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Phase behavior of a nematic liquid crystal in polybutadiene networks
1 May 1998
Chemical Physics Letters 287 Ž1998. 342–346
Phase behavior of a nematic liquid crystal in polybutadiene networks Kenji Urayama, Zhao-hui Luo 1, Takanobu Kawamura, Shinzo Kohjiya Institute for Chemical Research, Kyoto UniÕersity, Uji, Kyoto-fu 611, Japan Received 28 October 1997; in final form 11 December 1997
Abstract Phase behavior of polybutadiene ŽPB. networks swollen in nematic liquid crystal p-ethoxybenzylidene-p-n-butylaniline ŽEBBA. strongly depends on whether there is surrounding EBBA: without the surrounding EBBA, EBBA in PB networks fails to form the nematic phase; in the presence of the surrounding EBBA, the nematic phase is formed inside the gels. The results suggest that a high order of molecular alignment in the surrounding nematic EBBA induces the formation of nematic phase inside the gels and the inducement effects overcome a high dilution effect of the impurity ŽPB. on the interaction between EBBA molecules. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that the nematic–isotropic transition temperature ŽTNI . of a liquid crystal ŽLC. is in general depressed by adding a non-mesomorphic substance w1–4x. This subject has been investigated for the nematic LC systems containing low molecular mass molecules with various molecular shapes w2,3x or polymeric molecules w4x as a non-mesomorphic solute. The thermodynamical elucidation of the effects of solute quantity, size and shape on TNI has been mainly discussed in these early studies. In this Letter, we deal with the phase behavior of LC in the
1 Permanent address: Department of Chemistry, Central China Normal University, Wuhan 430070, People’s Republic of China.
systems composed of LC and polymer networks. The polym er networks are three-dim ensionally crosslinked polymer systems and they are not dissolved in solvent due to the presence of crosslinks. Instead of dissolution, a polymer network exhibits swelling, whose magnitude depends on the crosslinking density and the solubility of the constituent polymer against the solvent. The physicochemical properties of swollen polymer networks have often been compared with those of the corresponding uncrosslinked polymer solutions with an identical polymer concentration Žfor a review, see Ref. w5x.. In this study, we show that the phase behavior of a LC in polymer networks has several unique features which cannot be simply explained by the analogy with that in the system composed of the LC and the uncrosslinked polymer.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 4 3 9 - 5
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
2. Experimental 2.1. Samples Polybutadiene ŽPB. and p-ethoxybenzylidenep,n-buthylaniline ŽEBBA; Tokyo Kasei Co.. were employed as the constituent of the polymer network and the nematic LC, respectively. The PB networks were prepared by end-linking hydroxyl-terminated PB ŽAldrich Co.. with triphenylmethane-4,4X ,4Y-triisocyanate ŽSumitomo Bayer Co.. in p-xylene. The quoted weight average molecular mass of PB was 6.2 = 10 3 grmol. The solutions were poured into an end-capped glass tube with a diameter of ca. 1 mm. The end-linking reaction was performed at room temperature for 24 h. Three kinds of PB networks different in crosslinking density were made by varying the PB concentration in preparation. The samples prepared from 30, 50 and 70 wt% PB solutions were designated as PBN30, PBN50 and PBN70, respectively. The resulting cylindrical gels were immersed in p-xylene for 2 days and subsequently in toluene for 4 weeks. The p-xylene and toluene were renewed every day in order to wash out the unreacted materials thoroughly. The weight fractions of the unreacted materials were less than 5 wt%. The swollen networks were completely dried in air and thereafter immersed in EBBA at a certain temperature until the equilibrium swelling was achieved. The EBBA had a quoted purity of at least 99% and was employed without further purification.
