In the last decade, a variety of supramolecular liquid crystalline polymers, assembled by noncovalent bonds in either the effective side chain or the effective main chain, have been investigated.1–5 A large fraction of them are based on hydrogen bonds. Those based on ionic bonds most often involve polyelectrolytes with oppositely charged surfactants or functionalized polymers with complementarily functionalized alkanes that assemble by proton transfer interactions, giving side-chain polymer ionic complexes.6–11 There have been only a few reports on ionic complexes that involve what we term “surfactomesogens” – that is, surfactant-type molecules incorporating a thermotropic mesogenic motif.6, 12–18 Moreover, there is a dearth of studies that examine the effect of different molecular parameters on the liquid crystalline characteristics of these systems, especially the ionically complexed ones, and those that do focus principally on the effect of side-chain or spacer length or on the effect of the component stoichiometry.
In this work, we describe the thermal and structural properties of equimolar complexes formed by an azocontaining surfactomesogen (azo10Q, shown in Fig. 1) and various commercially available, oppositely charged polyelectrolytes (P) to evaluate the effect of the nature of the polymer backbone and associated parameters such as molecular weight. In addition, the effect of stoichiometry is studied for one of the complexes. The polyelectrolytes used and their acronyms are given in Table 1; the numbers associated with the acronym for the sulfonated polystyrenes (PSS) refer to the approximate number-average molecular weights, noting that two of these have relatively low polydispersities. The acronym, azo10Q, was chosen to indicate the chromophore nature, the spacer length, and the interacting functional group (Q for quaternized ammonium).
Table 1. Acronyms and Molecular Weight Characteristics of the Polyelectrolytes Used
A similar surfactomesogen (possessing a six-methylene spacer and a (bis(2-hydroxyethyl)ethyl)ammonium functionality) complexed with poly(vinyl sulfonate) (PVS) was actually the first example of a supramolecular side-chain liquid crystal polymer (supramolecular SCLCP) based on ionic bonding that was reported in the literature.12 The iodide-neutralized surfactomesogen was itself found to be liquid crystalline (smectic A), and complexation to PVS resulted in a partially interdigitated smectic A mesophase over an increased temperature range. We found that triethylammonium-functionalized alkoxy (10 or 12 carbons) methoxybiphenyl mesogens, whose Br-neutralized forms are not liquid crystalline, generate a single-layer smectic A phase over a wide temperature range when complexed with PVS or PSS.13, 19
The synthesis and characterization of the azo10Q surfactomesogen are described elsewhere.20, 21 One batch of sodium poly(4-styrene sulfonate) (PSS45) was supplied by Aldrich, and two other batches (PSS20 and PSS60), prepared by controlled free radical polymerization and therefore having lower polydispersities, were obtained from Dr. Michael Georges at the Xerox Research Center Canada, Mississauga; all were in the form of powders. Water solutions of sodium poly(vinyl sulfonate) (PVS) (25 wt %), sodium polyacrylate (PA) (40 wt %), and sodium polymethacrylate (PMA) (30 wt %) were obtained from Aldrich and freeze-dried before use. The molecular weights of these compounds are reported in Table 1. Sodium carboxymethyl cellulose (CMC-Na) in powder form, obtained from Aldrich, was reported by the supplier to have a degree of carboxylate substitution of 0.9 and a solution viscosity of 3000–6000 cP in 1% aqueous solution. Sodium cellulose sulfate (CS-Na) in powder form was purchased from Acros; no degree of substitution or other information was given. All polyelectrolytes were dried in vacuo at 65 °C for 2 days before use.
