ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия Б, 2012, том 54, № 9, с. 1475-1483
ЖИДКОКРИСТАЛЛИЧЕСКИЕ ПОЛИМЕРЫ
УДК 541(64+49):547.57
HYDROGEN-BONDED LIQUID-CRYSTALLINE COMPLEXES OF POLYESTER CONTAINING A PYRIDYL MOIETY WITH 4-(ALKOXY)BENZOIC ACID1
© 2012 г. Massoumeh Bagheri and Naser Jabbarvand Behrouz
Chemistry Department, Science Faculty, Azarbaijan University of Shahid Madani, 5375171379 Tabriz, Iran
e-mail: massoumehbagheri@yahoo.com Received December 23, 2012 Revised Manuscript Received March 29, 2012
Abstract — Novel hydrogen-bonded polyester complexes from mesogenic 4-(butyloxy)benzoic acid (2a), 4-(octyloxy)benzoic acid (2b), 4-(dodecyloxy)benzoic acid (2c) and 4-(tetradecyloxy) benzoic acid (2d) as the hydrogen bond donors and a polyester (3) based on 2,6-pyridine dicarboxylic acid as the hydrogen bond acceptor were prepared by the melt method. The association by hydrogen bonding was confirmed by means of FTIR. The components were miscible up to 0.1 mole ratio of 2d and 2c versus the polyester repeat unit. The limiting mole ratio for 2b and 2a to 3 were 0.2 and 0.4, respectively. Phase separation in the supramolecular complexes occurred above these limiting values and a two phase system consisting of a polyester supramolecular complex and 4-(alkyloxy)benzoic acid was formed. This was due to weak hydrogen bonding between the low molecular acid (especially the acid with longer terminal alkoxy group) and pyridyl unit of polyester in comparison with thermodynamically more favorable dimerization of acid molecules. The liquid crystalline behavior of these supramolecular polymeric complexes was confirmed by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). It was found that 2a exhibits no mesomorphism when mixed with polyester. However, among the other acid derivatives, 2b exhibited stable mesogenic complexes.
INTRODUCTION
Liquid-crystalline polymers (LCP) have attracted a great deal of interest in recent years because they found a number of commercial applications ranging from high-strength engineering plastics to optical display devices [1—4]. Side-chain liquid-crystalline polymers (SCLCP) have even received increasing attention during last decades due to their intriguing properties and broad range of potential applications especially in the field of microelectronics, optical data storage systems and non-linear optics [5—7]. These materials are traditionally prepared by covalently linking a rigid organic molecule to a polymeric backbone via a flexible aliphatic spacer.
However, liquid crystalline (LC) materials, formed through non-covalent interaction, have only recently received considerable attention [8—10]. In these systems, liquid-crystallinity is induced or stabilized through non-covalent interactions between complementary binding sites of the polymer main-chain and low molar mass rigid mesogens. Because of the reversibility of non-covalent forces, supramolecular materials have dynamic molecular structures more sensitive to external conditions and stimuli than conventional covalently bonded mesomorphic polymers. In addition, the relatively simple preparation of these materials allows a wide variety of functional systems to be designed [11].
1 Статья печатается в представленном авторами виде.
One class of these materials are supramolecular SCLCP, formed by hydrogen bonding interactions, [12—16] these group attracted more attention due to their promising applications as optical storage and display devices. In order to attain such SCLCP supramolecular structures, a low mass molecule and a polymer backbone must possess complementary binding sites, namely, H-bond donors and H-bond acceptors.
A number of supramolecular SCLCP assembled via pyridyl-acid hydrogen bonds have been prepared [12—14, 17]. However, the carboxylic acid moieties are strongly self-associated through the formation of intermolecular carboxylic acid dimers [18], which contains two hydrogen bonds for each dimer. In order to achieve hetero-intermolecular complexation, the formation of a more thermodynamically stable kind of hydrogen bonding than that of the acid dimers is required.
These hydrogen bonds are widely applicable to various types of functional polyacryl ate [19—22] and pol-ysiloxanes [23] as well as low molecular weight complexes. Poly [(meth)acrylic acid]s, poly(vinylpyri-dine)s, polyamides and polyurethanes are used as the backbones capable of hydrogen bonding for the preparation of supramolecular polymers [12]. It has been shown that 2,6-te-(amino)pyridine moieties can function as a molecular component for mesogenic complexes through the formation of double hydrogen bonds [24]. Kato, T. et al. described the examples of such supramolecular polymers obtained by self-assembly of 4-(alkoxy)benzoic acid derivatives and
4-(4'-(octyloxy) phenyl)benzoic acid [25]. Supramo-lecular mesogenic side-chain polymers have been prepared from a polyacrylate containing a benzoic acid side chain and 2,6-bis-(acylamino) pyridines [10].
