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Miscibility induced by confinement in mixed monolayer films
Colloids and Surfaces A: Physicochem. Eng. Aspects 367 (2010) 161–166
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Miscibility induced by confinement in mixed monolayer films Wen-Ping Hsu ∗ Department of Chemical Engineering, National United University, 1, Lien-Da, Kung-Ching Li, Miao-Li 36003, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 19 March 2010 Received in revised form 9 June 2010 Accepted 9 July 2010 Available online 16 July 2010 Keywords: Stereoregular PMMA PS-b-PEO Monolayer
a b s t r a c t Both poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) are known to be immiscible with poly(styrene). PMMA is found to be miscible with poly(ethylene oxide). Therefore, PMMA is predicted by the mean field theory to be immiscible with poly(styrene-b-ethylene oxide) (PS-b-PEO) in the bulk state. The miscibility of PMMA with PS-b-PEO may be different in the two-dimensional state. The mixed monolayer behavior of stereoregular (including isotactic, atactic and syndiotactic) PMMA and PS-b-PEO was investigated in this article on the basis of the measurements of surface pressure–area per molecule (–A) isotherms at three different temperatures. The miscibility and nonideality of the mixed monolayers were examined by calculating the excess area as a function of composition. Mostly negative deviations from ideality were observed in the mixed monolayers at 25 ◦ C and 32.5 ◦ C. This is likely because of favorable interaction between PMMA and PEO. However, positive deviations occurred at 32.5 ◦ C and 40 ◦ C with atactic PMMA (or syndiotactic PMMA) mixed monolayers. With confinement in the two-dimensional state, the miscibility between PMMA and PS-b-PEO was greatly improved in comparison with the bulk state. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It has been known for years that the stereoregularity of polymer chains influences polymer–polymer miscibility. Due to its availability in both syndiotactic and isotactic forms, poly(methyl methacrylate) (PMMA) has been used frequently in the investigation of the effect of tacticity on miscibility. Several articles have shown that the tacticity of PMMA influence blend compatibility when PMMA is blended with a chemically different polymer. One system worth noticing is PMMA/poly(ethylene oxide) (PEO) [1–3]. For the PMMA/PEO system, the results for miscibility are not consistent. One result is that isotactic PMMA is more miscible with PEO than syndiotactic PMMA [1]. The other results are that syndiotactic PMMA is more miscible with PEO than isotactic PMMA [2,3]. The reason may be the differences in the molecular weights and the preparation methods of the sample. Because atatctic PMMA is mainly composed of syndiotatic PMMA, the result of atactic one is often similar to syndiotactic PMMA. Kawaguchi and Nishida [4] reported surface pressure measurements of binary mixture of PMMA and PEO. The negative deviation of surface area in mixture was found and indicated that the intermolecular interaction between PMMA and PEO is attractive. The mixture at the air/water interface is non-ideally miscible and stable. To the best of my knowledge, no reports on isotactic and syndiotactic PMMA with
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PEO has been documented. However, the results are likely similar to atatcic PMMA and PEO. PS and PMMA are not likely to be miscible in the monolayer state similar to the bulk state. One of the most widely investigated diblock copolymers is certainly poly(styrene-b-ethylene oxide) (PS-b-PEO) [5–8]. PS-b-PEO at the air/water interface demonstrates variety of nanostructures (dots, spagetti, rings, chainlike aggregates etc.) resulting from the spontaneous copolymer aggregation. The hydrophilic PEO dissolves into the pure water subphase, while the hydrophobic blocks aggregate at the interface during compression. Various morphologies have been detected depending on the relative chain length of the hydrophilic and hydrophobic blocks, the concentration of spreading solution and the surface pressure. The possible formation of non-equilibrium states when PS-b-PEO that is spread has a block with a high glass temperature (PS in the present case), and another with a relatively low Tg has been pointed out by several authors [9–11]. To the best of our knowledge, there is no report on the mixing behavior of PS-b-PEO with stereoregular PMMA in the bulk state. The reason is likely as follows. PS and PMMA are known to be immiscible. PEO is miscible with PMMA. PEO is also immiscible with PS. Therefore PS-b-PEO is still immiscible with PMMA because of two immiscible pairs (PS/PMMA and PEO/PS) as predicted by the mean field theory. The miscibility of PMMA with PS-b-PEO in the two-dimensional state is likely different from the bulk state. Therefore in this article, mixed monolayers of PS-b-PEO with streoregular PMMA were prepared at the air/water interface. The surface pressure–area