Preparation of the Azo10Q/PolyelectrolyteComplexes
Mesogen–polyelectrolyte complexation was achieved by ion exchange between azo10Q and the Na-neutralized polymers, with elimination of the Na+Br− counterions. To accomplish this, accurately-weighed components, in view of the desired stoichiometry, were dissolved separately under stirring in warm deionized water (about 65 °C) at a concentration of 1 g/L, giving clear solutions. The azo10Q solution was then added slowly to the polyelectrolyte solution (resulting in a precipitate, except for the complexes of low stoichiometries), followed by stirring of the mixture at about 80 °C for 24 h and then freeze-drying. The precipitate obtained by mixing the components (itself a sign of complexation) was in the form of a fine suspension in most cases; it was more abundant for the azo10Q/PSS series and that for azo10Q/CMC was in the form of a gel-like film that adhered to the stirring bar. To eliminate the counterions, all freeze-dried products were dialyzed against deionized water for 2–3 days, replacing the water outside the dialysis bag daily. Finally, the product was freeze-dried, and then dried in vacuo at about 65 °C for 1 week or at about 80 °C for 3 days. To prepare the complex with CS, the azo10Q quantity was calculated as if the degree of sulfate substitution was 3, on the basis that excess (uncomplexed) azo10Q is eliminated by the dialysis. All of the complexes were orange to red in color.
Techniques of Analysis
Energy dispersive X-ray microanalysis (EDX) was carried out using a Jeol JSM-840A instrument equipped with an energy dispersive X-ray spectrometer, for which the samples were glued to an aluminum stub and coated with a thin layer of Pd/Au alloy. Elemental analysis was performed at Laval University (CHN analysis), at the University of Montreal (CHNS analysis) and at Guelph Chemical Laboratories (CHNO analysis). Thermogravimetric analysis was carried out under constant nitrogen flow (200 mL/min) at a heating rate of 10 °C/min using a Mettler TGA-50 balance, after first maintaining the sample at 50 °C for 10 min.
Thermal characterization was performed using a PerkinElmer DSC–7 differential scanning calorimeter, calibrated with indium and flushed with nitrogen. Unless otherwise specified, heating and cooling scans were performed at 10 °C/min, on 5–15 mg of sample packed into standard aluminum pans. First-order transition temperatures are given by the peak values, and glass transition temperatures (Tg) by the midpoint of the heat capacity jump of the second heating scan.
Observations of the complexes between crossed polarizers were made using a Zeiss Axioskop polarizing optical microscope equipped with a 25× Leica objective and a Hitachi 3 CCD HV-D27 camera. The temperature was regulated using a Mettler FP5 or FP80 temperature controller and a Mettler FP52 or FP82 hot stage.
X-ray diffractograms were obtained using a Bruker diffractometer equipped with a Kristalloflex 760 sealed-tube copper anode generator, operated at 40 kV and 40 mA, and a two-dimensional position-sensitive wire-grid detector (Bruker AXS). Collimation was effected by a graphite monochromator with a 0.8-mm pinhole. Data in the range of 2θ = 0.9–27° were obtained using a home-made off-centered 5-mm beam stop and a sample-to-detector distance of 9 cm. Temperature was regulated by an Instec HCS 400 oven coupled with a STC 200D controller. The samples were placed in sealed Mark-Röhrchen glass capillaries (Charles Supper) of 1.0 mm inner diameter. All images were treated by subtracting a baseline image obtained using an empty capillary tube. The two-dimensional image was then integrated (generally over 180°) to plot the intensity as a function of 2θ. The resulting curves were subsequently given flattened baselines by choosing baseline points here and there around the main diffraction features. The d-spacings were determined from the maximum of the diffraction peaks, using the Bragg equation, d = nλ /(2 sinθ), where n is an integer giving the order of the diffraction peak, 2θ is the diffraction angle, and λ is the wavelength of the X-ray beam used (Cu Kα, λ = 0.1542 nm). Molecular lengths (lC) were estimated with Hyperchem 5 (Hypercube) for the lowest energy conformation of the surfactomesogen, where the methylene units are in an all-trans conformation, to which the polymer repeat unit was added.
RESULTS AND DISCUSSION
The azo10Q surfactomesogen is a crystalline compound that melts directly into the isotropic phase with no intervening liquid crystalline mesophase (in contrast to the similar surfactomesogen reported in the literature12). It is polymorphic and shows two melting transitions at about 120 and 135 °C, related to solution-crystallized and melt-crystallized forms, respectively.21 The polyelectrolytes are simply amorphous materials. As will be shown later, the complexation of these two materials generates a (smectic A) liquid crystalline phase over a wide temperature range, with no evidence of any crystalline phase.
