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SAN blend in film
Materials Letters 60 (2006) 589 – 593 www.elsevier.com/locate/matlet
Phase behavior of near-critical PVME/SAN blend in film Kun Yang ⁎, Qi Yang, Guangxian Li, Yajie Sun, Yimin Mao College of Polymer Science and Engineering, The State Key Laboratory for Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China Received 23 March 2005; accepted 22 September 2005 Available online 13 October 2005
Abstract The phase behavior of near-critical PVME/SAN blend in thin film was studied by time-resolved small-angle light scattering (SALS) and atomic force microscopy (AFM). The phase diagram showed that PVME/SAN99 blend had the lower critical solution temperature (LCST), the LCST and the critical composition was 119 °C, 74 wt.% PVME, respectively. However, the cloud points of this system were hard to be obtained before the decomposition of PVME when the weight fraction of SAN99 was more than 50 wt.%. Both SALS and AFM results confirmed that, for PVME/SAN99 blend, there was certainly a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition in the twophase region of 3D Ising regime. For 80 / 20 PVME / SAN99 blend, during the phase separation, the small orientation of SAN99 domains took place at the early stage of phase separation, and finally disappeared when the phase separation finished. But for 70 / 30 PVME / SAN99 blend, the orientation did not appear. © 2005 Elsevier B.V. All rights reserved. Keywords: Phase separation mechanism; PVME/SAN blend; Orientation
1. Introduction Numerous theoretical and experimental studies have described the solubilization of block copolymer segments by homopolymers, where the homopolymer is of similar chemistry as one of the block copolymer segment. The studies on blends of random copolymer with homopolymer are relatively less. The earliest theoretical descriptions of random copolymer/homopolymer systems were based on an extension to random copolymer of Flory–Huggins (FH) theory. Later, Shimomai et al. [1] applied the equation of state theory (EOS), and Dudowicz and Freed [2] applied the general lattice cluster theory (LCT) to describe these systems. In the previous paper [3], we discussed the miscibility of polystyrene (PS) with random copolymer styrene–acrylonitrile (SAN). It had been found for the first time that PS/SAN blend showed the upper critical solution temperature (UCST) behavior. The miscibility of binary polymer blend of PS/SAN was predicted based on both Flory's equation of state theory (EOS) [4] and Wolf's theory on binary polymer blends of the
⁎ Corresponding author. Tel.: +86 28 85405122; fax: +86 28 85405402. E-mail address: [email protected] (K. Yang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.045
A/A–B type [5]. The prediction results of the two theories and the experiment results were in accordance with each other. For blends of random copolymer with homopolymer, where the homopolymer was not of similar chemistry as one of the random copolymer segment, poly(methyl methacrylate) (PMMA)/SAN was a common random system, which had been studied extensively [6–8]. Poly (vinyl methyl ether) (PVME)/ SAN was another rational system. Min and Paul [9] first studied the blends of PVME with SAN, and found the effect of acrylonitrile content on cloud points for blends with SAN copolymer at 80 wt.% PVME, but the phase behavior of this system was little studied. According to the mean-field theory, there exist two phase separation mechanisms, one is nucleation and growth mechanism, the other is spinodal decomposition mechanism. Some people [10–15] have confirmed, for near-critical blends, the phase separation is generally controlled by the spinodal decomposition mechanism, and for off-critical blends, the phase separation is generally controlled by the nucleation and growth mechanism. G. Müller et al. [16] have found if one takes the 3D Ising regime into consideration, the phase separation is described as a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition by
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strong fluctuation in the two-phase region of 3D Ising regime. To understand this phenomenon further, we used the small-angle light scattering and atomic force microscopy to study the phase behavior of near-critical PVME/SAN blend in film. 2. Experimental 2.1. Materials PVME (Mw = 75,000, Mw / Mn = 4.04, Tg = 243K) was purchased from Tokyo Kasei Kogyo Co. Ltd. Monomers, styrene and acrylonitrile were freed from inhibitor and distilled prior to use. Random copolymer SAN was synthesized by radical polymerization in toluene solution for 3–4 h at 347 K, using AIBN as an initiator. The resulting polymer solution was precipitated into a large excess amount of methanol and purified and dried in vacuum oven for at least 48 h. The molecular characteristics of SAN were given in Table 1. 2.2. Sample Preparation Blend of PVME/SAN99 was first weighed at desired composition, and then dissolved into toluene to make 5 wt.% solution. The solution stayed still for several hours in order to get clear, homogeneous liquid. The film was prepared by casting from solution on a glass substrate in a dry atmosphere, the toluene was slowly evaporated at ambient temperature for three days, then the resulting film (30 μm) was further dried in a vacuum oven for three days at 60 °C. 2.3. Measurements 2.3.1. Small-angle light scattering (SALS) The light scattering apparatus used in this study was discussed in the previous paper [3]. The light source was a polarized 10-mW He–Ne laser beam with a wavelength of 632.8 nm. A two-dimensional charge coupled device (CCD) camera was used to record the change of scattering patterns and the scattering intensity during the experiments, which provided a total angular range from 0.5° to 10°. A temperature-controlled chamber consisting of copper plate and heating bars is used to heat the sample at different heating rates. The accuracy of the temperature control is in the order of ± 0.1 °C. 2.3.2. Atomic force microscopy Atomic force microscopy (AFM) experiment was performed with a Nanoscope scanning probe microscope. The height and phase images were obtained simultaneously while operating the instrument in the tapping mode under ambient conditions.
