Article pubs.acs.org/Macromolecules
Novel Behavior in Shear Flow Orientation of Side-Chain Polymethacrylate Nematic Liquid Crystals Masatoshi Tokita,* Aya Ikoma, Toshinari Ishii, Sungmin Kang, and Junji Watanabe Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Tatsuro Matsuoka Department of Molecular Design and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ABSTRACT: Shear flow orientation of nematic liquid crystals (LCs) has been investigated for two side-chain polymethacrylates having the same methoxyazobenzene mesogens but different spacers of either four (PM4MA) or six (PM6MA) methylene groups. These two polymer nematic LCs exhibited orientation behaviors distinct from each other. PM4MA arranged the nematic director in the velocity direction (the b-orientation), whereas PM6MA pointed the director to the neutral (the a-orientation) or velocity gradient (the c-orientation) directions depending on both shear rate and molar mass. Decreasing shear rate or increasing molar mass gave rise to an orientation transition from a to c. Such anomalous nematic orientation behavior is related to the presence of fluctuating smectic clusters, and explainable if fluctuations, which couple to shear flow to induce the a-orientation, have a lifetime which decreases with increasing backbone chain length so as to be sufficiently short to be averaged out under shear flow.
1. INTRODUCTION Nematic liquid crystals (NLCs) are complex fluids that exhibit variety of orientation under shear flow.1 Three types of orientation can be assumed when the nematic director lies simply along the three principal directions of shear flow as shown in Figure 1. The a-, b-, and c-orientations refer to the
NLCs promotes macroscopic orientation, that of the nonflowaligning NLCs shows transient rheological responses such as damped time-dependent oscillations in the first normal stress difference and the shear stress, and results in a polydomain structure. X-ray scattering studies have shown that some of the “non-flow-aligning” NLCs have pretransitional smectic clusters and arrange the director direction preferentially along the v × ∇v direction to adopt the a-orientation. Such variety of shear flow behavior has been reported not only for monomeric nematogens4 but also for polymeric nematics whose mesogens are attached to polysiloxane backbones via alkyl spacers.5,6 The polysiloxane with mesogens connected four methylene spacers formed a nematic LC, which adopted the b orientation under shear flow. In contrast, the polysiloxane with six methylene spacers formed nematic and smectic LCs. The nematic LC exhibited transient rheological responses and adopted the a orientation under shear flow. In this paper, we report the orientation behavior of two types of side-chain polymethacrylate NLCs under shear flow. The polymethacrylates have methoxyazobenzene mesogens connected to a backbone by methylene spacers and are designated as PMnMA where n = 4 or 6 is the number of methylene units in the spacer.
Figure 1. Three simple orientations for the nematic director under shear flow.
nematic director pointing along the neutral (v × ∇v), velocity (v), and velocity gradient (∇v) directions, respectively. According to the Ericksen−Leslie theory of nematodynamics,2,3 NLCs are divided into two categories on the shear flow orientation behavior. The first category consists of “flowaligning” NLCs which can adopt the b-orientation, while the second consists of “non-flow-aligning” NLCs in which the hydrodynamic torque causes the director to rotate indefinitely in the v−∇v plane. While shear flowing of the flow-aligning © 2012 American Chemical Society
Received: April 3, 2012 Revised: May 15, 2012 Published: May 21, 2012 4857