The composition of the complexes was first determined. The absence of Na+ and Br− ions after 3 days of dialysis was confirmed by EDX in all of the equimolar complexes, including the CS complex. The purities and stoichiometries of the equimolar complexes and one nonequimolar complex were verified by elemental analysis. As shown in Table 2, the results are very satisfactory: the values found for the carboxylated complexes (azo10Q/CMC, azo10Q/PMA, and less so azo10Q/PA) are closest to the theoretical values, assuming no water molecules present, whereas those for the sulfonated complexes (azo10Q/PSS and azo10Q/PVS) are better approximated by assuming the presence of one water molecule per repeat unit. The complexes were also found to be thermally stable by TGA to over 200 °C, considering 5% weight loss values (Table 3). Unless otherwise noted, it was possible to perform the various thermal and structural analyses of the complexes below dynamic onset degradation temperatures (also given in Table 3).
Table 2. Elemental Analysis of the azo10Q/Polyelectrolyte Complexes
DSC thermograms of selected complexes are given in Figure 2. The various transition temperatures and enthalpies are listed in Table 3. All but one of the complexes (azo10Q/PMA) possess a high-temperature first-order peak that is generally located at a temperature above the mesogen melting point, which itself is suggestive of a new phase resulting from the complexation. The complexes are birefringent between crossed polarizers below this transition (an example of the kind of textures obtained is given in Fig. 3), and are isotropic above this transition. The low enthalpies, as well as the small supercoolings, are consistent with it being a transition between a liquid crystalline and the isotropic phase (LC-is transition or clearing point). The exact enthalpy appears to depend on the polyelectrolyte: the lowest values are obtained for the complexes with the carboxylate polyelectrolytes, which give weaker ionic bonds than do sulfonate or sulfate polyelectrolytes, and the highest one is obtained for the PVS complex.
The temperature of the LC-is transition is observed to depend on the nature of the polyelectrolyte as well as on its molecular weight. The highest values are obtained for the three azo10Q/PSS complexes, and they show a weak increase with increase in polyelectrolyte molecular weight.22 In contrast, the value for the PVS complex is much lower. This might be attributed to its much lower molecular weight (as observed also in covalently bonded SCLCPs below a certain threshold value23), although the presence of the phenyl group may contribute as well to the stability of the mesophase in the PSS complexes. The CS complex likewise has a low LC-is transition temperature, whereas the CMC complex gives a much higher transition temperature (this might again be a consequence mainly of a much lower molecular weight for the former; but this information is unavailable).
The PMA and PA complexes appear less stable. The PA complex shows the LC-is transition only on the first heating (where it is broad) and cooling curves, whereas it is not observed at all for the PMA complex. Birefringence between crossed polarizers for the latter was observed to disappear irreversibly in the 150–160 °C range, accompanied by air bubbles, suggesting decomposition. Indeed, Table 3 indicates that the onset of weight loss occurs at a particularly low temperature for this complex, which can therefore be associated with the beginning of decomposition rather than loss of water. The onset of weight loss is a little higher in the PA complex, which is what might allow the LC-is transition to be observed in this case (although the broadness of the transition in the DSC thermogram and its lack of reproducibility suggest that it is also affected by decomposition). In contrast to the PA and PMA complexes, the CMC complex is stable and its clearing point is reproducible (weight loss also begins well above the clearing point). This suggests that it is not the carboxylate group by itself that is responsible for the lack of thermal stability of the PA and PMA complexes, but more likely their proximity to one another: in CMC there is about one carboxylate group per ring, whereas in PA and PMA there is a carboxylate group on every second main-chain carbon.
All of the complexes show a clearly visible glass transition, generally a little above ambient temperature (it was found to be lower for the PA and PMA complexes under the standard conditions, but when these samples, placed in aluminum pans with pierced covers, were held overnight at 90 °C under N2 atmosphere in the DSC chamber, their Tgs were found to be in the same range as the other complexes24). It appears to be molecular weight independent, judging from the PVS and the three PSS complexes. The somewhat higher Tg of the CMC and CS complexes may be related to the greater rigidity of the cellulose backbone.
