What Is a Model Liquid Crystalline Polymer Solution? - ACS Publications

Chapter 23

Downloaded by CORNELL UNIV on August 1, 2016 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch023

What Is a Model Liquid Crystalline Polymer Solution?: Solvent Effects on the Flow Behavior of LCP Solutions 1

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S. Chidambaram , P. D. Butler , W. A. Hamilton , and M. D. Dadmun 1

Chemistry Department, University of Tennessee, Knoxville, TN 37996 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

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The flow-induced alignment of liquid crystalline solutions of poly (benzyl L-glutamate) (PBLG) in deuterated benzyl alcohol (DBA) and deuterated m-cresol (DMC) is determined using small angle neutron scattering. Surprisingly, the similar solutions show marked differences in their steady state and relaxation response to shear. During shear, the two solutions behave similarly at high shear rates (> 1 s ), however, at low shear rates the PBLG in DBA shows an increase in orientation with shear rate which is absent in DMC. Upon shear cessation, PBLG in DMC retains flow induced alignment for long times (> 6 hours) while the orientation of PBLG in DBA dissipates quickly (5-10min.). The results are particularly unexpected, as DBA and DMC are isotopes. Possible explanations for this anomalous behavior are discussed. The results exemplify the need for a more complete understanding of the important parameters that affect the flow of LCP solutions so that a more universal theory can be developed which can predict flow behavior of non-model LCP solutions. -1

Introduction Liquid Crystalline Polymers (LCPs) are of considerable interest inasmuch as their inherent molecular ordering has dramatic consequences on their macroscopic properties. LCPs are utilized in many high performance applications due to their superior strength and stiffness, excellent solvent resistance, low coefficient of thermal expansion, and low viscosity. Since molecular alignment affects the macroscopic properties and is easily achieved during flow, the rheology and relation between an applied flow field and molecular orientation of LCPs are an area of great interest. * Over twenty years ago, Kiss and Porter discovered the unique phenomenon of a 2

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Corresponding author.

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© 2000 American Chemical Society

Cebe et al.; Scattering from Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

357 negative fjrst normal stress difference (Ni) in LCPs. Other anomalies including low die swell, strong dej>£ndence of transient rheological behavior and structure on shear and thermal history, band structures on cessation of flow, and decrease in viscosities with increased shear rate at low shear rates (shear thinning) have also been observed. Subsequently, there has been considerable interest in understanding the flow of LCP in hopes of explaining the fundamental reasons for this unique behavior. Examples of experimental techniques that have examined the flow of liquid crystalline polymers include rheology, flow birefringence and scattering techniques (X-ray , light and neutron ). In particular, it has been demonstrated that birefringence, x-ray scattering and small angle neutron scattering (SANS) can be used to accurately characterize the extent of molecular ordering of an LCP, though each has it's advantages and disadvantages. Models and theories have also been proposed which account for many of the novel aspects of the rheology of LCPs. " However, a universal model that fully describes the flow characteristics of all LCPs is yet to be developed. For example, a theoretical model developed by Marrucci and coworkers has been successful in explaining the anomalous sign changes in Ni and has been extended to three dimensions by Larson and coworkers. In addition to confirming the predictions of Marrucci, Larson's model is successful in understanding the changes in the second normal stress difference, N . However, these theories do not capture all of the complexity of the flow of LCPs. One major deficiency is the inability to account for the existence of a shear thinning regime at very low shear rates, i.e. "Region I" behavior, which occurs in many LCP solutions. These theories predict a low shear rate Newtonian plateau. PBLG/m-cresol is known as a LCP model system for the study of the flow behavior of LCP solutions, primarily due to it's unusual normal stress behavior. Indeed, Burghardt recently defmed a model LCP solution as one that exhibits behavior predicted by the Doi model, among other factors. Thus, most of the experimental evidence which is utilized to illustrate the flow behavior of LCP comes from the poly(y-benzyl L-glutamate) (PBLG)/m-cresol solution. ' * * ' Though there seems to be very good correlation between theory and PBLG/m-cresol solution behavior including the appearance of a negative first normal stress difference, the flow behavior of many LCP solutions does not mimic that of PBLG/m-cresol. Some important aspects that are found in many LCP solutions but not PBLG/m-cresol is a regime at low shear rate where the solution is shear thinning at moderate concentrations, the absence of a negative Ni, and the expedient relaxation of shearinduced alignment after the removal of shear. Moreover, current theory does not allow for this behavior. Therefore, in the drive to develop a broad theoretical description of the flow behavior of LCP which can account for all of the behavior that is observed experimentally, a more specific understanding of the fundamental reasons for the differences between the flow of PBLG/m-cresol and other LCP solutions is needed. If the reasons for this variety of flow behavior can be determined, they can then be accounted for theoretically, and this will lead to a more complete theoretical description of the flow of LCP solutions. Thus, by this argument, a model LCP solutions should be one that allows the systematic study of all the factors which influence the flow of LCP solutions. Pursuant to this, small angle neutron scattering and rheology have been used to investigate the flow and alignment behavior of two very similar LCP solutions. The solutions are poly(y-benzyl L-glutamate) in deuterated m-cresol (DMC) and 6

