Macromolecules Volume 28, Number 13
June 19, 1995
0 Copyright 1995 by the American Chemical Society
Synthesis of Liquid Crystalline Phosphazenes Containing Chiral Mesogens Harry R. Allcock* and Eric H. Klingenberg Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received December 27, 1994; Revised Manuscript Received March 17, 1995@
ABSTRACT: Phosphazene cyclic trimers and high polymers containing biphenyl side groups with (S)2-methyl-1-butoxy as a terminal unit were synthesized. The chiral biphenyl units [4,4’-C,&C&40CHzCH(CH~)CHZCH~] were linked to the skeletal phosphorus atoms of the small-molecule phosphazene cyclic trimers and high polymers through -O(CH&H20),spacer linkages, where m = 2 or 3. Of the cyclic where m = trimers synthesized, with the formula N~P~[~(CHZCHZO),C~H~C~H~OCHZCH(CH~)CHZCH~I~ 2 (4) or 3 (5), only 5 was liquid crystalline, as determined by differential scanning calorimetry (DSC). Single-substituent high-polymeric poly(organophosphazenes) were synthesized that contained the chiral biphenyl unit linked to the backbone through spacer units m = 2 (6)and 3 (8). Cosubstituent polymers were also synthesized which contained both the chiral biphenyl derivative linked to the backbone through varying spacer lengths and with nonmesogenic cosubstituents, OCH~CHZOCHZCHZOCH~ and OCH2CF3. The liquid crystalline behavior of these polymers was studied with respect to spacer length and nonmesogenic cosubstituents. All the polymers synthesized showed enantiotropic liquid crystallinity. Morphologies and thermal behavior of these compounds were investigated with the use of DSC, X-ray diffraction, and optical hot stage microscopy.
Introduction Considerable interest exists in the synthesis and physical properties of polymeric liquid crystalline material~.l-~ Liquid crystalline polymers (LCPs) combine the properties of small-molecule liquid crystals with characteristics generally associated with polymers, such as strength and ease of fabrication. Small-molecule liquid crystalline (LC)molecules have the ability to form well-ordered mesophases. These mesophases can be diverse in structure and often respond to external electric and magnetic fields. Polymers are typically lightweight and durable, can be easily processed, and have a high degree of dimensional stability. The combination of these two sets of properties allows the development of technologically useful material^.^-^ Specifically, liquid crystalline polymers are of interest in the development of molecular switching devices, optoelectronic displays, and optical data storage devices.8-10 Polyphosphazenes are a broad class of macromolecules based on the repeating unit (NPRz),. The properties of these macromolecules vary over a broad range depending on the side group ~tructure.l’-~~ Poly@
Abstract published in Advance ACS Abstracts, May 15, 1995.
0024-9297/95/2228-4351$09.00/0
phosphazenes are well suited for the assembly of useful LCP systems because the phosphorus-nitrogen backbone is very flexible, and this can serve to enhance mesophase stability, thus increasing the useful temperature range over which these materials can be used.14-17 The polyphosphazene backbone is also optically transparent in the visible region, which is an important feature in the development of optical devices.16-22These properties, combined with the ease with which the macromolecular properties can be modified by choosing the appropriate cosubstituents, make polyphosphazenes of considerable interest for both fundamental and applied research. Earlier studies indicated that specific polyphosphazenes, especially those which contained fluoroalkoxy or aryloxy side groups, were capable of unusual mesophase b e h a v i ~ r . ~The ~ - exact ~ ~ nature of these mesophases is still not fully understood. However, singlesubstituent LC polyphosphazenes have been synthesized which contain rigid aromatic structures linked t o the polymer through flexible alkoxy or alkyl group^.^^-^^ The LC properties of these polymers are largely dependent on the nature of the mesogenic side group used and the length of the spacer between the mesogen and the polymer backbone. Three reports exist of LC polyphosphazenes that contain both mesogenic and 0 1995 American Chemical Society