Table 1 Molecular Characteristics of SAN99 Symbol
Mw
Mw / Mn
Styrene volume fraction
SAN99
82,300
1.49
0.99
Fig. 1. Cloud point curve for PVME/SAN99 blend. Heating rate = 1 °C/min.
Images were taken at the fundamental resonance frequency of the silicon cantilevers, which was typically around 300 kHz. Typical scan speeds during recording were 0.3–1 lines/s using san heads. The phase images represent the variations of relative phase shifts (i.e., the phase angle of the interacting cantilever relative to the phase angle of freely oscillating cantilever at the resonance frequency) and are thus able to distinguish materials by their material properties. 3. Results and discussion 3.1. Small-angle light scattering The phase diagram [17] of PVME/SAN99 is showed in Fig. 1, and when the weight fraction of SAN99 is more than 50 wt.%, the cloud points of this system are hard to be obtained before the decomposition of PVME. Fig. 1 shows that the cloud point curve is asymmetric, and PVME/SAN99 blend has the lower critical solution temperature (LCST), the LCST and the critical composition is 119 °C, 74 wt.% PVME, respectively. In order to make the experimental condition in the two-phase region of 3D Ising regime, the two near-critical blends, 80 / 20 PVME / SAN99 (80 wt.% PVME) and 70 / 30 PVME / SAN99 (70 wt.% PVME), with experimental temperature 130 °C are chosen for this study. The evolution of experimental SALS patterns for 70 / 30 PVME / SAN99 at 130 °C is showed in Fig. 2. The SALS patterns are consistent with the common results of near-critical copolymer/ homopolymer blend. In the beginning, the 70 / 30 PVME / SAN99 blend is homogenous, after 57 s, the phase separation has occurred, and the so-called “ spinodal ring ” [18,19] is obtained. The spinodal ring becomes smaller and brighter step by step along with the phase separation. Fig. 3 is the time evolution of scattering curves, a scattering peak stays first at a constant position, and the early stage is very short, during the late stage, it shifts to lower scattering angles. This is the same as other works [14,20]. From Figs. 2 and 3, it can be concluded that, at 130 °C, for 70 / 30 PVME / SAN99 blend, the phase separation is controlled by the spinodal decomposition mechanism. The evolution of experimental SALS patterns for 80 / 20 PVME / SAN99 at 130 °C is also showed in Fig. 4. Interestingly, the SALS patterns are different from the common results of near-critical copolymer/homopolymer blend. As the phase separation takes place from the initially homogenous stage, the so-called “spinodal ring”
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Fig. 4. Measured SALS patterns for 80 / 20 PVME / SAN99 blend at 130 °C. Fig. 2. Measured SALS patterns for 70 / 30 PVME / SAN99 blend at 130 °C.
