J. MATER. CHEM., 1995, 5( 12), 2225-2228 2225

Towards Supramolecular Side-chain Liquid-crystal Polymers Part 3.+-Miscibility of Mesogenic Acids and Amorphous Polymers

Katherine 1. Alder, David Stewart and Corrie T. Imrie" Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, Scotland, UK A69 2UE

The thermal behaviour of blends of 4-n-octyloxybenzoic acid with polystyrene, poly(4-vinyl pyridine) and poly(2-vinyl pyridine) has been characterised using differential scanning calorimetry and polarised light microscopy. The acid is essentially immiscible with polystyrene over the whole composition range. However, molecular mixing is observed for blends containing up to an acid mole fraction of ca. 0.3 with either poly(4-vinyl pyridine) or poly(2-vinyl pyridine). This miscibility is driven by hydrogen-bond formation between the acid and pyridine groups. At higher concentrations of the acid, phase separation occurs although the presence of hydrogen bonding is evident in the IR spectra. No pronounced differences are observed between the blends containing the differing isomers of poly(viny1 pyridine). A model is proposed which interprets these data in terms of two competing processes: the self-association of the acid- forming dimers and the specific interaction between the acid and pyridine groups, yielding hydrogen-bonded complexes.

The principles of supramolecular ~hemistryl-~ have, in recent years, been applied in the design of novel mesogenic polymer^.^ In main-chain systems, the general approach has been to blend bifunctional monomers containing two or more comp- lementary groups, i.e. hydrogen-bond donors and acceptors, to yield a linear polymer; in this context, polymer denotes a regular association of many molecules. Lehn and c o - ~ o r k e r s ~ . ~ assembled such a structure using triply hydrogen-bonded moieties, while Alexander et al.' showed that the same archi- tecture can also be achieved using just a single hydrogen bond as the chain extender. The principal approach in the design of supramolecular side-chain liquid-crystal polymers, pion- eered by Katu, Frechet and co-workers, has involved the construction of the mesogenic unit via the interaction of a preformed polymer containing a pendant binding site and a complementary low molar mass co rnpo~nd .~ - '~ More recently, Malik et ~ 1 . ' ~ attached the mesogenic unit to the flexible spacer via a hydrogen bond and observed liquid crystallinity in blends of otherwise amorphous materials. Another possibil- ity exists in the design of supramolecular side-chain liquid- crystal polymers, however, in which the spacer bearing the mesogenic unit is non-covalently connected to the amorphous polymer backbone. Bazuin and Brandys16 described such a system in which a mesogenic acid was blended with poly(4- vinyl pyridine) and the resulting complex was reported to be liquid crystalline. However, our own studies of such sys- t e m ~ ' ~ ~ ' ~ have shown that to interpret these results in terms of a supramolecular side-chain polymer is incorrect and Bazuin et ~ 1 . ' ~ have also modified their initial interpretation of the data. Specifically, the systems we studied, comprising mesogenic acids, 1, blended with polystyrene, poly( 2-vinyl pyridine) and poly (4-vinyl pyridine), revealed that the acids are immiscible with polystyrene but exhibit molecular mixing with both poly( 2-vinyl pyridine) and poly(4-vinyl pyridine) up to an acid mole fraction of ca. 0.2 in the blend. Phase separation occurs with both polymers for higher concen- trations of acid. Indeed, the thermal behaviour of the poly(2- vinyl pyridine) and poly( 4-vinyl pyridine) based mixtures is remarkably similar. These results are surprising for two reasons: (i) the alkanoic acid-pyridine interaction is known to stabilise liquid crystallinity,20 whereas in these systems it results in only partial miscibility and at levels insufficient to support liquid crystallinity, and (ii) polymer blends containing either poly(2-vinyl pyridine) or poly( 4-vinyl pyridine) with a

hydrogen-bond donor often behave differently; specifically poly(2-vinyl pyridine) tends to be less miscible than poly(4- vinyl pyridine) for steric rea~ons .~ ' -*~ In order to investigate these matters further, we have characterised the thermal behaviour of blends containing 4-n-octyloxybenzoic acid, 2, and polystyrene, poly( 2-vinyl pyridine) or poly( 4-vinyl pyri- dine). These particular systems were chosen for several reasons: (i) the acid and polystyrene are not expected to exhibit any significant interaction and hence this system can be used as a control against which the other two may be compared; (ii) the study of both isomers of poly(viny1 pyridine) will allow the steric issue to be considered; (iii) blends of derivatives of benzoic acid and pyridine are known to exhibit enhanced or induced liquid cry~tallinity.~-'~.~~-~~ Note that we did not expect the mixtures of acid 2 and the poly(viny1 pyridine)s to exhibit liquid crystallinity because it is the self-associated acid dimers that are responsible for mesogenic behaviour and these are necessarily destroyed upon complexation with the poly- mer. Instead, we consider the systems to be of use in enhancing our understanding of the role of secondary interactions in promoting self-assembly in monomer-polymer systems.