Macromolecules, Vol. 28, No. 13, 1995
4352 Allcock and Klingenberg
Scheme 1 HO
OH + B r T
KOH,KI EtOH
HO 1
m = 2 (2) or 3 (3)
nonmesogenic cosubstituents. In all cases the trifluoroethoxy side unit was employed as the nonmesogenic side group^.^^-^^ Most of the LC polyphosphazenes examined to date exhibit LC mesophases that are of the smectic type. It has been demonstrated that compounds with smectic mesophases, particularly the smectic C mesophase, can be helielectric and ferroelectric, provided the mesogens have a large dipole moment across the short axis of the molecule. The proper alignment of the molecular dipoles is commonly accomplished by incorporation of chiral centers in the mesogenic molecules. Since Meyer first reported the ability of LC materials to be ferroelectric,38many reports have appeared concerned with the synthesis and properties of these types of LC material^.^^*^^ Shibaev and co-workers were the first to report the synthesis of side-chain LCPs which were f e r r ~ e l e c t r i c .However, ~~ chiral LC materials which do not form a smectic C mesophase also show interesting electrooptical proper tie^.^^-^^ Liquid crystalline compounds which generate both nematic and nontilted smectic phases can be useful as either passive or active components. Such uses include polarizers, holographic elements, thermographical tools, optical switches, and image e1aborato1-s.~~ Polyphosphazenes containing chiral azoxybenzene derivatives as mesogenic side groups were studied earlier in our program.28 These polymers had very broad mesophase temperature ranges, exhibited tilted smectic mesophases, and had spontaneous polarization values similar to those observed for small-molecule LC compounds. Until now this has been the only report of polyphosphazenes which contained chiral mesogens. In this paper we report the synthesis of a new series of LC polyphosphazenes with chiral biphenyl derivatives as the mesogenic side group substituents. The effects of spacer length on the LC behavior of the polymers was investigated. Cosubstituent polymers wcre also synthesized which contained either mesogenic and nonmesogenic cosubstituents or the same mesogenic group connected to the polymer chain through spacer units of varying lengths. The examination of such cosubstituent polymers allows a better understanding to be obtained of the effects that macromolecular cosubstituents have on the LC properties of phosphazene polymer systems.
Results and Discussion Synthesis and Characterization. The commercially available chiral unit (S)-l-bromo-2-methylbutane was allowed to react with 4,4’-dihydroxybiphenyl to yield the monoetherification product (Scheme 1). Compound 1 was then coupled t o the ethyleneoxy spacer linkages by a similar reaction to yield compounds 2 and 3 (Scheme 1). The chiral carbon was not directly involved in either reaction and, as a consequence, it remained unaffected. All compounds were white crystalline materials that were characterized by mass
spectrometry, lH nuclear magnetic resonance (NMR) spectroscopy, and 13C NMR spectroscopy. The phosphazene cyclic trimers were synthesized by the procedure illustrated in Scheme 2. Cyclic trimers 4 ( m = 2) and 5 ( m = 3) are white crystalline solids that are soluble in most common organic solvents. The cyclic trimers were characterized by lH, 13C, and 31P NMR spectroscopy. Poly(dich1orophosphazene) was prepared by the thermal ring-opening polymerization of hexachlorocyclotriphosphazene.13 The desired poly(organophosphazenes) were then synthesized by the nucleophilic displacement of the chlorine atoms as illustrated in Scheme 2. Structural characterization of the polymers was performed by IH, 13C, and 31PNMR spectroscopy and by elemental analysis (Table 1and Chart 1). The complete displacement of the chlorine atoms was confirmed by the existence of a singlet resonance in the 31P NMR spectrum for each polymer and by elemental analysis where applicable. All the polymers synthesized were off-white powdery materials. The single-substituent polymers were soluble in hot THF solutions, while the solubility of the cosubstituent polymers