- Email: [email protected]
Viscosity and rubber-like properties of isotropic and liquid crystalline solutions of oxypropylcellulose in water
506
V . G . KULICHIKHINet al.
PFA-10 and PFMA-10 is evidently the smaller kinetic rigidity of the main chain in PFA, which promotes greater mobility of the side groups. All the observed laws are in agreement with the concept of the mesomorphic structure of these comb-like polymers, and justify the use of a model for the structure of PFA-n and PFMA-n (n = 8, 10, 12) having layers formed by anti-parallel packing of the side groups (Fig. 4). The stability of the mesomorphic structure is ensured, on the one hand, by the mobility of the side branching in the t o O - - bond of the ester group, and on the other hand by the fairly strong inter-molecular interactions, which are retained at high temperatures.
Translated by N. STANDEN
REFERENCES 1. N. A. PLATA and V. P. SHIBAEV, Grebneobraznye polimery i zhidkie kristally (Comb-like Polymers and Liquid Crystals). Moscow, 1980. 2. C. W. BUNN and E. R. HOWELL, Nature 174: 549, 1954. 3. T. P. STEPANOVA, L. L. BURSHTEIN, L. D. BUDOVSKAYA, Yu. G. BAKLAGINA, V. N. IVANOVA,
N. A. NIKOROVA and N. I. TKACHEVA, Zh. Fiz. Khim. 58: 1949, 1984. 4. Yu. A. SHARONOV and M. V. VOLKENSHTEIN, Vysokomol. soyed. 4: 917, 1962 (not translated in Polymer Sci. U.S.S.R.).
Polymer Science U.S.S.R. Vol. 32, No. 3, pp. 506-512, 1990 Printed in Great Britain.
0032-3950/90 $10.00 + .00 © 1991 Pergamon Press plc
VISCOSITY AND RUBBER-LIKE PROPERTIES OF ISOTROPIC AND LIQUID CRYSTALLINE SOLUTIONS OF OXYPROPYLCELLULOSE IN WATER* V. G. KULICHIKHIN, M. KH. MIRDZHANOV, O. V. VASILEVA, M. P. ZABUGINA, YE. K. BORISENKOVA and I. V. EKAEVA A. V. Topchiev Institute for Petrochemical Synthesis, Academy of Sciences of the U.S.S.R. (Received 9 November 1988)
The specific features of the theological properties of lyotropic liquid crystal systems are demonstrated, using aqueous solutions of propoxycellulose as an example. These properties include the existence of maxima on the plots of viscosity, elastic strain, normal stresses, and dynamic modulus against the concentration. These features are generally observed at concentrations corresponding to the appearance of the liquid crystal phase, although, in the case of elastic strain, the maximum is displaced into the region of higher concentrations. Accumulation of the liquid crystal phase is accompanied by an increase in non-linearity of the mechanical behaviour. The correlation between the static and dynamic characteristics is thus impaired. In the case of completely liquid solution the first difference in the normal stresses becomes negative.
SOLUTIONS of a number of cellulose derivatives, beginning from a specific concentration c, form a liquid crystal (LC) phase, which is characteristic of orientation effects during flow at low rates and * Vysokomol. soyed. A32: No. 3,566-571, 1990.
Solutions of oxypropylcellulose in water
507
low shear stresses. The results of these effects is that the solutions have a set of unusual rheological properties, which play an important part both from the point of view of understanding the fundamental problems of the physics of polymers, and also in the rational organization of processes for their reprocessing. In this connection, solutions of cellulose ethers and esters are especially important. On the one hand, since these are readily accessible, the study of problems associated with LC ordering is open to a large number of research workers. On the other hand, the practical importance of cellulose materials (fibres and films) is such that establishment of the optimum molecular orientation under the conditions of the production process must be undertaken simultaneously. Propoxycellulose (POC) is the most polar of the cellulose derivatives, and shows both thermotropic and lyotropic mesomorphism. Furthermore, the LC state in cellulose systems was first recorded for aqueous POC solutions [1]. Later, the viscosity properties of LC solutions of POC were studied in detail by rotational [2] and capillary rheometry [3]. The flow curves for POC solutions contain a characteristic branch, which is characterstic of specially unoriented liquid crystals, in which the viscosity is inversely related to the stress [3, 4]. Under transitional conditions, two-stage stress development is observed [4]. With rise in concentration and temperature the viscosity is changed irregularly: in the region where the LC phase appears a viscosity maximum is observed, whereas with further increase in concentration (decrease in temperature) the viscosity can acquire minimum values [2, 5]. Since the concentration for the appearance of a LC phase is fairly high ( < 30 wt%) anisotropic solutions have high absolute viscosity values. Under these conditions, the optical criteria for the appearance of the LC phase are not always identical (in obtaining the preparation the effect of photoelasticity is superimposed). The rheological properties can significantly add to the set of data on a particular transition and reveal its nature. • The characteristics of the high elasticity of POC solutions in the viscous flow (in the relaxational sense) and in the LC (phase) states are also essentially specific. As a rule, such parameters as extrudate swelling, the input pressure losses, and the primary difference of the normal stresses N1 are changed, with respect to the concentration and temperature scales, in the same way as the viscosity, i.e. with a maximum and minimum [5]. This information leads to the formation of a mechanism for the viscoelastic behaviour of LC systems. However, quantitative data on the viscosity and elastic characteristics of lyotropic LC systems based on cellulose derivatives (as a rule these have a cholesterol structure) are clearly inadequate. In view of this, the main object of this work was to study the rheological properties of aqueous solutions of POC, using various experimental techniques. The problem set was to obtain the widest possible information on the rheological properties of one and the same solutions, using equipment based on different principle of action and with different flow geometries. The problem of correlating certain fundamental rheological characteristics, which so far has not been solved for LC polymer systems was solved simultaneously. The material used was grade "Klyugel J" POC (made by Gerkules), M = 3.1 x 105, degree of substitution - 3 . The aqueous solutions, of concentration 25, 30, 36, 45, 50, and 55 wt%, were prepared manually over 1-2 weeks at room temperature. The solutions obtained were analysed by means of a MIN-8 polarization microscope. Stable birefringence, not associated with photoelasticity, is developed in the solutions, beginning from a concentration of 45%, so that the critical concentration for the appearance of a LC phase is 36-45 wt%. It was impossible to determine the concentration corresponding to complete disappearance of the isotropic phase by an optical method because of the extremely high viscosity of the solutions.
