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Liquid crystalline solutions of cellulose in phosphoric acid for preparing cellulose yarnsBoerstoel, H.

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6. Birefringence of solutions of cellulose in phosphoric acid in elongational flow H. Boerstoel, J.B. Westerink, S.J. Picken, G. Dubbeldam, J. Veurink, M. Ypma, H. Maatman

Abstract During spinning the birefringence of solutions of cellulose in phosphoric acid was recorded in the elongational flow field of the air gap. Use was made of an on-line laser method, capable of simultaneously measuring the velocity of the filament and the birefingence. At draw ratios smaller than unity, i.e. in the die swell, a strong increase of birefringence with draw ratio was found. Once the extrusion velocity was reached, the birefringence saturated. Solutions that were either isotropic or anisotropic in the quiescent state behaved similarly and it was concluded that the isotropic solution displayed a flow induced phase transition.

6.1 Introduction In air gap spinning liquid crystalline solutions structure formation takes place in the spinneret, in the air gap, and during coagulation. The shear and elongational flow fields account for the high orientation achieved during the process. The rheological behavior in shear flow is described in Chapter 4. The determination of the elongational viscosity is treated in Chapter 5. Use was made of a laser Doppler measurement for determining the velocity profile of the filaments during drawing in the air gap. Moreover, the equipment is also suitable for the on- line determination of the birefringence treated in this chapter. On-line birefringence measurements have been used before for monitoring the orientation development during wet-spinning of cellulose. In viscose spinning, on-line birefringence measurements, based on interference microscopy, were performed by Vroom as early as in 1963. Retardation profiles have been given as a hnction of the distance from the spinneret, with the filaments being drawn in the bath'. Mortimer and Peguy used an on-line birefringence device based on a method by Harris in the processing of Lyocell fibers spun from an isotropic solutionz4. In the present study simultaneous measurements of the velocity and the birefringence were performed of cellulose solutions in phosphoric acid during drawing. Use was made of solutions of cellulose in phosphoric acid that were either isotropic or anisotropic in the quiescent state'. In the previous chapter the results of the velocity measurements are described. Here the birefringence profiles will be presented, use being made of the velocity profiles for the calculation of the diameter of the filaments.

6.2 Deformation mechanisms in elongational flow In the elongational flow field of the air gap the liquid crystalline solution will orient. Picken et al.derived a model for the overall orientation, here referred to as S, which was based on domains, where the local orientational order is governed by thermal motion, and the order

parameter SI, can be obtained from equilibrium conditions of a mean field theory6. The director orientation of the various domains is described by a second order parameter, here Sdir. The overall orientational order is then described by:

S = S,, . S,,

The director orientation is affected by the elongational flow field. Due to the long relaxation times the orientation scales with strain, i.e. total elongational deformation, rather than with strain rate. For the evolution of the director orientation Sd; as a function of strain (a) use was made of the affine deformation model of Kuhn and ~riin':

where a is an apparent draw ratio, composed of a predraw h, , which accounts for the orientation induced in the spinneret, and of the draw ratio in the air gap X: a=& X. Kenig used a slightly different approach, with the effectiveness of the elongational flow field being accounted for by an orientability parameters. Kenig found an initial fast orienting part and a subsequent saturation of orientation. This saturation occurred at a draw ratio in the range of 2 to 8, depending on the polymer. The initial part is described as follows:

tan 8 = tan 8, XP (6.3)

Where p is the orientability parameter and 0 the azimuthal half-width of the distribution determined from X-ray diffraction.

6.3 Experimental 6.3.1 Determination of the birefringence with an Abbe refractometer Of a number of solutions the birefringence in the quiescent state was determined by making use of a type B Abbe refractometer. Measurements were performed at room temperature. Use was made of a polarizer in the ocular of the refractometer, which was rotated to determine both the parallel and the perpendicular refractive indices. The difference between the two values was the birefringence. This birefringence in the quiescent state was used as a reference for on-line determination.

