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Solubility of liquid crystalline copolybibenzoates
Eur. P&m. J. Vol. 32, No. 5, PD.631-634, 1996
- Copyright 0 1996Eisevier 8cience Ltd Printed in Great Britain. All rights reserved 00143057/96$15.00+ 0.00
Pergamon
SOLUBILITY OF LIQUID CRYSTALLINE COPOLYBIBENZOATES ERNEST0
PBREZ,* ZHEN ZHU, ANTONIO BELLO, JO& and ROSARIO BENAVENTE
M. PERENA
Instituto de Ciencia y Tecnologia de Polimeros (CSIC), Juan de la Cierva 3, 28006-Madrid, Spain (Received 9 March 1994; accepted in final form 23 May 1995)
Abstract-The solubility behaviour of several liquid crystalline copolymers based on poly(triethylene glycol p,p’-bibenzoate) and poly(octamethylene p,p’-bibenzoate), and the corresponding homopolymers, has been studied, analysing the influence of the proportion of oxyethylene units in the spacer. The value of the solubility parameter has also been estimated, from intrinsic viscosity measurements, for the different polymer samples, and compared with the calculations made from several methods. The solubility parameters for all the homopolybibenzoates and copolybibenzoates studied in this work lie in the range 6 = 9.5 & 0.2 (cal/cm3)i’r [19.4 + 0.4 (MPa)“*]. However, the solubility range increases very significantly with the proportion of oxyethylene units: while poly(octamethylenep,p’-bibenzoate) homopolymer is only soluble in tetrachloroethane, a fairly large number of solvents is found for copolymers with high proportions of oxyethylene units in the spacer.
INTRODUCl-ION
The presence of two oxygen atoms in the spacer of PTEB, replacing two methylene units of PSMB, introduces significant conformational differences between the two polymers, in such a way that their phase behaviour and transition temperatures are markedly different [A. A considerable increase in solubility is also found for PTEB with respect to PIMB, as will be shown in this work. The purpose of this article is the analysis of the influence on the solubility of the proportion of oxyethylene units in the spacer for several copolymers of P8MB and PTEB. The value of the solubility parameter, 6, has also been estimated for the different polymer samples.
Main-chain liquid crystalline polymers consisting of rigid mesogenic cores separated by flexible aliphatic spacers have attracted much attention in recent years. These polymers exhibit self-ordering properties which induce anisotropy. The reduced melting points and
increased solubility, caused by the introduction of flexible spacers, improve their processing capability. The thermotropicliquidcrystallinecharacterofpolyesters derived from bibenzoic acid has been reported in several papers [l-4] which have focused on the interesting behaviour of polybibenzoates, PBS, in the solid state. However, little information on solubility has been published, owing to the poor solubility of PBS with linear all-methylene spacers. The lower members of this series are only soluble in mixtures of phenols and tetrachloroethane [l-3]. However, a significant increase in solubility has been found for PBS with oxyethylene [5] or branched [6] spacers. We have previously performed a comparative analysis [7] of the thermotropic properties of poly(triethylene glycol p,p’-bibenzoate), PTEB and poly(octamethylene p,p’-bibenzoate), PSMB. These polybibenzoates have the following structural formula:
EXPERIMENTAL Several copolyesters based on poly(triethylene glycol p,p’bibenzoate) and poly(octamethylene p,p’-bibenzoate) were synthesized by melt transesterification of diethyl p,p’-bibenzoate and varying amounts of triethylene glycol, TEG, and octamethylene glycol, OMG, using isopropyl titanate as catalyst. The copolyesters were purified by precipitating into methanol their solutions in chloroform.
PTEB CO-0-CH2-CH2-0-CH2-CH2-0-CH2-CH2-0
P8MB CO-O-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-0
*To whom all correspondence should be addressed. 631
E. P&rez et al.
632
Table I. Molar compositions in TEG, molecular weights, and densities’ and solubility parameters of polybibenzoates calculated from different methods 6,,, (ca1/cm3)“2 Polymer P8MB c21 c44 C63 Cl6 C84 PTEB
Lo
10-4 M,
(g/cm’)
Small
Hay
VKH
6.7 4.8 6.0 8.0 I.5
1.16 I.18 1.20 1.21 1.22 I .23 1.24
9.88 9.91 9.93 9.93 9.96 9.91 9.99
10.26 10.38 10.46 10.54 10.60 10.64 10.71
9.33 9.45 9.52 9.60 9.66 9.69 9.16
0 27 44 63 76 84 100
“From the method of Van Krevelen and Hoftyzer, VKH [14].