[11,20] does not emerge. In fact, the pattern is a “ two-wing” [21] just as the SALS pattern for blend under shearing, and the shear rate is relatively low [18,22]. Finally, the pattern becomes circular gradually as the phase separation proceeds. It indicates that, for 80 / 20 PVME / SAN99 blend, the phase separation conforms to the nucleation and
Fig. 3. Time evolution of scattering curves of 70 / 30 PVME / SAN99 blend: early and late stage at 130 °C.
growth mechanism, and the SAN99 domains orient when the phase separates. But the orientation is very small, along with the phase separation, the orientation will disappear finally. Fig. 5 shows the evolution of scattering intensity vs. time in these two blends. It can be obtained that although the experimental temperature (130 °C) is higher than the phase separation temperature, the phase separation does not take place immediately. It takes a period of time before the phase separation, and this period of time (203 s) of 80 / 20 PVME / SAN99 blend is much more than that (40 s) of 70 / 30 PVME / SAN99 blend. And the phase separation rate of 70 / 30 PVME / SAN99 blend is faster than that of 80 / 20 PVME / SAN99 blend. The difference between nucleation and growth mechanism and spinodal decomposition mechanism is the activation barrier. For nucleation and growth mechanism, due to the activation barrier encountered during formation of a spherical nucleus, it will take a period of time to conquer the activation barrier before the phase separation taking place. But for spinodal decomposition mechanism, the activation barrier does not exist, the phase separation takes place at once when the phase separation temperature is reached. In our experiments, 80 / 20 PVME / SAN99 blend has a long period of time before the phase separation, which confirms the phase separation is
Fig. 5. Evolution of the scattering intensity as a function of time for 80 / 20 PVME / SAN99 and 70 / 30 PVME / SAN99 blends at 130 °C, q = 1.5 μ m− 1.
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Fig. 6. AFM phase images of 70 / 30 PVME / SAN99 blend. The insert is the corresponding light scattering.
controlled by the nucleation and growth mechanism. For 70 / 30 PVME / SAN99 blend, there is also a period of time before the phase separation, but it is very little, just 40 s, which is caused by the apparatus error of SALS. Because in the initially 40 s, the phase separation of 70 / 30 PVME / SAN99 blend is slight, the SALS cannot obtain the signals. For above reasons, it is confirmed that the phase separation is controlled by the spinodal decomposition mechanism in 70 / 30 PVME / SAN99 blend. 3.2. Atomic force microscopy SALS have showed the bulk phase separation of near-critical PVME/SAN99 blend in film, and to study further, the AFM experiments have been done. Fig. 6 are the AFM phase images of 70 / 30 PVME / SAN99 blend. Initially, only one phase exists in the film, as the time increases, the bicontinuous morphology appears [10]. This means the surface phase separation of this blend is controlled by the spinodal decomposition mechanism. Finally, the nearly monodisperse SAN99-rich droplets with diameter of 1.2 μm are formed [22].
Fig. 7. AFM phase images of 80 / 20 PVME / SAN99 blend. The insert is the corresponding light scattering.
The AFM phase images of 80 / 20 PVME / SAN99 blend are showed in Fig. 7. Also, there is initially only one phase in the film, along with the increase of time, the very small SAN99-rich droplets appear, the slight orientation of SAN99 domains occurs at the same time, but not obvious like in SALS patterns. When the time reaches 395 s, the droplets become large, and there is a tendency of coalescing two large droplets into one larger droplet. Finally, Fig. 7(d) shows the much larger droplets with broader droplet size distribution [10,23–27], the average diameter of droplets is about 1.5 μm. Fig. 7(d) also shows the orientation disappears. To see the droplets coalescence clearly, we magnify the AFM phase images of 80 / 20 PVME / SAN99 blend at t = 395 s and t = 603 s, as shown in Fig. 8. It is clear that two large droplets coalesce to form much larger droplet. From these AFM phase images, it can be concluded that the surface phase separation of 80 / 20 PVME / SAN99 blend is controlled by the nucleation and growth mechanism. In order to explain the results of SALS and AFM, with consideration of the 3D Ising regime, a schematic phase diagram for PVME/SAN99 blend is showed in Fig. 9. It is clear that, in the twophase region of 3D Ising regime, the phase separation of 70 / 30 PVME / SAN99 blend is controlled by the non-linear spinodal decomposition, and the phase separation of 80 / 20 PVME / SAN99 blend is controlled by the homogeneous nucleation and growth. So, for PVME/SAN99 blend, there is certainly a transition from homogeneous nucleation and growth to non-linear spinodal decomposition in the two-phase region of 3D Ising regime. Some works [28–31] have found the shear-induced coalescence in polymer blends, Other works [18,22,31–33] have found the shearinduced orientation of polymer blends. In our study, both the coalescence and the orientation are found in 20 / 80 SAN99 / PVME
Fig. 8. AFM phase images of 80 / 20 PVME / SAN99 blend at higher magnification.
