Liquid crystalline twin epoxy monomers based on azomethine mesogen: synthesis and curing with aromatic diamines

doi:10.1016/S0032-3861(00)00111-7

Polymer

Volume 41, Issue 21, October 2000, Pages 7617-7625

https://doi.org/10.1016/S0032-3861(00)00111-7 Get rights and content

Abstract

Liquid crystalline (LC) epoxy monomers with twin mesogenic azomethines connected by different alkylene spacers (x=6, 7, 8, and 9) were synthesized to prepare LC thermosets. The epoxy monomers were characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC) and polarized microscopy. All four epoxy monomers were found to form nematic phase in the melt, and their melting (T_m) and isotropization temperatures (T_i) as well as corresponding thermodynamic values (ΔH_m, ΔH_i, ΔS_m and ΔS_i) decreased with increase of x in a zig-zag fashion, revealing the even–odd effect. Cure behavior of a stoichiometric mixture of an epoxy monomer with x=9 and diaminodiphenlysulfone (DDS) was investigated by DSC for specimens cured under various cure conditions. Nematic LC texture for the cured specimen was identified by polarized microscopy and X-ray diffractometry. From the DSC results and microscopic observations, phase diagram of cure time vs. transition temperature was constructed for the epoxy (x=9)/DDS system. This diagram shows the changes of T_m, T_i and glass transition temperatures (T_g) as cure proceeds. Regions for nematic LC phase are indicated in the diagram.

Introduction

During the past decade, there was renewal of interest in liquid crystalline (LC) thermosets because of their electrical, mechanical, and optical properties for advanced applications such as microelectrical packing, matrix for composites, and non-linear optics, respectively [1], [2]. The excellent properties of LC thermosets stem from the preservation of molecular organization in the mesophase of LC precursors and monomers by means of crosslinking. The resulting highly crosslinked thermoset containing rigid rod molecules can offer improvement of rather poor properties in transverse direction to the chain orientation.

Of possible thermosetting polymers, epoxy resins are commercially important because of their superior adhesion, heat and corrosion resistance, and mechanical and electrical properties than other classical thermosets. Incorporating LC structure into epoxy network could enhance the properties [3]. Over last few years, several research groups have studied synthesis and curing of LC diepoxides with a flexible spacer between epoxide moiety and mesogenic group [4], [5], [6], with two mesogens linked by a central flexible spacer [7], [8], and with substituents in mesogenic units [9], [10], [11].

Aromatic poly(azomethines), also called polyimines or Schiff-base polymers, have been extensively studied, since the thermotropic LC poly(azomethines), which were readily spun into high-strength fibers, have been synthesized [12], [13]. As in LC aromatic polyesters [14], many of the poly(azomethines) are insoluble in common organic solvents [15], [16]. Even LC azomethine polymers with flexible spacers between mesogens have showed poor solubility but could maintain good thermal stability [17]. On the other hand, quite a few studies have been published on LC diepoxides containing azomethine mesogens. In an early paper published in the preparation of networks from oligomeric azomethines with epoxides groups [18], the mesogenic properties of the diepoxides did not appear. Synthesis of diepoxide monomers with diad or triad aromatic rings linked by azomethine groups have been recently published. However, in the paper the curing of the diepoxides was not described [11], and later the crosslinking anionic polymerization of the diepoxides has been reported [19].

In this work, we synthesized and characterized twin epoxy monomers having two mesogenic azomethines linked by a central alkylene spacer with different lengths. Thermal and mesomorphic properties of the epoxy monomers and their mixtures with amine curing agents were investigated. Time–temperature phase diagram during isothermal curing for an epoxy/amine system was presented.

Section snippets

Synthesis

Synthetic route to LC twin epoxy monomers with mesogenic azomethines varying the length of the central polymethylene spacers (x) from 6 to 9 is shown in Scheme 1. Since the synthetic procedures used to prepare the epoxy monomers were essentially same, a representative epoxy with x=9 is given in the following.

Nonamethylene 1,9-bis(4-oxybenzaldehyde). In a typical synthesis, 4-hydroxybenzaldehyde (18 g, 0.15 mol) was dissolved in 60 ml of DMF at 80°C. To this solution were added 1,9-dibromononane (21

Synthesis, thermal and mesomorphic properties of LC epoxy monomers

The molecular structures of the epoxy monomers shown in Scheme 1 were evaluated by means of FT/IR and NMR techniques. Fig. 1 shows the ¹H NMR spectrum of an epoxy monomer with x=9, including the peak assignments corresponding to the structure. As shown in the figure, of methylene protons in epoxy ring, proton 1 displayed two doublets at 2.79–2.98 ppm, while proton 2 exhibited a multiplet centered at 3.42 ppm. Proton 3 in glycidyl ether group was expected to reveal two doublets: one was found at

Conclusion

Twin epoxy monomers based on the azomethine mesogen having different alkylene spacers (x=6, 7, 8, and 9) were synthesized, and cure behavior of the epoxy monomers with amine curing agents was investigated in this study. From the FT/IR and NMR spectroscopic analysis, molecular structures of the epoxy monomer were identified. A polarized microscope was used to observe mesomorphic optical texture. From the thermal behavior of epoxy monomers and mixtures of epoxy/amine investigated by DSC, an epoxy

Acknowledgements

This paper was supported in 1999 by Research Fund from Kumoh National University of Technology. J.-I. Jin would like to acknowledge the support of this research by the Korean Science & Engineering Foundation.

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Liquid crystalline twin epoxy monomers based on azomethine mesogen: synthesis and curing with aromatic diamines

Abstract

Introduction

Section snippets

Synthesis

Synthesis, thermal and mesomorphic properties of LC epoxy monomers

Conclusion

Acknowledgements

Prog Polym Sci

Prog Polym Sci

Polymer

Polymer

Eur Polym J

Macromolecules

J Polym Sci: Part A: Polym Chem

Macromolecules

J Polym Sci: Part A: Polym Chem

J Polym Sci: Part A: Polym Chem

Macromol Chem Phys