Samples of the two homopolymers were also analysed. The P8MB specimen was the same as that previously reported [5,7,8], while a new sample of PTEB was synthesized for this work. The composition of the copolymers was determined from the I3C-NMR spectra in solutions of deuterated chloroform, using a Varian XL-300 spectrometer. The molar fractions of TEG J&G, are shown in Table 1. The chemical shifts corresponding to the different carbons in the homopolymers have been reported previously [7] (spectra of PIMB were recorded in solution of deuterated tetrachloroethane). The solubility of the polymers was tested in several common solvents (see Table 2). About 15 mg of polymer in 1Oml of solvent were used and three different degrees of solubility were considered: good solubility (+ in Table 2), partial solubility or solubility only in the hot liquid (+ -), and null solubility (-). The values of the intrinsic viscosity, [q], were determined at 25”C, using an Ubbelohde viscometer, in the solvents for which the solubility test was positive. The molecular weights of the samples were determined by Size Exclusion Chromatography in a Waters 15OC equip-
ment, using chloroform as eluent. The universal calibration plot was obtained from the data of several samples of monodisperse polystyrene (with molecular weights ranging from 1800 to 2,300,OOO).The molecular weights of the different polybibenzoate samples, represented by the peak maximum, Mp, are also shown in Table 1 (PIMB homopolymer is not soluble in chloroform). They were calculated from the elution volume of the peak and the intrinsic viscosity of the whole sample.
RESULTS
AND DISCUSSION
The influence of the composition on the solubility range and on the solubility parameter, 6, has been analysed. This parameter is defined, in the original formulation [9, lo], as the square root of the cohesive energy density, which represents the energy of vaporization per unit volume. For non-volatile substances, like polymers, indirect methods have to be used for the estimation of 6 [I 1, 121. Thus, the solubility parameter of a polymer is usually taken as that of the solvent corresponding to the solution for which the intrinsic viscosity is maximum. A test of the solubility of the different homopolymer and copolymer samples of Table 1 was carried out in several solvents covering a wide range of solubility parameters. The results of this test are presented in Table 2. It is observed, first, that P8MB is only soluble in tetrachloroethane, but the solubility is greatly enhanced by copolymerization with PTEB in such a way that the copolymer with a 27% of TEG is already soluble in several chlorinated compounds. There is a continued increase in the solubility from C27 to PTEB, but the most important change occurs on passing from PSMB to C27. Thus, the introduction of relatively small amounts of oxyethylene units produces a significant increase in the solubility. The corresponding increase in the flexibility of the spacer is reflected by a lowering of the glass transition temperature [13]. This lowering is not directly proportional to the TEG content. However, the isotropization temperatures show a steady decrease, approximately proportional to the composition, on passing from PSMB to PTEB. Thus, the isotropization temperature is about 190°C for P8MB [8], 172°C for C27, 143°C for C63 and 114°C for PTEB [7]. The values of the intrinsic viscosity of the different polymers in their actual solvents are shown in Table 3. It can be observed that the value for PTEB homopolymer is significantly lower than those for the other samples. Moreover, the molecular weight of the PTEB sample is also considerably lower (see Table 1). One might expect that the increase in
Table 2. Solubilitv test of oolvbibenzoates Test result Solvent Heptane Methylcyclohexane Cyclohexane Toluen. Benzene Carbon tetrachloride Trichloroethylene Chloroform Tetrachloroethane Methylene chloride Chlorobenzene o-Dichlorobenzene I ,2,4-Trichlorobenzene Tetrahydrofuran Ethyl acetate Methyl i-propyl ketone Methyl propyl ketone Cyclohexanone Acetone Dimethyl sulphoxide Ethanol Methanol
6, (cal/cm’)‘i2 7.4
7.8 8.2 8.9 9.2 8.6 9.2 9.3 9.1 9.8 9.5 10.0 9.1 9.1 8.5 8.1
9.9 9.9 12.0 12.7 14.5
___ PBMB
C27
c44
C63
C76
C84
PTEB
_
-
-
-
-
-
-
_ _ _ _ _ _ _ + _ _ _ _ _
+f++ + + f+++-
f++ + + +> + + + + -
_ _ +++ + + + + + + + _ _
