Polymer-Dispersed Liquid Crystals: Compositional Dependence of

Chapter 14

Polymer-Dispersed Liquid Crystals: Compositional Dependence of Structure and Morphology Andrew J. Lovinger, Karl R. Amundson, and D. D. Davis

Downloaded by TUFTS UNIV on November 23, 2016 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch014

AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

We have investigated the structure and morphology of polymer-dispersed liquid crystals (PDLCs). These materials are commonly prepared by UV-induced crosslinking of a compatible mixture of a prepolymer with a liquid-crystal (LC) eutectic and are of high interest for flat-panel display applications. We have found that the morphology of these materials varies greatly with UV-irradiation temperature and composition. Within a range of ca. 20-70 wt.% L C , lower irradiation temperatures and higher LC contents favor a two -phase dispersion of bipolar droplets within the polymeric matrix. At low LC contents and high irradiation temperatures a new, space-filling spherulitic morphology is seen. These spherulites have a tangential orientation of the nematic L C molecules and are characterized by a highly unusual radial proliferation of surface inversion walls. We have found these defects to be initiated consistently at s=+1/2 disclinations and to be terminated at s=-1/2. The spherulites as well as the disclinations survive heating above the nematic-isotropic transition with little change.

Polymer-dispersed liquid crystals (PDLCs) are μιη-sized dispersions of nematic liquid-crystalline droplets within a polymeric matrix (1-3). They are finding intense interest for applications such as light switches in flatpanel displays and windows (3, 4). They are usually prepared by U V induced phase separation and cross-linking of a prepolymer containing a compatible blend of liquid crystals (LCs), although solution-casting from a common solvent and cooling from the melt below the upper-critical-

0097-6156/96/0632-0216$15.00/0 © 1996 American Chemical Society Isayev et al.; Liquid-Crystalline Polymer Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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solution temperature are also occasionally used (5). Contrary to the usual twisted nematic liquid-crystal displays, PDLCs operate on a scattering principle and therefore do not require polarizers (which absorb over 50% of all transmitted light). The LC molecules are selected so that their ordinary refractive index matches that of the host polymer. Since they have a positive dielectric anisotropy, they are aligned with an applied electric field, causing the initially scattering, randomly oriented bipolar droplets to give rise to a transparent film. Detailed calorimetric studies of the phase separation and curing processes in UV-irradiated PDLC materials have been given by Smith (6, 7). The morphology of PDLCs has been observed by scanning electron microscopy of blends of nematics in aqueous emulsions of polymers (8-10). Very recently, we have published a detailed morphological investigation of UV-cured PDLCs (11). Here, we summarize our recent findings on the compositional dependence of phase separation and morphology, and extend them to the description of the temperature dependence of the new morphologies that we reported earlier (11). Experimental Section The polymeric matrices were prepared by photopolymerization of commercial mixtures containing primarily trimethylolpropane diallyl ether, trimethylolpropane tristhiol and a diisocyanate ester (6). They were obtained from Norland Products, Inc., New Brunswick, NJ, under the designation NOA65. Laboratory-made mixtures of different acrylates were also studied. The liquid crystals were in most cases commercial eutectic mixtures of cyanobiphenyls and -triphenyls having a nematic-isotropic transition at 59-60°C and obtained from Merck Industrial Chemicals under the designation E7. Other, halogenated LCs were also occasionally used. The materials (which had been stored in the dark and handled under diminished light) were blended and sandwiched between thin glass cover slips to yield films of thickness ca. 12-18 μπ\. Photopolymerization was performed with a 100-W H g lamp at selected temperatures and with doses of 4.5 J / c m or greater. Specimens were examined in the polarizing optical microscope during heating and cooling using a Mettler microscope oven. In some cases, the interior of the films was visualized by scanning electron microscopy of properly prepared surfaces. 2

Results and Discussion Typical PDLC morphologies of >50% L C blends of LCrpolymer (E7:NOA65) cured at ambient temperature consist of a bimodal