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phase region of 3D Ising regime. For 80 / 20 PVME / SAN99 blend, during the phase separation, the small orientation of SAN99 domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes. But for 70 / 30 PVME / SAN99 blend, the orientation does not appear.
Acknowledgement The authors wish to thank the National Natural Science Foundation of China (29934070) for Financial Support.
References Fig. 9. Schematic phase diagram for PVME/SAN99 blend.
blend, so there must be some shear. This is probably that some residual toluene exists in the film. When the phase separation is proceeding, the residual toluene is simultaneously volatilizing, the volatilization of toluene will shear the film to produce the shear-induced coalescence and shear-induced orientation. Because the residual toluene is mainly existing in the bulk, the orientation observed in SALS patterns is more obvious than that obtained by AFM. The shear force is so small, the orientation is not large, so the phase separation is able to make the orientation disappear finally. But whether the volatilization of toluene is the driving force of the orientation appearing at the early stage of phase separation should be studied further in the future. For 70 / 30 PVME / SAN99 blend, the orientation does not appear, this is likely that the phase separation of this blend is controlled by the spinodal decomposition mechanism, and in the early stage, the bicontinuous structure is formed, the shear force produced by the volatilization of residual toluene is not enough to make the bicontinuous structure orientation.
4. Conclusion We have applied the SALS and AFM to study the phase behavior of near-critical PVME/SAN blend in film. From the phase diagram, it is obtained that PVME/SAN99 blend has the lower critical solution temperature (LCST), the LCST and the critical composition is 119 °C, 74 wt.% PVME, respectively. When the weight fraction of SAN99 is more than 50 wt.%, the cloud points of this system are hard to be obtained before the decomposition of PVME. For 70 / 30 PVME / SAN99 blend, the AFM phase images show that, along with the increase of time, the bicontinuous morphology is formed first, and finally the nearly monodisperse SAN99-rich droplets with diameter of 1.2 μm are formed. The AFM phase images for 80 / 20 PVME / SAN99 blend show that, as the phase separation proceeds, the very small SAN99-rich droplets appear, the droplets become large gradually, then two large droplets coalesce into one larger droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results confirm that, for PVME/SAN99 blend, there is surely a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition in the two-
[1] K. Shimomai, N. Higashida, T. Ougizawa, T. Inoue, B. Rudolf, Polymer 37 (1996) 5877. [2] J. Dudowicz, K.F. Freed, Macromolecules 31 (1998) 5094. [3] Q. Yang, Y.M. Mao, G.X. Li, Mater. Lett. 58 (2004) 3939. [4] T.P. Ang, G.X. Li, J.S. Higgins, Polymer 43 (2002) 6661. [5] B.A. Wolf, J. Chem. Phys. 110 (1999) 7542. [6] H.Q. Feng, L.F. Shen, Z.L. Feng, Eur. Polym. J. 31 (1995) 243. [7] R. Pichet, T. Supakanok, C. Sirijutaratana, J. Polym. Sci., Polym. Phys. Ed. 42 (2004) 871. [8] R. Pichet, T. Supakanok, C. Sirijutaratana, J. Polym. Sci., Polym. Phys. Ed. 42 (2004) 886. [9] K.E. Min, D.R. Paul, J. Polym. Sci., Polym. Phys. Ed. 26 (1988) 2257. [10] B.D. Ermi, A. Karim, J.F. Douglas, J. Polym. Sci., Polym. Phys. Ed. 36 (1998) 191. [11] S. Nojima, K. Tsutsumi, Nose, Takuhei, Polym. J. 14 (1982) 225. [12] S. Nojima, Y. Ohyama, M. Yamaguchi, Nose, Takuhei, Polym. J. 14 (1982) 907. [13] J.K. Kim, H.W. Son, Y. Lee, J. Kim, J. Polym. Sci., Polym. Phys. Ed. 37 (1999) 889. [14] P.A. Oyanguren, B. Aizpurua, M.J. Galante, C.C. Riccardi, O.D. Cortazar, I. Mondragon, J. Polym. Sci., Polym. Phys. Ed. 37 (1999) 2711. [15] G.Y. Wen, X. Li, Y.G. Liao, L.J. An, Polymer 