Interestingly, another Tg-like transition with a much smaller heat capacity increment is apparent at higher temperatures in the PSS, PVS, and CMC complexes. It is located 35–40 °C higher for the PSS and CMC complexes when compared with the PVS complex (perhaps related again to the much lower molecular weight of the latter). The presence of two DSC-detectable Tgs has been reported occasionally for other comb-like homopolymers25–29—mostly SCLCPs—including by our own group for an amphiphilic tail-end pyridinium side-chain methacrylate polymer complexed (at a location far from the polymer backbone) with alkyl sulfonates.30 Generally, these polymers are characterized by a backbone and sidechains (or parts thereof) that are chemically quite dissimilar and associated with quite different mobilities, this in the context of a disordered lamellar mesophase packing structure, as described in greater detail by Vuillaume and Bazuin. 30 This is thought to give rise to distinct one-dimensional nanophase separation (i.e. mesophase layers or lamellae composed of two distinct subplanes of nanometric thicknesses), leading to one Tg associated with cooperative main-chain motion (which may involve part of the side-chain in some systems) and another to cooperative side-chain (or a part thereof) motion. The assignment of the lower Tg (TgL) and the upper Tg (TgU) to one or the other nanophase (subplane) depends on the system.25–30 In the system of the present study, given that ionic interactions are known to give rise to high Tgs, we can reasonably postulate (in the context of the above two-Tg nanophase-separation interpretation) that TgL is associated with the side-chain mesogenic groups and TgU with the main-chain and ionic groups. This could explain why no molecular weight dependence was observed for what we hereafter term TgL, whereas TgU appears at a much lower temperature for the PVS complex when compared with the PSS complexes. It is noteworthy that Arrighi et al.26 and Hiller et al.31 have recently been investigating comb-like poly(di-n-alkyl itaconates) and poly(n-alkylmethacrylates), respectively, in great detail using various experimental methods, including dielectric measurements and solid-state NMR studies. Their results, to date, corroborate the concept of nanophase separation, resulting in two distinct levels of cooperative motion that can give rise to two Tg-like transitions.
Equimolar Complexes: Structural Characterization
Ambient temperature X-ray diffractograms of representative azo10Q/polyelectrolyte complexes are shown in Figure 4, and associated data are given in Table 4. All profiles are characterized by a broad halo centered at about 2θ = 20°, corresponding to a Bragg spacing of about 4.4 Å, which is typical of the lateral distance between disordered side-chains. The absence of diffraction peaks in the wide angle region indicates that no crystalline phase is present in any of these complexes. At small angles, two and sometimes three diffraction peaks, with reciprocal spacings in the ratio 1:2(:3), are consistently observed, indicating that they are the first-, second-, and third-order diffraction peaks of a lamellar packing structure. The Bragg spacings associated with these peaks are comparable with the combined molecular length estimated for the mesogen (in its most extended conformation) and the polymer repeat unit for all of the complexes (Table 4). Thus, it can be concluded that the liquid crystalline phase of the complexes is a single-layer smectic A mesophase.
Table 4. X-ray Scattering Data of azo10Q/Polyelectrolyte Complexes at Ambient Temperaturea
lC is the estimated molecular length of the complex repeat unit; d1, d2, and d3 are the calculated Bragg spacings from the first-, second-, and third-order reflections, respectively.