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358 deuterated benzyl alcohol (DBA). Both m-cresol and benzyl alcohol are helicogenic solvents in which PBLG forms a cholesteric phase above approximately 12-13 wt% as determined by optical microscopy (for the molecular weights utilized in these experiments; 250,000-320,000). These solutions were chosen as the solvents are isomers, and therefore, it is not expected that there will be significantly different polymer/solvent interactions between the two solutions, yet it is known that they exhibit different alignment behavior. It is hoped that the results of this systematic examination will provide an explanation for the difference in flow behavior that is observed in these two similar solutions. This, in turn, may provide guidance regarding the choice of important parameters which influence the flow of LCP and must be accounted for in a universal theory. 12,13


Experimental PBLG was purchased from Sigma Chemical Company, stored in afreezer,and dried in vacuum prior to use. The deuterated benzyl alcohol (DBA) was bought from MSD Isotopes while deuterated m-cresol (DMC) was obtained from CDN Isotopes. A 20 wt% solution of PBLG (MW-288000) in DBA and an 18 wt% solution of PBLG (MW-318000) in DMC were used, well into the cholesteric phase in both solvents. The solutions were left to equilibrate for at least a fortnight, often longer, above the gel temperature before being used in the experiments. The rheology experiments were performed on a Bohlin VOR (dynamic modulus and viscosity measurements) and a Rheometrics Dynamic Stress rheometer (viscosity measurements) using cone and plate fixtures with 2.5° cone. To insure that the flow transient had been surpassed, the sample was pre-sheared for 200 strain units prior to collecting steady viscosity data, or prior to flow cessation in those experiments where evolution of the moduli was monitored. The sample chamber was sealed to prevent solvent loss at long times. The dynamic modulus was obtained using a 1.3% strain at 1 Hz. All SANS experiments were conducted at the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Oak Ridge, TN. In-situ shear experiments were performed on samples contained in a specially designed couette cell with provision for sample heating. The inner component of the couette cell (stator) is temperature controlled. The cup that holds the sample (rotor) is not directly heated but was kept enclosed within a heating jacket to minimize thermal gradients across the sample. With this apparatus, the temperature of the sample was maintained constant throughout the experiment with a precision of ±1°C. The wavelength of neutrons used was 4.75À and the sample to detector distance was 2.008m. A l l solutions were sheared for 300 strain units before a scattering pattern was recorded to ensure that the transients did not influence the data. The experimental setup is such that the neutron beam is incident perpendicular to the flow of the solution and along the shear gradient direction. The scattered pattern is thus recorded in the flowvorticity plane. Steady state scattering experiments were performed at different shear rates. At each shear rate, scattering patterns were also recorded after cessation of flow at different time intervals to study the relaxation of the flow induced alignment. Temperature effects in a narrow temperature window (60 - 75 °C) were investigated


