flow-induced anisotropy and its decay in polymeric liquid crystals

Flowinduced anisotropy and its decay in polymeric liquid crystalsP. Moldenaers, H. Yanase, and J. Mewis

Citation: Journal of Rheology (1978-present) 35, 1681 (1991); doi: 10.1122/1.550250 View online: http://dx.doi.org/10.1122/1.550250 View Table of Contents: http://scitation.aip.org/content/sor/journal/jor2/35/8?ver=pdfcov Published by the The Society of Rheology Articles you may be interested in The influence of inertia and elastic retraction on flow-induced crystallization of isotactic polypropylene J. Rheol. 57, 1281 (2013); 10.1122/1.4812671 Simulation of film blowing including flow-induced crystallization J. Rheol. 45, 1085 (2001); 10.1122/1.1392300 A continuum model for flow-induced crystallization of polymer melts J. Rheol. 43, 85 (1999); 10.1122/1.550978 Flowinduced sound J. Acoust. Soc. Am. 97, 3261 (1995); 10.1121/1.413114 FlowInduced Orientation in Monodomain Systems of Polymeric Liquid Crystals J. Rheol. 33, 537 (1989); 10.1122/1.550027

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Flow-induced anisotropy and its decayin polymeric liquid crystals

P. Moldenaers, H. Yanase,a) and J. Mewis

Department of Chemical Engineering, KatholiekeUniuersiteit Leuoen, De Croylaan 46, B-3001 Leuuen, Belgium

(Received 24 January 1991; accepted 26 July 1991)

Synopsis

Flow induces anisotropy in polymeric liquid crystals. This can be demonstratedby comparing the stress transients during a sudden increase in shear rate withthose during flow reversal: the damped oscillationsof the shear stress resultingfrom these two experimentsare shifted by nearly 180·. Flow-induced anisotropydecays after the flow stops. Its evolution in time is followed by stress growthexperiments in the flow direction and in the opposite one, with systematicchanges in the rest period. The phase shift only disappears after several thousands of seconds.This time is much longer than the time necessaryfor the shearstress to relax. Other rheological characteristics, such as the variation of thedynamic moduli after stopping the flow, occur on the same time scale as theanisotropy decay. The anisotropy decay is not affected by temperature andconsequentlyviscosity can be ruled out as a governingfactor. The results agreein part with recent polydomain models.

INTRODUCTION

Anisotropy is one of the key features of liquid crystalline materials. Itis caused by a preferential although imperfect alignment of the molecules in a particular direction, represented by the director. In generalthe detailed alignment in a liquid crystalline system depends on itsintrinsic characteristics as well as on the concentration and the thermomechanical history that the sample has experienced. Aside from theacademic interest in the phenomenon of anisotropy, it is of great practical importance to predict and control the final orientation that is

a)Current address: Kyoto University, Department of Polymer Chemistry, Kyoto 606,Japan.

@) 1991 by The Society of Rheology, Inc.J. Rheol 35(8), November 1991 0148-6055/91/081681-19$04.00 1681


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1682 MOLDENAERS. YANASE, AND MEWIS

obtained in objects processed from polymeric liquid crystalline solutionsor melts (PLCs). In addition the anisotropy has to be taken into account in injection moulding because of its effect on mould filling.

There are not many rheological data available in the literature on theanisotropic nature of the viscosity of PLCs. Miesowicz (1935) was thefirst to measure the anisotropic viscosities of a low molecular weightthermotropic system, using an oscillating plate viscometer. The molecular orientation was ensured in these experiments by a magnetic field. Ifone assumes a fixed molecular orientation, the Leslie-Ericksen (1979)and Doi (1981) theories can yield expressions for the Miesowicz viscosities (Prilutski, 1984). Recently, the anisotropic viscosities and theirconcentration dependency were measured on a well-aligned polybenzylglutamate PLC sample by means of quasielastic Rayleigh light scattering (Taratuta et al., 1988). The anisotropy of a polymeric liquid crystalunder normal flow conditions was demonstrated by Malkin et al.(1979), using a combination of a Couette and a falling sphere viscometer. They reported a transverse viscosity that was twice as large as thelongitudinal viscosity.