508
V . G . KULICHIKI-UNet al.
The dynamic mechanical characteristics of the solutions were measured with a VR-74 [7] vibrorheometer, with a cylinder--cylinder-type working unit, supplemented by a device for preventing water (solvent) evaporation. The frequency range of the measurements was 10-2-20 Hz, and the strain amplitude 0.1-2.6%. The tangential ~" and normal N1 stresses developed during steady-state flow, were determined with an Instron-3250 rheometer, with a cone-plane working unit (cone tip angle 1740°), over the range of shear rates ~, = 5 x 10-3-2 s -1. High elastic strains 7e, accumulated during shear flow, were evaluated from the resiliance on a EM-elastometer. The instrument had a cone-plane working unit, and operated with ~"= constant. After attaining steady-state flow the load is removed and the angle of rotation of the plane thus released is recorded, this being a measure of the elastic energy stored in the material during flow. When the plane returns to its original position the solution was subjected to strain in the opposite direction, and after a certain rest period the next test was made. The viscosity was evaluated simultaneously. All the measurements were at room temperature, with a 15-min rest period between the measurements. The plots for the elastic strain against the shear stress are steadily rising curves, almost parallel to each other for solutions of different concentrations. This enables the values of 3'e to be compared when z = constant. Figure 1 shows plots of % against c when log ~"= 2. It can be seen that in the region of isotropic compositions the elastic strain is directly proportional to the concentration, which, as usual, reflects the increase in density of the system of inter-molecular contacts. However, at a concentration >45% the elastic strain begins to be decreased with increase in concentration. This is unusual, and is obviously associated with a decrease in density of the physical network, using the terminology accepted in the rheology of isotropic polymers. Accordingly, the macroelasticity of LC systems is significantly lower than that of isotropic solutions, but liquid crystals have a specific orientational elasticity, which is determined by the resistance of the significant molecular orientation to strains of different type [8]. Isotropic polymers have no orientational elasticity. However, the problem of determining the anisotropic elastic moduli of anisotropic solutions is outside the scope of this work. The effect of prehistory on the rheological properties of anisotropic POC solutions is followed in the dynamic tests. In these solutions the dynamic moduli are decreased with time, and the main changes occur during the first 24 h. Furthermore, up to six days of holding in the working unit the values of the modulus remain unchanged. Relaxation of the internal stresses and the production of equilibrium under the given conditions of texture, with disappearance of some of the disclinations evidently occurring during this time, so that both the elastic and dissipative relaxations of the LC system are somewhat decreased. The decrease in the values of the dynamic moduli generally does not exceed 0.1-0.2 orders of magnitude. On comparing the frequency relations for the moduli for solutions of different concentration, values are generally used which are obtained after holding the solutions over one day. The experimental data of Fig. 2 refer to this. A non-steady change in the position of the curves with increase in concentration, a decrease in the slope of the plots of log G against log to on passing into the LC state (this feature is also noted for the elastic modulus), a tendency to the appearance of an additional plateau for the LC solutions on the plot of log G' against log to in the low frequency region, and the absence of maxima on the plots of log G" against log to in passing to the high elastic region are characteristic features of the plots of the moduli against frequency. Figure 1 shows plots of the elastic modulus and the losses as a function of the concentration of the
Solutions of oxypropylcellulose in water
log G'[Pa] j /
'°gjial
509
~..,~
2
(a)
Pal 1 g
2.5
Io, G"[Pa] 2 log "r{Pa] J
.! (b)
z~ ~f,6
J
1
1.5 9--"~r i
ZO
4sO
FlG. 1
2-
l 60
%
-I
0
1
Fio. 2
FIG. 1. Elastic strain (1), log r at ~ = 0.1 s -1 (2), elastic modulus G' (3) and elastic loss G" at to = 1 s 1 (4) as a function of concentration. The different points on the plot of log ~"against c correspond to data obtained with different instruments. FIG. 2. Elastic modulus (a) and elastic loss (b) for POC solutions of c = 25 (1), 30 (2), 36 (3), 45 (4), 50 (5), and 55 wt% (6) as a function of frequency. solutions (at to = 1 Hz). With isotropic solutions the moduli increase with concentration, but at c > 4 5 % they begin to decrease, attaining approximately constant values at c~>50%. Undoubtedly, these features of the plots of the moduli against concentration are associated with reorganization of the structure of the system during formation of the LC phase. With increase in frequency the comparison between the plots of log G against c b e c o m e m o r e complete. The rate of growth of the elastic modulus with increase in concentration is higher than that of the loss modulus; as a result of this, the tangent of the mechanical loss angle, tan a, is decreased up to a concentration