359 in the case of PBLG/DBA, however all experiments on PBLG/DMC were performed at room temperature. These temperatures were chosen as they represent very similar thermodynamic temperatures. PBLG/DBA forms a gel at ca. 45 °C while the gel temperature for the PBLG/m-cresol solution is approximately -10 °C (as determined gravimetrically). Thus, the conditions for these experiments are approximately 30 ° above the gel temperature. The collected scattering patterns were corrected for sample transmission, detector sensitivity, scattering due to the cell, and background and normalized to absolute units. The corrected data were then convertedfrom2D patterns to ID plots of I(q, φ) cm" vs. φ, where φ is the azimuthal angle around the detector. A quantitative measure of the degree of ordering was obtained by calculating the alignment factor, Af(q), as proposed by Walker et al. 1

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J Kq, Φ)άφ ο In the analysis by Walker, it was shown that Af(q) asymptotes to a constant value at q values greater than 0.07 A' . They denote this asymptotic value as the macroscopic alignment factor A°° which is related to the effective order parameter S by the relation - A f = S . In the current analysis, the intensity was averaged over q = 0.06-0.12 A" , not the whole q range. Therefore, the effective alignment factor reported for these experiments is not a measure of the order parameter of the systems, but is a measure of the orientation of the sample. However, as the relative changes in alignment that occur due to shear rates, temperature, and solvent are of interest, this approximation is justified and does not influence the interpretation of the data. 1

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Results Steady State The changes in alignment factor, and therefore molecular orientation, as a function of shear rate for PBLG/DBA (70 °C) and PBLG/DMC (room temperature) are shown in Figure 1. These results clearly demonstrate that there is a marked difference in the response of the two solutions to an applied shear flow. PBLG/DBA exhibits three distinct regions. At low shear rates, the alignment increases with shear rate until it reaches a critical shear rate after which it remains constant and then increases with shear rate again at higher shear rates. This behavior has been observed previously. On the other hand, PBLG/DMC exhibits two regions similar to the high shear rate behavior of PBLG/DBA , with the absence of the increase in alignment at low shear rates that is seen in PBLG/DBA. At low and intermediate shear rates (below 10.0 s") the solution shows substantial orientation, which is independent of 12

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Figure 1. Shear rate dependence of the steady state molecular alignment of PBLG in deuterated benzyl alcohol at 70 °C and deuterated m-cresol at room temperature.


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shear rate. At shear rates above 10.0 s* the alignment increases with shear rate, as in the PBLG/DBA solution. This also is in agreement with previous results. Upon examination of this data, a first thought is that the shear rates that were examined for the PBLG/m-cresol sample were not low enough to observe this increase in alignment at low shear rate, but that it does exist. To estimate where this behavior should be observed, the data of PBLG/m-cresol can be shifted to lower shear rates so that the transitionfromNewtonian to shear thinning behavior overlaps that of PBLG/DBA. Completion of this shifting shows that if PBLG/m-cresol were to exhibit behavior similar to PBLG/DBA, it should feature an increase in alignment with shear rate at shear rates below 0.1 s" . The lowest two shear rates in Figure 1 are below this limit, implying that this regime does not exist in PBLG/m-cresol. Additionally, previous results which examined the shear rate dependence of the alignment of PBLG in m-cresol show that the alignment does not decrease with a decrease in shear rate down to 0.01 s". More importantly, however, is that the current molecular theories " which describe the flow of LCP solutions do not predict that this behavior should occur. It is interesting to note that the extent of alignment is larger in m-cresol than in benzyl alcohol for all shear rates, suggesting an easier path to alignment for the tri cresol solution. Superficially, this is counter intuitive, as one would expect the sample that it is at higher temperature (PBLG/DBA) to align more readily than the one at lower temperature (PBLG/m-cresol). This indicates that the observed differences are due to effects which are subtler than a mere temperature shift. 9,13