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per molecule (–A) isotherms of the binary films of stereoregular PMMA/PS-b-PEO was measured. On the basis of the results of surface pressure–area per molecule (–A) isotherms, miscibility between stereoregular PMMA and PS-b-PEO was investigated. The effects of tacticity of PMMA, temperature and PS-b-PEO composition on the mixed monolayer films was expounded and reported in detail in this article. To the best of our knowledge, there is no systematic and detailed report on the miscibility of mixed stereoregular PMMA/PS-b-PEO monolayers at the air/water interface. Interestingly, confinement in the monolayer state became a driving force for miscibility between PMMA and PS-b-PEO. 2. Experimental 2.1. Materials Isotactic, atactic and syndiotactic PMMAs (designated as i-, aand sPMMA in this study) were purchased from Polysciences, Inc., Warrington, PA. According to the supplier information, the molecular weights (Mw ) of iPMMA, aPMMA and sPMMA are the same about 100,000 g/mol. The molecular weight (Mn ) of PS-b-PEO obtained from Polymer Source, Inc., Montreal, Canada is about 95,000 g/mol for each block. The polydispersity index is 1.07. According to the supplier information, the glass transition temperatures for the PS block and the PEO block are 89 ◦ C and −60 ◦ C, respectively. The polydispersities (Mw /Mn ) of the three PMMAs were not measured therefore not reported here. However, the molecular weight distribution effect is believed to be minimal in the current study when compared with the effect of tacticity. We did not characterize the tacticity of PMMA by NMR. Therefore a simple estimation of the fractions of meso (m) and racemic (r) diads was resorted. The meso diad fractions of PMMA were computed previously[12]. The m and r fractions of iPMMA, aPMMA and sPMMA are 68.7% and 31.3%, 33.8% and 66.2%, 9.3% and 90.7%, respectively. The error of estimation is 5–8%. The glass transition temperatures (Tg s) of bulk iPMMA, aPMMA and sPMMA were determined to be 75 ◦ C, 103 ◦ C and 122 ◦ C, respectively, with a DuPont 2000 thermal analyzer at a heating rate of 20 ◦ C/min. The inflection point of the specific heat jump of the second thermal scan was taken as Tg . The Tg of iPMMA is much higher than 100% iPMMA (30–40 ◦ C), therefore the crystallization of iPMMA in this study is minimal and also proven by IR. 2-Butanone purchased from Kanto Chemical Co. Inc. was used as the spreading solvent for the polymer films. The solvent was chosen to be the same as a previous study of PMMA and poly(vinyl phenol) [13]. Only highly pure water, which was purified by means of a Milli-Q plus water purification system, with a resistivity of 18.2 M cm was used in all experiments. Blank experiments using 2-butanone was carried out that there were no surface-active impurities.
during the compression of the monolayer. The surface pressure was measured by the Wilhelmy plate method. The resolution for surface measurement is 0.004 mN/m, and the inaccuracy of surface area regulation is less than 1%, according to the specifications of the instruments. A surface pressure–area per molecule (–A) isotherm was obtained by a continuous compression of a monolayer at the interface by two barriers. Before each isotherm measurement, the trough and barriers were cleaned with an ethanol solution and then rinsed by purified water. The sand blasted platinum plate used for surface pressure measurements was also rinsed with purified water and then flamed before use. In addition, all glassware was cleaned prior to use in the same manner as the trough and barrier. For starting the experiment, the freshly cleaned trough was placed into position in the apparatus first, then it was filled with purified water as the subphase with temperatures controlled at 25 ± 1 ◦ C, 32.5 ± 1 ◦ C and 40 ± 1 ◦ C. The clean platinum plate was hanged in the appropriate position for surface pressure measurements. The surface pressure fluctuation was estimated to be less than 0.2 mN/m during the compression of the entire trough surface area range. Then, the two barriers were moved back to their initial positions. The sample concentration of solution of polymer and solvent was set at 0.5 mg/mL. A 25 L sample containing monolayer-forming polymeric materials was spread on the subphase by using a Hamilton microsyringe to make the deposition of polymer molecules at almost the same condition. At least 30–45 min was allowed for evaporation of the spreading solvent. After the solvent was evaporated, the monolayer was compressed continuously at a rate of 3.5 mm/min to obtain a single –A isotherm. The –A isotherms of our studied polymers are dependent on the compression rate therefore the results were performed at the same compression speed.
3. Results and discussion 3.1. Polymers and PMMA/PS-b-PEO mixture at 25 ◦ C The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 25 ◦ C are shown in Fig. 1. The discussion for the three stereoisomers at three temperatures was reported previously [14] and therefore omitted here. The results are consistent with those reported by Beredjick et al. [15]. The only slight difference is sPMMA demonstrating a lower lift-off area than a previous investigation [8].
2.2. Surface pressure measurements A model minitrough (M 1200) was purchased from KSV Instruments Ltd., Finland. The Teflon trough was 320 mm long and 75 mm wide. Regulation of the trough temperature was controlled by circulating constant temperature water from an external circulator through the tubes attached to the aluminum-based plate of the trough. The trough was placed on an isolated vibration-free table and was enclosed in a glass chamber to avoid contaminants from the air. A computer with an interface unit obtained from KSV instruments Ltd. was used to control the Teflon barriers. One of the important characteristics of the trough system is that two barriers confining a monolayer at the interface are driven symmetrically
Fig. 1. Surface pressure–area per molecule isotherms of the polymers at 25 ◦ C.
