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Phase behavior of blends of poly (vinyl methyl ether) with styrene-acrylonitrile in film
Materials Letters 59 (2005) 2680 – 2684 www.elsevier.com/locate/matlet
Phase behavior of blends of poly (vinyl methyl ether) with styrene-acrylonitrile in film Kun Yang*, Qi Yang, Guangxian Li, Yajie Sun, Yimin Mao College of Polymer Materials Science and Engineering, The State Key Laboratory for Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China Received 31 October 2004; accepted 22 April 2005 Available online 23 May 2005
Abstract The phase behavior of blends of SAN/PVME in film is studied by Time-resolved Small-angle light scattering (SALS) and Atomic force microscopy (AFM). The phase diagram shows that both SAN99 (styrene volume fraction is 0.99)/PVME and SAN97 (styrene volume fraction is 0.97)/PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME, and the cloud points of these two systems are hard to be obtained before the decomposition of PVME when the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%. For 20/80 SAN97/PVME and 20/80 SAN99/PVME blends (80 wt% PVME), although the experimental temperature 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. The AFM phase images of 20/80 SAN99/PVME blend show that, as the phase separation proceeds, very small SAN99-rich droplets appear, the droplets become large step by step, then two large droplets coalesce into one larger droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results show that, during the phase separation, the very small orientation of SAN domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes. D 2005 Elsevier B.V. All rights reserved. Keywords: Phase behavior; SAN/PVME blend; Orientation
1. Introduction Numerous theoretical and experimental studies have described the solubilization of block copolymer segments by homopolymers, where the homopolymer was of similar chemistry as one of the block copolymer segments. The studies on blends of random copolymer with homopolymer were relatively less. The earliest theoretical descriptions of random copolymer/homopolymer systems were based on an extension to random copolymer of Flory –Huggins (FH) theory, which was widely used. Later, Shimomai et al. [1] applied the equation of state theory (EOS), and Dudowicz and Freed [2] applied the general lattice cluster theory
* Corresponding author. Tel.: +86 28 85405122; fax: +86 28 85405402. E-mail address: [email protected] (K. Yang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.04.019
(LCT) to describe random copolymer/homopolymer blends. 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 A/A-B type [5]. The prediction results of the two theories and the experiment results were in accordance with each other well. In this paper, we studied the phase behavior of SAN with another homopolymer poly (vinyl methyl ether) (PVME), where the homopolymer was not of similar chemistry as one of the random copolymer segments. Min and Paul [6] firstly studied the blends of PVME with SAN, and found the effect
K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684 Table 1 Molecular characteristics of SAN
2681
3.2. Atomic force microscopy
Symbols
Mw
Mw / Mn
Styrene volume fraction
SAN99 SAN97
82,300 55,100
1.49 1.35
0.99 0.97
of acrylonitrile content on cloud point for blends with SAN copolymer at 80% by weight PVME. But they did not get the phase diagram of PVME/SAN blends, so we used the small-angle light scattering to obtain the phase diagram, and studied the phase behavior of this system.
2. Experimental
Atomic force microscopy (AFM) experiments were performed with a Nanoscope III scanning probe microscope. The height and phase images were obtained simultaneously while operating the instrument in the tapping mode under ambient conditions. 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.
2.1. Materials PVME (M w = 75 000, M w / M n = 4.04, T g = 243 K) was purchased from Tokyo Kasei Kogyo Co. Ltd. Monomers, S and AN were freed from inhibitor and distilled prior to use. Random copolymers SAN were 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 Blends of SAN/PVME were first weighed at desired composition, and then dissolved into toluene to make 5 wt % solutions. The solution stayed still for several hours in order to get clear, homogeneous liquids. The film was prepared by casting from solution on a glass substrate in a dry atmosphere, and the toluene was slowly evaporated at ambient temperature for three days, then the resulting thin films (30 Am) were further dried in a vacuum oven for three days at 60 -C.
4. Results and discussion 4.1. Small-angle light scattering The phase diagrams of SAN99/PVME and SAN97/PVME blends are showed in Fig. 1, and when the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%, the cloud points of these two systems are hard to be obtained before the decomposition of PVME. Fig. 1 shows that the cloud point curves are asymmetric, and both SAN99/PVME and SAN97/PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME. The cloud points of SAN97/PVME are lower than those of SAN99/PVME except 30/ 70 SAN/PVME (70 wt% PVME). Min and Paul [6] studied the effect of acrylonitrile content on cloud point for blends with SAN copolymer at 80% by weight PVME. They found, below 6 wt% AN, the cloud point increased with the increment of AN content. The result is in accordance with 30/70 SAN/PVME in our experiment, but different with other compositions. This is likely that the weight average molecular weight of SANs in our experiment are much lower than those used in Min et al. It is
160
3. Measurements
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 T 0.1 -C.