_ _ + _ + + + + + + + + _ _
_ _ ++ _ + + + + + + + + _
_ _ ++ _ + + + + + + + + _
+ _
+ +_
_
-
-
_
_
_ _ -
+-
+_
+_ _
+ +_
Solubility Table 3. Intrinsic
Solvent
Tetrachloroethane Chloroform Methylene chloride Trichloroethylene Chlorobenzene o-Dichlorobenzene Tetrahydrofuran I ,2,4-Trichlorobenzene Benzene Cvclohexanone
of copolybibenzoates
viscosities of Dolvbihenzoates in different solvents
PSMB
c27
c44
C63
Cl6
C84
F’TEB
1.30 -
I .27
1.01 0.96 0.78 0.73 0.71 0.73 0.60 0.61
0.68 0.83 0.70 0.58 0.59 0.57 0.60 0.53 -
0.76 1.11 0.91 0.72 0.81 0.77 0.73 0.81 0.54 0.53
0.67 0.80 0.68 0.49 0.52 0.50 0.46 0.50 0.36 0.3 I
0.40 0.42 0.37 0.28 0.30 0.27 0.24 0.25 0.17
1.22 0.92 -
solubility would be partly due to the corresponding decrease in the molecular weight. We have reported the solubility behaviour in tetrachloroethane of a sample of PTEB with a considerably higher viscosity: 1.15 dl/g [5], finding that the solubility test is exactly the same as that for the actual sample of PTEB used in this work. We believe, therefore, that the solubility differences in Table 2 are mainly due to the effect of the presence of varying proportions of oxyethylene units in the spacer. The intrinsic viscosities listed in Table 3 can be used for the estimation of the solubility parameters. However, the original theory for this parameter [9] was developed only for non-polar substances, and the existence of specific interactions, like dipolar or hydrogen bonding, were not considered. Therefore, it is advisable to determine the solubility parameter in a range of solvents with similar polarity or capability of forming hydrogen bonds. In fact, it can be observed
1.6
1.4
0.6
0.4
0.2
633
I
I
I
,
9.2
9.4
9.6
9.8
I
6, (calknl~)“~
Fig. 1,Dependence of the intrinsic viscosity on the solubility parameter of the solvents for the indicated samples.
0.10
from Table 3 that the viscosity in chloroaliphatic solvents as a function of the solubility parameter of the solvent follows a clearly different trend than that for the other solvents. Thus, only chloroaliphatic liquids have been used for the representation in Fig. 1 (the solubility parameter of methylene chloride has been taken as 9.8 (cal/cm3)‘/2, the mean value between those reported in the literature [12]). The estimation of the solubility parameter of the polymer from this figure is not reliable, bearing in mind the reduced number of solvents and the dispersity of the data points. Nevertheless, smooth curves have been drawn through the experimental points, and the maxima were determined. The results indicate that the 6 values for all the analysed samples are in the interval 9.5 f 0.2 (cal/cm3)1’2 [19.4 f 0.4 (MPa)1’2]. It is not possible to establish significant differences between the samples. Group contributions can also be used to calculate the solubility parameter [12], provided that the density of the polymer is known. The density can also be estimated from group contributions [14]. The results for the polybibenzoate samples are shown in the fourth column of Table 1. The corresponding values of the solubility parameter, calculated by the methods of Small [15], Hoy [16] and Van Krevelen and Hoftyzer, VKH [14], are shown in the last columns of Table 1. It is observed that the most refined method of VKH gives very good agreement with the experimental determinations. A single solubility parameter cannot explain some aspects of the solubility behaviour, in particular the different trends of the viscosity values among the different families of solvents. Additional information is obtained from the three-dimensional solubility parameter approach [ 17, 181, divided into dispersion, a,, polar, 6,) and hydrogen-bonding capability, a,, contributions. It has been shown in other polybibenzoates that the three components of this parameter, which represents the centre of the solubility sphere [17, 181, are of the order of 8.7, 2.5 and 3.0 (cal/cm3)ii2 for a,, 6, and 6,, respectively. The same values have been estimated for the polymers in this work, considering the components of the solubility parameter [17, 181 for the solvents used. The polybibenzoates have very similar solubility parameter components, but the solubility range, that is the radius of the solubility sphere, is very sensitive to variations in the spacer. The radius of this sphere increases significantly with the TEG