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Correlation of Steady State Alignment to Viscosity The viscosity as a function of shear rate for PBLG/DMC at room temperature and PBLG/DBA at 70 °C, the conditions of the scattering experiments, are shown in Figure 2. The difference in the magnitude of viscosity of the two solutions can be accounted for by temperature effects on the viscosity of the solvents and is therefore not related to the relative alignment of the two samples. In other words, if the viscosity of PBLG/DBA at 25 °C is estimated by adjusting the viscosity of the solvent with an Arrhenius factor, the viscosity of the solution is shifted up to he dotted line showing qualitatively similar behavior to the PBLG/m-cresol data. It is interesting that the shear dependence of the viscosity of the two solutions is similar, though they exhibit different alignments at low shear rates. In particular, it is curious that the increased molecular alignment with shear rate that PBLG/DBA exhibits at low shear rate does not correlate to shear thinning behavior in the viscosity. Thus, the qualitative difference in molecular alignment that is observed by scattering does not manifest itself as different macroscopic (viscosity) behavior. This will be considered further in the discussion section.

Orientation Relaxation The change in molecular alignment at different time intervals after cessation of shear is presented in Figure 3 for the PBLG/DMC solution and Figure 4 for PBLG/DBA at 75 °C respectively. The data obtained at 60 °C and 65 °C are very



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Figure 2. Viscosity behavior of PBLG/DMC and PBLG/DBA under the same conditions as Figure 1.

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Figure 3. Relaxation of theflowinduced alignment in the PBLG/DMC solution after removal of shear for four shear rates.



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Time after removal of shear (min) Figure 4. The relaxation ofshear induced alignment in PBLG/DBA at 75 °C.


364 similar to those at 75 °C, and thus there is no temperature dependence of this behavior in this temperature window. These results show that there is a notable difference in the relaxation behavior of the two solutions. PBLG/DMC solutions possess significant orientation even after long relaxation times (~ 6 hours). The relaxation behavior for all steady state shear rates depicts the same trend; an initial increase in orientation followed by a slight decrease and a subsequent increase to a steady value. The post-shear alignment also scales with previously applied shear rate and exhibits a minimum in alignment at the same post shear reduced time (« 500). The degree of alignment that occurs after 6 hours of relaxation time also increases with previously applied shear rate. In contrast, the alignment in PBLG/DBA solutions is not as long lived as that of PBLG/DMC. A l l experiments show the same qualitative behavior; an initial increase in orientation that lasts for about 5-10 minutes and then a relatively sharp decrease that asymptotes to a small positive alignment. The orientation of the solutions sheared at 0.1 s' show a final alignment that is almost zero, suggesting no residual alignment. Additionally, during the relaxation following all shear rates except 0.1 s", there exists a small peak in orientation at long times. This response has not been observed in the case of PBLG/DMC or in other SANS studies involving PBLG/deuterated dimethyl formamide. It must also be noted that the relaxation kinetics do not scale with previous shear rate as has been observed in PBLG/MC solutions. '


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Dynamic Modulus after Shear Cessation To correlate the observed alignment relaxation behavior of the solutions to a macroscopic Theological property, the evolution of the dynamic modulus after shear cessation of the PBLG/DBA solutions were performed under similar shear rates as the neutron scattering experiment, though the temperature was limited to 65 °C. The results of the mechanical measurements at two shear rates have been plotted with the corresponding alignment factors (SANS data) in Figure 5 to show the correspondence between the changes in the two parameters with time. Logically, the dynamic modulus decreases with an increase in molecular orientation as there is a decrease in resistance to strain with increasing alignment. This correlation is found to be true in the case of PBLG/DBA solutions with a decrease in moduli corresponding very well to the increase in molecular alignment during relaxation. Similarly, the decrease in alignment at relaxation times greater than 10 minutes is found to agree well with the increase in modulus on the same time scale. Similar experiments on the PBLG/mcresol solution show good correspondence between the dynamic moduli and the evolving alignment structure of the solution after shear cessation. 23

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Discussion Clearly, these results show that the flow behavior of PBLG in deuterated tri cresol shows remarkably different behavior than a similar solution in deuterated benzyl alcohol. This is particularly surprising given the similarity in the chemical structure of the two solvents; they are isomers. For the steady state results, the