6.3.2 Optical method An on-line method for the determination of birefringence was developed by ~ a m s ' . Here use is made of a modified version extended with a Laser Doppler velocity measuring technique. Figure 5.4 gives a schematic representation of the set-up for simultaneous determination of the velocity and birefringence. The Laser Doppler measurements are described in Chapter 5. The light beam of a linearly polarized He-Ne laser (wavelength 632.8 nm) passes through a rotating 112 wave plate, which results in a linearly polarized light beam, the polarization direction of which rotates at twice the angular speed of the 112 wave plate. Then the beam passes through a 114 wave plate, with the axes being at an angle of 45" with the filament direction. Dependent on the polarization direction of the incident light, the polarization direction of the resulting beam varies from circular (clockwise or counter-clockwise) to linear and vice versa at a frequency of four times that of the rotating 112 wave plate. The light then passing through a birefringent filament will result in a phase shift of the periodically varying polarization state of the scattered light. A lens collimates the light to the detector surface. A polarization filter at an angle of 45" with the filament direction is placed in front of the detector monitoring the resulting sinusoidal intensity fluctuation, which has a frequency of four times the angular velocity of the 112 wave plate and is phase shifted with respect to an isotropic filament by an angle 4, determined by the difference between the vertical and horizontal optical

path lengths (And):

,where d is the diameter of the filament and 3, the wavelength of the laserlight (632.8 nrn). The sinusoidal intensity fluctuation is given in Figure 5.5. A description of the spinning experiments for this optical determination is given in Table 5.1.

6.3.3 Preparation of yarns A solution was prepared on a Linden Z-kneader, as described in Chapter 5. The presolvent had a PzOs-concentration of 74.3 % wlw. The solution had a dry polymer content of 15.2 % wlw. A 15-1 container was filled with the solution and mounted on the spinning machine. A booster pump was fed by forcing the solution out of the container under Nz-pressure. The solution was filtered through a 20 pm filter. A spinning pump provided for the spinning pressure. In a static mixer the solution was heated to 30°C. The spinneret, containing 50 holes with a diameter of 65pm (10" entrance angle, Lld 2) was placed in a double-walled housing, which was kept at approximately 4S°C. Above the spi-nneret was positioned a gauze pack with 325-mesh filters. In the air gap draw ratios between 0.7 and 3 8 were imposed. In most cases the air gap length was 40 mm, be it that at the lowest draw ratios the air gap was smaller in order to prevent the

filaments from elongating under their own weight. Use was made of a static coagulation bath filled with cold acetone. The yarns were washed with water, neutralized with a 2.5 % wlw Na2C03-solution, washed, dried on a heated godet of 150°C, and wound on a bobbing. The forces on the yarn were kept as low as possible. The set-up is depicted in Figure 6.1.

Filter

Mixer

I Washing Drying Winding -

I I

Figure 6.1: Spinning machine for the preparation of cellulose yarns.

In the preparation of the cellulose yarns the role of the non-solvent is very important. In our patent application three homologous series were tested: n-alkanols, ketones and esters5. The yarns were spun in accordance with the method described above.

6.4 Results and Discussion 6.4.1 Birefringence measurements For a number of concentrations the birefringence in the quiescent state measured with the Abbe refiactometer is given in Figure 6.2.

0 5 10 15 20 25

cellulose concentration (wlw96)

Figure 6.2: Birefringence as afunction of cellulose concentration for the Abbe refractometer (crosses) and the laser method (circles). Temperatures as indicated in Table 5. I

For the experiments described in Section 5.4.1 the phase angle was determined on-line as a function of the distance fiom the spinneret. Use was made of the optical technique described in Section 6.3.1. There are two problems to the interpretation of the phase angle. Due to the various positions of the 114 wave plate (-45" and 45") with respect to the filament axis the sign of the phase angle is indeterminate. Moreover, the measured values are inbetween -180" and +180° and a multiple of 360" should be added to or subtracted fiom the measured value which is provided or not with a minus sign. Several possibilities for the course of the phase angle in the air gap are displayed in Figure 6.3. For the calculation of the birefringence from the phase angle the diameter should be known at the same point in the air gap. Therefore, use was made of a combined technique for the determination of the velocity and the birefringence. The velocity profiles are discussed in Chapter 5. From these velocity profiles the diameters were calculated from:

, where Q is the volume flow and d, and v, are the diameter and velocity at a distance z from the spinneret.

draw ratio (-)

Figure 6.3: Possible phase angles as a function of draw ratio for a 14.3 % whv cellulose solution @p. I in Table 5. I), the measured angle ( (open circles), -( (solid circles), -&360°

(open squares), )360° (triangles), @+36O0 (crosses)

From the phase angles the birefringence was calculated as a function of distance. The various possibilities are displayed in Figure 6.4.

draw ratio (-)

Figure 6.4: Birefringence as a function of of draw ratio, for the various phase angles as indicated in Figure 6.3

