the reason why nitrogen and helium gases showed the opposite effect on phase behavior, we measured the amount of nitrogen and helium absorption into PS and PnPMA blocks at room temperature (Figure 3). Interestingly, the absorption

Figure 3. Absorption amounts of two gases (nitrogen (circles) and helium (squares)) into homopolymer PS (closed symbols) and PnPMA (open symbols) at various gas pressures.

Figure 4. (a) Temperature dependence of birefringence intensity for PS-b-PnPMA-BL with argon gas pressure, (b) plots of transition temperatures (TLDOT (●) and TUODT (○)) as a function of argon gas pressure, and (c) measured argon gas absorption into homopolymer PS (■) and PnPMA (□). 496

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SL theory, a system of polymer + polymer or polymer + oligomer mixture with solvent molecules on a lattice is first considered in a mean-field way. The solvent molecules are then treated as lattice vacancies to play a role as free volume. At a given set of temperature and pressure, the equilibrium number of those vacant sites dilutes the component densities to give the equation-of-state properties. The resultant equation of state can be categorized as a van der Waals fluid for the given system, which conserves the principle of corresponding states. Therefore, all the individual properties of identical molecules depend on three characteristic parameters (characteristic pressure P*, characteristic density ρ* (≡1/V*), and characteristic temperature T*). The calculated values for P*, ρ*, and T* for PS, PnPMA and the three gas molecules are given in Table 2. Table 2. Characteristic Parameters of each Block and Three Gases Used in This Study P* (MPa)a

1/ρ* (cm3/g)a

T* (K)a

r

448.6 435.6 617.3 814.7 31.8

0.876 24 0.909 61 0.527 91 0.320 10 1.886 77

647.6 621.7 162.2 195.2 7.8

∞ ∞ 6.8 6.4 3.7

PS PnPMA N2 Ar Hea

Figure 5. Plots of the calculated amount of three gases (argon, nitrogen and helium) absorption into PS (dashed line) and PnPMA (solid line) as a function of gas pressure. Inset is the calculated amount of He gas adsorption into PS and PnPMA at pressures ranging from 54 and 60 bar.

among three gases (helium, nitrogen and argon), and slightly selective absorption into the PnPMA block was found. Even though the amount of nitrogen absorption into each block was less than that of argon, the difference between absorption amounts into PnPMA and PS appeared to be similar to that of nitrogen. On the other hand, the absorption of helium gas into both blocks was essentially zero, which is consistent with our experiment (see Figure 3). The Flory−Huggins interaction parameter χ depends on the difference in the strength of selfcohesion (∼((T1*)1/2 − (T2*)1/2)2). Therefore, PS and PnPMA should accommodate argon most because of the largest van der Waals interactions among three gases. Likewise, nitrogen gas marks the second in absorption into the polymers. Because the self-cohesion of PnPMA is slightly less than that of PS, all the gases result in larger absorption into the former polymer than into the latter. In accord with the present calculations, the absorbed gas molecules in case of either argon or nitrogen swell the two polymers. In the given mixture of PS and PnPMA, the decreased volumes of the constituent polymers would contribute to segregation tendency. On the other hand, because helium cannot be absorbed into both PS and PnPMA blocks, it only pressurizes the mixture and thus decreases the free volume disparity. Therefore, the gas pressure effect on the phase behavior of PS-b-PnPMA is strongly dependent on the difference in the gas absorption amount into each block. In addition, the preferred gas absorption into PnPMA block compared with PS block can be explained as follows. Even though gases are not compatible with polymers energetically, the combinatorial entropy of a gas/polymer pair could induce the sorption of gases into polymer chains. The difference in absorption amount of a gas into either PS or PnPMA chains can be described by Hildebrand’s solubility parameter, which represents the cohesive energy per unit volume. Since the cohesive energy contains three interaction forces (dispersion, polar, and hydrogen forces), it is described by the characteristic energy parameter εii in the lattice fluid theory. Because the cross interactions between those components are given by εij = (εiiεjj)1/2, the compatibility (or incompatibility) between two different species is expressed by εAA + εBB − 2εAB = ((εAA)1/2 − (εBB)1/2)2. Because of the stronger self-interaction (and thus