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Figure 5. The evolution of the dynamic modulus of PBLG in DBA after theflowfield has been removed as well as the corresponding molecular alignment for previous shear rates of 1.0 s' and 10 s' . 1

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366 alignment behavior differs for these two solutions at low shear rates. This is the region where the current understanding of the flow behavior of LCP solutions suggests that the defect structure of the solution is very important in defining the response of the solution to shear flow. " Therefore, the difference that is observed in the low shear rate regime may be a manifest of texture variations (i.e. the structure of a defect in the DBA solution may be inherently different than the structure of a defect in the m-cresol solution). However, the fact that there is no difference in the viscosity behavior of the two solutions is puzzling as one would expect that texture differences would manifest themselves here also. Interestingly, in liquid crystalline hydroxypropylcellulose/D20 solutions, an increase in alignment with shear rate is exhibited at low shear rates, correlating very well to region I shear thinning viscosity behavior. This suggests that region I shear thinning in some LCP solutions does correlate to molecular alignment. These results demonstrate that the correlation between viscosity and low shear rate molecular alignment is complex. In particular, it must be emphasized that the viscosity behavior of an LCP solution at low shear rates does not correlate directly to, nor can be utilized to predict, the molecular alignment. At higher shear rates, the correlation between molecular orientation and viscosity appears to be much more robust. The relaxation of the alignment between the two solutions also differs. The alignment of the DBA solution relaxes back to a near isotropic state while the mcresol solution retains the shear induced alignment for very long times. However, the dynamic modulus and alignment responses agree very well for both solutions, with a decrease in alignment correlating very well to an increase in the dynamic modulus. This suggests that the dynamic modulus can be directly correlated to the molecular alignment of LCP solutions for the shear rates studied here. But what is the origin of the differences in the response of the two solutions to an applied shear flow? Thefirstthought is to ascribe the alignment differences to mere temperature effects. As the temperature is increased, the Frank Elastic constants of the solution texture will substantially decrease. This, in turn, should result in improved alignment of the sample that is at a higher temperature (PBLG/DBA) over that of the sample that is at a lower temperature (PBLG/m-cresol). This is exactly the opposite of the observed experimental results; the PBLG/DBA is aligned to a lesser extent in the steady state than PBLG/m-cresol. Additionally, as mentioned in the experimental section, the differences in the magnitude of the viscosity of the two samples can be accounted for with temperature effects on the solvent. Finally, the two temperatures that are utilized in the experiments are thermodynamically similar. The combination of all of these factors leads to the conclusion that the alignment effects observed in these experiments are due to more subtle factors than a change in temperature. Three other possibilities are also being examined. First, the PBLG molecule may be more stiff (rod-like) in m-cresol than in benzyl alcohol. If the PBLG molecule is more (semi)flexible in benzyl alcohol, it should be more difficult to align and this could account for the shear rate dependent alignment at low shear rates. Similarly, after the shear field is removed, the fluctuations in the rigidity of the polymer chain in benzyl alcohol could serve as the impetus for the relaxation of the shear induced alignment. Alternatively, the less rigid molecule could merely be more difficult to align and this would account for both the low shear rate and relaxation behavior of PBLG in benzyl alcohol. 25

