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carbon disulfide as revealed by neutron diffraction
HIYSICA ELSEVIER
Physica B 213&214 (1995) 724 726
Gelation mechanism of atactic polystyrene/carbon disulfide revealed by neutron diffraction
as
Y. Izumi a'*, J. Suzuki h, S. Katano b, S. Funahashi b aMacromolecular Research Laboratory, Facul~' of Engineering, Yamagata Universi~, Yonezawa, Yamagata 992, Japan b Tokai Research Establishment. Japan Atomie Energy Research Institute, Tokai, lbaraki 319-11, Japan
Abstract
The temperature dependence of the ordered structures appeared in a gel of atactic polystyrene in carbon disulfide has been studied using wide-angle neutron diffraction. All data are taken over a wide temperature range from 200 to 300 K under an off-stoichiometric condition, i.e., at the polymer concentration of 0.252 g/em 3. The gel far below the gelation temperature contains highly ordered structures, while the gel near the gelation temperature contains less distinct ordered structure. Based on these results, the molecular mechanism of the sol-gel transion of the present system is discussed.
1 Introduction
Atactic polystrene (aPS) should, by nature, be an uncrystallizable polymer, and so far there has been no evidence of any traces of crystallites in the bulk state. It has been widely believed that there were no new discoveries to be made with this polymer. Accordingly, the discovery that this polymer formed a physical gel in a solvent [1] came as a surprise, often followed by disbelief. However, various experiments carried out with different techniques [2-7] have all pointed to a genuine phenomenon. A thermal-analysis study of the gelation and gel melting [7] revealed the presence of exotherms and endotherms, respectively, that were interpreted as resulting from the existence of three-dimensional objects. It has been suggested that these objects arose from the formation of a polymer-solvent compound whose stoichiometry is defined somewhere between 40% and 50% of the polymer concentration. * Corresponding author.
According to this suggestion, the neutron-diffraction measurements have been limited to only one polymer concentration (40% w/w, the stoichiometric condition) and only two temperatures (313 K (sol) and 213 K (gel)). Although the result confirmed with no ambiguity the presence of ordered structures in this type of gel [8], the gelation model deduced from the thermal analysis is not necessarily supported. Further characterization of this physical gel has been carried out using small-angle neutron scattering (SANS) [9, 10]. The result of SANS was almost independent of polymer concentration but strongly dependent on the temperature. The lowering of temperature induced the appearance and successive growth of the branched polymer a so-called droplet. Regarding this droplet as a threedimensional object, it is evident that the gelation model presented based on the thermal analysis is not totally confirmed by SANS. Here, we report on a series of neutron-diffraction experiments intended to reveal the molecular mechanism of the sol gel transition in this system.
0921-4526/95/$09.50 1995 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 J 0 0 2 6 0 - X
E &umi et al. /Physica B 213&214 (1995) 724 726
2. Experimental
725
[0 7
i
i
i
2.1. Materials The deuterated atactic polystyrene sample (aPSD) (Mw = 90000 and M ~ / M , < 1.02) was purchased from Polymer Laboratories Ltd. Carbon disulfide was guaranteed reagent grade and purified. The polymer concentration of a P S D in carbon disulfide, c, was 0.252 g/cm 3, while the stoichiometry is defined somewhere between 40% and 50% of the polymer concentration [7]. The cross-over concentration c* is about 0.10g/cm 3 [11]. The gel-melting temperature corresponding to c = 0.252 g/cm 3 was about 276 K [12].
1¢
2.2. Neutron diffraction measurements The experiments were performed using the triple-axis spectrometer (TAS-2) installed at the research reactor JRR-3, at the Japan Atomic Energy Research Institute. For the present experiment, neutron wavelength 2 = 1.445 ~ was used. The momentum-transfer q range was the range 0.2 < q (A-1) < 2.5. Here, q is related to the Bragg angle 20 by q = (4rt/2) sin 0. The sample was tightly sealed in a cylindrical aluminum cell of diameter 10 mm. A blank sample containing only solvent was prepared under the same conditions as the sample. The sample cell was filled with helium. It was mounted on a cryostat and the temperature of the cell was controlled within 0.2 K. The diffraction patterns were obtained at five different temperatures: 300, 250, 240, 220 and 200 K. The experimental details have been reported elsewhere 1-13]. The net diffraction intensity at each temperature was then calculated according to the equation
l(q) = l(q)~amp (Tsolv/T~,~p)
kfl(q)soj,.