Its monolayer nature concords with what we observed for other complexes with sulfonated polyelectrolytes,13, 19 but it contrasts with the partially interdigitated smectic A mesophases formed by the similar complex reported by Ujiie and Iimura12 and by the proton-transfer surfactomesogen complexes described previously by us.18 On the other hand, analogous SCLCP polymers, where mesogenic side groups are attached covalently to ionic (quaternized) backbones, were found by Guillon and coworkers to also give rise to orthogonal single-layer mesophases,6, 32, 33 including for mesogenic groups with cyano and chiral tails (which are typically associated with partial bilayer and tilted layer mesophases, respectively).34 The latter result was attributed to the dominant effect of the ionic interactions on the packing structure. An exception to the monolayer structure for these kinds of polymers was reported by Ujiie and Iimura, who found, instead, a bilayer smectic A phase (preceded by a higher order, tilted bilayer mesophase).35
The profiles undergo no significant change as a function of temperature within the mesophase range until the isotropic phase is reached, and they are reversible on cooling (though with much decreased intensity for the PMA complex). It may be noted that, in some cases (notably the CMC and PVS complexes), the first-order diffraction peak is much less intense than the second-order peak. The crystalline phase of the azo10Q mesogen displays the same characteristic in the small-angle region (see inset in Fig. 4). A similar effect was observed in most of the systems of Guillon and coworkers mentioned earlier (they reported first- and second-order reflections of similar intensity generally), and was associated with an additional plane of symmetry in the lamellar electron density profile.32–34
Two possible models of the monolayer structure, proposed also by Guillon and coworkers for their materials32–34 and equally applicable to the present systems, are illustrated in Figure 5. The first model (left) shows a completely interdigitated lamellar packing of the side chains giving lateral alternating head-to-tail orientation. In the second model (right), only the rigid cores of the mesogens overlap, and the alkyl spacers on either side are extended to roughly half of their most extended conformations to fill in the space corresponding to the lateral molecular area of two mesogenic cores. Both models allow for favorable ionic interactions, but only the second one accommodates as well the (probably weaker) segregation tendency of the alkyl and aromatic parts. Furthermore, the second model gives an additional plane of symmetry that may be responsible for the stronger intensity of the second-order reflection.32–34 Neither model takes into account the possibility that the polar nitro groups may play a role in stabilizing the charges (although appropriate rearrangements of the charge positions might accommodate this possibility in the first model). More detailed studies using specialized techniques are necessary to verify or refine these models, and to understand why some of the complexes give a more intense second-order reflection and others do not.36
A SEM micrograph of the azo10Q/PSS45 complex, representative of all of the complexes, is shown in Figure 6. This image is characteristic of a homogeneous, amorphous morphology, consistent with the liquid crystalline nature of the material.
The effect of the stoichiometry of the complexed components was investigated using PSS45. The DSC thermograms for azo10Q/sulfonate molar ratios varying from 1.0 to 0.1, as well as the first derivatives of the thermograms (which highlight the two Tg-like transitions), are given in Figure 7. This figure indicates that there are two major consequences of reducing the proportion of surfactomesogen in these complexes. The first is that the LC-is transition decreases rapidly in intensity, and is no longer visible below a molar ratio of 0.7; it also shows some decrease in temperature. Thus, this transition appears to be quite sensitive to stoichiometry. Birefringence, however, was observed until much lower molar ratios, although with decreasing intensity and becoming barely visible for molar ratios of 0.2 and 0.1; it tends to disappear gradually in the region of 150 °C for molar ratios below 0.7, and is reversible as long as the temperature is not raised too much above this point, indicating a memory and/or viscosity effect. The gradual disappearance in birefringence to an apparent isotropic phase, with no corresponding transition observed by DSC, has been observed in other ionic mesomorphic polymer systems, and may be explained by a progressive decrease in the size or correlation length of the mesomorphic domains below the wavelength of visible light.8, 37 It was also noted that the lamellar spacings obtained from X-ray diffractograms are constant to a molar ratio of about 0.5 (with some apparent dependence on the thermal or sample history, which was not investigated in detail), suggesting block copolymer-like character for nonequimolar complexation in this system.17 X-ray data were less clear about the smectic nature of the complexes with compositions below 0.5 molar ratio.
The second consequence of the decrease in molar ratio is a concomitant increase in TgL. Figure 8 shows this increase to be approximately linear, at a rate of −0.9 °C/unit molar ratio. In contrast, TgU appears to remain approximately constant, although it becomes difficult to detect at lower molar ratios. However, the significant width of the Tg transition for the 0.4 molar ratio and the gradual decrease in this width with further decrease in molar ratio (particularly visible in the derivative thermograms in Fig. 7(b)) suggest a progressive merging of TgL with TgU as the former increases in value, and essentially complete merging at the 0.1 molar ratio.