E. Ptrez et al.
634
content, its value passing from less than one unit of the 6 components for C27 to approximately two units for PTEB. In conclusion, the solubility parameters for all the homopolybibenzoates and copolybibenzoates studied in this work are in the range 6 = 9.5 f 0.2 (cal/cm3)“* [19.4 k 0.4 (MPa)“*]. However, the solubility range increases very significantly with the TEG content, in such a way that while P8MB is only soluble in tetrachloroethane, copolymer C27 can be dissolved in several common solvents, and a fairly large number of solvents is found for copolymers with higher TEG proportions in the spacer.
acknowledge the financial support of the Comisibn Interministerial de Ciencia y Tecnologia, Project No. MAT94-0858-E, and of the Consejeria de Education y Cultura de la Comunidad de Madrid. Z. Zhu is indebted to the Ministerio de Education y Ciencia for a postdoctoral grant.
Acknowledgements-We
REFERENCES
1. P. Meurisse, C. Noel, L. Monnerie and B. Fayolle. Br. Polym. J. 13, 55 (1981). 2. J. Watanabe and M. Hayashi. Macromolecules 21, 278 (1988).
3. W. J. Jackson and J. C. Morris. ACS Symp. Ser. 435, 16 (1990). 4. A. Bello, J. M. Pereiia, E. Perez and R. Benavente. Macromol.
Symp. f&J, 297 (1994).
5. E. Perez, R. Benavente, M. M. Marugan, A. Bello and J. M. Pereiia. Polvm. Bull. 25. 413 (1991). 6. M. M. Marugan, E. Perez, R..Benavente,‘A. Bello and J. M. Pereiia. Eur. Polym. J. 2S, 1159 (1992). 7. E. Perez, E. Riande, A. Bello, R. Benavente and J. M. Per&a. Macromolecules 25, 605 (1992). 8. E. Perez, Z. Zhu, A. Bello, R. Benavente and J. M. Pereiia. Polymer 35, 4794 (1994). 9. J. Hildebrand and R. Scott. Solubility of Nonelectrolytes. Reinhold, New York (1949). 10. G. Scatchard. Chem. Rev. 8, 321 (1931). 11. J. L. Gardon. In Encyclopedia of Polymer Science and Technology (edited by H. F. Mark, N. G. Gaylord and N. M. Bikales), Vol. 3, p. 833. Wiley-Interscience, New York (1968). 12. E. A. Grulke. In Polymer Handbook (edited bv J. Brandrup and E. H. Immergut), 3rd edn, P. VII-519. Wilev. New York (1989). 13. R. Benavente, Z. ihu,’ J. M. Perefia, A. Bello and E. Perez. Polymer (in press). 14. D. W. Van Krevelen and P. J. Hoftyzer. Properties of Polymers. Correlation with Chemical Structure. Elsevier, Amsterdam (1972). 15. P. A. Small. J. appl. Chem. 3, 71 (1953). 16. K. L. Hoy. J. Paint Technol. 42, 76 (1970). 17. C. M. Hansen. J. Paint Technol. 39, 104 (1967). 18. C. M. Hansen. Ind. Engng Chem., Prod. Res. Dec. 8, 2 (1969).
- Copyright 0 1996Eisevier 8cience Ltd Printed in Great Britain. All rights reserved 00143057/96$15.00+ 0.00
Pergamon
SOLUBILITY OF LIQUID CRYSTALLINE COPOLYBIBENZOATES ERNEST0
PBREZ,* ZHEN ZHU, ANTONIO BELLO, JO& and ROSARIO BENAVENTE
M. PERENA
Instituto de Ciencia y Tecnologia de Polimeros (CSIC), Juan de la Cierva 3, 28006-Madrid, Spain (Received 9 March 1994; accepted in final form 23 May 1995)
Abstract-The solubility behaviour of several liquid crystalline copolymers based on poly(triethylene glycol p,p’-bibenzoate) and poly(octamethylene p,p’-bibenzoate), and the corresponding homopolymers, has been studied, analysing the influence of the proportion of oxyethylene units in the spacer. The value of the solubility parameter has also been estimated, from intrinsic viscosity measurements, for the different polymer samples, and compared with the calculations made from several methods. The solubility parameters for all the homopolybibenzoates and copolybibenzoates studied in this work lie in the range 6 = 9.5 & 0.2 (cal/cm3)i’r [19.4 + 0.4 (MPa)“*]. However, the solubility range increases very significantly with the proportion of oxyethylene units: while poly(octamethylenep,p’-bibenzoate) homopolymer is only soluble in tetrachloroethane, a fairly large number of solvents is found for copolymers with high proportions of oxyethylene units in the spacer.