Temperature ºC
3.1. Small-angle light scattering (SALS)
150
140
130 SAN99/PVME
120 SAN97/PVME
110 40
50
60
70
80
90
100
wt% of PVME Fig. 1. Cloud point curves for SAN99/PVME and SAN97/PVME blends. Heating rate = 1 -C/min.
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K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
Fig. 2. Measured SALS patterns for 20/80 SAN97/PVME blend at 130 -C.
clear that the obtained phase diagram of SAN/PVME are more useful than the result obtained just for 20/80 SAN/PVME. The critical composition of the SAN99/PVME is about 74 wt% PVME, and for the SAN97/PVME system, the critical composition is higher, about 77 wt% PVME. So the 20/80 SAN99/PVME and 20/ 80 SAN97/PVME blends, with experimental temperature 130 -C are chosen for this study. The evolution of experimental SALS patterns for 20/80 SAN97/PVME at 130 -C is showed in Fig. 2. To our surprise, the SALS patterns are different from the common results of copolymer/homopolymer blends. As the phase separation takes place from the initially homogenous stage, the so-called ‘‘spinodal ring’’ [7] does not emerge. In fact, the pattern is a ‘‘two-wing’’ [8], just as the SALS pattern for blend under shearing, and the shear
rate is relatively low [7,9]. Finally, the pattern becomes circular gradually as the phase separation proceeds. At the same time, we studied the 20/80 SAN99/PVME blend at 130 -C, Fig. 3 shows the evolution of experimental SALS patterns, and the patterns are the same as those showed in Fig. 2. From Figs. 2 and 3, it can be concluded that the small orientation of SAN domains takes place when the phase is separating, and as the phase separation proceeds, the orientation will disappear finally. Fig. 4 is 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
Fig. 3. Measured SALS patterns for 20/80 SAN99/PVME blend at 130 -C.
K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
140 120 SAN97/PVME
Intensity (a.u.)
100 80
SAN99/PVME
60 40 20 0 0
200
400
600
800
1000
1200
1400
Time (s) Fig. 4. Evolution of the scattering intensity as a function of time for 20/80 SAN99/PVME and 20/80 SAN97/PVME blends at 130 -C.
(203 s) of 20/80 SAN99/PVME blend is more than that (177 s) of 20/80 SAN97/PVME blend. This result is consistent with the cloud point curves showed in Fig. 1; the cloud point of 20/80 SAN97/ PVME is lower than that of 20/80 SAN99/PVME. 4.2. Atomic force microscopy SALS have showed the bulk phase separation of SAN/PVME blend in films, and to study further, the AFM experiments have
2683
been done. Fig. 5 is the AFM phase images of 20/80 SAN99/ PVME blend. Fig. 5(a) shows that there is initially only one phase in the film, along with the increase of time, the very small SAN99rich droplets appear, showed in Fig. 5(b), this means the phase separation takes place. The slight orientation of SAN99 domains occurs at the same time, but not obviously as showed in SALS patterns. Fig. 5(c) shows, when the time is 395 s, the droplets become large, and there is a tendency of coalescing two large droplets into one droplet. Finally, Fig. 5(d) shows the much larger droplets with broader droplet size distribution [10 – 14], and the average diameter of droplets is about 1.5 Am. Fig. 5(d) also shows this orientation disappears. These AFM results are in good agreement with those of SALS. To see the droplet coalescence clearly, we magnify the AFM phase images of 20/80 SAN99/ PVME blend at t = 395 s and t = 603 s, as shown in Fig. 6. From Fig. 6, it is clear that two large droplets coalesce to form much larger droplet finally. Some works [15 – 18] have found the shear-induced coalescence in polymer blends. Other works [7,9,18,19,20] have found the shear-induced orientation of polymer blends. In our study, both the coalescence and the orientation are found in 20/80 SAN99/PVME 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 bulk, the orientation observed in SALS patterns is more obvious than that obtained by AFM. The shear force is very small, the
Fig. 5. AFM phase images of 20/80 SAN99/PVME blend. The insert is the corresponding light scattering.