44 (2003) 4035. [16] G. Müller, D. Schwahn, T. Springer, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 55 (1997) 7267. [17] K. Yang, Q. Yang, G.X. Li, Y.J. Sun, Y.M. Mao, Mater. Lett. 59 (2005) 2680. [18] K. Matsuzaka, H. Jinnai, T. Koga, T. Hashimoto, Macromolecules 30 (1997) 1146. [19] T. Inoue, T. Ougizawa, J. Macromol. Sci., Chem. 26 (1989) 147. [20] T. Kyu, J.M. Saldanha, J. Polym. Sci., Polym. Phys. Ed. 28 (1990) 97. [21] Z. Hong, M.T. Shaw, R.A. Weiss, Macromolecules 31 (1998) 6211. [22] S.H. Kim, E.K. Hobbie, J.W. Yu, C.C. Han, Macromolecules 30 (1997) 8245. [23] M. Rabeony, D.T. Hseih, R.T. Garner, D.G. Peiffer, J. Chem. Phys. 97 (1992) 4505. [24] U. Jeong, D.Y. Ryu, D.H. Kho, D.H. Lee, J.K. Kim, Macromolecules 36 (2003) 3626. [25] C.D. Han, D.M. Baek, J. Kim, Macromolecules 25 (1992) 3052. [26] A. Adedeji, S.D. Hudson, A.M. Jamieson, Polymer 38 (1997) 737. [27] H.S. Jeon, J.H. Lee, N.P. Balsara, Macromolecules 30 (1997) 973. [28] C.M. Roland, J. Polym. Sci., Polym. Phys. Ed. 22 (1984) 79. [29] W. Gronski, B. Fries, C. Weis, C. Friedrich, Polymer 41 (1999) 3029. [30] O. Vorobyova, M.A. Winnik, J. Polym. Sci., Polym. Phys. Ed. 39 (2001) 2317. [31] H.S. Jeon, S. Kim, C.C. Han, Polym. Sci., Polym. Phys. Ed. 39 (2001) 819. [32] S.A. Madbouly, T. Chiba, T. Ougizawa, T. Inoue, Polymer 42 (2001) 1743. [33] Z.J. Chen, M.T. Shaw, R.A. Weiss, Macromolecules 28 (1995) 648.
Phase behavior of near-critical PVME/SAN blend in film Kun Yang ⁎, Qi Yang, Guangxian Li, Yajie Sun, Yimin Mao College of Polymer Science and Engineering, The State Key Laboratory for Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China Received 23 March 2005; accepted 22 September 2005 Available online 13 October 2005
Abstract The phase behavior of near-critical PVME/SAN blend in thin film was studied by time-resolved small-angle light scattering (SALS) and atomic force microscopy (AFM). The phase diagram showed that PVME/SAN99 blend had the lower critical solution temperature (LCST), the LCST and the critical composition was 119 °C, 74 wt.% PVME, respectively. However, the cloud points of this system were hard to be obtained before the decomposition of PVME when the weight fraction of SAN99 was more than 50 wt.%. Both SALS and AFM results confirmed that, for PVME/SAN99 blend, there was certainly a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition in the twophase region of 3D Ising regime. For 80 / 20 PVME / SAN99 blend, during the phase separation, the small orientation of SAN99 domains took place at the early stage of phase separation, and finally disappeared when the phase separation finished. But for 70 / 30 PVME / SAN99 blend, the orientation did not appear. © 2005 Elsevier B.V. All rights reserved. Keywords: Phase separation mechanism; PVME/SAN blend; Orientation
1. Introduction Numerous theoretical and experimental studies have described the solubilization of block copolymer segments by homopolymers, where the homopolymer is of similar chemistry as one of the block copolymer segment. The studies on blends of random copolymer with homopolymer are relatively less. The earliest theoretical descriptions of random copolymer/homopolymer systems were based on an extension to random copolymer of Flory–Huggins (FH) theory. Later, Shimomai et al. [1] applied the equation of state theory (EOS), and Dudowicz and Freed [2] applied the general lattice cluster theory (LCT) to describe these systems. In the previous paper [3], we discussed the miscibility of polystyrene (PS) with random copolymer styrene–acrylonitrile (SAN). It had been found for the first time that PS/SAN blend showed the upper critical solution temperature (UCST) behavior. The miscibility of binary polymer blend of PS/SAN was predicted based on both Flory's equation of state theory (EOS) [4] and Wolf's theory on binary polymer blends of the