Based on the value of the birefringence measured by the Abbe refiactometer and the orientation development as a function of draw ratio in the filaments, the correct profile is the

one marked by the open squares (4+360°). The orientation in the filaments will be treated in Section 6.4.2. In both Figures the draw ratio, calculated fiom the velocity at a certain point divided by the squirting speed, was chosen to be plotted on the x-axis, rather than the distance fiom the spinneret or the deformation rate. For an isotropic polymer system the latter would be an appropriate choice, because in that case the birefringence is expected to be a finction of deformation rate. For anisotropic systems the orientation was found to be a function of the total deformation, rather than the deformation rate and hence the birefringence was plotted as a function of draw r a t i ~ ~ ' ~ ' ' ~ . This is confirmed in Figure 6.5, where the birefringence is plotted as a function of draw ratio for various spinning speeds. Although the elongational velocity varies with spinning speed, (and distance from spinneret) a uniform birefringence profile is found for all the experiments. As was to be expected, the draw ratio is the factor determinative of the orientation due to the long relaxation times of the director field.

0 0 1 2 3 4 5 6

draw ratio (-)

Figure 6.5: Birefringence as afinction of draw ratio at the following spinning velocities: 80 (triangles), 100 (open circles), 125 (open squares), 150 (crosses) and 200 dmin (solid circles)

The shape of this master curve is rather special. The initial part of the curve, with draw ratios below unity (i.e. the die swell region), is very steep, whereas at draw ratios above unity the

birefringence will saturate, which will be discussed fhrther on. The same curve is found for other air gap lengths, as displayed in Figure 6.6.

0 0.5 I 1.5 2 2.5 3 3.5 4 draw ratio (-)

Figure 6.6: Birefringence as afinction of draw ratio for the following air gap lengths: 13 (crosses), 28 (squares) and 49 mm (circles)

0 0 0.5 1 1.5 2 2.5 3 3.5 4

draw ratio (-)

Figure 6.7: Birefringence as a finction of draw ratio for 9.5 (DP 2300; squares), 14.3 (circles) and 17.1 % whv (triangles)

The influence of cellulose concentration can be seen in Figure 6.7 : only the saturation level of the birefringence is affected and not the shape of the curve. This effect also holds for the lowest concentration, which was spun above the clearing temperature. Even in this situation very rapid orientation occurs and it may be concluded that a phase transition is induced by the flow above, in and below the capillary of the spinneret, which indicates that the difference between the isotropic and anisotropic phase will disappear under the influence of flow. The plateau level of the birefingence as a knction of concentration is given in Figure 6.2, in which a comparison is made with the measurement in the quiescent state with the Abbe- refractometer. Reasonable agreement is found, although the Laser method gives a birefringence which is consistently higher. Of course it is possible to speculate about the origin of these differences. Firstly, a systematic difference between both techniques cannot be ruled out. Secondly, the order parameter in the quiescent state might be lower than in the elongational flow field in the air gap. In that case an orientation factor of approximately 0.7- 0.8 would hold for the equilibrium situation. Thirdly due to the flow upon closing the refractometer, the director might align at an angle with one of the planes, thus changing the apparent birefringence.

6.4.2 Yam properties as a function of draw ratio The birefringence and the sonic modulus of the yarns as a hnction of the draw ratio are given in Figure 6.8.

0 1 2 3 4 5

draw ratio (-)

Figure 6.8: Birefringence as aajirnction of &aw ratio in the air gap, according to the laser method (open circles) and in the yarn (crosses). The sonic modulus in the yam is also indicated (solid circles)

For the yams the same trend is observed as in the birefringence profiles in the air gap. At draw ratios below unity a sharp decrease can be noticed of the modulus and the birefringence, though less pronounced than in the on-line determination, whereas at draw ratios above 1.5 the properties of the yams are hardly influenced by the degree of drawing. The point at which the mechanism changes is only sligthly higher than observed in the on-line birefringence measurement. This therefore confirms that the right profile is found by on-line determination.