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K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
Fig. 6. AFM phase images of 20/80 SAN99/PVME blend at higher magnification.
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.
5. Conclusion We have applied the SALS and AFM to study the phase behavior of blends of SAN/PVME in films. From the phase diagram, it is obtained that both SAN99/PVME and SAN97/ PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME. When the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%, the cloud points of these two systems are hard to be obtained before the decomposition of PVME. The SALS experiments show that, for 20/80 SAN97/PVME and 20/80 SAN99/PVME blends, although the experimental temperature is higher than the phase separation temperature, the phase separation does not occur at once. It takes a period of time before the phase separation. For 20/80 SAN99/PVME blend, the AFM phase images show, along with the increase of time, phase separation takes place, the very small SAN99-rich droplets appear, the droplets become large gradually, then two large droplets coalesce into one droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results show that, during the phase separation, the small orientation of SAN domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes.
Acknowledgement The authors wish to thank the National Natural Science Foundation of China (29934070) for financial support.
References [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 34 (2002) 6661. [5] B.A. Wolf, J. Chem. Phys. 110 (1999) 7542. [6] K.E. Min, D.R. Paul, J. Polym. Sci., Polym. Phys. 26 (1988) 2257. [7] K. Matsuzaka, H. Jinnai, T. Koga, T. Hashimoto, Macromolecules 30 (1997) 1146. [8] Z. Hong, M.T. Shaw, R.A. Weiss, Macromolecules 31 (1998) 6211. [9] S.H. Kim, E.K. Hobbie, J.W. Yu, C.C. Han, Macromolecules 30 (1997) 8245. [10] U. Jeong, D.Y. Ryu, D.H. Kho, D.H. Lee, J.K. Kim, Macromolecules 36 (2003) 3626. [11] B.D. Ermi, A. Karim, J.F. Douglas, J. Polym. Sci., Polym. Phys. 36 (1998) 191. [12] C.D. Han, D.M. Baek, J. Kim, Macromolecules 25 (1992) 3052. [13] A. Adedeji, S.D. Hudson, A.M. Jamieson, Polymer 38 (1997) 737. [14] H.S. Jeon, J.H. Lee, N.P. Balsara, Macromolecules 30 (1997) 973. [15] C.M. Roland, J. Polym. Sci., Polym. Phys. 22 (1984) 79. [16] W. Gronski, B. Fries, C. Weis, C. Friedrich, Polymer 41 (1999) 3029. [17] O. Vorobyova, M.A. Winnik, J. Polym. Sci., Polym. Phys. 39 (2001) 2317. [18] H.S. Jeon, S. Kim, C.C. Han, Polym. Sci., Polym. Phys. 39 (2001) 819. [19] S.A. Madbouly, T. Chiba, T. Ougizawa, T. Inoue, Polymer 42 (2001) 1743. [20] Z.J. Chen, M.T. Shaw, R.A. Weiss, Macromolecules 28 (1995) 648.
Phase behavior of blends of poly (vinyl methyl ether) with styrene-acrylonitrile in film Kun Yang*, Qi Yang, Guangxian Li, Yajie Sun, Yimin Mao College of Polymer Materials Science and Engineering, The State Key Laboratory for Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China Received 31 October 2004; accepted 22 April 2005 Available online 23 May 2005
Abstract The phase behavior of blends of SAN/PVME in film is studied by Time-resolved Small-angle light scattering (SALS) and Atomic force microscopy (AFM). The phase diagram shows that both SAN99 (styrene volume fraction is 0.99)/PVME and SAN97 (styrene volume fraction is 0.97)/PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME, and the cloud points of these two systems are hard to be obtained before the decomposition of PVME when the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%. For 20/80 SAN97/PVME and 20/80 SAN99/PVME blends (80 wt% PVME), although the experimental temperature 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. The AFM phase images of 20/80 SAN99/PVME blend show that, as the phase separation proceeds, very small SAN99-rich droplets appear, the droplets become large step by step, then two large droplets coalesce into one larger droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results show that, during the phase separation, the very small orientation of SAN domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes. D 2005 Elsevier B.V. All rights reserved. Keywords: Phase behavior; SAN/PVME blend; Orientation