of 35% (Fig. 3). The evolution of the plots of tan a against c at higher concentrations depends on the frequency selected for comparison. Thus, when log to = - 1 and c > 45% tan a falls even m o r e sharply than for isotropic solutions; when log to = 0 tan a remains constant over the concentration range 35-55%; when log to = 0.8 a not very m a r k e d minimum appears on the plot of tan ~ against c in the region of 35--45%. At concentrations greater than 55% the plot of tan a against frequency disappears. It can be assumed that in LC solutions there are different flow units [9], and depending on the concentration and frequency at which the reaction occurs, the reaction of a particular unit can be manifested. For this reason, in the two-phase region the elastic c o m p o n e n t of the overall modulus p r e d o m i n a t e s at low frequencies, evidently reflecting the motion of large formations (domains), since at high frequencies the rate of growth of the dissipative component, which is associated with molecular mobility, can be higher. Finally, in the region where the solutions consist entirely of LC, we pass to flow units of the same type at all concentrations, which are not ruptured under the action of frequency effects. F r o m this point of view it can be assumed that the second critical concentration for P O C solutions is - 5 5 % . A b o v e this concentration the isotropic phase does not remain in the system. H o w e v e r , the existence of larger flow units in the completely LC solutions also is not excluded. The presence of an arm on the plots of log G" against log to over the log to range - 0 . 5 to - 1 (Fig. 2) is an indication of this. By analogy with isotropic systems, for which the presence of such an arm is associated with the relaxational mobility of the macromolecules on a scale larger than the
510
V . G . KULICHIKI-IIN et al.
log ~[S- 11 0
/
tan f
2 Z.5
2 I 3# FIG. 3
FIG. 3.
¢,*/o I 55
log 'r[Pa] 1
I
g
3
,
FIG. 4
Tangent of mechanicalloss angle as a functionof concentrationof POC solutionsfor log to = - 1 (1), 0 (2), and 0.8 (3). FIG. 4. Flowplots for POC solutionsof c = 25 (1), 30 (2), 36 (3), 45 (4), and 50 wt% (5).
chain interlockings Me [10], in the case of LC solutions it can reflect the presence of large domains. This possibly explains the shift of the elastic strain maximum in the direction of the other rheological characteristics. The problem of the nature of the high frequency plateau for LC systems so far remains unexplained, and it is thus expedient to remain within the framework within which reliable experimental facts have been accumulated. These are as follows. The rapid decrease in the slope starts earlier (by 0.4--0.5 orders with respect to frequency) with the loss modulus. At lower frequencies the indeces in the power relations G'~to ~ and G"~to ~ are lower than for traditional polymer systems. Thus, the value of a for solutions of concentration 25-36% is 1.2-1.3, is lowered to 0.95 for c = 45%, and is as low as 0.65 for c = 50 and 55%. The value of/3 is close to 0.8-0.9 for isotropic solutions, and is 0.75 and 0.65 for concentrations of 45 and 50-55%, respectively. These values differ significantly from the theoretical value (for isotropic polymers): a = 2 and/3 = 1 [11]. The domain flow in LC systems is evidently the reason for this. As already noted, in the region of the high frequency plateau (on the elastic modulus plot) a maximum of log G" is not recorded for POC solutions, although both for isotropic and for two-phase systems there is a noticeable decrease in slope of the plots of log G" against log to. Since the maximum of the plot of log G" against log to is especially clear for monodisperse polymers [11], its absence in LC systems can be connected with the presence of different kinetic flow units. Completely LC solutions for which the plot of log G" against log to is almost linear over the whole frequency range behave quite differently. The forced transition into the high elastic state probably occurs only for the isotropic part of the solutions, whereas for completely LC systems showing domain flow such a transition either just does not occur or is of another nature. Further discussion of the rheological properties of LC solutions of POC will be based on experimental data obtained under steady-state shear conditions. Figure 4 shows parts of the flow plots for solutions of different concentrations. The isotropic solutions behave as Newtonian liquids, whereas with the anisotropic solutions there are viscosity anomalies. The location of the plots in relation to the concentration changes in the same way as for the frequency plot of the loss modulus. Figure 1 shows plots for log ~"(at log ~, = - 1 ) as a function of concentration, which reflect the shear viscosity. The figure shows a plot in the region of 36-45%, and a subsequent weak relation between
Solutions of oxypropylcellulose in water
log r, log G"[Pa]
N~ x lO-~[Pa]
f~" o.,..s~//''; 2
511
tZ
qo
20
log ~, log ~[s-71 -2
-1
FIG. 5
O
I
-20 -
q
-2
-f
log ~ [s -~1 I
0
Fxo. 6
Tangential stresses as a functionof shear rate (1-3) and loss modulusas a functionof frequency (1'-3') for POC solutionswith c = 36 (1, 1'), 45 (2, 2'), and 50 wt% (3, 3'). FIG. 6. First differencein normalstresses as a functionof shear rate for POC solutionsof c = 25 (1), 36 (2), 45 (3), and 50 wt% (4). FIG. 5.