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in the surrounding EBBA during the cooling process. TNI was measured as the temperature at which the texture characteristic of nematic phase appeared. The T NI s obtained were confirmed to be unaffected by the repeated heating and cooling cycles. ŽII. The samples fully swollen at a certain temperature were transferred to the cell without the surrounding EBBA and then, immediately, the phase of EBBA was observed at the temperature at which the equilibrium swelling had been conducted. The gel samples fully swollen at various swelling temperatures ŽTs . in the polarized microscope were photographed and the diameter of the cylindrical gels at each Ts was measured on the enlarged photographs. Polymer volume fraction in the equilibrium swollen network Ž fe . was calculated using the diameters in the dry and swollen state under the assumption of additivity for volume.
3. Results and discussion Fig. 1 shows the reduced swelling temperature ŽTs) . dependence of fe for PBN30. Here, Ts) s o o TsrTNI where T NI is the T NI for the pure EBBA used for swelling. It can be seen that the degree of equilibrium swelling increases Ži.e. fe decreases. continuously with the rise of Ts , although the Ts o dependence of fe appears to change around T NI . Significant change in gel volume accompanying the
2.2. Measurements The phase behavior of EBBA was observed by cross-polarized microscopy with a Nikon polarized microscope equipped with the Mettler Hot Stage FP-82 under an N2 atmosphere. The temperature was controlled with an accuracy of better than "0.05 K. The observation of the phase behavior and the measurements of TNI of EBBA in the PB networks were conducted under the following two conditions. ŽI. The cylindrical gels were laid on the lateral side in the quartz cell and immersed in pure EBBA Ži.e. surrounded by EBBA.. The phase behavior was observed at the temperatures ranging from 353 to 348 K with a cooling rate of 0.01 Krmin which was slow enough to achieve quasi-equilibrium swelling
Fig. 1. The reduced swelling temperature ŽTs) . dependence of the PB volume fraction in PB networks equilibrium swollen in EBBA o Ž fe . for PBN30. Ts) sTs r TNI where Ts is the swelling temperao ture and TNI is the nematic–isotropic phase transition temperature for the pure EBBA used for swelling.
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
344
N–I transition of LC, such as discontinuous-like volume change reported in other temperature-sensitive gel systems Žfor a review, see Ref. w6x., was not observed in this system. The results for TNI of EBBA in PB networks surrounded by EBBA Žunder condition I. are shown in Table 1. It is found that the TNI s of EBBA in the networks do not significantly depend on the crosslinking density in the range examined here, and they o are ca. 1 K lower than T NI . The differences in TNI between the gels and surrounding EBBA are not due to an effect of non-equilibrium swelling. They correspond to the results obtained under thermodynamic equilibrium conditions, which was confirmed by the following experiment: the ambient temperature was held at 350.4 K at which the surrounding EBBA is in the nematic phase, while the EBBA in the networks is in the isotropic phase. The picture by the crosspolarized microscopy that the appearances of the gel and surrounding are respectively dark and bright, remained unchanged overnight. As can be seen in Table 1, the values of fe at Ts f T NI ŽTs s TNI q 0.3 K. are not largely affected by the PB concentration in preparation Ži.e. crosslinking density., though the initial Young’s modulus for elongation in the dry state significantly increases with the increase in crosslinking density. The comparable LC contents at TNI may be an origin of the almost same TNI for the three samples with different in crosslinking density. It is noteworthy that the transition of the EBBA in the networks from isotropic to nematic phase during the cooling process started from the circumference in contact with the surrounding EBBA and proceeded toward the center. In the case of heating process, the transition proceeded conversely. This observation Table 1 Nematic–isotropic phase transition temperature ŽT NI . of EBBA in the PB networks with surrounding EBBA, polymer volume fraction at equilibrium swelling Ž fe . at Ts fTNI and initial Young’s modulus in the dry state Ž E . for each sample Sample PBN30 PBN50 PBN70 a
TNI ŽK. a .b
349.8 Ž350.7 349.7 Ž350.6. b 349.9 Ž350.7. b
fe at Ts fTNI
E ŽMPa.