Here, /(q)samp, Tsamp, l(q)~o~, and T~,otv are the scattering intensity and transmission of the sample and solvent at each temperature, respectively, and kf is a correction factor for the concentration.
3. Results and discussion Fig 1 shows the net diffraction intensity at each temperature. As can be seen, a broad peak around q = 1.35 1 observed at 3 0 0 K almost corresponds to the amorphous halo appeared in the pure solid of aPS. A shoulder around q = 0.57 /k 1 and two peaks at 1.43 and 1.96 ~ 1 were observed in the gels at 220 and 200 K. On the other hand, no distinct shoulder was observed but less distinct two broad peaks were observed
103 0
'
J
~
0.5
1
1.5
J 2
2.5
Fig. 1. The net diffraction intensity as a function of temperature of aPSD (Mw = 90000 and M,/M. < 1.02) in carbon disulfide at 0.252g/crn3. An arrow at 300K shows the position of q = 1.35 A ~. The arrows at 200 K show the positions corresponding to the shoulder around q = 0.57 A ~ and two peaks around q = 1.43 and 1.95 A-~, respectively: 1(300 K), 2(250 K), 3(240 K), 4(220 K), 5(200 K).
in the gel at 250 K. A shoulder is slightly visible and two less distinct broad peaks were observed in the gel at 240 K. Thus, the distinct shoulder and two peaks were clearly observed only in the gel far below the gelation temperature. In this point, it is noted that the previous diffraction data were collected on the gels far below the gelation temperature [8 10, 14]. The present result indicates without ambiguity the presence of highly ordered structures in the gels far below the gelation temperature, as the effect of the solvent on the diffraction profiles was subtracted. On the other hand, the gel near the gelation temperature contains a less distinct ordered structure. We now compare the present work with previous results. The previous data on a P S D (Mw = 87000 and M ~ / M , < 1.05) and aPSD (Mw = 150000 and M w / M , = 1.26) in carbon disulfide were taken at c = 0.134 g/cm 3 [9, 10] and at c = 0.47 g/cm 3 I-8, 14], respectively. Guenet et al. chose the polymer concentration of c = 0.47 g/cm 3 (40% w/w) based on the temperature~zoncentration phase diagram and concluded that physical gelation in carbon disulfide arose from the
726
E lzumi et al. / Physica B 213&214 (1995) 724 726
formation of a polymer-solvent compound, whose stoichiometry was defined somewhere between c = 40% and 50% and accordingly the maximum quantity of physical junctions should be in this concentration range [7]. However, comparison between these data suggests that both the position and intensity of the shoulder around q = 0.57 ~ 1 are almost independent of the polymer concentration in the range from 0.134 to 0.47 g/cm 3, if any effect of the differences of aPSD employed could be neglected. This conclusion is not consistent with above conclusions deduced by Guenet et al. If the distances associated with these peaks are calculated by means of Bragg's law, d = 27t/q, then the shoulder around q = 0 . 5 7 ~ 1 corresponds to d = 11.0 A. It should be noted that this distance is very close to that found in the gel coagulates of syndiotactic polystyrene (sPS) in deuterated o-dichlorobenzene [15]. It is possible that a T T G G conformation is contained in the aPS gel far below the gelation temperature [16], although the distances corresponding to the two peaks at q = 1.43 and 1.96 A - ~ are not yet confirmed in the sPS gel. Furthermore, carbon disulfide may play an important role in the formation and stability of the gel, because the diffraction pattern for an aPS sample lyophilzed from the quenched gel was characterized by that of an amorphous PS [10]. The present results indicate that the ordered structure formed in the aPS gel depends strongly on the drop in T below the gelation temperature. That is, the structure of the cross-linking region is very sensitive to the temperature, that is, 'soft'. Further studies are in progress to provide these conclusions with further support. This work was partly supported by a Grant-in-Aid for Scientific Research (B: 03452264) from the Ministry of Education, Science and Culture.