INTRODUCl-ION
The presence of two oxygen atoms in the spacer of PTEB, replacing two methylene units of PSMB, introduces significant conformational differences between the two polymers, in such a way that their phase behaviour and transition temperatures are markedly different [A. A considerable increase in solubility is also found for PTEB with respect to PIMB, as will be shown in this work. The purpose of this article is the analysis of the influence on the solubility of the proportion of oxyethylene units in the spacer for several copolymers of P8MB and PTEB. The value of the solubility parameter, 6, has also been estimated for the different polymer samples.
Main-chain liquid crystalline polymers consisting of rigid mesogenic cores separated by flexible aliphatic spacers have attracted much attention in recent years. These polymers exhibit self-ordering properties which induce anisotropy. The reduced melting points and
increased solubility, caused by the introduction of flexible spacers, improve their processing capability. The thermotropicliquidcrystallinecharacterofpolyesters derived from bibenzoic acid has been reported in several papers [l-4] which have focused on the interesting behaviour of polybibenzoates, PBS, in the solid state. However, little information on solubility has been published, owing to the poor solubility of PBS with linear all-methylene spacers. The lower members of this series are only soluble in mixtures of phenols and tetrachloroethane [l-3]. However, a significant increase in solubility has been found for PBS with oxyethylene [5] or branched [6] spacers. We have previously performed a comparative analysis [7] of the thermotropic properties of poly(triethylene glycol p,p’-bibenzoate), PTEB and poly(octamethylene p,p’-bibenzoate), PSMB. These polybibenzoates have the following structural formula:
EXPERIMENTAL Several copolyesters based on poly(triethylene glycol p,p’bibenzoate) and poly(octamethylene p,p’-bibenzoate) were synthesized by melt transesterification of diethyl p,p’-bibenzoate and varying amounts of triethylene glycol, TEG, and octamethylene glycol, OMG, using isopropyl titanate as catalyst. The copolyesters were purified by precipitating into methanol their solutions in chloroform.
PTEB CO-0-CH2-CH2-0-CH2-CH2-0-CH2-CH2-0
P8MB CO-O-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-0
*To whom all correspondence should be addressed. 631
E. P&rez et al.
632
Table I. Molar compositions in TEG, molecular weights, and densities’ and solubility parameters of polybibenzoates calculated from different methods 6,,, (ca1/cm3)“2 Polymer P8MB c21 c44 C63 Cl6 C84 PTEB
Lo
10-4 M,
(g/cm’)
Small
Hay
VKH
6.7 4.8 6.0 8.0 I.5
1.16 I.18 1.20 1.21 1.22 I .23 1.24
9.88 9.91 9.93 9.93 9.96 9.91 9.99
10.26 10.38 10.46 10.54 10.60 10.64 10.71
9.33 9.45 9.52 9.60 9.66 9.69 9.16
0 27 44 63 76 84 100
“From the method of Van Krevelen and Hoftyzer, VKH [14].
Samples of the two homopolymers were also analysed. The P8MB specimen was the same as that previously reported [5,7,8], while a new sample of PTEB was synthesized for this work. The composition of the copolymers was determined from the I3C-NMR spectra in solutions of deuterated chloroform, using a Varian XL-300 spectrometer. The molar fractions of TEG J&G, are shown in Table 1. The chemical shifts corresponding to the different carbons in the homopolymers have been reported previously [7] (spectra of PIMB were recorded in solution of deuterated tetrachloroethane). The solubility of the polymers was tested in several common solvents (see Table 2). About 15 mg of polymer in 1Oml of solvent were used and three different degrees of solubility were considered: good solubility (+ in Table 2), partial solubility or solubility only in the hot liquid (+ -), and null solubility (-). The values of the intrinsic viscosity, [q], were determined at 25”C, using an Ubbelohde viscometer, in the solvents for which the solubility test was positive. The molecular weights of the samples were determined by Size Exclusion Chromatography in a Waters 15OC equip-
ment, using chloroform as eluent. The universal calibration plot was obtained from the data of several samples of monodisperse polystyrene (with molecular weights ranging from 1800 to 2,300,OOO).The molecular weights of the different polybibenzoate samples, represented by the peak maximum, Mp, are also shown in Table 1 (PIMB homopolymer is not soluble in chloroform). They were calculated from the elution volume of the peak and the intrinsic viscosity of the whole sample.