1. Introduction Numerous theoretical and experimental studies have described the solubilization of block copolymer segments by homopolymers, where the homopolymer was of similar chemistry as one of the block copolymer segments. The studies on blends of random copolymer with homopolymer were relatively less. The earliest theoretical descriptions of random copolymer/homopolymer systems were based on an extension to random copolymer of Flory –Huggins (FH) theory, which was widely used. Later, Shimomai et al. [1] applied the equation of state theory (EOS), and Dudowicz and Freed [2] applied the general lattice cluster theory
* Corresponding author. Tel.: +86 28 85405122; fax: +86 28 85405402. E-mail address: [email protected] (K. Yang). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.04.019
(LCT) to describe random copolymer/homopolymer blends. 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 A/A-B type [5]. The prediction results of the two theories and the experiment results were in accordance with each other well. In this paper, we studied the phase behavior of SAN with another homopolymer poly (vinyl methyl ether) (PVME), where the homopolymer was not of similar chemistry as one of the random copolymer segments. Min and Paul [6] firstly studied the blends of PVME with SAN, and found the effect
K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684 Table 1 Molecular characteristics of SAN
2681
3.2. Atomic force microscopy
Symbols
Mw
Mw / Mn
Styrene volume fraction
SAN99 SAN97
82,300 55,100
1.49 1.35
0.99 0.97
of acrylonitrile content on cloud point for blends with SAN copolymer at 80% by weight PVME. But they did not get the phase diagram of PVME/SAN blends, so we used the small-angle light scattering to obtain the phase diagram, and studied the phase behavior of this system.
2. Experimental
Atomic force microscopy (AFM) experiments were performed with a Nanoscope III scanning probe microscope. The height and phase images were obtained simultaneously while operating the instrument in the tapping mode under ambient conditions. 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.
2.1. Materials PVME (M w = 75 000, M w / M n = 4.04, T g = 243 K) was purchased from Tokyo Kasei Kogyo Co. Ltd. Monomers, S and AN were freed from inhibitor and distilled prior to use. Random copolymers SAN were 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 Blends of SAN/PVME were first weighed at desired composition, and then dissolved into toluene to make 5 wt % solutions. The solution stayed still for several hours in order to get clear, homogeneous liquids. The film was prepared by casting from solution on a glass substrate in a dry atmosphere, and the toluene was slowly evaporated at ambient temperature for three days, then the resulting thin films (30 Am) were further dried in a vacuum oven for three days at 60 -C.
4. Results and discussion 4.1. Small-angle light scattering The phase diagrams of SAN99/PVME and SAN97/PVME blends are showed in Fig. 1, and when the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%, the cloud points of these two systems are hard to be obtained before the decomposition of PVME. Fig. 1 shows that the cloud point curves are asymmetric, and both SAN99/PVME and SAN97/PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME. The cloud points of SAN97/PVME are lower than those of SAN99/PVME except 30/ 70 SAN/PVME (70 wt% PVME). Min and Paul [6] studied the effect of acrylonitrile content on cloud point for blends with SAN copolymer at 80% by weight PVME. They found, below 6 wt% AN, the cloud point increased with the increment of AN content. The result is in accordance with 30/70 SAN/PVME in our experiment, but different with other compositions. This is likely that the weight average molecular weight of SANs in our experiment are much lower than those used in Min et al. It is
160
3. Measurements
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 T 0.1 -C.
Temperature ºC
3.1. Small-angle light scattering (SALS)
150
140
130 SAN99/PVME
120 SAN97/PVME
110 40
50
60
70
80
90
100
wt% of PVME Fig. 1. Cloud point curves for SAN99/PVME and SAN97/PVME blends. Heating rate = 1 -C/min.
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K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
Fig. 2. Measured SALS patterns for 20/80 SAN97/PVME blend at 130 -C.