References [l] S. Wellinghoff, J. Shaw and E. Baer, Macromolecules 12 (1979) 932. [2] H.-M. Tan, A. Hiltner, E. Moet and E. Baer, Macromolecules 16 (1983) 28. [3] J.M. Guenet, N.F.F. Willmott and P.A. Ellesmore, Polym, Commun. 24 (1983) 230. [4] J. Clark, S.T. Wellinghoffand W.G. Miller, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 24(2) (1983) 86. [5] Y.S. Gan, J. Francois, J.M. Guenet, C. Allain and G. Gautier-Manuel, Makromol. Chem. Rapid Commun. 6 (1985) 225. [6] Y.S. Gan, J. Francois, J.M. Guenet, Macromolecules 19 (1986) 173. [7] J. Francois, J.Y.S. Gan, J.M. Guenet, Macromolecules 19 (1986) 2755. [8] J.M. Guenet, M. Klein and A. Menelle, Macromolecules 22 (1989) 494. [9] Y. lzumi, S. Katano, S. Funahashi, M. Furusaka and M. Arai, Rept. Prog. Polym. Phys. Jpn. 33 (1990) 15. [10] Y. lzumi, S. Katano, S. Funahashi, M. Furusaka and M. Arai, Physica B 180 & 181 (1992) 539, 545. [1 l] B. Farnoux, Ann. Phys. 1 (1976) 73. [12] Y. lzumi, K. Narisada and K. Okamura, Rept. Prog. Polym. Phys. Jpn. 36 (1993) 49. [13] S. Katano and M. lizumi, Phys. Rev. Lett. 52 (1984) 835. [14] J.M. Guenet and M. Klein, Makromol. Chem., Macromol. Syrnp. 39 (1990) 85. [15] M. Kobayashi, K. Tashiro, T. Nakaoki, T. Kozasa, Y. Izumi, J. Suzuki and S. Funahashi, Rept. Prog. Polym. Phys. Jpn. 35 (1992) 133. [16] T. Nakaoki and M. Kobayashi, J. Mol. Struc. 242 (1991) 315.
Physica B 213&214 (1995) 724 726
Gelation mechanism of atactic polystyrene/carbon disulfide revealed by neutron diffraction
as
Y. Izumi a'*, J. Suzuki h, S. Katano b, S. Funahashi b aMacromolecular Research Laboratory, Facul~' of Engineering, Yamagata Universi~, Yonezawa, Yamagata 992, Japan b Tokai Research Establishment. Japan Atomie Energy Research Institute, Tokai, lbaraki 319-11, Japan
Abstract
The temperature dependence of the ordered structures appeared in a gel of atactic polystyrene in carbon disulfide has been studied using wide-angle neutron diffraction. All data are taken over a wide temperature range from 200 to 300 K under an off-stoichiometric condition, i.e., at the polymer concentration of 0.252 g/em 3. The gel far below the gelation temperature contains highly ordered structures, while the gel near the gelation temperature contains less distinct ordered structure. Based on these results, the molecular mechanism of the sol-gel transion of the present system is discussed.
1 Introduction
Atactic polystrene (aPS) should, by nature, be an uncrystallizable polymer, and so far there has been no evidence of any traces of crystallites in the bulk state. It has been widely believed that there were no new discoveries to be made with this polymer. Accordingly, the discovery that this polymer formed a physical gel in a solvent [1] came as a surprise, often followed by disbelief. However, various experiments carried out with different techniques [2-7] have all pointed to a genuine phenomenon. A thermal-analysis study of the gelation and gel melting [7] revealed the presence of exotherms and endotherms, respectively, that were interpreted as resulting from the existence of three-dimensional objects. It has been suggested that these objects arose from the formation of a polymer-solvent compound whose stoichiometry is defined somewhere between 40% and 50% of the polymer concentration. * Corresponding author.