It is the aim of this work to investigate systematically the effect ofshear history on the flow-induced anisotropy of a lyotropic model system. For this purpose the effect of the direction of flow will be assessedby comparing the transient shear stresses in intermittent forward shearflow experiments with those during intermittent shear flow reversal.Varying the length of the rest period makes it possible to investigate thememory of the sample for its flow-induced anisotropy.

According to the Leslie-Ericksen theory (1979) or the Doi theory(1981), transient rheological behavior is due to changes in either thedirector orientation or the degree of orientation around this averagedirection. The observed rheological response cannot be explained by ahomogeneous molecular orientation alone. The presence of supermolecular structures (domains, defects) can have a great influence and mayoften dominate the rheological behavior of polymeric liquid crystals(Viola and Baird, 1986; Moldenaers et al., 1989). No attempt will bemade in this study to generate a monodomain structure in the samplebut rather the texture, as created by the flow, will be investigated.Consequently, the effects reported can be due to molecular as well assupermolecular structures. This combination of effectsis, however, whatis expected when utilizing these materials for practical purposes.


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LIQUID CRYSTALS IN TRANSIENT SHEAR 1683

EXPERIMENT

A solution of poly(y-benzyl-D-glutamate) (PBDG from SigmaChemical Company) in m-cresol has been used as a model system. ThePBDG has an average molecular weight of 310 000, yielding an aspectratio of the molecules of approximately 140.The concentration amountsto 25% by weight. It is about four times the critical concentration forthe onset of liquid crystallinity in this particular system. Hence noisotropic zones are expected to be present in the sample. At rest thesample is known to be cholesteric but during flow it develops readilyinto a nematic structure (Kiss and Porter, 1978). Equilibrium andtransient rheological characteristics of the material under investigationhave already been reported elsewhere (Moldenaers et al., 1990a). Extensive sets of rheological data are available for other liquid crystallinepolybenzylglutamates (Kiss and Porter, 1978; Kiss and Porter, 1980;Moldenaers and Mewis, 1986; Mewis and Moldenaers, 1987a and1987b; Taratuta et al., 1988; Larson and Mead, 1989), making this classof materials attractive for more detailed experimental and theoreticalinvestigations.

The rheological experiments were performed on a Rheometries Mechanical Spectrometer RMS 705F, equipped with either a 100 or a 10g em transducer. Cone and plate geometry has been used in all theexperiments (a = 0.02 rad, radius 25 mm). This ensures a constantshear history throughout the sample, which is a prerequisite to obtainanalyzable transient results. Data points can be sampled every 2 ms.

Transient rheological experiments constitute a sensitive probe forflow-inducedstructural changes (Moldenaers and Mewis, 1987; Guskeyand Winter, 1991). In the present work start-up experiments will beused. The sample will be presheared at a fixed rate until steady-stateconditions are reached. It is then allowed to rest for a specified period,after which shear flow is resumed in the same direction and at the sameshear rate (intermittent forward flow: IFF). The effect of the rest timeon the transient response renders information about the decay of theflow-induced structure after the shearing has been stopped. The flowcanalso be resumed at the same shear rate but in the opposite direction(intermittent flow reversal: IFR). In the limit of a zero rest period thelatter experiment reduces to simple flow reversal. The effectof the flowdirection on the stress transients can be used to probe the memory of thematerial for the flow-induced anisotropy. Using a Newtonian fluid witha similar viscosity as the PBDG solution it has been verified that instrument inertia is only significant during the first 0.1 s after start-up.


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1684

fa'

(b)

MOLDENAERS, YANASE, AND MEWIS

t r_ - ---I

III

time

time

FIG. 1. Transient shear flow experiments (a) Intermittent forward flow (IFF); (b)intermittent flow reversal (lFR); t, = variable rest period.

The types of experiments that have been performed are representedschematically in Fig. 1. The effect of temperature on the transient response will also be assessed in order to clarify the possible mechanismsresponsible for the observed behavior.

The stress response in intermittent forward flow experiments is illustrated in Fig. 2. Apart from the first sharp peak, the curves look like adamped oscillation superimposed on a monotonically increasing curve.The sharp initial peak appears when the rest period is 300 s or longer.It requires 100 strain units or more for the stress to reach steady state.


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o 50strain

100 150

FIG. 2. Reproducibility of intermittent forward flow experiments on PBDG: it = 0.4 s - I;t, = 750 s; 0: first experiment;.: second experiment. The symbols identify the extrema.Open symbols: maxima: tJ.: 1st; 0: 2nd; 0: 3rd; 0: 4th; filled symbols: minima: .: 1st;.: 2nd; .: 3rd; .: 4th.