clear that the obtained phase diagram of SAN/PVME are more useful than the result obtained just for 20/80 SAN/PVME. The critical composition of the SAN99/PVME is about 74 wt% PVME, and for the SAN97/PVME system, the critical composition is higher, about 77 wt% PVME. So the 20/80 SAN99/PVME and 20/ 80 SAN97/PVME blends, with experimental temperature 130 -C are chosen for this study. The evolution of experimental SALS patterns for 20/80 SAN97/PVME at 130 -C is showed in Fig. 2. To our surprise, the SALS patterns are different from the common results of copolymer/homopolymer blends. As the phase separation takes place from the initially homogenous stage, the so-called ‘‘spinodal ring’’ [7] does not emerge. In fact, the pattern is a ‘‘two-wing’’ [8], just as the SALS pattern for blend under shearing, and the shear
rate is relatively low [7,9]. Finally, the pattern becomes circular gradually as the phase separation proceeds. At the same time, we studied the 20/80 SAN99/PVME blend at 130 -C, Fig. 3 shows the evolution of experimental SALS patterns, and the patterns are the same as those showed in Fig. 2. From Figs. 2 and 3, it can be concluded that the small orientation of SAN domains takes place when the phase is separating, and as the phase separation proceeds, the orientation will disappear finally. Fig. 4 is 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
Fig. 3. Measured SALS patterns for 20/80 SAN99/PVME blend at 130 -C.
K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
140 120 SAN97/PVME
Intensity (a.u.)
100 80
SAN99/PVME
60 40 20 0 0
200
400
600
800
1000
1200
1400
Time (s) Fig. 4. Evolution of the scattering intensity as a function of time for 20/80 SAN99/PVME and 20/80 SAN97/PVME blends at 130 -C.
(203 s) of 20/80 SAN99/PVME blend is more than that (177 s) of 20/80 SAN97/PVME blend. This result is consistent with the cloud point curves showed in Fig. 1; the cloud point of 20/80 SAN97/ PVME is lower than that of 20/80 SAN99/PVME. 4.2. Atomic force microscopy SALS have showed the bulk phase separation of SAN/PVME blend in films, and to study further, the AFM experiments have
2683
been done. Fig. 5 is the AFM phase images of 20/80 SAN99/ PVME blend. Fig. 5(a) shows that there is initially only one phase in the film, along with the increase of time, the very small SAN99rich droplets appear, showed in Fig. 5(b), this means the phase separation takes place. The slight orientation of SAN99 domains occurs at the same time, but not obviously as showed in SALS patterns. Fig. 5(c) shows, when the time is 395 s, the droplets become large, and there is a tendency of coalescing two large droplets into one droplet. Finally, Fig. 5(d) shows the much larger droplets with broader droplet size distribution [10 – 14], and the average diameter of droplets is about 1.5 Am. Fig. 5(d) also shows this orientation disappears. These AFM results are in good agreement with those of SALS. To see the droplet coalescence clearly, we magnify the AFM phase images of 20/80 SAN99/ PVME blend at t = 395 s and t = 603 s, as shown in Fig. 6. From Fig. 6, it is clear that two large droplets coalesce to form much larger droplet finally. Some works [15 – 18] have found the shear-induced coalescence in polymer blends. Other works [7,9,18,19,20] have found the shear-induced orientation of polymer blends. In our study, both the coalescence and the orientation are found in 20/80 SAN99/PVME 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 bulk, the orientation observed in SALS patterns is more obvious than that obtained by AFM. The shear force is very small, the
Fig. 5. AFM phase images of 20/80 SAN99/PVME blend. The insert is the corresponding light scattering.
2684
K. Yang et al. / Materials Letters 59 (2005) 2680 – 2684
Fig. 6. AFM phase images of 20/80 SAN99/PVME blend at higher magnification.
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.
5. Conclusion We have applied the SALS and AFM to study the phase behavior of blends of SAN/PVME in films. From the phase diagram, it is obtained that both SAN99/PVME and SAN97/ PVME have the lower critical solution temperature (LCST), the LCST of SAN97/PVME is (117 -C) lower than that (119 -C) of SAN99/PVME. When the weight fraction of SAN (SAN99 or SAN97) is more than 50 wt%, the cloud points of these two systems are hard to be obtained before the decomposition of PVME. The SALS experiments show that, for 20/80 SAN97/PVME and 20/80 SAN99/PVME blends, although the experimental temperature is higher than the phase separation temperature, the phase separation does not occur at once. It takes a period of time before the phase separation. For 20/80 SAN99/PVME blend, the AFM phase images show, along with the increase of time, phase separation takes place, the very small SAN99-rich droplets appear, the droplets become large gradually, then two large droplets coalesce into one droplet, and the obtained droplet size distribution is broader. Both SALS and AFM results show that, during the phase separation, the small orientation of SAN domains takes place at the early stage of phase separation, and finally disappears when the phase separation finishes.
Acknowledgement The authors wish to thank the National Natural Science Foundation of China (29934070) for financial support.
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