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Fig. 3. Surface pressure–area per molecule isotherms of the polymers at 32.5 ◦ C.
layer isotherms in Fig. 2(b) demonstrate a complex behavior. Two mixed isotherms have surface pressure lower than PS-b-PEO. The –A isotherms of sPMMA/PS-b-PEO (75/25) monolayer follow the sPMMA –A isotherms closely but with even higher collapse pressure. For aPMMA/PS-b-PEO mixed monolayers (shown in Fig. 2(c)), the –A isotherms of two mixed monolayers are in between those of pure polymers. However, when PS-b-PEO composition is higher than aPMMA, the –A isotherms locate even lower than PS-b-PEO isotherms. 3.2. Polymers and PMMA/PS-b-PEO mixture at 32.5 ◦ C
Fig. 2. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 25 ◦ C.
The –A isotherms of PS-b-PEO (presented in Fig. 1) show a transition at lower than 10 mN/m similar to the findings of Cox et al. [8]. However, the collapse pressure (33–34 mN/m) in this study is smaller than the results of Cox et al. (43 mN/m) likely because of differences in molecular weights of PEO and PS and spreading solvent. PS-b-PEO demonstrates the lowest collapse pressure in the studied polymers as shown in Fig. 1. Fig. 2 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO and (c) aPMMA/PS-b-PEO at 25 ◦ C. For iPMMA/PS-b-PEO monolayers (shown in Fig. 2(a)), the –A isotherms of mixed monolayers demonstrate smaller surface area than either iPMMA or PS-b-PEO. However, the mixed mono-
The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 32.5 ◦ C are shown in Fig. 3. The obvious difference for PS-b-PEO is the transition pressure (7–8 mN/m at 25 ◦ C) moves to a higher surface pressure (9–10 mN/m at 32.5 ◦ C) and lower surface area resulting from temperature elevation as expected in most polymers. All four polymers demonstrate larger lift-off areas than those at 25 ◦ C. Fig. 4 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO and (c) aPMMA/PS-b-PEO at 32.5 ◦ C. The iPMMA/PS-b-PEO and sPMMA/PS-b-PEO monolayers show a quite different behavior at this temperature likely due to PMMA stereoregularity and possible PEO segments crystallization in the block copolymer. The –A isotherms of mixed monolayers have an order not according to its composition. The one with PMMA/PS-b-PEO(50/50) is on the top followed by PMMA/PS-bPEO(75/25) then PMMA/PS-b-PEO(25/75). The results will become clear after excess area analysis shown later. For the aPMMA/PS-bPEO monolayers, the situation is quite normal because of its mostly irregular and atactic chain conformations. The –A isotherms follow the composition trend and are located mostly in between two pure polymers. 3.3. Polymers and PMMA/PS-b-PEO mixture at 40 ◦ C The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 40 ◦ C are shown in Fig. 5. The collapse pressures of the four studied polymers decrease due to temperature elevation from 32.5 ◦ C to 40 ◦ C. However, the transition pressure of PS-b-PEO becomes lower likely because of less crystalline effect of PEO segments. Fig. 6 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PSb-PEO at 40 ◦ C. iPMMA/PS-b-PEO mixed monolayers in Fig. 6(a)
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Fig. 5. Surface pressure–area per molecule isotherms of the polymers at 40 ◦ C.
3.4. The excess areas of mixture at three different temperatures A recent study of Monroy et al. [16] on monolayers of hydrogenbonded polymer blends indicated that the calculation of the excess Gibbs energy provides a similar result as the excess area. Therefore in this study only the excess areas were calculated. At a given surface pressure, the excess area is defined as the difference between the average area per molecule of a mixed monolayer consisting of components 1 and 2 and that of an ideal mixed monolayer [17]. Aex = A12 − Aideal = A12 − (X1 A1 + X2 A2 )
Fig. 4. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 32.5 ◦ C.
show quite typical behavior with most blends’ –A isotherms located in between pure polymers. The situation is a little complex for the sPMMA/PS-b-PEO mixed monolayers (as presented in Fig. 6(b)). The results will become clear after excess area analysis in the following section. For the mixed aPMMA/PS-b-PEO monolayers illustrated in Fig. 6(c), only aPMMA/PS-b-PEO(50/50) mixed monolayer show a different behavior. The –A isotherm is shifted even to the right of aPMMA –A isotherm. The other two compositions have their –A isotherms between aPMMA and PSb-PEO.