Despite the complex nature of the transient, it is clear that the reproducibility is very good. The successivemaxima and minima are labeledto facilitate the detailed discussion of the shape of the transients in thefollowing sections. The preshear was always applied for at least 200strain units, which was found to be adequate to wipe out differences inthe kinematic history the sample might have experienced before.

RESULTS AND DISCUSSION

Equilibrium behavior

The steady-state shear behavior of the PBDG sample at a temperature of 293 K is shown in Fig. 3. All the transient experiments reportedin the next sections will be limited to the shear rate region where theviscosity is constant. The first normal stress difference shows the wellestablished phenomenon of negative values over a limited shear rateregion (Kiss and Porter, 1978, 1980; Gotsis and Baird, 1985; Navard,1986; Mewis and Moldenaers, 1987b). At low shear rates the first normal stress difference is positive and exhibits a linear dependency onshear rate. This proportionality is predicted by the Leslie-Ericksen


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1686 MOLDENAERS, YANASE, AND MEWIS

:'2_- - 0

~zCJo... .- -_._ 0-0-0-.-_._-_._-.- --- \/.-.-.----0/0 .--.

/0o

/

FIG. 3. Steady-state shear flow results at 293 K (.: viscosity; 0: positive N1; . : negativeNIl.

( 1979) and the Doi (1981) theories, although their basic assumption ofa monodomain structure is not satisfied here. A recent two-dimensionalanalysis of the Doi model, using a Maier-Saupe potential but avoidingthe mathematical decoupling approximation (Marrucci and Maffettone,1989), associates the negative normal stress with an increase of theorientational spread of the molecules due to the shear field. At highershear rates this effect disappears, causing NI to become positive. Thepositive normal stresses at low shear rates are attributed to directortumbling (Marrucci and Maffettone, 1990b). A similar but three-dimensional analysis of the model, using an Onsager potential, confirmsthe two-dimensional findings qualitatively (Larson, 1990). In the 3Danalysis three regions of director kinematics are established with increasing shear rate: tumbling, wagging, and steady state. Over theseregions of shear rate, but not strictly coinciding with them, positive,negative, and finally again positive normal forces are obtained.


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t r= : lOs:'106 e: 0

200 a: •394 s: 0.

981 s: •.1989.: 0

42600.: •


o 50strain

100 150

FIG. 4. Scaled shear stress for various rest times in intermittent forward flow experiments(1'=0.4s- 1; T=283K).

Intermittent forward flow: Effect of rest time

In a first series of intermittent experiments, stress growth curves wereobtained by resuming the shear flow in the same direction as appliedduring the preshearing. A typical start-up curve has already been shownin Fig. 2. The change of these curves with rest period is displayed in Fig.4 for rest periods varying between 10 and 42 600 seconds at a temperature of 283 K. The shear rate is kept at 0.4 s - I during the preshearingand during the actual experiment. In order to avoid any interferencefrom the total duration of the experiment, the rest times were changedin a random order. No irreversible changes in the sample behavior wereobserved in the present experiments. The stresses have been plottedversus strain rather than time. This is convenient because it is knownthat total deformation rather than time dominates many slow transientsin PLCs (Moldenaers and Mewis, 1986; Mewis and Moldenaers, 1987a;Doppert and Picken, 1987; Larson and Mead, 1989; Moldenaers et al.,1989). The transient stresses have been rescaled, by dividing by theequilibrium value, to facilitate the comparison of the intrinsic shapes ofthe curves when the test parameters are changed.

The stress growth curves in Fig. 4 reflect the structural recovery upto very long rest periods: it takes about 104 s of rest before the curves


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1688 MOLDENAERS. YANASE, AND MEWIS

reach their ultimate shape. The behavior reported here is totally unlikethat of isotropic polymeric materials. In the Newtonian region the shearstress would then monotonically approach its steady-state value, athigher shear rates only a single overshoot would normally be recorded.