Isayev et al.; Liquid-Crystalline Polymer Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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LIQUID-CRYSTALLINE POLYMER SYSTEMS

distribution of droplets having sizes of 4-8 μπι and 0.2-1 μ η ι , respectively (see Figure la). This figure is a scanning electron micrograph from a blend containing 65% L C , which is close to the maximum that can be incorporated at room temperature within this widely-used prepolymer. The smaller droplets are seen to be concentrated in the interstices between the larger ones and are probably generated at the later stages of curing when the matrix has become quite viscous and highly cross-linked. This morphology, which incorporates largje amounts of polymer matrix between droplets, obviously affects the transparency of the thin films, resulting in the well-known off-axis haze, which is associated with the refractive index mismatch between polymer and LC. One way to minimize this is through mixtures that can accommodate larger amounts of LC. Such materials may contain, for example, over 80% of a halogenated multiphenyl liquid-crystal mixture in a blend of alkyl acrylates (such as TL205:PN393 from Merck). We have found (12) that their morphology is strikingly different (see Figure lb): the polymer wall thickness is much smaller and the droplets are now arranged in a polygonal "foam texture". As a result, scattering occurs overwhelmingly among droplets rather than between L C and matrix, leading to favorable reduction in turbidity (13). Returning now to the E7:NOA65 blends, we examine the compositional dependence of their morphologies in the optical microscope. The droplet morphology is by no means exclusive or even prevalent, but depends on both composition and U V irradiation temperature (11). For example, Figure 2 shows that the two-phase PDLC morphology in 50:50 blends is replaced by a spherulitic one at irradiation temperatures between 40 and 50°C The spherulites are space-filling and are seen to have profuse radial striations. Overall, they exhibit a striking similarity to the typical spherulites of crystalline polymers. In addition to this morphological similarity, we also found that they have the same molecular orientation (i.e. tangential), as evidenced by their negative birefringence upon insertion of a first-order red plate at 45° to the crossed polars (see figure 3). This is somewhat surprising because the elastic constant for splay distortions in this liquid crystal is smaller than that for bend (K3/Ki=1.54), which would then favor a radial director orientation. However, these constants are provided from the manufacturer for the L C component alone, and it is not known how the presence of large amounts of flexible polymer molecules might modulate the elastic distortions experienced by the LC rods. Polymeric spherulites have been known (14, 15) to grow with a radially directed and non-crystallographically branched lamellar orientation. The molecules are generally perpendicular to the lamellar




14. LOVINGER ET AL.

Figure 1. Scanning electron micrographs (secondary electrons) showing the typical morphologies of PDLCs after extraction of the liquid crystal components, (a) 65:35 blend of LOpolymer, (b) 80:20 blend of (a different) LOpolymer. See text for further details.


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Figure 2. Typical morphologies of 50:50 blends of LOpolymer recorded in the polarizing optical microscope. Both were U V irradiatedwith4.5J/cm at (a)40°C and (b)50°C 2



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crystallites, which in turn are separated from each other by amorphous regions (see Figure 4a). In the novel LC spherulites described here, the rod-like molecules are again tangentially disposed, but there is no analogue of radially continuous lamellae. Instead, the molecules are expected to be aligned to each other within a network of cross-linked amorphous polymeric chains (see Figure 4b). The centers of the polymeric vs. nematic spherulites are also different. In crystalline polymers, nucleation occurs most commonly heterogeneously (e.g. on impurities) or quasi-homogeneously (see Figure 5a). In our LC case, the central region must involve a singularity: either an s=+l disclination line anchored at the two interior glass surfaces or an escaped point singularity (see Figure 5b). The above general behavior of 50:50 LOpolymer mixtures can now be extended to other compositions and temperatures. We found that higher concentrations of the nematic component favor the phaseseparated PDLC, e.g. for 65% LC samples the spherulitic morphology is not seen at any irradiation temperature. On the other hand, lower LC contents favor formation of spherulites. For example, a 25% L C mixture no longer yields PDLC droplets above room temperature: only by irradiation at low temperatures (e.g., 10°C) is such a morphology obtained, but it is not stable upon return to ambient, leading instead to a continuous nematic phase (11). We can summarize our findings with the aid of a ''morphological map" as in Figure 6, which shows the regions of PDLC versus spherulitic phases. Beyond ca. 72%, the liquid crystal component is no longer miscible with the initial photopolymerizable mixture. Since electrical and optical properties improve with L C content (13), it is desirable to be able to incorporate more than the ca. 72% that is possible with this mixture. It is for this reason that matrix polymers and liquid crystals of the general types referred to in Figure l b are becoming increasingly more useful, since they allow L C contents as high as ca. 84% and also lead to the advantageous scattering mechanism mentioned above (12, 13). Having discussed the general morphological map of these PDLC materials we now concentrate on the new spherulitic morphologies, specifically their internal microstructural features. Of these, the most obvious are the radial striations that are already discernible in Figure 2b. At high magnification between crossed polarizers, these striations generally exhibit closed dark loops; the material inside the loops has opposite birefringence to that outside. We therefore identify these features as surface inversion walls (16, 17): These are initiated and terminated by s=+l/2 or -1/2 disclination lines that are attached to the glass surfaces and are generally normal to them (see Figure 7). A related possibility consists of half-integral disclination lines running along the surface of one of the walls, but these have been associated