One of the structural parameters defining the molecular packing in some supramolecular liquid crystalline systems is the length of the terminal oligomethyl-ene group in the low molar mass component [19]. However, contrary to covalently bonded liquid crystalline polymers in supramolecular systems there is no set of general rules acceptable as a general guideline for the preparation of other macromolecular complexes, since mesomorphic behavior varies according to the
individual polymer. For example, Zigon et al. [13] showed that in supramolecular polyurethane complexes based on a polyurethane with a pendant pyridyl unit and 4-(octyloxy)benzoic acid or 4-(buty-loxy)benzoic acid, the complexes with shorter side-chains, crystallization of the main-chain takes place through the urethane C=O, N—H and pyridine groups, while systems with longer alkoxy groups exhibit a high degree of side-chain ordering via the more effective van der Waals interactions between the aliphatic tails. On the other hand, in the case of polymeric systems formed from polyamide with a 2,6-bis-(amino)pyridine moiety and 3-substituted 4-(alkoxy)benzoic acids (n = 6, 8 and 10), the highest melting temperatures were obtained for complexes with 4-(hexyloxy)benzoic acid derivatives possessing the shortest alkoxy groups [25].
In this work, we report on the preparation and characterization of new supramolecular SCLC polyester obtained by self-assembling of the polyester backbone containing pyridyl unit and 4-(alky-loxy)benzoic acid through hydrogen bonding. The purpose of this study was to investigate the effect of the polymer backbone on the hydrogen bonding interaction and miscibility of components of the supramolecular polyester complexes. It is important to investigate if the ratio of a mesogenic acid per pyridine unit of polyester would be necessary to create the mesomorphic supramolecular complex. We also studied the influence of the length of alkyl tails on the stabilization of the supramolecular polyester complexes.
EXPERIMENTAL
Materials
The purification and drying of compounds and solvents was carried out according to the common procedures. 2,6-Pyridindicarboxilic acid, 4-(hydroxyl)ben-zoic acid (1), 4-dibromobutane, 1-bromobutane, 1-bromooctane, 1-bromododecane, 1-bromotetrade-cane were purchased from Merck and used as received. 4-(Alkoxy)benzoic acids containing 4, 8, 12 and 14 number of carbon atoms in alkoxy group (2a—2d) were synthesized according to a literature procedure [18, 25] (Scheme 1). The polymer was synthesized ac-
cording to the route outlined in Scheme 2. Further details are given below.
Measurements
Spectroscopic characterization was carried out by the following procedure and instrumentation: melting points were recorded with an electrothermal 9100 apparatus. FTIR Spectra were recorded on a Brucker PS-15 spectrometer. 1H NMR Spectra were taken on a 400 MHz Brucker SP-400 AVANC spectrometer using chloroform as solvent with tetramethylsilane as internal standard. A differential scanning calorimeter Mettler 822 was used to determine phase transition temperatures at the heating and cooling rates of 10 grad/min. The instrument was calibrated with indium regarding temperature and enthalpy. An optical Zeiss polarizing microscope equipped with long working distance objective was also used to observe phase transitions. The samples were heated and cooled with a TMS94 hot stage and associated temperature controller. Thermogravimetric analysis (TGA) was carried out by using a Mettler-Toledo 822.
Synthesis
2,6-Pyridinedicarboxylic acid chloride. 2,6-Pyri-dine dicarboxylic acid (5 g, 37 mmol) was refluxed with thionyl chloride (8 ml) and dry dimethylforma-mide (DMF, 2 drops). After 5 h the excess of thionyl chloride was distilled off under reduced pressure and the residue was washed with dry hexane three times. The obtained acryl chloride was white solid in 91% yield and mp 57-59°C.
FTIR (KBr): v 3416 (NH, pyridine), 3085 (aromatic C-H), 1753 (C=O), 1637 and 1575 (C=C) cm-1.
Preparation of polyester 3. Polyester 3 was synthesized according to the route outlined in Scheme 2. 1,4-Butandiol (0.9 g, 10 mmol), triethylamine hydrochloride (0.14 mg, 1 mmol) and o-dichlorobenzene (10 ml) was placed in a flask. 2,6-Pyridinedicarboxylic acid chloride (2.04 g, 10 mmol) in o-dichlorobenzene (10 ml) was added dropwise through a funnel. This mixture was heated slowly until 150°C for a period of 2 h under nitrogen flow. The mixture was stirred at 170°C until the evolution of hydrogen chloride ceased. The crude polymer was precipitated in n-hex-ane, washed with methanol and dried under vacuum for 48 h to afford 3 as white powder with 82% yield. 1H NMR (CDCl3), ppm: 8H 2.04 (s, 2H, CH2CH2OCOAr), 4.52 (s, 2H, CH2CH2OCOAr), 8.03 (t, 1H, ArH ortho to -N), 8.29 (d, 2H, ArH meta to -N) ppm. FTIR (KBr): V 3063 (aromatic C-H), 2985 (CH2), 1740 (C=O), 1577 (C=C), 1290 and 242 (C-O) cm-1.
Preparation of hydrogen-bond
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