0.236 0.240 0.259
0.91 1.8 2.1
The temperature at which the nematic texture appears in cooling process. b T NI for the pure EBBA used for swelling.
implies that the phase of the EBBA inside the gels is strongly influenced by that outside the gels. This consideration is supported by the experimental results under condition II as shown below. The results of the cross-polarized microscopy of the samples without the surrounding EBBA Žunder condition II. were quite different from those with the surrounding EBBA. The appearances of the gel samo Ž ples equilibrium swollen at Ts - TNI 313, 323, 333 and 343 K. were dark in the cross-polarized microscopy at T s Ts , meaning that the nematic phase is not formed. The measurements below 313 K failed due to the occurrence of the crystallization of EBBA used for swelling. The same result as that observed under condition II was obtained by the following simple way. At the instant that the surrounding EBBA was removed from the gels in the nematic phase immersed in EBBA, the cross-polarized microscopy for the gels showed that the typical texture of nematic phase disappeared and the appearance became dark. These results strongly suggests that EBBA in the networks cannot form the mesophase without the surrounding EBBA, and that in the presence of the surrounding EBBA, the formation of the nematic phase inside the gels is induced by a high order of alignment of EBBA molecules in the nematic phase outside the gels. In this system as well as LC in general, the nematic phases both inside and outside the gel have a so-called polydomain structure where the orientation of the director is macroscopically random, but a high order of alignment of LC molecules within each domain is maintained. In a strict sense, the systems under condition II are not in equilibrium due to the removal of the surrounding LC, and the gels tend to shrink. However, the shrinking process was so slow that the loss of the volume of the gels could be negligible during the measurements at T s Ts . We consider that the failure of the formation of nematic phase under condition II is not due to an effect of non-equilibrium for swelling. It is naturally assumed that as the LC content increases and the temperature is lowered, the formation of the nematic phase becomes easier. As can be seen in Fig. 1, the content of EBBA inside the gels increases with the rise in Ts . However, it should be noted that it was impossible to measure TNI of the gels at T - Ts in the absence of the surrounding
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
EBBA. This is because a certain amount of EBBA leaked from the gel in the cooling process and the EBBA content inside the gel was not maintained during the measurements. o . Fig. 2 shows the plots of the T ) Žs TNI rTNI against the polymer volume fraction Ž f . for the system composed of uncrosslinked PB and EBBA. The measurements at f ) 0.12 were not successful due to the macroscopic phase separation of PB from EBBA. It is seen that TNI decreases with the increase of impurity ŽPB., which is qualitatively similar to the results for linear polymersq LC systems in Ref. w4x. On the basis of the results in Fig. 2, no occurrence of the phase transition of EBBA in the networks without the surrounding EBBA Žunder condition II. is attributed to the dilution effects of the impurity ŽPB. on the interaction between EBBA to form nematic phase, although some differences in the details of the depression effects on TNI will exist between the linear polymer and polymer network system. In the o , the polyPB networkq EBBA system at Ts - TNI mer content is rather high Ž fe ) 0.25., i.e. the volume fraction of LC inside the gels is not so high Ž0.75 at most.. The interaction among EBBA molecules inside the gels is not strong enough to form the mesophase solely. It is therefore expected that if a polymer with a higher solubility against a
345
LC is used as the constituent of network, which causes a higher degree of swelling, the LC inside the gel may form the mesophase without the surrounding LC. It should be noted that T NI for the PB gels with the surrounding LC is not so depressed as the ones for the uncrosslinked PB q LC system. Though the impurity contents in the gels at Ts f TNI are fairly large Ž fe f 0.25., T NI s for the gels are comparable to the one for the uncrosslinked PB q LC system with a dilute polymer concentration f f 0.03 Žsee Fig. 2.. Of course this comparison is superficial, because the former is an open system in which the LC molecules inside and outside the gels are apparently separated, but they can interact each other, while the latter is a closed system. However, from this comparison, we recognize that the driving force caused by the nematic phase of the surrounding LC, which induces the formation of nematic phase of LC inside the gels, is so strong that it remarkably reduces the depression effects of the impurity on T NI . Though the existence of crosslinks is a significant difference between the two systems, the fairly high T NI of the gels with the surrounding LC would not be explained by only this difference. The quantitative interpretation for TNI for the PB gels with the surrounding LC needs quantification of the following two factors: the power of the surrounding nematic LC to induce the formation of the mesophase inside the gels; the activity of LC in polymer networks which gives the degree of depression in T NI in the case without the surrounding LC. These subjects are the purposes of our subsequent work, together with the investigation on the effects of uniaxial elongation of gels on TNI .