Experiment a1 Materials

4-n-Octyloxybenzoic acid and polystyrene (molecular weight standard, M , = 45 730) were obtained from Aldrich and used without further purification. Poly( 2-vinyl pyridine) ( M , = 200 000) was obtained from Polysciences while poly( 4-vinyl pyri-

2 ~ ~~~~

Part 2: D. Stewart and C. T. Imrie, Liq. Cryst., 1995, submitted.

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2226 J. MATER. CHEM., 1995, VOL. 5

dine) was prepared via a free radical mechanism as described previ0us1y.l~

Blends

Blends of the acid with poly( 4-vinyl pyridine) were prepared by codissolving the components in pyridine and allowing the solvent to evaporate slowly. For the polystyrene and poly( 2- vinyl pyridine) based systems, THF was used as the solvent. All the mixtures were subsequently dried under vacuum for several days prior to characterisation.

Thermal Characterisation

The thermal behaviour of the blends was characterised by differential scanning calorimetry using a Polymer Laboratories PL-DSC equipped with an autocool accessory and calibrated using indium. The time-temperature pro- gramme used for each sample was identical: heated from 25 to 200 "C, held at 200 "C for 3 min, cooled from 200 to -5O"C, held at -50 "C for 3 min and finally reheated from -50 to 200°C. The heating and cooling rates in all cases were 10 "C min-l. Phase identification was performed by polarised light microscopy using an Olympus BH-2 optical microscope equipped with a Linkam THMS 600 heating stage and a TMS 91 control unit. The IR spectra of the blends were recorded using an AT1 Mattson Genesis Series FTIR spec- trometer. These samples were prepared as thin films on NaCl discs and were heat-treated under vacuum at 150 "C for 20 min prior to characterisation.

Results and Discussion The 4-n-octyloxybenzoic acid used in these investigations exhibited a crystal-smectic C transition at 103 "C, a smectic C-nematic transition at 108 "C and cleared at 144 "C; these temperatures are in good agreement with the literature values.30 Nematic phases were assigned on the basis of schlieren optical textures which flashed when subjected to mechanical stress, combined with the high mobility of the phase. On cooling the nematic phase, a sanded schlieren texture was obtained which did not flash when subjected to mechanical stress; in consequence, this was assigned as a smectic C phase. The phase diagrams for blends of this acid with polystyrene, poly (4-vinyl pyridine) and poly( 2-vinyl pyri- dine) are shown in Fig. 1-3, respectively. Fig. 1 shows that the acid and polystyrene are essentially immiscible over the

0.0 0.2 0.4 0.6 0.8 1.0 Xacid

Fig. 1 Dependence of the glass transition temperature (0), melting point (a), smectic C-nematic transition (0) and nematic-isotropic transition (H) on the composition of the acid-polystyrene blend; a crystal-crystal transition has been omitted for the sake of clarity. I, isotropic; N, nematic; K, crystal.

T I

120 1 0 ' N = = v fq 60 0 0

I ' K

0-l . 1 - I . I . r ' 4 0.0 0.2 0.4 0.6 0.6 1.0

Xacid

Fig. 2 Dependence of the glass transition temperature (0), melting point (a), smectic C-nematic transition (0) and nematic-isotropic transition (U) on the composition of the acid-poly(Cviny1 pyridine) blend; a crystal-crystal transition has been omitted for the sake of clarity. I, isotropic; N, nematic; K, crystal.