(1)
where A12 and Aideal are the mean and ideal areas per molecule of the mixed monolayer at a given surface pressure, respectively, X1 and X2 imply the mole fractions of components 1 and 2, respectively, and A1 and A2 are the areas per molecule of each pure monolayer at the same surface pressure. Based on Eq. (1), the Aex values of mixed PMMA/PS-b-PEO monolayers can be estimated from the data shown in Figs. 2(a)–(c), 4(a)–(c) and 6(a)–(c), individually. In Figs. 7(a)–(c) and 8(a)–(c) the normalized quantities, Aex /Aideal are shown as a function of PS-b-PEO mole fraction and surface pressure at 25 ◦ C and 32.5 ◦ C, respectively. The results at 40 ◦ C were not shown because of similarity to those at 25 ◦ C. The reason is likely due to similar mushroom to brush transition at 25 ◦ C and 40 ◦ C. The mushroom to brush transition at 32.5 ◦ C is higher than those at 25 ◦ C and 40 ◦ C. The surface pressures at 3 mN/m and 6 mN/m were chosen below the phase transition (8–10 mN/m) of PS-bPEO to estimate the excess surface area. For high surface pressure, the monolayer would be essentially formed by patches of PMMA and PS, with PEO chains mostly in water. The excess area at pressure of 9 mN/m close to the transition was also presented to show little difference from those at surface pressures of 3 mN/m and 6 mN/m. Since the average repeat unit of PS-b-PEO is smaller than PMMA, the mole fraction of PS-b-PEO is larger than the original weight fractions of 0.25, 0.50 and 0.75. For Fig. 7(a) the Aex /Aideal values are all negative. Negative excess area deviation shows favorable interaction between iPMMA and PS-b-PEO. The interaction is likely because of dipole– dipole interaction or hydrophobic interaction. The crystallization effect of iPMMA on mixed monolayer is considered to be insignificant since this would interfere with the interaction between iPMMA and PEO. Surface pressure difference does not cause any obvious change in the excess area. The Aex /Aideal values shown in Fig. 7(b) are close to or smaller than zero. The results at 50 wt% and 75 wt% PS-b-PEO are similar to Fig. 7(a). For Fig. 7(c) the Aex /Aideal values are mostly positive. At 50 wt% PS-b-
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Fig. 7. Aex /Aideal as a function of composition for mixed monolayers of (a) iPMMA/PSb-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at various surface pressures and 25 ◦ C. (calculated from Fig. 2).
Fig. 6. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 40 ◦ C.
PEO, the Aex /Aideal values are dependent on surface pressure. When the surface pressures are at 3 mN/m and 6 mN/m, the Aex /Aideal values are positive. However, the Aex /Aideal values become negative at 9 mN/m surface pressure. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 25 ◦ C can be listed as follows: iPMMA > sPMMA > aPMMA. The elevation of temperature from 25 ◦ C to 32.5 ◦ C probably causes PMMA to show less favorable interaction with PS-b-PEO. Thus less negative or more positive excess areas were observed in Fig. 8(a)–(c). For iPMMA/PS-b-PEO mixed monolayer at 32.5 ◦ C, the –A isotherms still demonstrate negative excess surface area as at
25 ◦ C. Totally speaking, the negative trend is less because of temperature increase. sPMMA/PS-b-PEO mixed monolayers at 32.5 ◦ C shown in Fig. 8(b) demonstrate positive deviations at mid composition different from those at 25 ◦ C. For aPMMA/PS-b-PEO mixed monolayers at 32.5 ◦ C (Fig. 8(c)), the –A isotherms are similar as at 25 ◦ C. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 32.5 ◦ C has the same order as at 25 ◦ C: iPMMA > sPMMA > aPMMA. The iPMMA/PS-b-PEO and sPMMA/PS-b-PEO mixed monolayers at 40 ◦ C demonstrate mostly negative deviation. iPMMA (or sPMMA) shows favorable interaction with PS-b-PEO at this temperature. However, for the aPMMA/PS-b-PEO mixed monolayers the excess surface areas are in the majority larger than or equal to zero. The temperature effect on the aPMMA mixed monolayers is marked likely because of its heterotactic segments becoming more disoriented at higher temperatures. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 40 ◦ C is the same as at 25 ◦ C and 32.5 ◦ C.
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from the –A isotherms of mixed stereoregular PMMA/PS-b-PEO monolayers show mostly the negative Aex /Aideal values at 25 ◦ C and 32.5 ◦ C. Therefore in the confined two-dimensional environment, PMMA interacts favorably with PS-b-PEO. The reason is likely because of attractive interaction between PMMA and PEO and nonequilibrium states formed by high Tg PS with PEO. The positive deviation is observed at 32.5 ◦ C and 40 ◦ C with aPMMA (or sPMMA) mixed monolayers. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility of stereoregular PMMA with PS-bPEO at the three studied temperatures can be ranked as follows: iPMMA > sPMMA > aPMMA. Acknowledgement The partial support by the National Science Council of Taiwan through Grant NSC-94-2216-E239-005 and NSC-95-2216-E239023 is greatly appreciated. References
Fig. 8. Aex /Aideal as a function of composition for mixed monolayers of (a) iPMMA/PSb-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at various surface pressures and 32.5 ◦ C (calculated from Fig. 4).