Acknowledgements Fig. 2. The plots of the reduced nematic–isotropic phase transition o o temperature T ) Ž sTNI r TNI where TNI is the TNI for pure EBBA. against PB volume fraction Ž f . for the uncrosslinked PBqEBBA system Ž`.. T ) for PB networkqEBBA system with the surrounding EBBA are also shown ŽI.. The PB network qEBBA systems without the surrounding EBBA fail to form the nematic phase.
This work was partly supported by the Grant-inAid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan ŽNo. 09212217. and by the grants for encouragement of young scholars from Kyoto University and Kyoto University–Venture Business Laboratory.
346
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
References w1x A.C. De Kock, Z. Phys. Chem. 48 Ž1904. 129. w2x B. Kronberg, D.F.R. Gilson, D. Patterson, J. Chem. Soc., Faraday Trans. II 72 Ž1976. 1673. ˚ w3x D.E. Martire, G.A. Oweimreem, G.I. Agren, S.G. Ryan, H.T. Peterson, J. Chem. Phys. 64 Ž1976. 1456.
w4x B. Kronberg, I. Bassignara, D. Patterson, J. Phys. Chem. 82 Ž1978. 1714. w5x J.P. Cohen Addad ŽEd.., Physical Properties of Polymeric Gels ŽJohn Wiley, New York, 1996.. w6x K. Dusek ŽEd.., Responsive Gels: Volume Transition I, Adv. Polym. Sci., 109 Ž1993..
Chemical Physics Letters 287 Ž1998. 342–346
Phase behavior of a nematic liquid crystal in polybutadiene networks Kenji Urayama, Zhao-hui Luo 1, Takanobu Kawamura, Shinzo Kohjiya Institute for Chemical Research, Kyoto UniÕersity, Uji, Kyoto-fu 611, Japan Received 28 October 1997; in final form 11 December 1997
Abstract Phase behavior of polybutadiene ŽPB. networks swollen in nematic liquid crystal p-ethoxybenzylidene-p-n-butylaniline ŽEBBA. strongly depends on whether there is surrounding EBBA: without the surrounding EBBA, EBBA in PB networks fails to form the nematic phase; in the presence of the surrounding EBBA, the nematic phase is formed inside the gels. The results suggest that a high order of molecular alignment in the surrounding nematic EBBA induces the formation of nematic phase inside the gels and the inducement effects overcome a high dilution effect of the impurity ŽPB. on the interaction between EBBA molecules. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that the nematic–isotropic transition temperature ŽTNI . of a liquid crystal ŽLC. is in general depressed by adding a non-mesomorphic substance w1–4x. This subject has been investigated for the nematic LC systems containing low molecular mass molecules with various molecular shapes w2,3x or polymeric molecules w4x as a non-mesomorphic solute. The thermodynamical elucidation of the effects of solute quantity, size and shape on TNI has been mainly discussed in these early studies. In this Letter, we deal with the phase behavior of LC in the