From the point of view of the complex as a whole, an increase in Tg with decrease in azo10Q content would certainly be expected by virtue of the decreasing plasticizing effect of the azo10Q molecules, which act as long, flexible (noncrystallising) side chains. It is interesting that the linear extrapolation of the data in Figure 7 suggests a Tg of about 125 °C for PSS-Na (zero azo10Q content), which is much below than what would be expected for dry PSS-Na. This may be the consequence of bound water in the complexes, which can be very difficult to remove; but it is just as likely that the linear extrapolation below 0.1 is not justified.
From the point of view of nanophase separation, where TgL is attributed to the side-chain sublayer and TgU to the main-chain sublayer, it is less obvious how to rationalize the trends observed, especially if the complexation tends to occur in block-like fashion. It can be speculated that the decreasing sizes and correlation lengths of the lamellar domains with decreasing surfactomesogen content result in greater proximity of these domains to the less mobile and increasingly predominant Na-neutralized regions (to which they are intimately connected via the polymer backbone), and this may reduce the overall mobility of the lamellar domains along the lines of what is understood for microphase-separated ionomers.38 However, a much more detailed study of the present system, including by solid-state NMR to detect the specific mobilities of different molecular parts, is warranted to elucidate these questions.
We previously investigated these same complexes at the air–water interface, where the complexation takes place by spreading the surfactomesogen on the water subphase containing the polyelectrolyte.20 This led to monolayers that could be transferred repeatedly to solid substrates, giving Z-type multilayers. These gave a second harmonic generation (SHG) signal that was found to be stable up to temperatures as high as 130 °C (and at room temperature for at least 4 months). Correlations may be drawn with the present investigation. First, there may be a connection between the Z-type multilayers and the single-layer smectic A phase in the bulk, although the models shown in Figure 5 would not be compatible with Z-type multilayers (possibly due to over-riding interactions of the ultrathin films with the hydrophilic solid substrate). Second, the stability of the SHG signal may be related to the presence of the second Tg-like transition observed in the bulk, although a high viscosity and/or strong interactions with the substrate may also contribute (all three being consequences of the presence of the ionic groups). We are currently beginning to investigate how these complexes respond to photoinduced motion and birefringence, and the role that the upper Tg-like transition may play in the behavior.
An azobenzene-containing surfactomesogen with a quaternary ammonium headgroup was ionically complexed to a variety of oppositely charged polyelectrolytes in equimolar proportions. In all cases, this generates a single-layer smectic A mesophase over a wide temperature range. The clearing temperatures of the complexes are generally higher than the melting point (crystal-isotropic transition) of the surfactomesogen. The precise value depends moderately on the nature of the polyelectrolyte, as well as on its molecular weight. Complexes with polyacrylate and polymethacrylate are thermally unstable, whereas those with sulfonated polyelectrolytes as well as carboxymethylcellulose (studied for a degree of substitution of 0.9) are stable.
A clearly visible glass transition is located a little above the ambient temperature. It has no apparent dependence on molecular weight, but is somewhat higher for the cellulose polyelectrolytes, possibly due to their greater rigidity. A second glass transition-like event, with a much smaller heat capacity, was found at higher temperatures, usually above 100 °C, in several of the complexes. In line with a few other such reports in the literature, this was tentatively rationalized by nanophase separation between the mesogenic side-chains and the ionic backbone, giving distinct sublayers within the lamellar planes; these may then account for the lower and upper glass transitions, respectively. In nonequimolar complexes, it was found that the lower glass transition increases significantly with decrease in surfactomesogen content (below equimolar), whereas the upper one remains approximately constant. It is not immediately clear how to rationalize this in terms of the nanophase separation concept; however, the increasing presence of Na-neutralized polymer repeat units should certainly lead to a rise in glass transition.
The financial support of NSERC (Canada) and FCAR (Québec) is gratefully acknowledged. We thank Drs. Michael Georges and Danièle Boils, along with Xerox Research Center Canada, for the gift of sulfonated polystyrenes and the GPC measurements, respectively.