222


s = +1 φ=0

s = +1 φ = 45°

s = +1 φ = 90°

Figure 3. Schematic representations of the molecular orientations of liquid-crystal molecules in various types of spherulitic morphologies originating at an s=+l disclination. The ordinary and extraordinary refractive indices are designated n and n , respectively. (Reproduced from Ref. 11; Copyright American Chemical Society). 0

amorphous 9 r e

I 0 n

crystalline mol. stems

e

polymer molecules

LC molecules

Figure 4. Comparison of (a) lamellar growth in spherulites of semicrystalline polymers with (b) possible growth of liquid crystals in spherulites containing partly cross-linked amorphous polymeric molecules. (Reproduced from Ref. 11; Copyright American Chemical Society).


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(a)

(b) I


spherulitic nucleus

I

(c)

•

I

1

s = +1 disclination line

point singularity

Figure 5. Comparison of spherulitic centers in (a) semi-crystalline polymers versus liquid crystals growing in a partly cross-linked amorphous polymeric matrix (b and c) for samples confined to thin films between two flat surfaces. (Reproduced from Ref. 11; Copyright American Chemical Society).

80

s ο

60

L A

50 40 30 20 10

A M

Ό R Ρ

Ή Ο

υ s

ι I

I

70

SINGLE PHASE / (spherulitic) / / / /

τ Ε D

PHASE SEPA

.r

S Ρ Η Ε R

MACRO

RATION

υ L I Τ Ε S

20

/

TWO PHASES (PDLC droplets)

40

60

80

100

% LIQUID CRYSTAL

Figure 6. Approximate morphological map of the E7:NOA65 LCipolymer blend system as a function of composition and U V irradiation temperature. (Reproduced from Ref. 11; Copyright American Chemical Society).


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Figure 7. Schematic representations of inversion wall defects in L C spherulites and of their possible radial orientation modes. Initiation at (a)ans=+l/2 and (b)ans=-l/2 disclination line. The polarizer and analyzer directions are denoted by Ρ and A , respectively, and the spherulitic growth direction by G. (Reproduced from Ref. 11; Copyright American Chemical Society).



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with free surfaces (18) and exhibit difuseness at their s=-l/2 termination points that is not seen in our samples. A number of different manifestations of these surface inversion walls is seen in the polarizing micrographs of Figure 8. Figure 8a shows the detailed morphology of these inversion wall defects. This is one case where successive proliferation of such defects occurs during growth, leading to crankshaft or zigzag appearance of extinction lines in the radial direction. Figure 8b demonstrates initiation of such inversion walls along a line that traverses obliquely through one spherulite. This shows that factors extrinsic to the growing LC/polymer mixtures can cause these defects to be introduced. Such extrinsic factors are probably scratches in the inner surfaces of the coverglass or other discontinuities. In Figure 8c we observe the detailed microstructure of inversion walls as they reach the interfaces between growing spherulites. The wall defects are seen to be reoriented along the interspherulitic boundaries instead of being terminated at the intersections. Remarkably, the orientation of the LC molecules remains the same within these walls (i.e., parallel to the original growth direction), as we ascertained through introduction of a first-order red plate into the optical path of the microscope, between sample and analyzer. Preservation of the overall nematic director within these regions is dictated by the packing requirements for the rod-like molecules. Finally, Figure 8d shows the typical morphology of LC/polymer spherulites growing in "confined" regions, i.e. those remaining after the surrounding material has already grown spherulitically. The inversion wall defects are now seen to form irregularly curved, coarse channels that imply growth from regions enriched in amorphous polymer. This is entirely analogous to the phenomena explained many years ago by Keith and Padden (14) in their classic studies of spherulites of crystalline materials (polymeric, organic, and inorganic) growing from "impure" melts. A remarkable feature of these inversion wall defects involves their uniformity in generation and termination. As seen schematically in Figure 7, these defects could be initiated at s=+l/2 disclination lines and terminated at s=-l/2 ones (Fig. 7a), or vice versa (Fig. 7b). Through rotation of spherulites about the axis of a polarizing microscope and examination of the concurrent optical changes in their radially oriented inversion walls, we found a uniform behavior: Always the inner nonbirefringent boundary (i.e., the one closest to the polarizer or analyzer direction) narrows as the defect approaches that polarizer or analyzer while the outer boundary becomes wider. This can be seen even in the stationary case of Figure 8b by comparing the widths of the dark brushes defining each radial defect in relation to their positions vis-à-vis the spherulitic Maltese cross. By examination of the schematics in Figure 7,