According to this suggestion, the neutron-diffraction measurements have been limited to only one polymer concentration (40% w/w, the stoichiometric condition) and only two temperatures (313 K (sol) and 213 K (gel)). Although the result confirmed with no ambiguity the presence of ordered structures in this type of gel [8], the gelation model deduced from the thermal analysis is not necessarily supported. Further characterization of this physical gel has been carried out using small-angle neutron scattering (SANS) [9, 10]. The result of SANS was almost independent of polymer concentration but strongly dependent on the temperature. The lowering of temperature induced the appearance and successive growth of the branched polymer a so-called droplet. Regarding this droplet as a threedimensional object, it is evident that the gelation model presented based on the thermal analysis is not totally confirmed by SANS. Here, we report on a series of neutron-diffraction experiments intended to reveal the molecular mechanism of the sol gel transition in this system.
0921-4526/95/$09.50 1995 Elsevier Science B.V. All rights reserved SSD1 0 9 2 1 - 4 5 2 6 ( 9 5 J 0 0 2 6 0 - X
E &umi et al. /Physica B 213&214 (1995) 724 726
2. Experimental
725
[0 7
i
i
i
2.1. Materials The deuterated atactic polystyrene sample (aPSD) (Mw = 90000 and M ~ / M , < 1.02) was purchased from Polymer Laboratories Ltd. Carbon disulfide was guaranteed reagent grade and purified. The polymer concentration of a P S D in carbon disulfide, c, was 0.252 g/cm 3, while the stoichiometry is defined somewhere between 40% and 50% of the polymer concentration [7]. The cross-over concentration c* is about 0.10g/cm 3 [11]. The gel-melting temperature corresponding to c = 0.252 g/cm 3 was about 276 K [12].
1¢
2.2. Neutron diffraction measurements The experiments were performed using the triple-axis spectrometer (TAS-2) installed at the research reactor JRR-3, at the Japan Atomic Energy Research Institute. For the present experiment, neutron wavelength 2 = 1.445 ~ was used. The momentum-transfer q range was the range 0.2 < q (A-1) < 2.5. Here, q is related to the Bragg angle 20 by q = (4rt/2) sin 0. The sample was tightly sealed in a cylindrical aluminum cell of diameter 10 mm. A blank sample containing only solvent was prepared under the same conditions as the sample. The sample cell was filled with helium. It was mounted on a cryostat and the temperature of the cell was controlled within 0.2 K. The diffraction patterns were obtained at five different temperatures: 300, 250, 240, 220 and 200 K. The experimental details have been reported elsewhere 1-13]. The net diffraction intensity at each temperature was then calculated according to the equation
l(q) = l(q)~amp (Tsolv/T~,~p)
kfl(q)soj,.
Here, /(q)samp, Tsamp, l(q)~o~, and T~,otv are the scattering intensity and transmission of the sample and solvent at each temperature, respectively, and kf is a correction factor for the concentration.
3. Results and discussion Fig 1 shows the net diffraction intensity at each temperature. As can be seen, a broad peak around q = 1.35 1 observed at 3 0 0 K almost corresponds to the amorphous halo appeared in the pure solid of aPS. A shoulder around q = 0.57 /k 1 and two peaks at 1.43 and 1.96 ~ 1 were observed in the gels at 220 and 200 K. On the other hand, no distinct shoulder was observed but less distinct two broad peaks were observed
103 0
'
J
~
0.5
1
1.5
J 2
2.5
Fig. 1. The net diffraction intensity as a function of temperature of aPSD (Mw = 90000 and M,/M. < 1.02) in carbon disulfide at 0.252g/crn3. An arrow at 300K shows the position of q = 1.35 A ~. The arrows at 200 K show the positions corresponding to the shoulder around q = 0.57 A ~ and two peaks around q = 1.43 and 1.95 A-~, respectively: 1(300 K), 2(250 K), 3(240 K), 4(220 K), 5(200 K).