log z and c. Accordingly, a decrease of internal friction in systems which have passed into the LC state is also recorded on the plots of steady-state continuous flow. In Fig. 5 the flow plots and the plot of log G" against log to (to = "~) are compared for 36, 45, and 50% solutions. Whereas for the isotropic (36%) solutions the relation between the tangential stress and the loss modulus is fairly good, with LC systems the dynamic characteristics are always above the static ones, this difference widening with concentration. An increase in the dynamic characteristics over the static ones has also been noted for other LC systems [9], and Kulichikhin et al. [12] associated it with the different levels of orientation attained in these and other experiments. This explanation is probable in that in each cycle of the dynamic experiment it is impossible to obtain the orientational situation realized under conditions of steady-state shear flow. Figure 6 shows the first difference N1 as a function of ,~ for solutions of 25, 36, 45 and 50% concentration. For isotropic systems the values of N1 increase with shear rate, and the concentration remains always >0. The position is different with LC solutions. Whereas in the region of the mixture of isotropic and anisotropic phases (c = 45%) the plot of N1 against log ~, at high shear rate changes sign, with completely LC solutions (50%) the normal stresses have negative values and increase with respect to the modulus with increased shear rate. Such unusual characteristics of the change in N~ with concentration prevents testing of the applicability of the fundamental relations between the viscoelastic characteristics for LC systems. However, on a first impression the appropriate correlation applies only to isotropic POC solutions. As regards anisotropic solutions, it is possible only to speak of qualitative agreement between the nature of the changes in such viscoelasticity parameters as the tangential and normal stresses, the elastic loss modulus, and high-elastic strains. All these characteristics pass through a maximum on the concentration scale, they increase with concentration in the region of isotropic solutions, and are decreased in the region of the LC systems. A quantitative correlation is not observed, as especially indicated by the fact that for 50% solutions of POC N1 < 0. The unusual nature and the lack of continuity of the main results obtained in studying the rheological properties of the isotropic and LC solutions of POC require tests on systems with a preset and accurately controllable orientation. This would provide a means of evaluating the anisotropic coefficients of viscosity and the elastic constants, and of testing the applicability of
512
V . G . KULICHIKHINel al.
existing theories to describing the experimental data, and of establishing a general picture of the rheological behaviour of LC polymers.
Translated by N. STANDEN
REFERENCES 1. R. WERBOWYJ and D. GRAY, Molec. Cryst. Liquid. Cryst. Letters. 34: 97, 1976. 2. S. SUTO, K. OBARA, S. NISHANTI and M. KARASAWA,J. Polymer Sci. Polymer Phys. Ed. 24: 1849, 1986. 3. S. SUTO, M. OHSHIRO, R. ITO and M. KARASAWA,Polymer 28: 23, 1987. 4. T. ASADA, S. HAYASHIDAand S. ONOGI, Repts. Progr. Polymer Phys. 23: Japan, 145, 1980. 5. S. SUTO and M. KARASAWA,Proc. Intern. Syrup. Fiber Sci. and Technol. Hacone, 280, 1985. 6. V. G. KULIC KHI , L. V. PETROVA, O. A. KHANCHICH, A.K. DIBROVA and E.G. KOGAN, Khim, Volokna, 42, 1985. 7. V. I. BRIZITSKII, Yu. G. YANOVSKII and D.A. MUSTAFAEV, Tez. Vsesoyuz. seminara "Instrumentalnye metody reologii (Thesis of All-Union Seminar "Instrumental Methods in Rheology). Moscow, 1972. 8. P. De GENNES, Fizika zhidkikh kristallov (Physics of Liquid Crystals, Translation from English (editor A. S. Sonin). Moscow, 1977. 9. V. G. KULICHIKHIN, A. Ya. MALKIN and S.P. PAPKOV, Vysokomol. soyed. A26: 451, 1984 (translated in Polymer Sci. U.S.S.R. 26: 3, 499, 1984). 10. V. S. VOLKOV, Mezhdunarodnaya konferetsiya 13o kauchuku i rezine (International Conference on Rubber and Rubber Products), Preprint A67. Moscow, 1984. 11. G. V. VINOGRADOV and A. Ya. MALKIN, Reologiya polimerov (Rheology of Polymers). Moscow, 1977. 12. V. G. KULICHIKHIN, L. P. BRAVERMAN, Z. V. KHANIN and A.V. VOLOKIIINA, Vysokomol. soyed. A30: 1386, 1988 (translated in Polymer Sci. U.S.S.R. 30: 7, 1451, 1988).