RESULTS
AND DISCUSSION
The influence of the composition on the solubility range and on the solubility parameter, 6, has been analysed. This parameter is defined, in the original formulation [9, lo], as the square root of the cohesive energy density, which represents the energy of vaporization per unit volume. For non-volatile substances, like polymers, indirect methods have to be used for the estimation of 6 [I 1, 121. Thus, the solubility parameter of a polymer is usually taken as that of the solvent corresponding to the solution for which the intrinsic viscosity is maximum. A test of the solubility of the different homopolymer and copolymer samples of Table 1 was carried out in several solvents covering a wide range of solubility parameters. The results of this test are presented in Table 2. It is observed, first, that P8MB is only soluble in tetrachloroethane, but the solubility is greatly enhanced by copolymerization with PTEB in such a way that the copolymer with a 27% of TEG is already soluble in several chlorinated compounds. There is a continued increase in the solubility from C27 to PTEB, but the most important change occurs on passing from PSMB to C27. Thus, the introduction of relatively small amounts of oxyethylene units produces a significant increase in the solubility. The corresponding increase in the flexibility of the spacer is reflected by a lowering of the glass transition temperature [13]. This lowering is not directly proportional to the TEG content. However, the isotropization temperatures show a steady decrease, approximately proportional to the composition, on passing from PSMB to PTEB. Thus, the isotropization temperature is about 190°C for P8MB [8], 172°C for C27, 143°C for C63 and 114°C for PTEB [7]. The values of the intrinsic viscosity of the different polymers in their actual solvents are shown in Table 3. It can be observed that the value for PTEB homopolymer is significantly lower than those for the other samples. Moreover, the molecular weight of the PTEB sample is also considerably lower (see Table 1). One might expect that the increase in
Table 2. Solubilitv test of oolvbibenzoates Test result Solvent Heptane Methylcyclohexane Cyclohexane Toluen. Benzene Carbon tetrachloride Trichloroethylene Chloroform Tetrachloroethane Methylene chloride Chlorobenzene o-Dichlorobenzene I ,2,4-Trichlorobenzene Tetrahydrofuran Ethyl acetate Methyl i-propyl ketone Methyl propyl ketone Cyclohexanone Acetone Dimethyl sulphoxide Ethanol Methanol
6, (cal/cm’)‘i2 7.4
7.8 8.2 8.9 9.2 8.6 9.2 9.3 9.1 9.8 9.5 10.0 9.1 9.1 8.5 8.1
9.9 9.9 12.0 12.7 14.5
___ PBMB
C27
c44
C63
C76
C84
PTEB
_
-
-
-
-
-
-
_ _ _ _ _ _ _ + _ _ _ _ _
+f++ + + f+++-
f++ + + +> + + + + -
_ _ +++ + + + + + + + _ _
_ _ + _ + + + + + + + + _ _
_ _ ++ _ + + + + + + + + _
_ _ ++ _ + + + + + + + + _
+ _
+ +_
_
-
-
_
_
_ _ -
+-
+_
+_ _
+ +_
Solubility Table 3. Intrinsic
Solvent
Tetrachloroethane Chloroform Methylene chloride Trichloroethylene Chlorobenzene o-Dichlorobenzene Tetrahydrofuran I ,2,4-Trichlorobenzene Benzene Cvclohexanone
of copolybibenzoates
viscosities of Dolvbihenzoates in different solvents
PSMB
c27
c44
C63
Cl6
C84
F’TEB
1.30 -
I .27
1.01 0.96 0.78 0.73 0.71 0.73 0.60 0.61
0.68 0.83 0.70 0.58 0.59 0.57 0.60 0.53 -
0.76 1.11 0.91 0.72 0.81 0.77 0.73 0.81 0.54 0.53
0.67 0.80 0.68 0.49 0.52 0.50 0.46 0.50 0.36 0.3 I
0.40 0.42 0.37 0.28 0.30 0.27 0.24 0.25 0.17
1.22 0.92 -
solubility would be partly due to the corresponding decrease in the molecular weight. We have reported the solubility behaviour in tetrachloroethane of a sample of PTEB with a considerably higher viscosity: 1.15 dl/g [5], finding that the solubility test is exactly the same as that for the actual sample of PTEB used in this work. We believe, therefore, that the solubility differences in Table 2 are mainly due to the effect of the presence of varying proportions of oxyethylene units in the spacer. The intrinsic viscosities listed in Table 3 can be used for the estimation of the solubility parameters. However, the original theory for this parameter [9] was developed only for non-polar substances, and the existence of specific interactions, like dipolar or hydrogen bonding, were not considered. Therefore, it is advisable to determine the solubility parameter in a range of solvents with similar polarity or capability of forming hydrogen bonds. In fact, it can be observed