4. Conclusions Miscibility deduced from the –A isotherms of mixed stereoregular PMMA/PS-b-PEO monolayers is in the majority different from that for the corresponding polymer blends in the bulk state in spite of possible minor complication caused by crystallization of PEO. aPMMA shows miscibility with PEO. PMMA and PS-bPEO are predicted to be immiscible in the bulk state because of two immiscible pairs (PMMA/PS and PS/PEO). The results deduced
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Miscibility induced by confinement in mixed monolayer films Wen-Ping Hsu ∗ Department of Chemical Engineering, National United University, 1, Lien-Da, Kung-Ching Li, Miao-Li 36003, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 19 March 2010 Received in revised form 9 June 2010 Accepted 9 July 2010 Available online 16 July 2010 Keywords: Stereoregular PMMA PS-b-PEO Monolayer
a b s t r a c t Both poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) are known to be immiscible with poly(styrene). PMMA is found to be miscible with poly(ethylene oxide). Therefore, PMMA is predicted by the mean field theory to be immiscible with poly(styrene-b-ethylene oxide) (PS-b-PEO) in the bulk state. The miscibility of PMMA with PS-b-PEO may be different in the two-dimensional state. The mixed monolayer behavior of stereoregular (including isotactic, atactic and syndiotactic) PMMA and PS-b-PEO was investigated in this article on the basis of the measurements of surface pressure–area per molecule (–A) isotherms at three different temperatures. The miscibility and nonideality of the mixed monolayers were examined by calculating the excess area as a function of composition. Mostly negative deviations from ideality were observed in the mixed monolayers at 25 ◦ C and 32.5 ◦ C. This is likely because of favorable interaction between PMMA and PEO. However, positive deviations occurred at 32.5 ◦ C and 40 ◦ C with atactic PMMA (or syndiotactic PMMA) mixed monolayers. With confinement in the two-dimensional state, the miscibility between PMMA and PS-b-PEO was greatly improved in comparison with the bulk state. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It has been known for years that the stereoregularity of polymer chains influences polymer–polymer miscibility. Due to its availability in both syndiotactic and isotactic forms, poly(methyl methacrylate) (PMMA) has been used frequently in the investigation of the effect of tacticity on miscibility. Several articles have shown that the tacticity of PMMA influence blend compatibility when PMMA is blended with a chemically different polymer. One system worth noticing is PMMA/poly(ethylene oxide) (PEO) [1–3]. For the PMMA/PEO system, the results for miscibility are not consistent. One result is that isotactic PMMA is more miscible with PEO than syndiotactic PMMA [1]. The other results are that syndiotactic PMMA is more miscible with PEO than isotactic PMMA [2,3]. The reason may be the differences in the molecular weights and the preparation methods of the sample. Because atatctic PMMA is mainly composed of syndiotatic PMMA, the result of atactic one is often similar to syndiotactic PMMA. Kawaguchi and Nishida [4] reported surface pressure measurements of binary mixture of PMMA and PEO. The negative deviation of surface area in mixture was found and indicated that the intermolecular interaction between PMMA and PEO is attractive. The mixture at the air/water interface is non-ideally miscible and stable. To the best of my knowledge, no reports on isotactic and syndiotactic PMMA with
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PEO has been documented. However, the results are likely similar to atatcic PMMA and PEO. PS and PMMA are not likely to be miscible in the monolayer state similar to the bulk state. One of the most widely investigated diblock copolymers is certainly poly(styrene-b-ethylene oxide) (PS-b-PEO) [5–8]. PS-b-PEO at the air/water interface demonstrates variety of nanostructures (dots, spagetti, rings, chainlike aggregates etc.) resulting from the spontaneous copolymer aggregation. The hydrophilic PEO dissolves into the pure water subphase, while the hydrophobic blocks aggregate at the interface during compression. Various morphologies have been detected depending on the relative chain length of the hydrophilic and hydrophobic blocks, the concentration of spreading solution and the surface pressure. The possible formation of non-equilibrium states when PS-b-PEO that is spread has a block with a high glass temperature (PS in the present case), and another with a relatively low Tg has been pointed out by several authors [9–11]. To the best of our knowledge, there is no report on the mixing behavior of PS-b-PEO with stereoregular PMMA in the bulk state. The reason is likely as follows. PS and PMMA are known to be immiscible. PEO is miscible with PMMA. PEO is also immiscible with PS. Therefore PS-b-PEO is still immiscible with PMMA because of two immiscible pairs (PS/PMMA and PEO/PS) as predicted by the mean field theory. The miscibility of PMMA with PS-b-PEO in the two-dimensional state is likely different from the bulk state. Therefore in this article, mixed monolayers of PS-b-PEO with streoregular PMMA were prepared at the air/water interface. The surface pressure–area