1 Permanent address: Department of Chemistry, Central China Normal University, Wuhan 430070, People’s Republic of China.
systems composed of LC and polymer networks. The polym er networks are three-dim ensionally crosslinked polymer systems and they are not dissolved in solvent due to the presence of crosslinks. Instead of dissolution, a polymer network exhibits swelling, whose magnitude depends on the crosslinking density and the solubility of the constituent polymer against the solvent. The physicochemical properties of swollen polymer networks have often been compared with those of the corresponding uncrosslinked polymer solutions with an identical polymer concentration Žfor a review, see Ref. w5x.. In this study, we show that the phase behavior of a LC in polymer networks has several unique features which cannot be simply explained by the analogy with that in the system composed of the LC and the uncrosslinked polymer.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 4 3 9 - 5
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
2. Experimental 2.1. Samples Polybutadiene ŽPB. and p-ethoxybenzylidenep,n-buthylaniline ŽEBBA; Tokyo Kasei Co.. were employed as the constituent of the polymer network and the nematic LC, respectively. The PB networks were prepared by end-linking hydroxyl-terminated PB ŽAldrich Co.. with triphenylmethane-4,4X ,4Y-triisocyanate ŽSumitomo Bayer Co.. in p-xylene. The quoted weight average molecular mass of PB was 6.2 = 10 3 grmol. The solutions were poured into an end-capped glass tube with a diameter of ca. 1 mm. The end-linking reaction was performed at room temperature for 24 h. Three kinds of PB networks different in crosslinking density were made by varying the PB concentration in preparation. The samples prepared from 30, 50 and 70 wt% PB solutions were designated as PBN30, PBN50 and PBN70, respectively. The resulting cylindrical gels were immersed in p-xylene for 2 days and subsequently in toluene for 4 weeks. The p-xylene and toluene were renewed every day in order to wash out the unreacted materials thoroughly. The weight fractions of the unreacted materials were less than 5 wt%. The swollen networks were completely dried in air and thereafter immersed in EBBA at a certain temperature until the equilibrium swelling was achieved. The EBBA had a quoted purity of at least 99% and was employed without further purification.
343
in the surrounding EBBA during the cooling process. TNI was measured as the temperature at which the texture characteristic of nematic phase appeared. The T NI s obtained were confirmed to be unaffected by the repeated heating and cooling cycles. ŽII. The samples fully swollen at a certain temperature were transferred to the cell without the surrounding EBBA and then, immediately, the phase of EBBA was observed at the temperature at which the equilibrium swelling had been conducted. The gel samples fully swollen at various swelling temperatures ŽTs . in the polarized microscope were photographed and the diameter of the cylindrical gels at each Ts was measured on the enlarged photographs. Polymer volume fraction in the equilibrium swollen network Ž fe . was calculated using the diameters in the dry and swollen state under the assumption of additivity for volume.
3. Results and discussion Fig. 1 shows the reduced swelling temperature ŽTs) . dependence of fe for PBN30. Here, Ts) s o o TsrTNI where T NI is the T NI for the pure EBBA used for swelling. It can be seen that the degree of equilibrium swelling increases Ži.e. fe decreases. continuously with the rise of Ts , although the Ts o dependence of fe appears to change around T NI . Significant change in gel volume accompanying the
2.2. Measurements The phase behavior of EBBA was observed by cross-polarized microscopy with a Nikon polarized microscope equipped with the Mettler Hot Stage FP-82 under an N2 atmosphere. The temperature was controlled with an accuracy of better than "0.05 K. The observation of the phase behavior and the measurements of TNI of EBBA in the PB networks were conducted under the following two conditions. ŽI. The cylindrical gels were laid on the lateral side in the quartz cell and immersed in pure EBBA Ži.e. surrounded by EBBA.. The phase behavior was observed at the temperatures ranging from 353 to 348 K with a cooling rate of 0.01 Krmin which was slow enough to achieve quasi-equilibrium swelling
Fig. 1. The reduced swelling temperature ŽTs) . dependence of the PB volume fraction in PB networks equilibrium swollen in EBBA o Ž fe . for PBN30. Ts) sTs r TNI where Ts is the swelling temperao ture and TNI is the nematic–isotropic phase transition temperature for the pure EBBA used for swelling.