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Shear alignments were conducted with a rheometer (UBM Rheosol G3000) using a cone-and-plate fixture (25 mm diameter and 5.642°) within its torque range (∼2 kg cm). Prior to each shear alignment, to erase any prior thermal/mechanical history, the sample was heated to the isotropic phase (150 °C), held there for 5 min, and then cooled to the nematic phase at a predetermined temperature, resulting in random polydomain. The shear flow period was typically 1−2 h within which the stress decreases and converges to a constant value. After the shar flow was stopped, the sample was cooled to room temperature and then removed from the rheometer fixture by putting it into liquid nitrogen. Orientation of the nematic LC was determined by 2-D wideangle X-ray diffraction (WAXD) patterns recorded on a flat imaging plate by irradiating X-ray beam (Cu Kα, Rigaku UltraX18) along three characteristic directions, the velocity gradient (∇v), velocity (v), and neutral (v × ∇v) directions.9−12
As in the nematic polysiloxanes, PM4MA and PM6MA exhibited phase transitions depending on n. PM4MA formed only a nematic phase, whereas PM6MA formed a nematic and smectic phases in order of decreasing temperature. Shear flowing of the NLCs was conducted at rates ranging from 10−3 to 10 s−1. The PM4MA nematic LC adopted the b-orientation similarly to the corresponding polysiloxane. Interestingly, the type of orientation achieved by shear flowing is dependent on the molecular weight of polymer (Mn). While the NLC of PM6MA with Mn lower than 13 200 adopted the a-orientation over the whole shear range, the NLCs of the polymers with Mn larger than 29 400 exhibited the orientation transition from a to c on decreasing the shear rate. The polymers with intermediate Mn showed an a-nonorientation transition. Thus, the nematic orientation achieved at low shear rate alters from a to c with increasing Mn. The origin of such peculiar orientation behaviors of PM6MA nematic will be discussed.
3. RESULTS 3.1. X-ray Diffraction Pattern. PM6MA formed a welloriented smectic LC by cooling the isotropic melt at a rate of 1 °C min−1 to ambient temperature at a magnetic field of 3 T. However, the orientation was disturbed by heating the sample to a nematic temperature without applying a field, with the extent of the disturbance dependent on Mn. Figure 2 shows the diffraction pattern of PM6MA-132, PM6MA-242, and PM6MA-294 in the smectic and nematic phases. The diffraction patterns of these three polymers in the smectic phase are similar to each other. All the patterns include two spots on the meridian and the crescent halo concentrated on the equator, showing that the smectic phase was well aligned
2. EXPERIMENTAL SECTION 2.1. Materials. PM4MA and PM6MA were prepared by atomtransfer radical polymerization (ATRP).7−11 The number and weightaverage molecular weights (Mn and Mw) of the polymers were determined by gel permeation chromatography using polystyrene standards. Each polymer is designated as PMnMA-x where x is equal to the whole-number part of Mn/100. PM4MA formed isotropic liquid and nematic phases on decreasing temperature, whereas PM6MA formed a smectic A1 phase on cooling from a nematic phase. Both of the polymers did not crystallize and the
Table 1. Characterization of Polymers sample
Mn/ 103
Mw/ 103
Mw/ Mn
Tg/ °C
PM4MA-182
18.2
24.8
1.36
75.0
PM4MA-333
33.3
44.0
1.32
76.5
PM4MA-446
44.6
78.1
1.75
75.9
PM6MA-68
6.8
8.9
1.31
69.0
PM6MA-132
13.2
15.8
1.20
77.8
PM6MA-242
24.2
32.4
1.34
76.9
PM6MA-294
29.4
51.5
1.75
73.7
PM6MA-411
41.1
62.1
1.51
76.4
TSN/°Ca ΔHSN/kJ mol−1b
TNI/°Cc ΔHNI/kJ mol−1d
85.3 3.00 91.7 3.65 95.7 2.20 92.4 1.22 93.8 1.53
138.0 2.83 142.8 2.75 140.1 1.07 125.6 2.35 131.5 2.70 136.1 2.36 134.0 3.28 134.3 3.01
a
Smectic−nematic transition temperature. bEnthalpy change on smectic−nematic transition. cNematic−isotropic transition temperature. dEnthalpy change on nematic−isotropic transition temperature.
LC phases vitrify. Table 1 provides the temperatures and enthalpy changes on these transitions which were determined by differential scanning calorimetry (DSC) thermograms (Perkin−Elmer Pyris1 DSC) measured at a heating rate of 10 °C min−1. 2.2. Methods. X-ray diffraction measurements were carried out using synchrotron radiation at the 4C2 Beamline of the Pohang Light Source, Korea. The diffraction patterns were recorded using a Princeton two-dimensional charge-coupled device (CCD) camera. The sample temperature was controlled by a Mettler FP90/82HT hotstage with an accuracy of ±0.1 °C.