⁎ Corresponding author. Tel.: +86 28 85405122; fax: +86 28 85405402. E-mail address: [email protected] (K. Yang). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.09.045
A/A–B type [5]. The prediction results of the two theories and the experiment results were in accordance with each other. For blends of random copolymer with homopolymer, where the homopolymer was not of similar chemistry as one of the random copolymer segment, poly(methyl methacrylate) (PMMA)/SAN was a common random system, which had been studied extensively [6–8]. Poly (vinyl methyl ether) (PVME)/ SAN was another rational system. Min and Paul [9] first studied the blends of PVME with SAN, and found the effect of acrylonitrile content on cloud points for blends with SAN copolymer at 80 wt.% PVME, but the phase behavior of this system was little studied. According to the mean-field theory, there exist two phase separation mechanisms, one is nucleation and growth mechanism, the other is spinodal decomposition mechanism. Some people [10–15] have confirmed, for near-critical blends, the phase separation is generally controlled by the spinodal decomposition mechanism, and for off-critical blends, the phase separation is generally controlled by the nucleation and growth mechanism. G. Müller et al. [16] have found if one takes the 3D Ising regime into consideration, the phase separation is described as a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition by
590
K. Yang et al. / Materials Letters 60 (2006) 589–593
strong fluctuation in the two-phase region of 3D Ising regime. To understand this phenomenon further, we used the small-angle light scattering and atomic force microscopy to study the phase behavior of near-critical PVME/SAN blend in film. 2. Experimental 2.1. Materials PVME (Mw = 75,000, Mw / Mn = 4.04, Tg = 243K) was purchased from Tokyo Kasei Kogyo Co. Ltd. Monomers, styrene and acrylonitrile were freed from inhibitor and distilled prior to use. Random copolymer SAN was synthesized by radical polymerization in toluene solution for 3–4 h at 347 K, using AIBN as an initiator. The resulting polymer solution was precipitated into a large excess amount of methanol and purified and dried in vacuum oven for at least 48 h. The molecular characteristics of SAN were given in Table 1. 2.2. Sample Preparation Blend of PVME/SAN99 was first weighed at desired composition, and then dissolved into toluene to make 5 wt.% solution. The solution stayed still for several hours in order to get clear, homogeneous liquid. The film was prepared by casting from solution on a glass substrate in a dry atmosphere, the toluene was slowly evaporated at ambient temperature for three days, then the resulting film (30 μm) was further dried in a vacuum oven for three days at 60 °C. 2.3. Measurements 2.3.1. Small-angle light scattering (SALS) The light scattering apparatus used in this study was discussed in the previous paper [3]. The light source was a polarized 10-mW He–Ne laser beam with a wavelength of 632.8 nm. A two-dimensional charge coupled device (CCD) camera was used to record the change of scattering patterns and the scattering intensity during the experiments, which provided a total angular range from 0.5° to 10°. A temperature-controlled chamber consisting of copper plate and heating bars is used to heat the sample at different heating rates. The accuracy of the temperature control is in the order of ± 0.1 °C. 2.3.2. Atomic force microscopy Atomic force microscopy (AFM) experiment was performed with a Nanoscope scanning probe microscope. The height and phase images were obtained simultaneously while operating the instrument in the tapping mode under ambient conditions.
Table 1 Molecular Characteristics of SAN99 Symbol
Mw
Mw / Mn
Styrene volume fraction
SAN99
82,300
1.49
0.99
Fig. 1. Cloud point curve for PVME/SAN99 blend. Heating rate = 1 °C/min.