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367 Second, the differences that are seen in the flow behavior may be due to phase behavior differences between the two samples. Though the temperature of the two sets of experiments is approximately 30° C above the gelation temperature of the two solutions, it is possible that PBLG in DBA may not be as molecularly dispersed as in the PBLG/m-cresol solution. If this is true, it may be that the low shear rate alignment changes are a result of the shear field breaking up aggregates. Similarly, the relaxation back to an isotropic alignment state could be the re-aggregation of aligned molecules after the shear field is removed. It is possible to test both of these predictions as both the polymer flexibility and aggregation state dramatically affect the phase diagram of LCP solutions. Liquid crystalline polymers exhibit a phase behavior in solution that can be approximated by Figure 6. At high temperature and low concentration, the solution is isotropic. As the concentration is increased, the solution passes through a narrow biphasic region (known as the chimney) to form a liquid crystalline phase. The concentration where the biphasic region is entered (isotropic -> biphasic) is often denoted as C* and where it is exited (biphasic -> liquid crystalline) is often denoted C**. As the temperature is lowered from the isotropic or liquid crystalline phase, the solution enters into a broad biphasic regime. As the flexibility of the polymer chain increases, the chimney region shifts to higher concentrations. In the case of aggregation, if it occurs side-by-side, aggregation will result in a decrease in the aspect ratio while end-to-end aggregation results in an increase in the aspect ratio. In either case, the position of the chimney region (i.e. C* and C**) will depend intimately on the aspect ratio of chain. Thus, if PBLG in BA is more flexible or aggregated side-by-side, then the chimney region of the phase diagram should be shifted dramatically to higher concentrations from that of the PBLG/m-cresol system. Alternatively, if PBLG in BA is aggregated end-to-end, then the chimney region of the phase diagram should be measurably shifted to lower concentrations from that of the PBLG/m-cresol system Early calculations by Flory have quantified how flexibility and aspect ratio alters this phase diagram. For this study, the most important change in the phase diagram is that the chimney region shifts in a fairly dramatic fashion. Thus, Flory's original equations were utilized to calculate the phase diagram of a semiflexible polymer in solution near the chimney region. From this calculation, C* and C** as a function of polymer flexibility and aspect ratio were determined. The calculated phase diagrams for three rigidities are shown in Figure 7. To evaluate the data in this study, plots which quantify how C* and C** changes with flexibility and aspect ratio were created. This effect is shown in Figure 8 which plots C*(/)/C*(rod) and C**(/)/C**(rod) as a function of/ C*(rod) and C**(rod) are the critical concentrations for a rigid, rodlike molecule, while C*(f) and C**(/) are similar concentrations for a semiflexible polymer, and/is a measure of the flexibility of a chain. One way to view/is as afractionof bonds along the polymer chain that are flexible. Figure 8 shows that the chimney region shifts to measurably higher concentrations with a very small change in flexibility. Quantitatively, by increasing the amount of flexibility by only 1%, the chimney region shifts up in concentration by a factor of 1.5. Thus, if PBLG in benzyl alcohol is only 1% more flexible than in mcresol, (from the data below) the C* of PBLG/BA should occur at ca. 14-15%. Similar calculations were completed to demonstrate the affect of aggregation or aspect ratio on the chimney region of the phase diagram. Figure 9 shows quantitatively how altering the aspect ratio of the polymer chain alters the chimney 29

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Figure 6. Diagrammatic representation of the phase diagram of a rodlike liquid crystalline polymer in solution.