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Figure 8. Appearance between crossed polars of various morphological features associated with the inversion wall defects in LC spherulites. (a) Successive radial initiation, (b) initiation by extrinsic factors, (c) rejection and reorientation of inversion walls in interspherulitic boundaries, and (d) spherulitic growth in confined regions. The material is a 25:75 E7:NOA65 LQpolymer blend U n irradiated with 4.5 J/cm . (Reproduced from Ref. 11; Copyright American Chemical Society). 2



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in

this narrowing and widening behavior is consistent with case (a), i.e. initiation at s=+l/2 disclination lines. Reasons for this propensity may have to do with the orientation of L C molecules with respect to polymeric cross-links or other singularities. The nematic rods are not expected to exhibit preferential wetting or other favorable interactions with such inhomogeneities (Figure 9b), and will thus adopt a radial orientation toward them (Figure 9a) leading to s=+l/2 disclinations. The isotropization ("melting") of these nematic spherulites during heating is also remarkable. The general behavior is seen in Figure 10. The spherulites (and their inversion wall defects) survive essentially unchanged until ca. 61.5°C (Figure 10a). Above that temperature, isotropic droplets appear throughout the samples. In some cases (11) these are concentrated near the spherulitic boundaries; in some others isotropization is initiated at the spherulitic peripheries (see spherulites marked "1" and "2" in Fig. 10b and c) and grows inward. Upon further increase in temperature by only a few tenths of one °C the nonbirefringent droplets coalesce and the spherulites "melt" completely at 62°C (Figure 10c). However, even after further heating to 65°C (Figure lOd), subsequent cooling below the isotropic-to-nematic transition temperature leads to re-formation of the very same spherulites. This includes reappearance of the original inversion walls with minimal or no disruptions. In Figure lOd, these disruptions are seen to be associated with the locations of the original isotropic droplets. A l l of these results indicate that a memory of the spherulitic morphology and substructure survives even above the isotropization temperature. Possibly, this is a result of an imprinting of the L C molecular arrangements onto the partly cross-linked polymeric matrix, as well as of the confinement of the nematic rods by the polymer in such a way that regeneration of their original orientations is highly favored. Conclusions Polymer-dispersed liquid crystals are seen to exhibit exceptionally complex morphologies ranging from L C droplets phase-separated within the polymeric matrix to nematic spherulites incorporating the polymer. The spherulites have been examined in detail and are seen to be characterized by radially oriented defects, which we attribute to inversion walls; these are initiated uniformly through s=+l/2 disclination lines. The molecular orientation in these L C spherulites appears to be tangential, similar to the case of semicrystalline polymers. The isotropization behavior of these spherulites is also complex, beginning with nucleation of non-birefringent regions at the interiors (and occasionally the peripheries) and followed by reappearance of the main morphological features upon subsequent re-cooling.



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(a) Reduced Nematic Wetting

(b) Preferred Nematic Wetting

s = +1/2

s = -1/2

Figure 9. Hypothetical modes of initiation of (a)s=+l/2 and (b)s=1/2 disclinations during radial growth in L C spherulites depending upon mutual orientations of the LC molecules on regions of polymeric cross-links or other inhomogeneities. (Reproduced from Ref. 11; Copyright American Chemical Society).



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Figure 10. Polarizing micrographs depicting the isotropization behavior during heating of L C spherulites in a 25:75 E7:NOA65 LCpolymer blend UV-irradiated with 4.5 J/cm at 25°C (a) Heated to61.5°C, (b)61.6°C, and (c) 61.8°C. (d) Cooled to ambient after complete isotropization at 62°C and further heating to 65°C. 2


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Polymer-Dispersed Liquid Crystals: Compositional Dependence of

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