depended mainly on the nonmesogenic macromolecular cosubstituent. In general, all the cosubstituent polymers synthesized were more soluble in organic solvents than were the single-substituent polymers. This difference in solubility can be attributed to the ability of the cosubstituents to disrupt side group crystallinity. Liquid Crystalline Behavior. Compounds 1-3 and the phosphazene cyclic trimer 4 were not liquid crystalline as determined by differential scanning calorimetry (DSC) and optical hot stage microscopy experiments. However, cyclic trimer 5 does exhibit monotropic LC behavior between 115 and 109 “C as determined by DSC experiments and optical hot stage examination. The schlieren texture observed in optical hot stage experiments with -l/2 and +l/2 singularities suggests that the mesophase is nematic in ~ t r u c t u r e . ~ All the high polymers synthesized were enantiotropic LC materials. Attachment of the biphenyl groups to the polymer chain hinders the melting process and allows the appearance of a mesophase. This difference between the thermal behavior of the polymers and the small-molecule compounds is known as the “polymeric e f f e ~ t ” . l -The ~ thermal transitions measured for the polymers by DSC were all relatively broad in range, ca. 10-15 “C. The width of the thermal transitions is most likely due to the molecular weight polydispersity of the polymer samples. All the polymers synthesized had polydispersities of ca. 1.9-3.5. In such a disperse system the lower molecular weight species are more likely to undergo the thermal transitions at lower temperatures than are the higher molecular weight components. The differences in these transition temperatures cause the transitions to occur over a wider
Macromolecules, Vol. 28, No. 13, 1995
LC Phosphazenes Containing Chiral Mesogens 4363
4(m=2) S(m=3) m=2or3
R' = CH2CH20CH2CH20CH3or CH~CFJ range of temperatures than would be the case for polymers having a low polydispersity. There were no glass transitions detected for these polymer systems. This is most likely due to the mesogenic side chains crystallizing before the polymers reach their glass transition temperature. This was not unexpected since polyphosphazenesgenerally have very low glass transition temperatures.13 Polymers with the Biphenyl Unit Linked to the Backbone through Diethyleneoxy Spacer Groups. Polymers 6-8, which contain the biphenyl unit linked to the polymer backbone through diethyleneoxy spacer linkages, all showed similar LC mesophases. These polymers displayed cholesteric (Ch) and smectic A (Sa) mesophases as identified by optical hot stage microscopy and X-ray diffraction. Polymer 6 was liquid crystalline between the temperatures of 125 and 226 "C (Tables 2 and 3 and Figure 1). The Sa to Ch transition occurred at 174 "C on heating. The X-ray diffraction pattern of 6, which was quenched at 180 "C, showed only a diffuse ring. This is consistent with nematic type ordering. However, because the side groups contain a chiral center, this type of mesophase is then designated as either a twisted nematic or a cholesteric phase: for the purposes of this paper it will be referred to as the cholesteric phase (Ch). The optical micrographs taken during the cooling of 6 from the isotropic phase appeared as long thin batonnets or "cracks". This texture is consistent with those of previously identified polymeric Ch me so phase^.^ With further cooling these batonnets appear to coalesce to form a focal conical fan texture. X-ray diffraction experiments on polymer 6 quenched at 135 "C revealed that the fan texture is a layered structure which corresponds to the Sa type ordering. Side group length was calculated using Quanta 3.048and was found to be 22.04 A. The layer spacing determined from X-ray diffraction%xperiments was calculated to be approximately 42.5 A (Table 4). This suggests that the layered planes consist of two sets