Images were taken at the fundamental resonance frequency of the silicon cantilevers, which was typically around 300 kHz. Typical scan speeds during recording were 0.3–1 lines/s using san heads. The phase images represent the variations of relative phase shifts (i.e., the phase angle of the interacting cantilever relative to the phase angle of freely oscillating cantilever at the resonance frequency) and are thus able to distinguish materials by their material properties. 3. Results and discussion 3.1. Small-angle light scattering The phase diagram [17] of PVME/SAN99 is showed in Fig. 1, and when the weight fraction of SAN99 is more than 50 wt.%, the cloud points of this system are hard to be obtained before the decomposition of PVME. Fig. 1 shows that the cloud point curve is asymmetric, and PVME/SAN99 blend has the lower critical solution temperature (LCST), the LCST and the critical composition is 119 °C, 74 wt.% PVME, respectively. In order to make the experimental condition in the two-phase region of 3D Ising regime, the two near-critical blends, 80 / 20 PVME / SAN99 (80 wt.% PVME) and 70 / 30 PVME / SAN99 (70 wt.% PVME), with experimental temperature 130 °C are chosen for this study. The evolution of experimental SALS patterns for 70 / 30 PVME / SAN99 at 130 °C is showed in Fig. 2. The SALS patterns are consistent with the common results of near-critical copolymer/ homopolymer blend. In the beginning, the 70 / 30 PVME / SAN99 blend is homogenous, after 57 s, the phase separation has occurred, and the so-called “ spinodal ring ” [18,19] is obtained. The spinodal ring becomes smaller and brighter step by step along with the phase separation. Fig. 3 is the time evolution of scattering curves, a scattering peak stays first at a constant position, and the early stage is very short, during the late stage, it shifts to lower scattering angles. This is the same as other works [14,20]. From Figs. 2 and 3, it can be concluded that, at 130 °C, for 70 / 30 PVME / SAN99 blend, the phase separation is controlled by the spinodal decomposition mechanism. The evolution of experimental SALS patterns for 80 / 20 PVME / SAN99 at 130 °C is also showed in Fig. 4. Interestingly, the SALS patterns are different from the common results of near-critical copolymer/homopolymer blend. As the phase separation takes place from the initially homogenous stage, the so-called “spinodal ring”
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Fig. 4. Measured SALS patterns for 80 / 20 PVME / SAN99 blend at 130 °C. Fig. 2. Measured SALS patterns for 70 / 30 PVME / SAN99 blend at 130 °C.
[11,20] does not emerge. In fact, the pattern is a “ two-wing” [21] just as the SALS pattern for blend under shearing, and the shear rate is relatively low [18,22]. Finally, the pattern becomes circular gradually as the phase separation proceeds. It indicates that, for 80 / 20 PVME / SAN99 blend, the phase separation conforms to the nucleation and
Fig. 3. Time evolution of scattering curves of 70 / 30 PVME / SAN99 blend: early and late stage at 130 °C.
growth mechanism, and the SAN99 domains orient when the phase separates. But the orientation is very small, along with the phase separation, the orientation will disappear finally. Fig. 5 shows the evolution of scattering intensity vs. time in these two blends. It can be obtained that although the experimental temperature (130 °C) is higher than the phase separation temperature, the phase separation does not take place immediately. It takes a period of time before the phase separation, and this period of time (203 s) of 80 / 20 PVME / SAN99 blend is much more than that (40 s) of 70 / 30 PVME / SAN99 blend. And the phase separation rate of 70 / 30 PVME / SAN99 blend is faster than that of 80 / 20 PVME / SAN99 blend. The difference between nucleation and growth mechanism and spinodal decomposition mechanism is the activation barrier. For nucleation and growth mechanism, due to the activation barrier encountered during formation of a spherical nucleus, it will take a period of time to conquer the activation barrier before the phase separation taking place. But for spinodal decomposition mechanism, the activation barrier does not exist, the phase separation takes place at once when the phase separation temperature is reached. In our experiments, 80 / 20 PVME / SAN99 blend has a long period of time before the phase separation, which confirms the phase separation is
Fig. 5. Evolution of the scattering intensity as a function of time for 80 / 20 PVME / SAN99 and 70 / 30 PVME / SAN99 blends at 130 °C, q = 1.5 μ m− 1.
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Fig. 6. AFM phase images of 70 / 30 PVME / SAN99 blend. The insert is the corresponding light scattering.
controlled by the nucleation and growth mechanism. For 70 / 30 PVME / SAN99 blend, there is also a period of time before the phase separation, but it is very little, just 40 s, which is caused by the apparatus error of SALS. Because in the initially 40 s, the phase separation of 70 / 30 PVME / SAN99 blend is slight, the SALS cannot obtain the signals. For above reasons, it is confirmed that the phase separation is controlled by the spinodal decomposition mechanism in 70 / 30 PVME / SAN99 blend. 3.2. Atomic force microscopy SALS have showed the bulk phase separation of near-critical PVME/SAN99 blend in film, and to study further, the AFM experiments have been done. Fig. 6 are the AFM phase images of 70 / 30 PVME / SAN99 blend. Initially, only one phase exists in the film, as the time increases, the bicontinuous morphology appears [10]. This means the surface phase separation of this blend is controlled by the spinodal decomposition mechanism. Finally, the nearly monodisperse SAN99-rich droplets with diameter of 1.2 μm are formed [22].