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370 region of the phase diagram. Again, a slight change in the aspect ratio will result in a dramatic shift in the chimney region in either direction. Thus, these data show that if PBLG is slightly more flexible in DBA than in mcresol or aggregated end-to-end or aggregated side-to-side in DBA, for the conditions of the scattering experiments, the chimney region of PBLG/DBA should occur at a much different concentration regime than in m-cresol. To test this assumption, the chimney region of the phase diagram of 20 wt.% PBLG (MW = 287,000) in benzyl alcohol and in m-cresol were determined by polarized optical microscopy. For this study, both BA and m-cresol were vacuum distilled immediately prior to sample preparation to minimize water contamination. Poiydispersity effects on the chimney region were eliminated in this analysis by using PBLG that came from the same batch. Figure 10 shows the resultant plot of the phase behavior of these two solutions. For PBLG in BA, C* « 8-9 %; C** « 13-14% while for PBLG in m-cresol, C* « 9-10 %; C** « 12-13%. This shows that the chimney is not shifted relative to PBLG/m-cresol, therefore, PBLG in BA can not be more flexible nor less molecularly dispersed than in m-cresol. Therefore, it is highly unlikely that the anomalous alignment behavior described above is due to either of these two factors. It should be emphasized here that these experiments do not quantify flexibility and thus do not unequivocally eliminate polymer flexibility as a cause for the observed behavior. To more fully quantify the role of polymer flexibility on the results of this study, neutron scattering experiments are underway to quantify the persistence length of PBLG in these two solvents. The last possible explanation is that there may be an inherent solvent dependence on the structure of the solutions defects or disclinations. It may be that the defects in PBLG/DBA resist alignment or annihilation more than those in m-cresol. If this is the case, one could envision a process where the * stronger texture of DBA limits the extent of molecular alignment at low shear rate. In other words, the defects are more long lived, thus the LCP in the immediate vicinity of the defect is not aligned by the shear flow at low shear rates. This would explain the shear rate dependent alignment that is observed in the PBLG/DBA solution. A 'stronger' texture would also explain the alignment relaxation behavior, as surviving defects could act as nucleation points for post shear isotropization. The stronger texture should manifest itself as increase in the Frank elastic constants of PBLG/BA over those of PBLG/m-cresol even at the increased temperature. Qualitative differences between the defect structure of PBLG/BA and PBLG/m-cresol during and after shear are indeed observed by in-situ light scattering. To verify and quantify this effect, the correlation between defect structure and molecular alignment for these two structures is currently under investigation using in-situ shear small angle light scattering and polarized optical microscopy. One additional possibility to explain these results must also be mentioned. Recently, Burghardt et. al. have suggested that differences between the alignment behavior of HPC and PBLG may be due to a 'stronger' tendency to form the cholesteric phase in HPC. This cholesteric structure limits the ability of the solution to align and remain aligned after the shear field has been removed. The completion of similar experiments to those described in this publication with a nematic PBG solution (formed from equimolar amounts of PBLG and PBDG) would provide substantial evidence regarding the importance of the cholesteric nature of PBLG in BA on its alignment behavior. 5

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It is well known that many parameters can affect the flow of liquid crystalline polymers such as polymer rigidity, polymer-solvent interactions, and polymer concentration. The results presented in this article demonstrate that there exist other, less obvious, parameters which also can dramatically affect the alignment of LCP under shear flow. The current understanding of the reported results suggest that the defect structure of an LCP solution is one such parameter. Finally, this article documents the need for a more thorough understanding of all of the molecular parameters which influence the flow behavior of liquid crystalline polymers. A universal theory which describes the flow behavior of all LCP will only be developed when all characteristics are defined and accounted for which impact the rheology of LCP solutions. Moreover, a 'model' liquid crystalline polymer solution should be one in which these factors can be systematically studied.

Conclusion The orientation of PBLG induced by flow in two solvents, deuterated benzyl alcohol (DBA) and deuterated m-cresol (DMC), has been studied by small angle neutron scattering. Unexpectedly, results show marked differences in the steady state and relaxation responses of the solutions to shear. Steady state results show that the solutions behave similarly at high shear rates (> 1 s") with an initial nonlinear region followed by a linear increase in alignment. However, at low shear rates the PBLG in DBA shows an increase in orientation with shear rate which is absent in DMC. Relaxation results further contrast the difference in ordering of the two solutions. The PBLG/DMC sample retains the flow induced ordering of molecules even after long periods of time («6 hours). Conversely, the ordering of PBLG in DBA by shear decays to very low orientation rather quickly. The results also show that the correlation between molecular alignment and rheological parameters is not trivial. The shear rate dependence of the viscosity is not readily related to the molecular orientation particularly at low shear rates, however, the relation between molecular alignment and dynamic modulus is more robust. These differences are surprising as benzyl alcohol and m-cresol are isomers. Clearly, there exist fundamental distinctions that account for their diverse behavior. A difference in the defect structure of PBLG in BA versus PBLG in m-cresol may prove to be the underlying cause of this alignment behavior. Regardless, the results exemplify the need for a broader choice of 'model' liquid crystalline polymer solutions when examining the flow-induced structure in liquid crystalline polymer solutions. Moreover, a more complete understanding of the important parameters that affect the flow of LCP solutions is needed so that a more universal theory can be developed which can predict flow behavior of non-model LCP solutions. 1

Acknowledgment We would like to thank Wes Burghardt for obtaining the dynamic modulus data and for useful discussions. The Division of Materials Sciences, U. S. Department of Energy, supports the research at Oak Ridge under contract No. DE-AC05960R22464 with Lockheed Martin Energy Research Corp.