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per molecule (–A) isotherms of the binary films of stereoregular PMMA/PS-b-PEO was measured. On the basis of the results of surface pressure–area per molecule (–A) isotherms, miscibility between stereoregular PMMA and PS-b-PEO was investigated. The effects of tacticity of PMMA, temperature and PS-b-PEO composition on the mixed monolayer films was expounded and reported in detail in this article. To the best of our knowledge, there is no systematic and detailed report on the miscibility of mixed stereoregular PMMA/PS-b-PEO monolayers at the air/water interface. Interestingly, confinement in the monolayer state became a driving force for miscibility between PMMA and PS-b-PEO. 2. Experimental 2.1. Materials Isotactic, atactic and syndiotactic PMMAs (designated as i-, aand sPMMA in this study) were purchased from Polysciences, Inc., Warrington, PA. According to the supplier information, the molecular weights (Mw ) of iPMMA, aPMMA and sPMMA are the same about 100,000 g/mol. The molecular weight (Mn ) of PS-b-PEO obtained from Polymer Source, Inc., Montreal, Canada is about 95,000 g/mol for each block. The polydispersity index is 1.07. According to the supplier information, the glass transition temperatures for the PS block and the PEO block are 89 ◦ C and −60 ◦ C, respectively. The polydispersities (Mw /Mn ) of the three PMMAs were not measured therefore not reported here. However, the molecular weight distribution effect is believed to be minimal in the current study when compared with the effect of tacticity. We did not characterize the tacticity of PMMA by NMR. Therefore a simple estimation of the fractions of meso (m) and racemic (r) diads was resorted. The meso diad fractions of PMMA were computed previously[12]. The m and r fractions of iPMMA, aPMMA and sPMMA are 68.7% and 31.3%, 33.8% and 66.2%, 9.3% and 90.7%, respectively. The error of estimation is 5–8%. The glass transition temperatures (Tg s) of bulk iPMMA, aPMMA and sPMMA were determined to be 75 ◦ C, 103 ◦ C and 122 ◦ C, respectively, with a DuPont 2000 thermal analyzer at a heating rate of 20 ◦ C/min. The inflection point of the specific heat jump of the second thermal scan was taken as Tg . The Tg of iPMMA is much higher than 100% iPMMA (30–40 ◦ C), therefore the crystallization of iPMMA in this study is minimal and also proven by IR. 2-Butanone purchased from Kanto Chemical Co. Inc. was used as the spreading solvent for the polymer films. The solvent was chosen to be the same as a previous study of PMMA and poly(vinyl phenol) [13]. Only highly pure water, which was purified by means of a Milli-Q plus water purification system, with a resistivity of 18.2 M cm was used in all experiments. Blank experiments using 2-butanone was carried out that there were no surface-active impurities.
during the compression of the monolayer. The surface pressure was measured by the Wilhelmy plate method. The resolution for surface measurement is 0.004 mN/m, and the inaccuracy of surface area regulation is less than 1%, according to the specifications of the instruments. A surface pressure–area per molecule (–A) isotherm was obtained by a continuous compression of a monolayer at the interface by two barriers. Before each isotherm measurement, the trough and barriers were cleaned with an ethanol solution and then rinsed by purified water. The sand blasted platinum plate used for surface pressure measurements was also rinsed with purified water and then flamed before use. In addition, all glassware was cleaned prior to use in the same manner as the trough and barrier. For starting the experiment, the freshly cleaned trough was placed into position in the apparatus first, then it was filled with purified water as the subphase with temperatures controlled at 25 ± 1 ◦ C, 32.5 ± 1 ◦ C and 40 ± 1 ◦ C. The clean platinum plate was hanged in the appropriate position for surface pressure measurements. The surface pressure fluctuation was estimated to be less than 0.2 mN/m during the compression of the entire trough surface area range. Then, the two barriers were moved back to their initial positions. The sample concentration of solution of polymer and solvent was set at 0.5 mg/mL. A 25 L sample containing monolayer-forming polymeric materials was spread on the subphase by using a Hamilton microsyringe to make the deposition of polymer molecules at almost the same condition. At least 30–45 min was allowed for evaporation of the spreading solvent. After the solvent was evaporated, the monolayer was compressed continuously at a rate of 3.5 mm/min to obtain a single –A isotherm. The –A isotherms of our studied polymers are dependent on the compression rate therefore the results were performed at the same compression speed.
3. Results and discussion 3.1. Polymers and PMMA/PS-b-PEO mixture at 25 ◦ C The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 25 ◦ C are shown in Fig. 1. The discussion for the three stereoisomers at three temperatures was reported previously [14] and therefore omitted here. The results are consistent with those reported by Beredjick et al. [15]. The only slight difference is sPMMA demonstrating a lower lift-off area than a previous investigation [8].
2.2. Surface pressure measurements A model minitrough (M 1200) was purchased from KSV Instruments Ltd., Finland. The Teflon trough was 320 mm long and 75 mm wide. Regulation of the trough temperature was controlled by circulating constant temperature water from an external circulator through the tubes attached to the aluminum-based plate of the trough. The trough was placed on an isolated vibration-free table and was enclosed in a glass chamber to avoid contaminants from the air. A computer with an interface unit obtained from KSV instruments Ltd. was used to control the Teflon barriers. One of the important characteristics of the trough system is that two barriers confining a monolayer at the interface are driven symmetrically
Fig. 1. Surface pressure–area per molecule isotherms of the polymers at 25 ◦ C.
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Fig. 3. Surface pressure–area per molecule isotherms of the polymers at 32.5 ◦ C.
layer isotherms in Fig. 2(b) demonstrate a complex behavior. Two mixed isotherms have surface pressure lower than PS-b-PEO. The –A isotherms of sPMMA/PS-b-PEO (75/25) monolayer follow the sPMMA –A isotherms closely but with even higher collapse pressure. For aPMMA/PS-b-PEO mixed monolayers (shown in Fig. 2(c)), the –A isotherms of two mixed monolayers are in between those of pure polymers. However, when PS-b-PEO composition is higher than aPMMA, the –A isotherms locate even lower than PS-b-PEO isotherms. 3.2. Polymers and PMMA/PS-b-PEO mixture at 32.5 ◦ C
Fig. 2. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 25 ◦ C.