Not many data on stress growth experiments on PLCs, with a welldefined kinematic history of the sample, are available in the literature.Hence no thorough comparison is possible at present. Wissbrun (1980)reported multiple overshoots in a start-up experiment on a thermotropiccopolyester. A similar behavior was reported by Metzner and Prilutski(1986) for a lyotropic system of hydroxypropylcellulose (HPC) in acetic acid. The previous shear history of the sample was not specified inthese experiments, but the data indicate that multiple overshoots are nota unique feature of the polybenzylglutamates studied here. Moreover anequilibrium value for the shear stress was reached in these studies aftershearing for 60--200 strain units, which are values comparable to theones observed here for PBDG. A lyotropic solution of aromatic polyamides in sulphuric acid has been investigated by Viola and Baird( 1986). They measured an overshoot in the shear stress upon restartingthe flow. No second overshoot was recorded by these investigators. Itshould however be noted that their solution was obtained by redissolving aramid fibers. Doppert and Picken (1987) experimented with afresh solution of a similar polymer. They reported a shear stress thatexhibits multiple overshoots when the shear flow is resumed. The overall shape of their curves is much more in line with the ones recordedhere for polybenzylglutamates. Similar experiments were performed byViola and Baird (1986) on a thermotropic copolyester. Also in thissample a sharp peak developed after long rest periods. Recently Grizzuti et al. (1990) investigated the effect of the rest period on the firstsharp peak of the stress growth curve for a solution of hydroxypropylcellulose in water. The initial response was found to be sensitive to restperiods for up to 104 s. The remainder of the curve was not studied indetail by these authors because the oscillations are not so pronounced inHPC as they are in PBDG.

A cholesteric-nematic transition occurs in the PBDG sample uponcessation or inception of the flow. It is not expected to have a majoreffect on the rheological results presented in Fig. 4 (Kiss and Porter,1978; Larson and Mead, 1989). This is confirmed by an investigation ona racemic mixture of PBLG and PBDG (Berghmans, 1989). Racemicmixtures are nematic under all conditions. The stress growth curves andtransient dynamic moduli after shearing revealed a similar behavior forthe nematic solution as for the single enantiomer system studied here.


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•c

•o

•A

5

FIG. S. Occurrence of the successive extrema in intermittent forward flow as a function oftrest (t = 0.4 s -I; T= 283 K, symbols as in Fig. 2).

Because of the multiplicity of features of the curves in Fig. 4, a moresystematic analysis of the transients seems indicated. Figure 5 shows thevalues of the strain, measured from the inception of flow, at which theoscillatory stress reaches extrema. The labeling of the minima and maxima is indicated in Fig. 2. The strains are plotted versus the duration oftime the sample has been at rest before resuming the shear. Figure 5demonstrates clearly that the transients depend on rest time up to periods as long as 104 s. Other peculiarities of the stress growth curves alsoshow up in Fig. 5. Up to 300 s of structural recovery of the dampedoscillatory shear stress essentially displays two oscillations beforesteady-state conditions are reached. However, if the sample is allowed torecover for a longer time, the resulting curves become much more complicated. First of all the damped pattern is preceded by a sharp initialpeak in the stress. The latter is reached after approximately 0.3 strainunits, irrespective of rest time. As a comparison, in isotropic polymericmaterials the first and only maximum is typically reached after a strainof the order 2-3. The level of the sharp peak decreases with increasingrest time. This again is unlike the behavior of isotropic polymeric systems. The decrease agrees however with the decrease of the dynamicmoduli upon cessation of flow (see further). For HPC solutions Griz-


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zutti et al. (1990) recorded an increase of the first peak with rest time.It is known that for these solutions the dynamic moduli also increaseupon cessation of flow (Moldenaers and Mewis, 1988). Hence the twophenomena might be related.

Similarly to the sharp peak, the level of the first broad maximumdecreases drastically with rest time (Fig. 4). However its position remains unchanged at 8 strain units (open circles in Fig. 5). The secondbroad peak (open squares in Figs. 2 and 5) shows a remarkably different behavior. After a rest period of about 300 s it divides into two peaks.One maximum remains at a strain of approximately 35, whereas theother gradually shifts to higher strains. The result is a transient shearstress curve with four peaks. After very long rest times the first tworeduce to a single one. The approximate wavelength over the wide rangeof rest periods is about 3Q-40 strain units. These features will be compared in the next section with those obtained in intermittent flow reversal experiments.