6.4.3 Discussion It was found that the orientation profile can be divided into two regions, viz. one with a draw ratio above unity , where the orientation level will saturate, and one below unity, which corresponds to the die swell , where a sharp decrease of birefringence was found. For yarns spun from anisotropic solutions of poly (para-phenylene terephthalamide) Ciferri and Valenti also found that above a draw ratio of unity the modulus level would saturate, whereas a sharp decrease of the modulus was found towards lower draw ratioslO. This was ascribed to the low tension in the spinning line upon the filaments being relaxed instead of drawn. There are two ways of looking at the initial steep part of the curve. First of all a broad distribution of orientation angles might be considered. Then this broad distribution, which corresponds to the low birefringence in the die swell, will rapidly narrow. Two mechanisms were given in order to describe the orientation development during drawing the affine deformation model by Kuhn and Griin, with supplements by Picken, and the orientability model by ~ e n i ~ ~ " . In Figure 6.9 some possibilities are given for the use of these models. For the afline deformation model the curves were drawn in the case of absence of predraw and a predraw of 10. The predraw represents the apparent strain imposed by the elongational flow field in the spinneret. It should be noted that in the case of absence of predraw the predicted birefringence starts low, as found in the experiments, but does not increase fast enough to fit the experimental values. In the case of a predraw of 10 the saturation level is well described, but fails to predict the low birefringence levels in the die swell. For the orientability model also two curves were given, viz for an orientability parameter of 0.6 and 2, as defined by Equation 6.3. The value of 0.6 is the highest value given by Kenig, and applies to poly(para-phenylene terephthalamide). The birefringence profile based thereon is far too slow to fit the experiments. A value of 2 would be more appropriate, but it seems unlikely that the present system of cellulose dissolved in phosphoric acid would orient much faster than the systems given by Kenig. We therefore introduce the second possibility. Suppose the orientation of the molecules is already on a high level in the capillary. Then it does not seem likely that the molecules desorient to a random state. If we assume that the director field is incompressible, a buckling deformation will occur under axial compression as sketched in Figure 6.10.

0 1 2 3 4 5 6

draw ratio (-)

Figure 6.9: Birefringence as a function of &aw ratio. Comparison of theories with experiment F (circles), 1: aDne &formation, without predraw, 2: Kenig withp=0.6 3: Kenig

withp=2, 4: affine deformation with uprehaw of 10. In all cases Anm=O. 0115.

Figure 6.10: The assumed buckling of the director field; the director is at a constant angle 8 with the filament axis

The director is then at a constant angle 8 with the filament axis. Due to conservation of length

of the director, the angle 0 in the die swell can be calculated, for a uniform velocity profile in the spinneret, from the draw ratio h:

Then the measured birefringence is:

, where in this case 0 is a fixed angle instead of the average of the distribution. By combining Equations 6.6 and 6.7 the birefringence can be written as a hnction of the draw ratio:

A n 3 1 = - a 2 -- An, 2 2

0

0 1 2 3 4 5 6 draw ratio (-)

Fzgure 6. I I : Bzrefiingence as a Jirnctzon of draw ratio, experiment F (circles) and the description according to the helicoidal direc for pattern , as in Equation 6.8, An,,=O. 01 15.

Other configurations with a preferred orientation angle 8 are: piled cones and a helical arrangement of the director. The last option was suggested by ~ e ~ e r " . Both options would lead to the same dependence on the draw ratio of the birefringence. In view of the polarization micrograph a choice was made for the buckling deformation. The birefiingence profile calculated from Equation 6.8 is given in Figure 6.11. Excellent agreement is found with the results of the experiment. The occurrence of this kind of deformational behavior seems likely. In the yarns the same trend was observed for the sonic modulus and the birefringence. In polarization microscopy the occurrence of a periodically varying structure could be discerned. Figure 6.12 shows the micrographs of the yarns spun at various draw ratios. No preferred orientation angle could be identified by X-ray scattering. However, upon the draw ratio being increased from 0.7 to 3.8, the azimuthal half-width of the orientation distribution decreased only from 12" to 7.S0, with a saturation level being obtained at a draw ratio of 1.5. The crystalline orientation therefore covers only a very small part of the total orientation effect. The preferred orientation angle can thus not be seen in the crystalline fraction. as determimed by X-ray scattering. Fried and Sixou observed the same phenomena in liquid crystalline solutions of hydroxypropyl cellufose in acetic acid, where relaxation was allowed after an elongational m~t ion '~ . The effect was ascribed to the elastic behavior of the solutions and a zig-zag director orientation was also proposed. In the description it is assumed that in the spinneret there occurs a rather perfect alignment, and that the presumed reversibility of the deformation process will result in a replica of the situation in the capillary being found at draw ratio unity. However, beyond this point an additional orienting effect might be brought about by drawing. Therefore, this description could be combined with the &ne deformation model, or the empirical model by Kenig. This has not been done, though, for in this range of orientation levels the birefringence is too insensitive. Moreover, the sonic modulus, which, unlike the birefringence, should be sensitive towatds an improved orientation, will also saturate after a draw ratio of 1.5. This could either mean that the elongational flow field does not impose an improved orientation, or that the conditions downstream, viz. the type of non-solvent and the temperature and tensions used during coagulation, washing and drying will be determinative of the level of (des-)orientation. The effects thereof also account for the fact that the development of the birefringence in the yarn as a function of draw ratio is less pronounced than that measured in the air gap. The fluxes of solvent and non-solvent into and out of the filament might have an orienting or a desorienting effect. Due to the additional drawing having a marginal effect, as demonstrated here, the emphasis of further investigation should be on the coagulation process. The effect of the type of non-solvent is demonstrated in our patent application, where alcohols, ketones and esters were used in the coagulation bath5. To give an impression of the importance of the type