180 I 1

1 20 I I

* ' N =

K

0.0 0.2 0.4 0.6 0.8 1.0 Xacid

Fig. 3 Dependence of the glass transition temperature (0), melting point (a), smectic C-nematic transition (0) and nematic-isotropic transition ( W) on the composition of the acid-poly( 2-vinyl pyridine) blend; a crystal-crystal transition has been omitted for the sake of clarity. I, isotropic; N, nematic; K, crystal.

entire composition range; specifically, for the blends with 0.20-0.89 mole fraction of acid, the transition temperatures are identical to those of the pure acid. For the 0.10 mole fraction acid mixture, the clearing temperature is reduced by 19°C while the C-Sc and Sc-N transition temperatures are unchanged. The most dilute blend, i.e. 0.05 mole fraction acid, does not exhibit liquid crystallinity and instead undergoes a sluggish crystallisation directly from the isotropic phase. This dependence of the transition temperatures upon blend com- position is essentially identical to that observed for blends of polystyrene with acids 117,18 and indicates a high degree of immiscibility between the two components. This view is supported by the observation of phase separation in the isotropic phase when viewed through the polarising micro- scope. However, as was noted for the acids 1 , 1 7 9 1 8 uniform crystalline and liquid-crystalline textures are obtained over the whole preparation. This observation presumably implies that droplets of the acid are dispersed in the polystyrene-rich phase, the size of which lies below the resolving power of the microscope, but give rise to uniform textures. A similar explanation accounts for the textures obtained for polymer dispersed liquid-crystal films.31

The modification of the thermal behaviour of the acid in the most dilute polystyrene-based mixtures indicates that, to a very limited degree, the two components are miscible. The polystyrene acts as an isotropic solute in the acid-rich phase,

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J. MATER. CHEM., 1995, VOL. 5 2227

so reducing the transition temperatures. The clearing trans- ition is thermodynamically considerably weaker than the melting transition and hence more susceptible to being affected by the presence of a solute. It is surprising, therefore, that the Sc-N transition temperature is not reduced for the 0.10 mole fraction acid blend. The absence of liquid crystallinity for the most dilute acid blend indicates that proportionally a greater amount of polystyrene is molecularly dissolved in the acid- rich phase, while the reduction of Tg for polystyrene reflects a plasticization by the acid.

The two components in the polystyrene-acid mixtures are essentially non-interacting and this view is supported by IR spectroscopy. The spectra obtained for the blends are simply the combined spectra of the two individual components. Thus this system may be used as a control against which mixtures containing interacting components may be compared.

Fig. 2 shows the dependence of the transition temperatures upon blend composition for the poly( 4-vinyl pyridine)-based system. In contrast to the behaviour of the polystyrene-based mixtures (see Fig. 1), a marked dependence on composition is observed. The thermal behaviour of the 0.89, 0.80 and 0.71 mole fraction acid mixtures is essentially identical to that of the pure acid. In the 0.61 and 0.53 mole fraction blends, the clearing temperature is reduced by 7 and 14"C, respect- ively, while the melting and s,-N transition temperatures are unchanged. This suggests that a limited amount of molecular mixing occurs in these mixtures. For the 0.39mole fraction acid blend, the clearing temperature has been reduced by 35 "C, the melting point is unchanged but the smectic C phase has been extinguished. The blends containing acid mole fractions of 0.31 or less remain isotropic upon cooling to - 50 "C; only a glass transition is evident in the DSC traces for these mixtures. Tg for poly( 4-vinyl pyridine) decreases rapidly upon increasing the acid concentration. These results suggest that molecular mixing occurs between the components up to an acid mole fraction of 0.31, and increasing the acid concentration further results in phase separation. Optical microscopy did not detect phase separation in any of these mixtures, but presumably the domain sizes lie below the resolving power of the microscope, i.e. the two components have a higher degree of compatibility than polystyrene and the acid.

The miscibility observed for the poly (4-vinyl pyridine)- based mixtures is driven by hydrogen-bond formation between the acid and pyridine groups. Evidence for this specific interaction may be found in the IR spectra of the blends; specifically, the spectra of the blends contain new bands at ca. 1930 and 2550 crn-', indicative of strong hydrogen bond- ing,32 and the carbonyl band at 1685 cm-' for the pure acid is shifted to 1700 cm-' in the more dilute blends. The spectra contain evidence to support the occurrence of hydrogen bonding in all the blends but it is most pronounced in the more dilute samples for which molecular mixing was observed. For example, the carbonyl stretch in the equimolar blend is very similar to that observed for the pure acid.