V . G . KULICHIKHINet al.
PFA-10 and PFMA-10 is evidently the smaller kinetic rigidity of the main chain in PFA, which promotes greater mobility of the side groups. All the observed laws are in agreement with the concept of the mesomorphic structure of these comb-like polymers, and justify the use of a model for the structure of PFA-n and PFMA-n (n = 8, 10, 12) having layers formed by anti-parallel packing of the side groups (Fig. 4). The stability of the mesomorphic structure is ensured, on the one hand, by the mobility of the side branching in the t o O - - bond of the ester group, and on the other hand by the fairly strong inter-molecular interactions, which are retained at high temperatures.
Translated by N. STANDEN
REFERENCES 1. N. A. PLATA and V. P. SHIBAEV, Grebneobraznye polimery i zhidkie kristally (Comb-like Polymers and Liquid Crystals). Moscow, 1980. 2. C. W. BUNN and E. R. HOWELL, Nature 174: 549, 1954. 3. T. P. STEPANOVA, L. L. BURSHTEIN, L. D. BUDOVSKAYA, Yu. G. BAKLAGINA, V. N. IVANOVA,
N. A. NIKOROVA and N. I. TKACHEVA, Zh. Fiz. Khim. 58: 1949, 1984. 4. Yu. A. SHARONOV and M. V. VOLKENSHTEIN, Vysokomol. soyed. 4: 917, 1962 (not translated in Polymer Sci. U.S.S.R.).
Polymer Science U.S.S.R. Vol. 32, No. 3, pp. 506-512, 1990 Printed in Great Britain.
0032-3950/90 $10.00 + .00 © 1991 Pergamon Press plc
VISCOSITY AND RUBBER-LIKE PROPERTIES OF ISOTROPIC AND LIQUID CRYSTALLINE SOLUTIONS OF OXYPROPYLCELLULOSE IN WATER* V. G. KULICHIKHIN, M. KH. MIRDZHANOV, O. V. VASILEVA, M. P. ZABUGINA, YE. K. BORISENKOVA and I. V. EKAEVA A. V. Topchiev Institute for Petrochemical Synthesis, Academy of Sciences of the U.S.S.R. (Received 9 November 1988)
The specific features of the theological properties of lyotropic liquid crystal systems are demonstrated, using aqueous solutions of propoxycellulose as an example. These properties include the existence of maxima on the plots of viscosity, elastic strain, normal stresses, and dynamic modulus against the concentration. These features are generally observed at concentrations corresponding to the appearance of the liquid crystal phase, although, in the case of elastic strain, the maximum is displaced into the region of higher concentrations. Accumulation of the liquid crystal phase is accompanied by an increase in non-linearity of the mechanical behaviour. The correlation between the static and dynamic characteristics is thus impaired. In the case of completely liquid solution the first difference in the normal stresses becomes negative.
SOLUTIONS of a number of cellulose derivatives, beginning from a specific concentration c, form a liquid crystal (LC) phase, which is characteristic of orientation effects during flow at low rates and * Vysokomol. soyed. A32: No. 3,566-571, 1990.
Solutions of oxypropylcellulose in water
507
low shear stresses. The results of these effects is that the solutions have a set of unusual rheological properties, which play an important part both from the point of view of understanding the fundamental problems of the physics of polymers, and also in the rational organization of processes for their reprocessing. In this connection, solutions of cellulose ethers and esters are especially important. On the one hand, since these are readily accessible, the study of problems associated with LC ordering is open to a large number of research workers. On the other hand, the practical importance of cellulose materials (fibres and films) is such that establishment of the optimum molecular orientation under the conditions of the production process must be undertaken simultaneously. Propoxycellulose (POC) is the most polar of the cellulose derivatives, and shows both thermotropic and lyotropic mesomorphism. Furthermore, the LC state in cellulose systems was first recorded for aqueous POC solutions [1]. Later, the viscosity properties of LC solutions of POC were studied in detail by rotational [2] and capillary rheometry [3]. The flow curves for POC solutions contain a characteristic branch, which is characterstic of specially unoriented liquid crystals, in which the viscosity is inversely related to the stress [3, 4]. Under transitional conditions, two-stage stress development is observed [4]. With rise in concentration and temperature the viscosity is changed irregularly: in the region where the LC phase appears a viscosity maximum is observed, whereas with further increase in concentration (decrease in temperature) the viscosity can acquire minimum values [2, 5]. Since the concentration for the appearance of a LC phase is fairly high ( < 30 wt%) anisotropic solutions have high absolute viscosity values. Under these conditions, the optical criteria for the appearance of the LC phase are not always identical (in obtaining the preparation the effect of photoelasticity is superimposed). The rheological properties can significantly add to the set of data on a particular transition and reveal its nature. • The characteristics of the high elasticity of POC solutions in the viscous flow (in the relaxational sense) and in the LC (phase) states are also essentially specific. As a rule, such parameters as extrudate swelling, the input pressure losses, and the primary difference of the normal stresses N1 are changed, with respect to the concentration and temperature scales, in the same way as the viscosity, i.e. with a maximum and minimum [5]. This information leads to the formation of a mechanism for the viscoelastic behaviour of LC systems. However, quantitative data on the viscosity and elastic characteristics of lyotropic LC systems based on cellulose derivatives (as a rule these have a cholesterol structure) are clearly inadequate. In view of this, the main object of this work was to study the rheological properties of aqueous solutions of POC, using various experimental techniques. The problem set was to obtain the widest possible information on the rheological properties of one and the same solutions, using equipment based on different principle of action and with different flow geometries. The problem of correlating certain fundamental rheological characteristics, which so far has not been solved for LC polymer systems was solved simultaneously. The material used was grade "Klyugel J" POC (made by Gerkules), M = 3.1 x 105, degree of substitution - 3 . The aqueous solutions, of concentration 25, 30, 36, 45, 50, and 55 wt%, were prepared manually over 1-2 weeks at room temperature. The solutions obtained were analysed by means of a MIN-8 polarization microscope. Stable birefringence, not associated with photoelasticity, is developed in the solutions, beginning from a concentration of 45%, so that the critical concentration for the appearance of a LC phase is 36-45 wt%. It was impossible to determine the concentration corresponding to complete disappearance of the isotropic phase by an optical method because of the extremely high viscosity of the solutions.