Figure 2. X-ray diffraction pattern of (a) PM6MA-132, (b) PM6MA242, and (c) PM6MA-411 at ambient temperature. The sample was cooled from the isotropic phase at a rate of 1 °C min−1 with an applyed magnetic field of 3 T. The magnetic field direction is vertical. (d−f) The diffraction pattern measured at a nematic temperature of 100 °C for the sample whose diffraction pattern at ambient temperature is shown in the left side. 4858
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with the layer normal parallel to the magnetic field direction. Other diffuse scatterings on and off the meridian have been attributed to short-range orders.13 The streaks on the meridian show d-spacings equal to the integral submultiples of the smectic layer spacing and suggest that the mesogens tend to align themselves in rows over a short-range. The streaks off the meridian have been explained by transverse modulation of the smectic layers with a short correlation length as large as 50 Å and labeled (102). On increasing temperature to the nematic phase, the smectic layer reflection disappears and the crescent halo spreads azimuthally, indicating that the long-range layer ordering of mesogens disappears and the orientational order decreases. It is noteworthy that with increasing Mn the crescent halo spreads more widely. The (102) streaks in the smectic phase become concentrated on the meridian correspondingly without changing the d-spacing. Similar treatment for PM4MA yielded a highly oriented glassy nematic LC which displays a diffraction pattern typical of a side-chain polymer nematic LC. The pattern is shown in Figure 3 and includes two outer diffuse crescents on the
Figure 3. X-ray diffraction pattern of PM4MA-333 measured at ambient temperature. The sample was cooled from the isotropic phase at a rate of 1 °C min−1 with an applied magnetic field of 3 T.
equator and some streaks on the meridian. Heating the sample to a temperature higher than Tg expanded the crescents in the azimuthal direction to some extent, but the degree of spread depended on temperature rather than Mn. 3.2. Shear Flow Orientations of Nematic LCs. The nematic phases of PM4MA and PM6MA differ in their orientations under shear flow, although these polymers have a very similar chemical structure. The nematic LC of PM4MA arranged the director in the v direction as expected from hydrodynamics, whereas that of PM6MA arranged the director in either the v × ∇v or ∇v direction depending on the shear rate and Mn. Shear flow aligned the nematic director of PM4MA in the v direction in the entire investigated range of shear rate, temperature, and Mn. Typical X-ray diffraction patterns are shown in Figure 4. The crescent-shaped halos are concentrated on the axes perpendicular to the v direction on the v−v × ∇v and v−∇v planes, whereas the halo is ring-shaped on the ∇v−v × ∇v plane. These three diffraction patterns indicate that the shear flow aligned the nematic director parallel to the v direction. This nematic orientation is the so-called the borientation which has been usually observed for nematic LCs. In contrast, PM6MA subjected to shear flow adopted two other nematic orientations. One is the a-orientation in which
Figure 4. 2D-WAXD results for PM4MA-182 sheared for 2 h at 130 °C with shear rate of 1.06 × 10−2 s−1. Diffraction patterns were measured by irradiating an X-ray beam along three orthogonal directions: velocity (v), velocity gradient (∇v), and neutral (v × ∇v).