Fig. 3 shows the dependence of the transition temperatures on composition for the poly (2-vinyl pyridine)-based mixtures and reveals qualitatively identical behaviour to that observed for the poly(4-vinyl pyridine) mixtures (see Fig. 2). Thus for the blends containing mole fractions of acid of 0.68 or greater, the thermal behaviour observed is identical to that of the pure acid, i.e. phase separation has occurred. For the 0.59 and 0.47 blends the clearing temperature is reduced by ca. 11 "C, while the melting and Sc-N transition temperatures are unchanged. For the 0.44 mole fraction acid blend, the clearing temperature has been reduced by 14°C and the smectic C phase was extinguished. The reduction in the clearing temperature is not as great as that seen for the analogous

poly (4-vinyl pyridine) blend, suggesting a steric effect, albeit a small one. Phase separation was not detected optically in these blends. The blends containing an acid mole fraction of 0.29 or less mix molecularly and remain amorphous to -50°C. The IR spectra obtained for these blends are also essentially identical to those described for the poly( 4-vinyl pyridine) mixtures and hence support the view that hydrogen- bond formation drives miscibility.

Molecular mixing appears to occur, therefore, up to an acid mole fraction of ca. 0.3 in the poly(2-vinyl pyridine) and poly( 4-vinyl pyridine) blends and is driven by hydrogen-bond formation. This prompts the question, however, as to why the equimolar blends, in which hydrogen-bond formation should be maximised, appear to undergo phase separation. To under- stand this, we need to consider two competing equilibria: self- association of the acid to form dimers and the inter-association of the acid and pyridine to form the hydrogen-bonded com- plex. It is the balance between these that determines the degree of molecular mixing in the blends. Unfortunately, the IR spectra are too complex to allow for even a semi-quantitive estimate of the relative amounts of each species. Studies of polymeric systems have shown, however, that the equilibrium constant for the self-association of carboxylic acids is greater than that for complex formation, despite the fact that the hydrogen bond between the unlike components is the stronger i n t e r a ~ t i o n . ~ ~ This suggests that the entropic contribution to hydrogen-bond formation can play a decisive role in determin- ing the phase behaviour. For the polymer-monomer systems described here, we propose that at low concentrations of acid, the acid-pyridine complexes are randomly dispersed amongst non-complexed pyridine groups, and this randomisation of the acid units assists miscibility. As the concentration of acid is increased, acid self-association becomes entropically more favourable. The resulting acid dimers are immiscible with the polymer and hence phase separation occurs. However, by increasing the strength of the hydrogen-bond interaction, the concentration at which this separation occurs can be shifted upwards. For the benzoic acid derivative reported here, mol- ecular mixing occurs up to an acid mole fraction of ca. 0.3, whereas for alkanoic acid derivatives molecular mixing occurs below an acid mole fraction of ca. 0.2.17,18 On the basis of the pK, values, the benzoic acid derivative would be expected to form a stronger hydrogen bond with pyridine than an alkanoic acid derivative. This view is further supported by the IR spectra of these systems, which reveal more intense bands associated with hydrogen-bond association for the benzoic acid systems than for the alkanoic acids. The occurrence of hydrogen-bond formation for all the blend compositions indicates that the interphase region is stabilised via hydrogen bonding and hence a high degree of compatibility is observed. This model also accounts for the absence of any pronounced differences between the poly( 4-vinyl pyridine) and poly( 2- vinyl pyridine) systems arising from steric considerations because molecular mixing occurs only at relatively low concen- trations of acid. As the concentration of acid is increased steric difficulties would become more pronounced, and differ- ences in behaviour would be expected, but phase separation occurs prior to such an arrangement.

A consequence of this model is that in order to utilise hydrogen bonding in the construction of supramolecular side- chain liquid-crystal polymers, in which the side-chain is non- covalently bound to the backbone, the strength of the hydro- gen bond should be increased. Simply increasing this, however, is likely to result in proton transfer. An alternative approach is to use complementary units containing multiple binding sites, and this has been used, for example, to solubilise melamine in a copolymer-containing maleimide33 as well as

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2228 J. MATER. CHEM., 1995, VOL. 5

to construct mesogenic ~ n i t s . ~ , ~ * ~ ~ Such materials are currently under investigation.

We are pleased to acknowledge support from EPSRC, grant number GR/J32701, and the University of Aberdeen Research Committee for the award of a grant to purchase the PL-DSC.

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Paper 5103535J; Received 2nd June, 1995

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