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
344
N–I transition of LC, such as discontinuous-like volume change reported in other temperature-sensitive gel systems Žfor a review, see Ref. w6x., was not observed in this system. The results for TNI of EBBA in PB networks surrounded by EBBA Žunder condition I. are shown in Table 1. It is found that the TNI s of EBBA in the networks do not significantly depend on the crosslinking density in the range examined here, and they o are ca. 1 K lower than T NI . The differences in TNI between the gels and surrounding EBBA are not due to an effect of non-equilibrium swelling. They correspond to the results obtained under thermodynamic equilibrium conditions, which was confirmed by the following experiment: the ambient temperature was held at 350.4 K at which the surrounding EBBA is in the nematic phase, while the EBBA in the networks is in the isotropic phase. The picture by the crosspolarized microscopy that the appearances of the gel and surrounding are respectively dark and bright, remained unchanged overnight. As can be seen in Table 1, the values of fe at Ts f T NI ŽTs s TNI q 0.3 K. are not largely affected by the PB concentration in preparation Ži.e. crosslinking density., though the initial Young’s modulus for elongation in the dry state significantly increases with the increase in crosslinking density. The comparable LC contents at TNI may be an origin of the almost same TNI for the three samples with different in crosslinking density. It is noteworthy that the transition of the EBBA in the networks from isotropic to nematic phase during the cooling process started from the circumference in contact with the surrounding EBBA and proceeded toward the center. In the case of heating process, the transition proceeded conversely. This observation Table 1 Nematic–isotropic phase transition temperature ŽT NI . of EBBA in the PB networks with surrounding EBBA, polymer volume fraction at equilibrium swelling Ž fe . at Ts fTNI and initial Young’s modulus in the dry state Ž E . for each sample Sample PBN30 PBN50 PBN70 a
TNI ŽK. a .b
349.8 Ž350.7 349.7 Ž350.6. b 349.9 Ž350.7. b
fe at Ts fTNI
E ŽMPa.
0.236 0.240 0.259
0.91 1.8 2.1
The temperature at which the nematic texture appears in cooling process. b T NI for the pure EBBA used for swelling.
implies that the phase of the EBBA inside the gels is strongly influenced by that outside the gels. This consideration is supported by the experimental results under condition II as shown below. The results of the cross-polarized microscopy of the samples without the surrounding EBBA Žunder condition II. were quite different from those with the surrounding EBBA. The appearances of the gel samo Ž ples equilibrium swollen at Ts - TNI 313, 323, 333 and 343 K. were dark in the cross-polarized microscopy at T s Ts , meaning that the nematic phase is not formed. The measurements below 313 K failed due to the occurrence of the crystallization of EBBA used for swelling. The same result as that observed under condition II was obtained by the following simple way. At the instant that the surrounding EBBA was removed from the gels in the nematic phase immersed in EBBA, the cross-polarized microscopy for the gels showed that the typical texture of nematic phase disappeared and the appearance became dark. These results strongly suggests that EBBA in the networks cannot form the mesophase without the surrounding EBBA, and that in the presence of the surrounding EBBA, the formation of the nematic phase inside the gels is induced by a high order of alignment of EBBA molecules in the nematic phase outside the gels. In this system as well as LC in general, the nematic phases both inside and outside the gel have a so-called polydomain structure where the orientation of the director is macroscopically random, but a high order of alignment of LC molecules within each domain is maintained. In a strict sense, the systems under condition II are not in equilibrium due to the removal of the surrounding LC, and the gels tend to shrink. However, the shrinking process was so slow that the loss of the volume of the gels could be negligible during the measurements at T s Ts . We consider that the failure of the formation of nematic phase under condition II is not due to an effect of non-equilibrium for swelling. It is naturally assumed that as the LC content increases and the temperature is lowered, the formation of the nematic phase becomes easier. As can be seen in Fig. 1, the content of EBBA inside the gels increases with the rise in Ts . However, it should be noted that it was impossible to measure TNI of the gels at T - Ts in the absence of the surrounding