The –A isotherms of PS-b-PEO (presented in Fig. 1) show a transition at lower than 10 mN/m similar to the findings of Cox et al. [8]. However, the collapse pressure (33–34 mN/m) in this study is smaller than the results of Cox et al. (43 mN/m) likely because of differences in molecular weights of PEO and PS and spreading solvent. PS-b-PEO demonstrates the lowest collapse pressure in the studied polymers as shown in Fig. 1. Fig. 2 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO and (c) aPMMA/PS-b-PEO at 25 ◦ C. For iPMMA/PS-b-PEO monolayers (shown in Fig. 2(a)), the –A isotherms of mixed monolayers demonstrate smaller surface area than either iPMMA or PS-b-PEO. However, the mixed mono-
The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 32.5 ◦ C are shown in Fig. 3. The obvious difference for PS-b-PEO is the transition pressure (7–8 mN/m at 25 ◦ C) moves to a higher surface pressure (9–10 mN/m at 32.5 ◦ C) and lower surface area resulting from temperature elevation as expected in most polymers. All four polymers demonstrate larger lift-off areas than those at 25 ◦ C. Fig. 4 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO and (c) aPMMA/PS-b-PEO at 32.5 ◦ C. The iPMMA/PS-b-PEO and sPMMA/PS-b-PEO monolayers show a quite different behavior at this temperature likely due to PMMA stereoregularity and possible PEO segments crystallization in the block copolymer. The –A isotherms of mixed monolayers have an order not according to its composition. The one with PMMA/PS-b-PEO(50/50) is on the top followed by PMMA/PS-bPEO(75/25) then PMMA/PS-b-PEO(25/75). The results will become clear after excess area analysis shown later. For the aPMMA/PS-bPEO monolayers, the situation is quite normal because of its mostly irregular and atactic chain conformations. The –A isotherms follow the composition trend and are located mostly in between two pure polymers. 3.3. Polymers and PMMA/PS-b-PEO mixture at 40 ◦ C The –A isotherms of monolayers for the three PMMA stereoisomers and PS-b-PEO at 40 ◦ C are shown in Fig. 5. The collapse pressures of the four studied polymers decrease due to temperature elevation from 32.5 ◦ C to 40 ◦ C. However, the transition pressure of PS-b-PEO becomes lower likely because of less crystalline effect of PEO segments. Fig. 6 presents the –A isotherms of mixed monolayers for (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PSb-PEO at 40 ◦ C. iPMMA/PS-b-PEO mixed monolayers in Fig. 6(a)
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Fig. 5. Surface pressure–area per molecule isotherms of the polymers at 40 ◦ C.
3.4. The excess areas of mixture at three different temperatures A recent study of Monroy et al. [16] on monolayers of hydrogenbonded polymer blends indicated that the calculation of the excess Gibbs energy provides a similar result as the excess area. Therefore in this study only the excess areas were calculated. At a given surface pressure, the excess area is defined as the difference between the average area per molecule of a mixed monolayer consisting of components 1 and 2 and that of an ideal mixed monolayer [17]. Aex = A12 − Aideal = A12 − (X1 A1 + X2 A2 )
Fig. 4. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 32.5 ◦ C.
show quite typical behavior with most blends’ –A isotherms located in between pure polymers. The situation is a little complex for the sPMMA/PS-b-PEO mixed monolayers (as presented in Fig. 6(b)). The results will become clear after excess area analysis in the following section. For the mixed aPMMA/PS-b-PEO monolayers illustrated in Fig. 6(c), only aPMMA/PS-b-PEO(50/50) mixed monolayer show a different behavior. The –A isotherm is shifted even to the right of aPMMA –A isotherm. The other two compositions have their –A isotherms between aPMMA and PSb-PEO.