The dependency of the stress transient on rest time reflects a slowvariation of the flow-induced structure during rest. This result can becompared with other measures of structural evolution upon stoppingthe flow, such as stress relaxation. After shearing at 0.4 s - I, as in theprevious experiments, the shear stress initially relaxes fast: it decays to20% of its original value in 8 s. After 100 s only 2% of the stress stillpersists. Clearly the major part of the structural evolution which showsup in stress growth experiments takes place after the stress has essentially relaxed.

Measuring the linear dynamic moduli upon cessation of flow hasproven to be a more sensitive measure for structural changes than stressrelaxation (Moldenaers and Mewis, 1986; Larson and Mead, 1989).For the present sample the moduli first increase after stopping the flow,then they go through a broad maximum and finally decay to a finalvalue (Moldenaers et al., 1990a). For a shear rate of 0.4 s -1, as in thestress growth experiments discussed here, the moduli reach their maximum after 600 s and their equilibrium values after approximately10000 s. The latter value is comparable, within experimental error, withthe time after which the stress growth becomes independent of rest time.

The overall shape of the present stress transients is similar to thosereported in the literature for other liquid crystalline materials (Wissbrun, 1980; Metzner and Prilutski, 1984; Viola and Baird, 1986; Doppert and Picken, 1987; Grizzuti et al., 1990). In the present work thesharp initial peak of the stress growth curve is followed by two to threeperiods of the damped oscillation. Only one or two oscillations have


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IIIIII

e!~"tl..!!cUIII

t r= : 0 s:.·127 0: 0

334 s: •756 s: •

4920.: •

~~"'W'I""'-. ~ .1§ ~ ,

18 It: /II . ·

f 8 Iiif ! :

." p :~A"" I :~~- ..,p. I''/

~ ....~o.o 50

strain100 150

FIG. 6. Scaled shear stress for various rest times in intermittent flow reversal experiments(1'=O.4s- l ; T=283K).

been recorded for most PLCs, with the first broad maximum appearingafter 70-100 strain units. For the PBDG sample this maximum appearsafter 35 strain units. The total deformation to reach steady state ishowever of the order of 100 strain units in all cases. The rigidity of thepolybenzylglutamate molecules, compared to the relative flexibility ofhydroxypropylcellulose, aromatic copolyesters and aromatic polyamides, might explain the quantitative difference. With other materialsthe absolute value of the sharp peak as well as of the broad maximum isusually larger than the steady-state stress, whereas in the present datathey are often smaller (Fig. 4). Changing the molecular weight of thePBG or the concentration of the solutions does not drastically affect thenumber of oscillations or their period (Berghmans, 1989;Moldenaers etal., 199Oa).

Intermittent flow reversal: Effect of rest time

A selection of reduced stress growth curves recorded in intermittentflow reversal experiments at 283 K is presented in Fig. 6. The restperiods range from 0 s, where the experiment reduces to ordinary flowreversal, to 4920 s. At first glance the curves bearmany similarities withthose measured in intermittent forward flow (Fig. 4). The reduced


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1692

100

oo

MOLDENAERS, YANASE, AND MEWIS

••• <>• <>

<>• o• • • •<> •<> <> •• 0 0 0

• • 0

00 0

.. ... ... ...2 :3 4 5log trest

FIG. 7. Occurrence of the successive extrema in intermittent flow reversal as a function oft,cst (1'=0.4s- 1; T=283 K). Maxima: Is: 1st; D: 2nd; 0: 3rd. Minima: ... : 1st;.: 2nd;.: 3rd.

shear stress curves in Fig. 6 also consist of a damped oscillatory patternpreceded by a sharp initial peak. In the flow reversal experiment andafter short rest times the initial peak reduces rather to a shoulder in thecurve. The equilibrium stress in intermittent flow reversal is againreached after shearing has been applied for a hundred strain units andthe curves are also sensitive to rest time for periods of up to 104 s.

There are some very important differences between the sets of curvesrecorded in intermittent forward flow and intermittent flow reversal.This can be illustrated by plotting the strain values at which the extremaoccur for the intermittent flow reversal experiments (Fig. 7), similar tothe results for intermittent forward flow experiments presented in Fig.5. In Fig. 7 the data between brackets represent the results for instantaneous flowreversal. It is clear that the number of oscillations does notdepend on rest period in the intermittent flow reversal experiments.Apart from a sharp initial peak, two broad maxima are recordedthroughout, no splitting of the second oscillation as observed in IFF ispresent in the IFR experiments.