The values of P*, ρ*, and T* are calculated by fitting the SL equation of state to the volume data available in the literature (ref 28 for PS, ref 29 for PnPMA, and ref 30 for Ar, He, and N2). bThe segment sizes σ of PS, PnPMA, N2, and Ar are determined, respectively, as 2.71, 2.70, 1.54, and 1.49 +. Although σ for for He varies from 0.3726 to 2.11,27 in this study, we chose σ of 1.50. a

These three characteristic parameters are also related to three molecular parameters (the number of segments r, segmental (or monomeric) size σ, and segment−segment interaction ε, where kT* = P*/ρ* = ε, r = MP*/kT*ρ*, and 1/ρ* = V* = NArσ3/M with molecular weight M and the Avogadro’s number NA). The phase behavior of the mixture is governed by an interaction parameter α [unit = mole cm−3] which is equated to α = ((P1*)1/2 − (P2*)1/2)2/RT for nonpolar interactions per segmental volume, without any adjustable parameter, where R is the gas constant. Thus, the Flory−Huggins interaction parameter (χ) is given by αv*with v* (=σ3) being the theoretical segmental volume. A detailed description of SL theory can be found elsewhere.22−24 A polymer is assumed to make contact with a gas, which can be treated in the SL theory as the mixture of a polymer and gasforming oligomer molecules exhibiting two-phase vapor−liquid equilibrium. According to Pope et al.,25 the weight fraction w1 of a gas absorbed by the polymer at a partial pressure of P1 (in MPa) is estimated with Henry’s law constant (kH) at low pressure as

w1 = kH −1P1 The kH is obtained from the SL theory as kH −1 =

M1 exp[r1{ρ2 T1*/ρ2 *T1 − 1 − (ρ2 */ρ2 − 1) ρ2 RT ln(1 − ρ2 /ρ2 *)} − ρ2 M1χ /ρ2 *ρ1*]

where subscripts 1 and 2 denote gas and polymer, respectively, and M1 is the molecular weight of the gas molecule. Figure 5 gives plots of theoretically estimated gas absorptions as a function of gas pressure. Argon absorption was the highest 497

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higher T*) of PS compared with PnPMA, the former is denser than the latter, indicating that the overall cohesive energy of PS is higher than that of PnPMA. Therefore, PS becomes less compatible with the gases than PnPMA, which results in higher sorption of the gases into PnPMA than into PS. The above results indicate that the phase behavior is also dramatically influenced by gas pressure. For instance, argon gas could be used to enhance the phase segregation between PS and PnPMA. Then, a PS-b-PnPMA with fully disordered state could be microphase-separated at a higher argon gas pressure. Figure 6 shows the birefringence intensity variation of PS-b-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.K.K.) [email protected]; (J.C.) jhcho@dankook. ac.kr. Author Contributions ‡

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF). J.C. acknowledges the financial support from NRF through Basic Science Research Program (No. 2012011583).

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Figure 6. Birefringence intensity variation of fully disordered PS-bPnPMA-L at different argon gas pressures.

PnPMA-L having fully disordered state at different argon gas pressures. At an argon pressure of 20 bar, PS-b-PnPMA-L exhibits a closed-loop phase behavior with TLDOT of 148 °C and TUODT of 212 °C. Also, with further increase of gas pressure to 40 bar, the closed loop size was expanded (TLDOT = 139 °C and TUODT = 223 °C). This indicates that the phase behavior of PSb-PnPMA was successfully tuned by employing various kinds of gases not only to be more miscible but also more microphasesegregated.

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CONCLUSIONS We investigated the effect of inert gas pressure on the phase behavior of block copolymers such as PS-b-PnPMA showing closed-loop phase behavior with high baroplasticity. We chose nitrogen, helium and argon gases because they are widely used as inert gases. When we applied helium gas pressure, it enhanced the miscibility between two blocks similar to hydrostatic pressure. On the other hand, nitrogen and argon gas pressure enhanced the immiscibility of PS-b-PnPMA. As a result, closed-loop size was decreased with helium gas pressure but enlarged with nitrogen and argon gas pressure. In addition, we successfully tuned the fully disordered PS-b-PnPMA-L with argon gas pressure to show closed-loop phase behavior with both of TLDOT and TUODT. Our observation implies that even well-known and widely employed inert gases such as nitrogen and argon can affect the phase behavior of weakly interacting block copolymers at high pressures. The effects of gas pressure depending on a kind of gas could be one of the simple and facile methodologies to tune the phase behavior of weakly interacting block copolymers. 498

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