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
EBBA. This is because a certain amount of EBBA leaked from the gel in the cooling process and the EBBA content inside the gel was not maintained during the measurements. o . Fig. 2 shows the plots of the T ) Žs TNI rTNI against the polymer volume fraction Ž f . for the system composed of uncrosslinked PB and EBBA. The measurements at f ) 0.12 were not successful due to the macroscopic phase separation of PB from EBBA. It is seen that TNI decreases with the increase of impurity ŽPB., which is qualitatively similar to the results for linear polymersq LC systems in Ref. w4x. On the basis of the results in Fig. 2, no occurrence of the phase transition of EBBA in the networks without the surrounding EBBA Žunder condition II. is attributed to the dilution effects of the impurity ŽPB. on the interaction between EBBA to form nematic phase, although some differences in the details of the depression effects on TNI will exist between the linear polymer and polymer network system. In the o , the polyPB networkq EBBA system at Ts - TNI mer content is rather high Ž fe ) 0.25., i.e. the volume fraction of LC inside the gels is not so high Ž0.75 at most.. The interaction among EBBA molecules inside the gels is not strong enough to form the mesophase solely. It is therefore expected that if a polymer with a higher solubility against a
345
LC is used as the constituent of network, which causes a higher degree of swelling, the LC inside the gel may form the mesophase without the surrounding LC. It should be noted that T NI for the PB gels with the surrounding LC is not so depressed as the ones for the uncrosslinked PB q LC system. Though the impurity contents in the gels at Ts f TNI are fairly large Ž fe f 0.25., T NI s for the gels are comparable to the one for the uncrosslinked PB q LC system with a dilute polymer concentration f f 0.03 Žsee Fig. 2.. Of course this comparison is superficial, because the former is an open system in which the LC molecules inside and outside the gels are apparently separated, but they can interact each other, while the latter is a closed system. However, from this comparison, we recognize that the driving force caused by the nematic phase of the surrounding LC, which induces the formation of nematic phase of LC inside the gels, is so strong that it remarkably reduces the depression effects of the impurity on T NI . Though the existence of crosslinks is a significant difference between the two systems, the fairly high T NI of the gels with the surrounding LC would not be explained by only this difference. The quantitative interpretation for TNI for the PB gels with the surrounding LC needs quantification of the following two factors: the power of the surrounding nematic LC to induce the formation of the mesophase inside the gels; the activity of LC in polymer networks which gives the degree of depression in T NI in the case without the surrounding LC. These subjects are the purposes of our subsequent work, together with the investigation on the effects of uniaxial elongation of gels on TNI .
Acknowledgements Fig. 2. The plots of the reduced nematic–isotropic phase transition o o temperature T ) Ž sTNI r TNI where TNI is the TNI for pure EBBA. against PB volume fraction Ž f . for the uncrosslinked PBqEBBA system Ž`.. T ) for PB networkqEBBA system with the surrounding EBBA are also shown ŽI.. The PB network qEBBA systems without the surrounding EBBA fail to form the nematic phase.
This work was partly supported by the Grant-inAid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan ŽNo. 09212217. and by the grants for encouragement of young scholars from Kyoto University and Kyoto University–Venture Business Laboratory.
346
K. Urayama et al.r Chemical Physics Letters 287 (1998) 342–346
References w1x A.C. De Kock, Z. Phys. Chem. 48 Ž1904. 129. w2x B. Kronberg, D.F.R. Gilson, D. Patterson, J. Chem. Soc., Faraday Trans. II 72 Ž1976. 1673. ˚ w3x D.E. Martire, G.A. Oweimreem, G.I. Agren, S.G. Ryan, H.T. Peterson, J. Chem. Phys. 64 Ž1976. 1456.
w4x B. Kronberg, I. Bassignara, D. Patterson, J. Phys. Chem. 82 Ž1978. 1714. w5x J.P. Cohen Addad ŽEd.., Physical Properties of Polymeric Gels ŽJohn Wiley, New York, 1996.. w6x K. Dusek ŽEd.., Responsive Gels: Volume Transition I, Adv. Polym. Sci., 109 Ž1993..