From the comparison of Figs. 5 and 7 several conclusions can be


09:23:25


drawn with respect to the flow-induced anisotropy. First of all, aftershort rest times, the periods of the oscillatory stress in IFF and IFR arethe same. They both amount approximately to 33 strain units. In Fig.8(a) the stress growth curves are compared for IFF and IFR after restperiods of 16 and 18 s, respectively. It is obvious that the curves areshifted by approximately 180·, thus clearly demonstrating the anisotropy induced by the previous shearing. Figure 8(b) compares the transient shear stress after 2000 s of rest for the two kinds of experiments.Although the two curves are not identical yet, they are now in phase.

From the experiments important conclusions concerning the flowinduced structure and the memory of the sample with respect to itsanisotropy can be drawn. The data in both kinds of experiments (IFFand IFR) show unambiguously that structural changes take place uponcessation of flows which require several thousands of seconds to reachtheir equilibrium conditions. Moreover, after stopping the flow the sample keeps a memory during thousands of seconds for the direction inwhich it has been sheared, after which the memory is gradually lost.

No other systematic data on intermittent flow reversal are availablein the literature to compare with the present results. Viola and Baird(1986) recorded the stress growth in IFF and IFR for a thermotropiccopolyester. They could not detect an effect of flow direction in theirexperiments at a single rest time of 6 s.

Effect of temperature

Changing the temperature can be a useful method to gain insight inthe interrelationship between various rheological parameters. It can alsohelp to clarify the physical interpretation of the data. In the presentinvestigation the intermittent forward flow as well as the intermittentflow reversal experiments at 283 K discussed above have been supplemented with data at 293 and 303 K. The shear rate was kept at 0.4 s - I

in all the experiments.The appearance of the curves at 293 and 303 K was very much like

those measured at 283 K. The effect of flow direction looked very similar to that discussed above for the IFF and IFR experiments at 283 K.The strains at which the oscillatory stress reaches its extrema in intermittent forward flow are plotted in Fig. 9 for the three temperatures(same figure as Fig. 5 at 283 K). It is obvious that no effect of temperature can be detected for the positions of the maxima and minima. Allthe phenomena reported at 283 K, including the splitting of the secondbroad maximum into two oscillations, are present in the three series of


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(a)o 50

strain100 150

lb)o 50

strain100 150

FIG. 8. Comparison of the scaled shear stress in intermittent forward dow and intermittent dow reversal experiments (t = 0.4 s -1; T= 283 K), (a) 6: frest = 16 s (IFF); .:frest = 18 s (lFR); (b) 6: trest = 2000 s (IFF); .A.: Irest = 2000 s (IFR).

experiments. Likewise no effectof temperature could be observed for thepositions of the maxima and minima in intermittent flow reversal experiments. The viscosity changes by a factor of three between 283 and


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100• II I •• .6.

....o&g0

.~ •• •" ~* *' IB0 I-fI" • .6.•c:d~&A~ ~~ 8'0

0 0...1il

• •••*- •IZU!'iI .aD IZ9 I2!Zi • •0 0

00 2 :3 4 5

log trest

FIG. 9. Effect of temperature on the occurrence of the successive extrema in intermittentforward flow as a function of Ires. (r= 0.4 s - 1); open symbols: maxima filled symbols:minima; 1J. and .: 283 K; 0 and.: 293 K; 0 and .: 303 K.

303 K. Clearly it does not noticeably affect the kinetics of the structuralchanges that are responsible for the stress growth curves.

The absence of temperature effects is not unexpected. It has also beenreported for slow transients in another solution of polybenzylglutamatesin m-cresol (PBLG: Mw = 250000, 12% concentration) (Moldenaersand Mewis, 1986; Mewis and Moldenaers, 1987b). In the latter investigations no effect of temperature on the kinetics of the dynamic moduliupon cessation of flow could be detected. It was also established that thetail of the relaxation curve was independent of temperature whereas theinitial part shifted to shorter times at higher temperatures (Moldenaersand Mewis, 1990b). The latter shift scaled with zero-shear viscosity.The fact that all the prolonged transient experiments upon cessation offlow, performed so far on polybenzylglutamates, are independent oftemperature suggests that they might be governed by the same structural changes. In addition these transients are often inversely proportional to the previous shear rate.