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References 1. National Material Advisory Board Liquid Crystalline Polymers National Academy Press 1990. 2. Kiss, G.; Porter, R. S. J. Polym. Sci. Symp. 1978 65, 193. 3. Jerman, R.E.; Baird, D.G. J. Rheol. 1981, 25, 275. 4. Mewis, J.; Moldenaers, P. Mol. Cryst. Liq. Cryst. 1987, 153, 291. 5. Grizzuti, N.; Cavella, S.; Cicarelli, P. J. Rheol. 1990, 34, 1293. 6. Navard, P. J. Polym. Sci., Polym. Phys. Ed. 1986, 24, 435. 7. Ernst, B..; Navard, P. Macromolecules 1989, 22, 1419. 8. Kiss, G.; Porter, R. S. Mol. Cryst. Liq. Cryst. 1980b 60, 267. 9. Hongladarom, K.; Burghardt, W. R. Macromolecules 1993, 26, 772. 10. Picken, S. J.; Aerts, J.; Visser, R.; Northolt, M . G. Macromolecules 1990, 23, 3849. 11. Ernst, B.; Navard, P.; Hashimoto, T.; Takebe, T. Macromolecules 1990, 23, 1370. 12. Dadmun, M . D.; Han, C. C. Macromolecules 1994, 27, 7522. 13. Hongladarom, K.; Ugaz, V. M.; Cinader, D. K.; Burghardt, W. R.; Quintana, J. P.; Hsiao, B. S.; Dadmun, M. D.; Hamilton, W. Α.; Butler, P. D. Macromolecules 1996, 29, 5346. 14. Marrucci, G.; Maffettone, P. L. Macromolecules 1989, 22, 4076. 15. Onogi, S.; Asada, T. inRheology;Astarita, G.; Marrucci, G.; Nicolais, L.; Eds.; Plenum: New York 1980, 3, 947. 16. Larson, R. Macromolecules 1990, 23, 3983. 17. Larson, R. G.; Mead, D. W. J. Rheol. 1989. 33. 1251. 18. Larson, R. G.; Prominslow, J.; Baek, S. -G.; Magda, J. J. Ordering in Macromolecular Systems; Springer-Verlag: Berlin 1994, p. 191. 19. Walker, L. M.; Kernick, W. A. III.; Wagner, N . J. Macromolecules 1997, 30, 508. 20. Walker, L. M.; Wagner, N. J. Macromolecules 1996, 29, 2298. 21. Burghardt, W.; Bedford, B.; Hongladarom, K.; Mahoney, M . in Flow Induced Structures in Polymers; Nakatani, A.I., Dadmun, M.D., Eds., ACS Symposium Series 597, American Chemical Society: Washington, D.C., 1995. 22. Moldanaers, P.; Mewis, J. J. Rheol. 1986, 30, 567. 23. Larson, R.G.; Mead, D.W. J. Rheol. 1989, 33, 1251. 24. DeGennes, P.G.; Prost, J.The Physics of Liquid Crystals; Oxford Science Publications: Oxford, 1993, p. 103. 25. Walker, L.M.; Wagner, N.J. J. Rheol. 1994, 38, 1525 and references therein. 26. Marrucci, G. in Proc. IX International Conference on Rheology; Mena, Β., Garcia-Rejon, Α., Rangle-Nafaile, C. Eds.; Universidad Nacional Autonoma de Mexico: 1984; Vol. 1. 27. Larson, R.G. Rheol. Acta 1996, 35, 150. 28. Dadmun, M.D. in Flow Induced Structures in Polymers; Nakatani, A.I., Dadmun, M.D., Eds., ACS Symposium Series 597, American Chemical Society: Washington, D.C., 1995. 29. Flory, P.J. Proc. Roy. Soc. (London) 1956, A234, 60. 30. Hongladarom, K.; Secakusuma, V.; Burghardt, W. J. Rheol. 1994, 38, 1505.


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