of side group molecules which overlap slightly, and this is consistent with the structure of the smectic mesophases of previously studied LC polyphosphazenes (Figure 2).28-31 Polymers 7 and 8 show the same mesophases as 6. This is in contrast to previously studied phosphazene systems in which the LC mesophases for the copolymers were less ordered than the mesophases observed for the homopolymer^.^^,^^,^^ However, the previously studied systems contained larger amounts of the nonmesogenic cosubstituents than those copolymers studied here. The relatively low amounts of nonmesogenic cosubstituents contained in the polymer systems presented here, less than 27% of the total substitution, may be the reason for polymers 6 and 7 exhibiting the same mesophases. X-ray diffraction experiments on 7 indicate that the layer-like order is lost as the temperature is raised, similar to 6 (Table 4). This behavior was also observed with 8. The isotropization temperatures for 7 and 8 are lower than those observed for 6. This is probably due to the nonmesogenic side groups breaking up the crystalline packing of the biphenyl substituents. The Sa to Ch transition for 7 occurs at 135 "C and is consistent with the idea that the nonmesogenic side groups interfere with the crystalline packing of the mesogenic groups. However, 8 shows the same transition at 195 "C, approximately 20 "C higher than the same transition for 6, which occurs at 174 "C. The difference in the Sa-Ch transition temperatures for 6 and 8 is not easily explained. The higher Sa-Ch transition temperature of 8 (195 "C) could be a result of an ordering of the trifluoroethoxy substituents. It is known that poly[bis(trifluoroethoxy)phosphazenel is capable of unusual mesophase f ~ r m a t i o n . ~Also, ~ - ~ it~ has been observed in polymer-dispersed LC systems that the mixing of small-molecule LC compounds with polymers is considerably less when the polymer is the differences in the strucf l ~ o r i n a t e d .Therefore, ~~ tural characteristics of the trifluoroethoxy group and the
4354 Allcock and Klingenberg
Macromolecules, Vol. 28, No. 13, 1995
Table 1. Characterization Data for the Phosphazene Cyclic Trimers and High Polymers compd
31P NMR (ppm)
4
18.5 (s)
11, 17,27, 35, 73, 115, 116, 129, 129.5, 133, 134, 154, 158
5
18.0 (s)
11, 17,27,35,62, 67.5,
elemental anal. I3C NMR (ppm)
69.5, 72, 72.5, 115, 127, 133, 134, 150
6
-7.2 (s)
7
-6.8
8
-7.3 (s)
9
-6.8 (s)
10
-6.3 (s)
11
-6.9 (s)
12
-7.2“
13
-6.1”
14
-4.9“
a
(9)
11, 17, 27, 35, 62, 67,
69, 70, 70.5, 71, 72, 72.5, 115, 127, 132, 133, 157,158 11.0, 17.2, 25.4, 34.9, 64.9, 65.1,67.5, 70.0, 71.8, 72.3, 114.2, 126.4, 132.5, 133.0, 157.7, 158.1 12.1, 17.0, 26.4, 35.0, 65.1,67.1, 69.5, 71.2, 73.2, 114.1, 128.0, 133.1, 157.5, 158.2 11.6, 16.5, 26.1, 34.8, 62.1, 68.2, 70.2, 72.6, 73.2, 114.7, 127.8, 133.9, 156.5, 158.2 11.7, 16.4,26.2,35.0, 62.1, 68.2, 70.2, 73.1, 73.4, 74.0, 114.6, 127.9, 133.1, 134.0, 156.7, 158.4 11.7, 16.5, 26.1, 34.7, 62.1, 64.2, 68.2, 70.2, 73.1, 73.5, 114.6, 127.9, 133.1, 134.0, 156.5, 158.2 11.0, 16.5, 26.3, 35.0, 65.1, 66.0, 69.7, 70.5, 72.7, 114.5, 127.0, 131.9, 132.5, 157.5, 158.2 11.5, 14.6, 26.4, 34.8, 65.1,65.9, 67.0,69.1, 70.4, 72.5, 114.5, 127.2, 132.5, 157.5, 158.2 11.5, 14.6, 26.4, 34.8, 65.1, 65.9, 67.0,69.1, 70.4, 72.5, 114.5, 127.2, 132.5, 157.5, 158.2
IH NMR (ppm) 0.98 (t, CH31, 1.04 (d, CH3), 1.30 (m, CHd, 1.60 (m, CHd, 1.89 (m, CHI, 3.82-4.20 (m, OCH2), 7.45-6.97 (ArH) 0.96 (t, CH31, 1.05 (d, CHd, 1.27 (m, CHd, 1.60 (m, CH2), 1.86 (m, CHI, 3.74-4.16 (m, OCHd, 7.45-6.97 (ArH) 0.65-0.98 (CH3), 1.02-1.15 (CH2), 1.20-1.32 (CHz), 1.51-1.70 (CHI, 3.2-4.1 (CHz), 6.3-7.2 (ArH) 0.70-1.10 (CH3), 1.20-1.40 (CH2), 1.45-1.61 (CHz), 1.70-1.80 (CH), 3.00-4.70 (OCH2),6.20-7.64 (ArH) 0.80-1.15 (CH3), 1.21-1.43 (CH2), 1.53-1.74 (CH2), 1.76-1.98 (CH), 3.00-4.81 (OCHz),6.30-7.53 (ArH) 0.80-1.12 (CH3), 1.22-1.31 (CH2), 1.32-1.39 (CH21, 1.81-1.90 (CHI, 3.01-4.95 (OCH21, 6.45-7.62 (ArH) 0.85-1.12 (CH3), 1.14-1.35 (CH21, 1.46-1.68 (CHz), 1.72-1.91 (CHI, 3.18-4.56 (OCH21, 6.45-7.49 (ArH) 0.85-1.15 (CH31, 1.17-1.28 (CH21, 1.47-1.69 (CH2), 1.75-1.96 (CHI, 3.42-4.45 (OCHz),6.54-7.53 (ArW 0.71-1.08 (CH3), 1.11-1.24 (CHz), 1.42-1.63 (CH2), 1.65-1.89 (CHI, 3.37-4.55 (OCH21, 6.30-7.41 (ArH) 0.75-1.05 (CH3), 1.06-1.20 (CH21, 1.32-1.65 (CHz), 1.71-1.95 (CH), 2.81-4.45 (OCH2),6.05-7.43 (ArH)
% calc (9% found)
C, 69.0 (68.4) N, 2.00 (1.84) H, 7.17 (7.20) C1, < 1 (0.03) C, 65.9 (65.1) N, 1.90 (2.16) H, 7.43 (7.77) C1,