508
V . G . KULICHIKI-UNet al.
The dynamic mechanical characteristics of the solutions were measured with a VR-74 [7] vibrorheometer, with a cylinder--cylinder-type working unit, supplemented by a device for preventing water (solvent) evaporation. The frequency range of the measurements was 10-2-20 Hz, and the strain amplitude 0.1-2.6%. The tangential ~" and normal N1 stresses developed during steady-state flow, were determined with an Instron-3250 rheometer, with a cone-plane working unit (cone tip angle 1740°), over the range of shear rates ~, = 5 x 10-3-2 s -1. High elastic strains 7e, accumulated during shear flow, were evaluated from the resiliance on a EM-elastometer. The instrument had a cone-plane working unit, and operated with ~"= constant. After attaining steady-state flow the load is removed and the angle of rotation of the plane thus released is recorded, this being a measure of the elastic energy stored in the material during flow. When the plane returns to its original position the solution was subjected to strain in the opposite direction, and after a certain rest period the next test was made. The viscosity was evaluated simultaneously. All the measurements were at room temperature, with a 15-min rest period between the measurements. The plots for the elastic strain against the shear stress are steadily rising curves, almost parallel to each other for solutions of different concentrations. This enables the values of 3'e to be compared when z = constant. Figure 1 shows plots of % against c when log ~"= 2. It can be seen that in the region of isotropic compositions the elastic strain is directly proportional to the concentration, which, as usual, reflects the increase in density of the system of inter-molecular contacts. However, at a concentration >45% the elastic strain begins to be decreased with increase in concentration. This is unusual, and is obviously associated with a decrease in density of the physical network, using the terminology accepted in the rheology of isotropic polymers. Accordingly, the macroelasticity of LC systems is significantly lower than that of isotropic solutions, but liquid crystals have a specific orientational elasticity, which is determined by the resistance of the significant molecular orientation to strains of different type [8]. Isotropic polymers have no orientational elasticity. However, the problem of determining the anisotropic elastic moduli of anisotropic solutions is outside the scope of this work. The effect of prehistory on the rheological properties of anisotropic POC solutions is followed in the dynamic tests. In these solutions the dynamic moduli are decreased with time, and the main changes occur during the first 24 h. Furthermore, up to six days of holding in the working unit the values of the modulus remain unchanged. Relaxation of the internal stresses and the production of equilibrium under the given conditions of texture, with disappearance of some of the disclinations evidently occurring during this time, so that both the elastic and dissipative relaxations of the LC system are somewhat decreased. The decrease in the values of the dynamic moduli generally does not exceed 0.1-0.2 orders of magnitude. On comparing the frequency relations for the moduli for solutions of different concentration, values are generally used which are obtained after holding the solutions over one day. The experimental data of Fig. 2 refer to this. A non-steady change in the position of the curves with increase in concentration, a decrease in the slope of the plots of log G against log to on passing into the LC state (this feature is also noted for the elastic modulus), a tendency to the appearance of an additional plateau for the LC solutions on the plot of log G' against log to in the low frequency region, and the absence of maxima on the plots of log G" against log to in passing to the high elastic region are characteristic features of the plots of the moduli against frequency. Figure 1 shows plots of the elastic modulus and the losses as a function of the concentration of the
Solutions of oxypropylcellulose in water
log G'[Pa] j /
'°gjial
509
~..,~
2
(a)
Pal 1 g
2.5
Io, G"[Pa] 2 log "r{Pa] J
.! (b)
z~ ~f,6
J
1
1.5 9--"~r i
ZO
4sO
FlG. 1
2-
l 60
%
-I
0
1
Fio. 2
FIG. 1. Elastic strain (1), log r at ~ = 0.1 s -1 (2), elastic modulus G' (3) and elastic loss G" at to = 1 s 1 (4) as a function of concentration. The different points on the plot of log ~"against c correspond to data obtained with different instruments. FIG. 2. Elastic modulus (a) and elastic loss (b) for POC solutions of c = 25 (1), 30 (2), 36 (3), 45 (4), 50 (5), and 55 wt% (6) as a function of frequency. solutions (at to = 1 Hz). With isotropic solutions the moduli increase with concentration, but at c > 4 5 % they begin to decrease, attaining approximately constant values at c~>50%. Undoubtedly, these features of the plots of the moduli against concentration are associated with reorganization of the structure of the system during formation of the LC phase. With increase in frequency the comparison between the plots of log G against c b e c o m e m o r e complete. The rate of growth of the elastic modulus with increase in concentration is higher than that of the loss modulus; as a result of this, the tangent of the mechanical loss angle, tan a, is decreased up to a concentration of 35% (Fig. 3). The evolution of the plots of tan a against c at higher concentrations