the nematic director lies along the v × ∇v direction. Figure 5 shows the X-ray patterns measured for PM6MA-132 sheared at 5 s−1 and 120 °C. A pair of sharp arcs attributed to smectic layers is visible on the v × ∇v axis because the nematic phase transformed into the smectic phase on cooling after cessation of shearing. The crescent-shaped halos are concentrated on the axes perpendicular to the v × ∇v direction on the v−v × ∇v and ∇v−v × ∇v planes, whereas the halo is ring-shaped on the v−∇v plane. These three diffraction patterns indicate that the nematic LC adopts the a-orientation in which the nematic director is parallel to the v × ∇v direction. The other is the corientation in which the nematic director lies along the ∇v direction. Figure 6 shows the WAXD patterns of PM6MA-411 sheared and at 1 × 10−2 s−1 at 100 °C. While only ring-shaped reflection was observed on the v−v × ∇v plane, two outer diffuse crescents are concentrated on the v × ∇v and v axes on the ∇v−v × ∇v and v−∇v planes, respectively. This set of 4859
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Figure 5. 2D-WAXD results for PM6MA-132 sheared for 1 h at 120 °C with a shear rate of 5.32 s−1. Diffraction patterns were measured as described in Figure 4. Figure 6. 2D-WAXD results for PM6MA-411 sheared for 2 h at 100 °C with a shear rate of 1.06 × 10−2 s−1. Diffraction patterns were measured as described in Figure 4.
patterns indicates that the nematic LC adopts the c-orientation where the director axis is parallel to the ∇v direction. This is the first case of clear observation of the c-orientation for the nematic LCs as far as we know. The c-orientation is typical of the smectic LC. Figure 7 shows how the orientation depends on shear rate and temperature for five polymers with different Mn. The aorientation is attained invariably by shear flowing of the NLCs of all the polymers at rates higher than 0.1 s−1, whereas the type of nematic orientation attained at rates lower than 0.1 s−1 depends on Mn. The a-orientation is attained in low-molecularweight PM6MA-68 and PM6MA-132 polymers, whereas the corientation is adopted by high-molecular-weight PM6MA-294 and PM6MA-411 polymers, and no preferential orientation appeared in the intermediate-molecular-weight PM6MA-242 polymer. The nematic LCs of the high-molecular-weight PM6MA polymers thus show an orientation transition from c to a with increasing shear rate.
units in the spacer moiety, shear flowing of their nematic phases resulted in orientations orthogonal with each other. While PM4MA exhibited the b-orientation as expected for nematic LCs, PM6MA achieved either an a- or c-orientation which is expected for smectic LCs rather than nematic LCs. An important difference between these polymers is whether the polymer forms a smectic phase at lower temperature. The nematic phase of PM6MA transforms to a smectic phase, whereas that of PM4MA vitrifies without any phase transition. The a-orientation of the nematic phase has been reported for monomeric and polymeric nematics which transform to smectics on cooling, and has been attributed to pretransitional smectic fluctuations existing in the nematics.4−6 In the same way, the anomalous orientations of the PM6MA nematic may be attributed to the existence of pretransitional smectic fluctuations, which can preserve the intensities of the (102) and meridian streaks in the X-ray diffraction pattern measured at nematic temperature (Figures 2d−f).
4. DISCUSSION Although these PM4MA and PM6MA polymers are similar in chemical structure and differ only in the number of methylene 4860
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Figure 7. Orientation diagram for the director in five PM6MA nematic LCs with different molecular weights; (a) PM6MA-68, (b) PM6MA-132, (c) PM6MA-242, (d) PM6MA-294, and (e) PM6MA-411. Orientations are a (square), c (circle), or no preferential orientation (triangle). Shear rates and temperatures in the upper left were not accessible because the apparent torque was beyond the mechanical range of the rheometer.
transition on increasing shear rate, this orientation transition implies that the smectic fluctuations in higher-molecular-weight PM6MA-294 and PM6MA-411 polymers undulate more rapidly than those in the lower-molecular-weight PM6MA-68 and PM6MA-132 polymers. In other words, the undulation relaxation rate (1/τ) of the smectic fluctuations increases with increasing Mn. Such a difference in 1/τ can influence the nematic structure. It is known that the smectic layer undulation with a wavelength λ shows a relaxation rate of 1/τ = 4π2K1/(η3λ2), where K1 is the Frank constant for splay deformation of the layer and η3 is the viscosity of the smectic phase in Poiseuille flow in the layer direction.23 The Mn-dependence of 1/τ implies that λ decreases with increasing Mn, assuming that the Mn-dependence of K1/η3 is negligible. Because the undulation of smectic fluctuations involves splay of the nematic director and increases its density proportionally to 1/λ, the macroscopic nematic order will decrease with increasing value of Mn. Such a trend can be seen clearly by comparing the diffraction patterns of three PM6MA polymers having different value of Mn (see Figure 2, parts d−f).