in the gel at 250 K. A shoulder is slightly visible and two less distinct broad peaks were observed in the gel at 240 K. Thus, the distinct shoulder and two peaks were clearly observed only in the gel far below the gelation temperature. In this point, it is noted that the previous diffraction data were collected on the gels far below the gelation temperature [8 10, 14]. The present result indicates without ambiguity the presence of highly ordered structures in the gels far below the gelation temperature, as the effect of the solvent on the diffraction profiles was subtracted. On the other hand, the gel near the gelation temperature contains a less distinct ordered structure. We now compare the present work with previous results. The previous data on a P S D (Mw = 87000 and M ~ / M , < 1.05) and aPSD (Mw = 150000 and M w / M , = 1.26) in carbon disulfide were taken at c = 0.134 g/cm 3 [9, 10] and at c = 0.47 g/cm 3 I-8, 14], respectively. Guenet et al. chose the polymer concentration of c = 0.47 g/cm 3 (40% w/w) based on the temperature~zoncentration phase diagram and concluded that physical gelation in carbon disulfide arose from the
726
E lzumi et al. / Physica B 213&214 (1995) 724 726
formation of a polymer-solvent compound, whose stoichiometry was defined somewhere between c = 40% and 50% and accordingly the maximum quantity of physical junctions should be in this concentration range [7]. However, comparison between these data suggests that both the position and intensity of the shoulder around q = 0.57 ~ 1 are almost independent of the polymer concentration in the range from 0.134 to 0.47 g/cm 3, if any effect of the differences of aPSD employed could be neglected. This conclusion is not consistent with above conclusions deduced by Guenet et al. If the distances associated with these peaks are calculated by means of Bragg's law, d = 27t/q, then the shoulder around q = 0 . 5 7 ~ 1 corresponds to d = 11.0 A. It should be noted that this distance is very close to that found in the gel coagulates of syndiotactic polystyrene (sPS) in deuterated o-dichlorobenzene [15]. It is possible that a T T G G conformation is contained in the aPS gel far below the gelation temperature [16], although the distances corresponding to the two peaks at q = 1.43 and 1.96 A - ~ are not yet confirmed in the sPS gel. Furthermore, carbon disulfide may play an important role in the formation and stability of the gel, because the diffraction pattern for an aPS sample lyophilzed from the quenched gel was characterized by that of an amorphous PS [10]. The present results indicate that the ordered structure formed in the aPS gel depends strongly on the drop in T below the gelation temperature. That is, the structure of the cross-linking region is very sensitive to the temperature, that is, 'soft'. Further studies are in progress to provide these conclusions with further support. This work was partly supported by a Grant-in-Aid for Scientific Research (B: 03452264) from the Ministry of Education, Science and Culture.
References [l] S. Wellinghoff, J. Shaw and E. Baer, Macromolecules 12 (1979) 932. [2] H.-M. Tan, A. Hiltner, E. Moet and E. Baer, Macromolecules 16 (1983) 28. [3] J.M. Guenet, N.F.F. Willmott and P.A. Ellesmore, Polym, Commun. 24 (1983) 230. [4] J. Clark, S.T. Wellinghoffand W.G. Miller, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 24(2) (1983) 86. [5] Y.S. Gan, J. Francois, J.M. Guenet, C. Allain and G. Gautier-Manuel, Makromol. Chem. Rapid Commun. 6 (1985) 225. [6] Y.S. Gan, J. Francois, J.M. Guenet, Macromolecules 19 (1986) 173. [7] J. Francois, J.Y.S. Gan, J.M. Guenet, Macromolecules 19 (1986) 2755. [8] J.M. Guenet, M. Klein and A. Menelle, Macromolecules 22 (1989) 494. [9] Y. lzumi, S. Katano, S. Funahashi, M. Furusaka and M. Arai, Rept. Prog. Polym. Phys. Jpn. 33 (1990) 15. [10] Y. lzumi, S. Katano, S. Funahashi, M. Furusaka and M. Arai, Physica B 180 & 181 (1992) 539, 545. [1 l] B. Farnoux, Ann. Phys. 1 (1976) 73. [12] Y. lzumi, K. Narisada and K. Okamura, Rept. Prog. Polym. Phys. Jpn. 36 (1993) 49. [13] S. Katano and M. lizumi, Phys. Rev. Lett. 52 (1984) 835. [14] J.M. Guenet and M. Klein, Makromol. Chem., Macromol. Syrnp. 39 (1990) 85. [15] M. Kobayashi, K. Tashiro, T. Nakaoki, T. Kozasa, Y. Izumi, J. Suzuki and S. Funahashi, Rept. Prog. Polym. Phys. Jpn. 35 (1992) 133. [16] T. Nakaoki and M. Kobayashi, J. Mol. Struc. 242 (1991) 315.