1.6
1.4
0.6
0.4
0.2
633
I
I
I
,
9.2
9.4
9.6
9.8
I
6, (calknl~)“~
Fig. 1,Dependence of the intrinsic viscosity on the solubility parameter of the solvents for the indicated samples.
0.10
from Table 3 that the viscosity in chloroaliphatic solvents as a function of the solubility parameter of the solvent follows a clearly different trend than that for the other solvents. Thus, only chloroaliphatic liquids have been used for the representation in Fig. 1 (the solubility parameter of methylene chloride has been taken as 9.8 (cal/cm3)‘/2, the mean value between those reported in the literature [12]). The estimation of the solubility parameter of the polymer from this figure is not reliable, bearing in mind the reduced number of solvents and the dispersity of the data points. Nevertheless, smooth curves have been drawn through the experimental points, and the maxima were determined. The results indicate that the 6 values for all the analysed samples are in the interval 9.5 f 0.2 (cal/cm3)1’2 [19.4 f 0.4 (MPa)1’2]. It is not possible to establish significant differences between the samples. Group contributions can also be used to calculate the solubility parameter [12], provided that the density of the polymer is known. The density can also be estimated from group contributions [14]. The results for the polybibenzoate samples are shown in the fourth column of Table 1. The corresponding values of the solubility parameter, calculated by the methods of Small [15], Hoy [16] and Van Krevelen and Hoftyzer, VKH [14], are shown in the last columns of Table 1. It is observed that the most refined method of VKH gives very good agreement with the experimental determinations. A single solubility parameter cannot explain some aspects of the solubility behaviour, in particular the different trends of the viscosity values among the different families of solvents. Additional information is obtained from the three-dimensional solubility parameter approach [ 17, 181, divided into dispersion, a,, polar, 6,) and hydrogen-bonding capability, a,, contributions. It has been shown in other polybibenzoates that the three components of this parameter, which represents the centre of the solubility sphere [17, 181, are of the order of 8.7, 2.5 and 3.0 (cal/cm3)ii2 for a,, 6, and 6,, respectively. The same values have been estimated for the polymers in this work, considering the components of the solubility parameter [17, 181 for the solvents used. The polybibenzoates have very similar solubility parameter components, but the solubility range, that is the radius of the solubility sphere, is very sensitive to variations in the spacer. The radius of this sphere increases significantly with the TEG
E. Ptrez et al.
634
content, its value passing from less than one unit of the 6 components for C27 to approximately two units for PTEB. In conclusion, the solubility parameters for all the homopolybibenzoates and copolybibenzoates studied in this work are in the range 6 = 9.5 f 0.2 (cal/cm3)“* [19.4 k 0.4 (MPa)“*]. However, the solubility range increases very significantly with the TEG content, in such a way that while P8MB is only soluble in tetrachloroethane, copolymer C27 can be dissolved in several common solvents, and a fairly large number of solvents is found for copolymers with higher TEG proportions in the spacer.
acknowledge the financial support of the Comisibn Interministerial de Ciencia y Tecnologia, Project No. MAT94-0858-E, and of the Consejeria de Education y Cultura de la Comunidad de Madrid. Z. Zhu is indebted to the Ministerio de Education y Ciencia for a postdoctoral grant.
Acknowledgements-We
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