It was unexpected that the nematic of PM6MA-294 and PM6MA-411 achieved the c-orientation, and showed an orientation transition from c to a with increasing shear rate. This c−a orientation transition has been found for smectic (or lamellar) phases11,12,14−17 and discussed theoretically as follows.18−22 While the c-orientation remains stable under a shear flow which is so slow that layer undulations are averaged out, it becomes unstable when the shear flow is fast so as to dilate and compress the undulated layers, and consequently the preferential layer orientation alters from c to a. These orientation behaviors imply that the nematic phase of PM6MA preserves a short-range smectic structure so that it exerts a great influence on the nematic orientation under shear flow. Another interesting fact is that the nematic orientation is also affected by Mn. By comparing the orientation maps for the nematics of PM6MA polymers with different Mn (Figure 7), we know that the nematic orientation which is preferred under shear flows at a lower rate than 0.1 s−1 alters from a to c with increasing Mn. According to the explanation of the orientation 4861
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Azimuthal spread of the crescent halo becomes larger with increasing Mn. The undulation of smectic fluctuations in the PM6MA nematics might take its origin from the influence of the backbone length on subtle smectic layering of the mesogenic cores (i.e., smectic fluctuations). The chain backbones are confined between the layers consisting of side-chain mesogens and extend preferentially along the layers. The layering of sidechain mesogens thus conflicts with the inherent tendency of the polymer to adopt random-coiled conformation to maximize its conformation entropy. To compensate for the loss of chain entropy, long backbones can both cross and distort even the solid layers in smectic phases.24−27 The same confliction can be considered for the PM6MA nematic phase where short-range layer ordering of side-chain mesogens (i.e., smectic fluctuations) can influence backbone chain conformation. This causes the smectic fluctuations to undulate at a wavelength decreasing with increasing backbone length. The relaxation rate of the undulation hence increases with the molecular weight of polymer.
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5. CONCLUSIONS Under shear flow, the PM6MA nematic LC exhibited orientation behavior distinct from that of the PM4MA nematic LC, although these two nematics are formed by side-chain polymethacrylates having the same methoxyazobenzene mesogens but different spacers of either four (PM4MA) or six (PM6MA) methylene groups. PM4MA arranged the nematic director in the velocity direction (the b-orientation) as expected from hydrodynamics of nematic LC. In contrast, PM6MA pointed the director either to the neutral (the a-orientation) or velocity gradient (the c-orientation) directions. Such orientation behavior of PM6MA nematics is similar to that of smectic LCs and attributed to pretransitional layering of side-chain mesogens, so-called smectic fluctuations. The orientation transition from a to c with increasing Mn is found for PM6MA polymers sheared at a rate lower than 0.1 s−1. It implies that on increasing Mn, the relaxation rate (1/τ) of the undulation becomes such that the undulation is averaged out under shear flow at rate slower than 0.1 s−1 and the corientation is preferred. Because the undulation at faster relaxation rate has a shorter wavelength it is expected that the macroscopic nematic order decreases with increasing Mn. It agrees with the feature of X-ray diffraction pattern which includes crescent halo spreading azimuthally larger on increasing Mn. The undulation of the smectic fluctuations may be promoted by compensation of the loss of the chain conformation entropy of the backbones confined and elongated between short-range mesogens layers.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a JSPS Grant-in-Aid for Scientific Research (B) (22340120). The synchrotron radiation X-ray measurements at Pohang Light Source (PLS) were supported by MOST and POSCO. 4862
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