Fig. 7. AFM phase images of 80 / 20 PVME / SAN99 blend. The insert is the corresponding light scattering.
The AFM phase images of 80 / 20 PVME / SAN99 blend are showed in Fig. 7. Also, there is initially only one phase in the film, along with the increase of time, the very small SAN99-rich droplets appear, the slight orientation of SAN99 domains occurs at the same time, but not obvious like in SALS patterns. When the time reaches 395 s, the droplets become large, and there is a tendency of coalescing two large droplets into one larger droplet. Finally, Fig. 7(d) shows the much larger droplets with broader droplet size distribution [10,23–27], the average diameter of droplets is about 1.5 μm. Fig. 7(d) also shows the orientation disappears. To see the droplets coalescence clearly, we magnify the AFM phase images of 80 / 20 PVME / SAN99 blend at t = 395 s and t = 603 s, as shown in Fig. 8. It is clear that two large droplets coalesce to form much larger droplet. From these AFM phase images, it can be concluded that the surface phase separation of 80 / 20 PVME / SAN99 blend is controlled by the nucleation and growth mechanism. In order to explain the results of SALS and AFM, with consideration of the 3D Ising regime, a schematic phase diagram for PVME/SAN99 blend is showed in Fig. 9. It is clear that, in the twophase region of 3D Ising regime, the phase separation of 70 / 30 PVME / SAN99 blend is controlled by the non-linear spinodal decomposition, and the phase separation of 80 / 20 PVME / SAN99 blend is controlled by the homogeneous nucleation and growth. So, for PVME/SAN99 blend, there is certainly a transition from homogeneous nucleation and growth to non-linear spinodal decomposition in the two-phase region of 3D Ising regime. Some works [28–31] have found the shear-induced coalescence in polymer blends, Other works [18,22,31–33] have found the shearinduced orientation of polymer blends. In our study, both the coalescence and the orientation are found in 20 / 80 SAN99 / PVME
Fig. 8. AFM phase images of 80 / 20 PVME / SAN99 blend at higher magnification.
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phase region of 3D Ising regime. For 80 / 20 PVME / SAN99 blend, during the phase separation, the small orientation of SAN99 domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes. But for 70 / 30 PVME / SAN99 blend, the orientation does not appear.
Acknowledgement The authors wish to thank the National Natural Science Foundation of China (29934070) for Financial Support.
References Fig. 9. Schematic phase diagram for PVME/SAN99 blend.
blend, so there must be some shear. This is probably that some residual toluene exists in the film. When the phase separation is proceeding, the residual toluene is simultaneously volatilizing, the volatilization of toluene will shear the film to produce the shear-induced coalescence and shear-induced orientation. Because the residual toluene is mainly existing in the bulk, the orientation observed in SALS patterns is more obvious than that obtained by AFM. The shear force is so small, the orientation is not large, so the phase separation is able to make the orientation disappear finally. But whether the volatilization of toluene is the driving force of the orientation appearing at the early stage of phase separation should be studied further in the future. For 70 / 30 PVME / SAN99 blend, the orientation does not appear, this is likely that the phase separation of this blend is controlled by the spinodal decomposition mechanism, and in the early stage, the bicontinuous structure is formed, the shear force produced by the volatilization of residual toluene is not enough to make the bicontinuous structure orientation.
4. Conclusion We have applied the SALS and AFM to study the phase behavior of near-critical PVME/SAN blend in film. From the phase diagram, it is obtained that PVME/SAN99 blend has the lower critical solution temperature (LCST), the LCST and the critical composition is 119 °C, 74 wt.% PVME, respectively. When the weight fraction of SAN99 is more than 50 wt.%, the cloud points of this system are hard to be obtained before the decomposition of PVME. For 70 / 30 PVME / SAN99 blend, the AFM phase images show that, along with the increase of time, the bicontinuous morphology is formed first, and finally the nearly monodisperse SAN99-rich droplets with diameter of 1.2 μm are formed. The AFM phase images for 80 / 20 PVME / SAN99 blend show that, as the phase separation proceeds, the very small SAN99-rich droplets appear, the droplets become large gradually, then two large droplets coalesce into one larger droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results confirm that, for PVME/SAN99 blend, there is surely a transition from homogeneous nucleation and growth to nonlinear spinodal decomposition in the two-
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