of non-solvent the sonic modulus of these yarns is given in Figure 6.13. In each spinning

wperfment acetone was used a3 a reference, and the souic modulus of h e yarns i s taken

relative to d~ai of the yarn, spun in acetone. It is observed tha! the type of non-solvent detwrrrim to some extent the mechanical properties of the yams.

0 1 2 3 4 5 number of C-atoms

Figure 6.13: Sonic modulus of cellulose yarns spun in various coagulation media, relative to the sonic modulus of yams spun in acetone. Use was mnde of alcohols (solid circles), ketones (open circles) and esters (crosses).

6.5. Conclusions Due to a combined velocity and birefringence measuring technique we were able to determine the orientation profile during stretching in the air gap of isotropic and anisotropic solutions of cellulose in phosphoric acid. A uniform pattern was found when plotting the birefringence as a finction of draw ratio. No distinction was found between isotropic and anisotropic solutions indicating a flow induced phase transition in the former case. The orientation profile could be split up into a very steep initial part, which occurred in the die-swell, and a constant birefringence beyond the die-swell. The same pattern, though less pronounced, was observed in the sonic modulus and birefringence of yarns spun at draw ratios between 0.7 and 3.8. From the on-line birefringence measurements it seems very likely that in the die-swell a director buckling phenomenon occurs, e.g. as in a concertina, rather than a random distribution of angles. The idea was supported by polarization micrographs showing a periodically varying structure at low draw ratios. When draw ratios higher than 1.5 were applied, the structure disappeared. Excellent agreement between the theoretical and experimental orientation development was observed. If it is assumed that at a draw ratio of one an image is found of the situation in the capillary it must be concluded that in the spinneret a very high degree of orientation is already present. Further drawing in the air gap did not increase the sonic modulus

of the yarns. Possibly the contribution of the additional stretching are ruled out by coagulation and tension effects.

6.6 Acknowledgment The authors wish to thank Mr Ir. E. Thiiss for his preparatory work on the optical measurements, Mr G. Ten Broeke for the practical support, Dr. R van der Hout for stimulating discussions, and Dr. C.J.M. Van den Heuvel and Dr. R.H. Huisman and coworkers for the structural analysis of the yarns.

6.6 References 1. R.A. Vroom, "Kinetic aspects of the viscose rayon spinning process", thesis Delft (1963) 2. P.H. Harris, GB 2049 175A (ICI) 3. S.A. Mortimer, A.A. Peguy, Textile Res. J. 64(9), 544 (1994) 4. S.A. Mortimer, A.A. Peguy, J. Appl. Pol. Sci. (60), 1747 (1996) 5. H. Boerstoel, B.M. Koenders, J.B. Westerink, W09606208 (Akzo Nobel) 6. S.J. Picken, S. van der Zwaag, M.G. Northolt, Polymer 33 (14), 2998 (1992) 7. W. Kuhn, F. Griin, Kolloid Zeitschrift 101, 248 (1942) 8. S. Kenig, Pol. Eng. Sci., 27(12), 887 (1987) 9. H. Boerstoel, M.Ypma, WO 9606207 10.A. Ciferri, B. Valenti in " Ultra high modulus polymers" (ed Cifem and Ward), Chapter 7,

Applied Science Publishers London (1977) 1l.R.B. Meyer, in "Polymer Liquid Crystals" (ed A. Cifferi, W.R. Krigbaum, R.B. Meyer)

Chapter 6, Academic Press, NewYork (1982) 12.F. Fried, P. Sixou, Mol Cryst. Liq. Cryst. I$&B, 163 (1988)