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Comparison with theory

The oscillating stress transients upon inception of shear flow are notadequately described by the classical theories for PLCs. On the otherhand, a damped oscillating pattern for the reduced viscosity was obtained by Manke and Williams (1989) with an internal viscosity modelfor solutions of flexible chains with a Gaussian spatial mass distribution.However, this latter case differs strongly from the physical nature ofPLCs. A suitable physical picture to describe the measured transientscould be that of anisotropic inclusions with convectiveflow effectsdominating the rotational Brownian motion (Hinch and Leal, 1973). ForPLCs, however, the complex transients also occur in the Newtonianregion.

Recently the effectsof nonhomogeneous and time-dependent directororientation have been considered. This leads to oscillating stress transients that scale with strain (Marrucci and Maffettone, 1990a; Larson,1990). Burghardt and Fuller (1990) have supplemented this by assuming an Ericksen number that saturates with increasing shear rate. Thisnumber contains a length scale. The gap size turns out not to be asuitable length scale in the Ericksen number as it is too large a value toexplain the damping of the oscillating part of the stress transient. Thedefect or domain size provides apparently a logical alternative whichcorrelates several of the available data for PBGs. The model requires theoscillating stress transients to regain their final shape after a dimensional rest time of 10--100 for PBGs, similar to what is found for recoil.The present intermittent shear flow experiments require larger valuesfor y't to reach their ultimate shape.

Larson and Doi (1991) have presented a somewhat similar theorywhich averages the results of the Doi theory (1981) over all domains,the kinetics of which are specified in a phenomenological fashion. For avanishing rest time, their model predicts a phase shift of nearly 180·between IFF and IFR experiments. This is in agreement with the experimental results. Dimensional arguments lead to a characteristic timescale for the material which equals rrd2/K (Marrucci, 1985). Thisprinciple has been used in the polydomain models by Larson and Doi(1991) and Burghardt and Fuller (1990). Based on experimental evidence, the Ericksen number reaches a constant value above a criticalshear rate. Combining these two results leads to a time scaling inverselyproportional to shear rate and independent of temperature. Earlier results (Moldenaers and Mewis, 1986, 1990a, and 199Gb) are in agreement with these predictions. The present results indicate that the global


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anisotropy obeys a similar law. There are at present no computationalresults about the decay of the flow-induced anisotropy in the literature.

It should be pointed out that the experiments deviate from the modelpredictions in a significant manner. The stress transients show an initialsharp peak, which does not fit in the oscillatory pattern. It is not predicted by the available models and has been logically associated withmolecular relaxation mechanisms (Burghardt and Fuller, 1990). Thepresent results indicate that it changes over the same time period as theother extrema. Therefore it seems to be also affected by the supermolecular structure. A second discrepancy with theory appears after restperiods of 200-300 s, when the second broad peak starts to split in twodifferent peaks, resulting in a nonperiodic transient. It becomes periodicagain when the second oscillation finallyreaches the same wavelength asthe first one. It seems as if the sample can only sustain a maximumwavelength. If this value is reached it "breaks" in two smaller parts.

CONCLUSIONS

Stress growth experiments are a sensitive method to probe the changing structure in PLCs after the flow has ceased. The flow can be restarted in the same direction as during preshearing (intermittent forward flow, IFF) or in the opposite one (intermittent flow reversal,IFR). In the case ofPLCs the resulting stress transients are different forthe two experiments, thus providing one of the very few possible measurements of the flow-induced anisotropy and its decay under rest conditions. After very short rest periods the oscillatory stress transients inIFF and IFR are nearly shifted 180·. This fits in with recent modelcalculations based on the kinetics of the full orientational distributionfunction for the director. When the rest period increases, the oscillationsbecome more pronounced. At the same time, the phase shift between thecurves for IFF and IFR gradually vanishes. The rate of change does notdepend on temperature, which can be rationalized on the basis of adimensional argument. This fits in with earlier results on slow phenomena in PLCs. The stress transients for intermittent forward flow show anonperiodic pattern after rest periods of several hundreds of seconds.There is not yet an explanation for this phenomenon.

ACKNOWLEDGMENTS

This work was partially supported by a grant from AKZO International Research, Arnhem, the Netherlands and by the Onderzoeksfonds


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ofthe K.u. Leuven. The authors wish to thank Dr. Doi and Dr. Larsonfor stimulating discussions.

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