(1)
where A12 and Aideal are the mean and ideal areas per molecule of the mixed monolayer at a given surface pressure, respectively, X1 and X2 imply the mole fractions of components 1 and 2, respectively, and A1 and A2 are the areas per molecule of each pure monolayer at the same surface pressure. Based on Eq. (1), the Aex values of mixed PMMA/PS-b-PEO monolayers can be estimated from the data shown in Figs. 2(a)–(c), 4(a)–(c) and 6(a)–(c), individually. In Figs. 7(a)–(c) and 8(a)–(c) the normalized quantities, Aex /Aideal are shown as a function of PS-b-PEO mole fraction and surface pressure at 25 ◦ C and 32.5 ◦ C, respectively. The results at 40 ◦ C were not shown because of similarity to those at 25 ◦ C. The reason is likely due to similar mushroom to brush transition at 25 ◦ C and 40 ◦ C. The mushroom to brush transition at 32.5 ◦ C is higher than those at 25 ◦ C and 40 ◦ C. The surface pressures at 3 mN/m and 6 mN/m were chosen below the phase transition (8–10 mN/m) of PS-bPEO to estimate the excess surface area. For high surface pressure, the monolayer would be essentially formed by patches of PMMA and PS, with PEO chains mostly in water. The excess area at pressure of 9 mN/m close to the transition was also presented to show little difference from those at surface pressures of 3 mN/m and 6 mN/m. Since the average repeat unit of PS-b-PEO is smaller than PMMA, the mole fraction of PS-b-PEO is larger than the original weight fractions of 0.25, 0.50 and 0.75. For Fig. 7(a) the Aex /Aideal values are all negative. Negative excess area deviation shows favorable interaction between iPMMA and PS-b-PEO. The interaction is likely because of dipole– dipole interaction or hydrophobic interaction. The crystallization effect of iPMMA on mixed monolayer is considered to be insignificant since this would interfere with the interaction between iPMMA and PEO. Surface pressure difference does not cause any obvious change in the excess area. The Aex /Aideal values shown in Fig. 7(b) are close to or smaller than zero. The results at 50 wt% and 75 wt% PS-b-PEO are similar to Fig. 7(a). For Fig. 7(c) the Aex /Aideal values are mostly positive. At 50 wt% PS-b-
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Fig. 7. Aex /Aideal as a function of composition for mixed monolayers of (a) iPMMA/PSb-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at various surface pressures and 25 ◦ C. (calculated from Fig. 2).
Fig. 6. Surface pressure–area per molecule isotherms for mixed monolayers of (a) iPMMA/PS-b-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at 40 ◦ C.
PEO, the Aex /Aideal values are dependent on surface pressure. When the surface pressures are at 3 mN/m and 6 mN/m, the Aex /Aideal values are positive. However, the Aex /Aideal values become negative at 9 mN/m surface pressure. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 25 ◦ C can be listed as follows: iPMMA > sPMMA > aPMMA. The elevation of temperature from 25 ◦ C to 32.5 ◦ C probably causes PMMA to show less favorable interaction with PS-b-PEO. Thus less negative or more positive excess areas were observed in Fig. 8(a)–(c). For iPMMA/PS-b-PEO mixed monolayer at 32.5 ◦ C, the –A isotherms still demonstrate negative excess surface area as at
25 ◦ C. Totally speaking, the negative trend is less because of temperature increase. sPMMA/PS-b-PEO mixed monolayers at 32.5 ◦ C shown in Fig. 8(b) demonstrate positive deviations at mid composition different from those at 25 ◦ C. For aPMMA/PS-b-PEO mixed monolayers at 32.5 ◦ C (Fig. 8(c)), the –A isotherms are similar as at 25 ◦ C. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 32.5 ◦ C has the same order as at 25 ◦ C: iPMMA > sPMMA > aPMMA. The iPMMA/PS-b-PEO and sPMMA/PS-b-PEO mixed monolayers at 40 ◦ C demonstrate mostly negative deviation. iPMMA (or sPMMA) shows favorable interaction with PS-b-PEO at this temperature. However, for the aPMMA/PS-b-PEO mixed monolayers the excess surface areas are in the majority larger than or equal to zero. The temperature effect on the aPMMA mixed monolayers is marked likely because of its heterotactic segments becoming more disoriented at higher temperatures. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility degree with PS-b-PEO at 40 ◦ C is the same as at 25 ◦ C and 32.5 ◦ C.
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from the –A isotherms of mixed stereoregular PMMA/PS-b-PEO monolayers show mostly the negative Aex /Aideal values at 25 ◦ C and 32.5 ◦ C. Therefore in the confined two-dimensional environment, PMMA interacts favorably with PS-b-PEO. The reason is likely because of attractive interaction between PMMA and PEO and nonequilibrium states formed by high Tg PS with PEO. The positive deviation is observed at 32.5 ◦ C and 40 ◦ C with aPMMA (or sPMMA) mixed monolayers. If the Aex /Aideal values are used as an indicator for miscibility, the miscibility of stereoregular PMMA with PS-bPEO at the three studied temperatures can be ranked as follows: iPMMA > sPMMA > aPMMA. Acknowledgement The partial support by the National Science Council of Taiwan through Grant NSC-94-2216-E239-005 and NSC-95-2216-E239023 is greatly appreciated. References
Fig. 8. Aex /Aideal as a function of composition for mixed monolayers of (a) iPMMA/PSb-PEO, (b) sPMMA/PS-b-PEO, and (c) aPMMA/PS-b-PEO at various surface pressures and 32.5 ◦ C (calculated from Fig. 4).
4. Conclusions Miscibility deduced from the –A isotherms of mixed stereoregular PMMA/PS-b-PEO monolayers is in the majority different from that for the corresponding polymer blends in the bulk state in spite of possible minor complication caused by crystallization of PEO. aPMMA shows miscibility with PEO. PMMA and PS-bPEO are predicted to be immiscible in the bulk state because of two immiscible pairs (PMMA/PS and PS/PEO). The results deduced
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