depends on the frequency selected for comparison. Thus, when log to = - 1 and c > 45% tan a falls even m o r e sharply than for isotropic solutions; when log to = 0 tan a remains constant over the concentration range 35-55%; when log to = 0.8 a not very m a r k e d minimum appears on the plot of tan ~ against c in the region of 35--45%. At concentrations greater than 55% the plot of tan a against frequency disappears. It can be assumed that in LC solutions there are different flow units [9], and depending on the concentration and frequency at which the reaction occurs, the reaction of a particular unit can be manifested. For this reason, in the two-phase region the elastic c o m p o n e n t of the overall modulus p r e d o m i n a t e s at low frequencies, evidently reflecting the motion of large formations (domains), since at high frequencies the rate of growth of the dissipative component, which is associated with molecular mobility, can be higher. Finally, in the region where the solutions consist entirely of LC, we pass to flow units of the same type at all concentrations, which are not ruptured under the action of frequency effects. F r o m this point of view it can be assumed that the second critical concentration for P O C solutions is - 5 5 % . A b o v e this concentration the isotropic phase does not remain in the system. H o w e v e r , the existence of larger flow units in the completely LC solutions also is not excluded. The presence of an arm on the plots of log G" against log to over the log to range - 0 . 5 to - 1 (Fig. 2) is an indication of this. By analogy with isotropic systems, for which the presence of such an arm is associated with the relaxational mobility of the macromolecules on a scale larger than the
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V . G . KULICHIKI-IIN et al.
log ~[S- 11 0
/
tan f
2 Z.5
2 I 3# FIG. 3
FIG. 3.
¢,*/o I 55
log 'r[Pa] 1
I
g
3
,
FIG. 4
Tangent of mechanicalloss angle as a functionof concentrationof POC solutionsfor log to = - 1 (1), 0 (2), and 0.8 (3). FIG. 4. Flowplots for POC solutionsof c = 25 (1), 30 (2), 36 (3), 45 (4), and 50 wt% (5).
chain interlockings Me [10], in the case of LC solutions it can reflect the presence of large domains. This possibly explains the shift of the elastic strain maximum in the direction of the other rheological characteristics. The problem of the nature of the high frequency plateau for LC systems so far remains unexplained, and it is thus expedient to remain within the framework within which reliable experimental facts have been accumulated. These are as follows. The rapid decrease in the slope starts earlier (by 0.4--0.5 orders with respect to frequency) with the loss modulus. At lower frequencies the indeces in the power relations G'~to ~ and G"~to ~ are lower than for traditional polymer systems. Thus, the value of a for solutions of concentration 25-36% is 1.2-1.3, is lowered to 0.95 for c = 45%, and is as low as 0.65 for c = 50 and 55%. The value of/3 is close to 0.8-0.9 for isotropic solutions, and is 0.75 and 0.65 for concentrations of 45 and 50-55%, respectively. These values differ significantly from the theoretical value (for isotropic polymers): a = 2 and/3 = 1 [11]. The domain flow in LC systems is evidently the reason for this. As already noted, in the region of the high frequency plateau (on the elastic modulus plot) a maximum of log G" is not recorded for POC solutions, although both for isotropic and for two-phase systems there is a noticeable decrease in slope of the plots of log G" against log to. Since the maximum of the plot of log G" against log to is especially clear for monodisperse polymers [11], its absence in LC systems can be connected with the presence of different kinetic flow units. Completely LC solutions for which the plot of log G" against log to is almost linear over the whole frequency range behave quite differently. The forced transition into the high elastic state probably occurs only for the isotropic part of the solutions, whereas for completely LC systems showing domain flow such a transition either just does not occur or is of another nature. Further discussion of the rheological properties of LC solutions of POC will be based on experimental data obtained under steady-state shear conditions. Figure 4 shows parts of the flow plots for solutions of different concentrations. The isotropic solutions behave as Newtonian liquids, whereas with the anisotropic solutions there are viscosity anomalies. The location of the plots in relation to the concentration changes in the same way as for the frequency plot of the loss modulus. Figure 1 shows plots for log ~"(at log ~, = - 1 ) as a function of concentration, which reflect the shear viscosity. The figure shows a plot in the region of 36-45%, and a subsequent weak relation between
Solutions of oxypropylcellulose in water
log r, log G"[Pa]
N~ x lO-~[Pa]
f~" o.,..s~//''; 2
511
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qo
20
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-1
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O
I
-20 -
q
-2
-f
log ~ [s -~1 I
0
Fxo. 6
Tangential stresses as a functionof shear rate (1-3) and loss modulusas a functionof frequency (1'-3') for POC solutionswith c = 36 (1, 1'), 45 (2, 2'), and 50 wt% (3, 3'). FIG. 6. First differencein normalstresses as a functionof shear rate for POC solutionsof c = 25 (1), 36 (2), 45 (3), and 50 wt% (4). FIG. 5.
log z and c. Accordingly, a decrease of internal friction in systems which have passed into the LC state is also recorded on the plots of steady-state continuous flow. In Fig. 5 the flow plots and the plot of log G" against log to (to = "~) are compared for 36, 45, and 50% solutions. Whereas for the isotropic (36%) solutions the relation between the tangential stress and the loss modulus is fairly good, with LC systems the dynamic characteristics are always above the static ones, this difference widening with concentration. An increase in the dynamic characteristics over the static ones has also been noted for other LC systems [9], and Kulichikhin et al. [12] associated it with the different levels of orientation attained in these and other experiments. This explanation is probable in that in each cycle of the dynamic experiment it is impossible to obtain the orientational situation realized under conditions of steady-state shear flow. Figure 6 shows the first difference N1 as a function of ,~ for solutions of 25, 36, 45 and 50% concentration. For isotropic systems the values of N1 increase with shear rate, and the concentration remains always >0. The position is different with LC solutions. Whereas in the region of the mixture of isotropic and anisotropic phases (c = 45%) the plot of N1 against log ~, at high shear rate changes sign, with completely LC solutions (50%) the normal stresses have negative values and increase with respect to the modulus with increased shear rate. Such unusual characteristics of the change in N~ with concentration prevents testing of the applicability of the fundamental relations between the viscoelastic characteristics for LC systems. However, on a first impression the appropriate correlation applies only to isotropic POC solutions. As regards anisotropic solutions, it is possible only to speak of qualitative agreement between the nature of the changes in such viscoelasticity parameters as the tangential and normal stresses, the elastic loss modulus, and high-elastic strains. All these characteristics pass through a maximum on the concentration scale, they increase with concentration in the region of isotropic solutions, and are decreased in the region of the LC systems. A quantitative correlation is not observed, as especially indicated by the fact that for 50% solutions of POC N1 < 0. The unusual nature and the lack of continuity of the main results obtained in studying the rheological properties of the isotropic and LC solutions of POC require tests on systems with a preset and accurately controllable orientation. This would provide a means of evaluating the anisotropic coefficients of viscosity and the elastic constants, and of testing the applicability of
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existing theories to describing the experimental data, and of establishing a general picture of the rheological behaviour of LC polymers.
Translated by N. STANDEN
REFERENCES 1. R. WERBOWYJ and D. GRAY, Molec. Cryst. Liquid. Cryst. Letters. 34: 97, 1976. 2. S. SUTO, K. OBARA, S. NISHANTI and M. KARASAWA,J. Polymer Sci. Polymer Phys. Ed. 24: 1849, 1986. 3. S. SUTO, M. OHSHIRO, R. ITO and M. KARASAWA,Polymer 28: 23, 1987. 4. T. ASADA, S. HAYASHIDAand S. ONOGI, Repts. Progr. Polymer Phys. 23: Japan, 145, 1980. 5. S. SUTO and M. KARASAWA,Proc. Intern. Syrup. Fiber Sci. and Technol. Hacone, 280, 1985. 6. V. G. KULIC KHI , L. V. PETROVA, O. A. KHANCHICH, A.K. DIBROVA and E.G. KOGAN, Khim, Volokna, 42, 1985. 7. V. I. BRIZITSKII, Yu. G. YANOVSKII and D.A. MUSTAFAEV, Tez. Vsesoyuz. seminara "Instrumentalnye metody reologii (Thesis of All-Union Seminar "Instrumental Methods in Rheology). Moscow, 1972. 8. P. De GENNES, Fizika zhidkikh kristallov (Physics of Liquid Crystals, Translation from English (editor A. S. Sonin). Moscow, 1977. 9. V. G. KULICHIKHIN, A. Ya. MALKIN and S.P. PAPKOV, Vysokomol. soyed. A26: 451, 1984 (translated in Polymer Sci. U.S.S.R. 26: 3, 499, 1984). 10. V. S. VOLKOV, Mezhdunarodnaya konferetsiya 13o kauchuku i rezine (International Conference on Rubber and Rubber Products), Preprint A67. Moscow, 1984. 11. G. V. VINOGRADOV and A. Ya. MALKIN, Reologiya polimerov (Rheology of Polymers). Moscow, 1977. 12. V. G. KULICHIKHIN, L. P. BRAVERMAN, Z. V. KHANIN and A.V. VOLOKIIINA, Vysokomol. soyed. A30: 1386, 1988 (translated in Polymer Sci. U.S.S.R. 30: 7, 1451, 1988).