Poly(ether imide)s from Diamines with <i>Para</i>-, <i>Meta</i>-, and <i>Ortho</i>-Arylene Substitutions: Synthesis, Characterization, and Liquid Crystalline Properties

Abstract

We have synthesized homologous series of para -, meta -, and ortho -substituted aryl ether diamine monomers, with either 2, 3, or 4 ether linkages per monomer unit, and prepared their corresponding poly(ether imide)s with 3,3′,4,4′-biphenyl dianhydride (BPDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). All polymers were obtained in high molecular weights and gave good quality films with expected mechanical and thermal properties. The ortho - and meta -substituted diamines gave fully amorphous polymers, whereas the para -based diamines resulted in semicrystalline polymers. The glass-transition temperatures ( T g ) drop in the order of para > ortho > meta, and the T g drops considerably as a function of the aryl ether content and appears to level off at four ether linkages. The T g values of our polymers were contrasted with a simple quantitative model and found to be in good agreement with the experimental results (±10 °C). BPDA in combination with an all para -substituted, aryl ether-based diamine (BPDA- P 3 ) forms a thermotropic liquid crystalline phase. Optical microscopy experiments confirm the presence of a nematic melt. Highly aligned films could easily be obtained by stretching the films in the liquid crystal phase, and XRD analysis of quenched films confirmed the presence of a highly aligned smectic A phase (SmA) with an order parameter ⟨ P 2 ⟩ = 0.87, indicating a high degree of molecular alignment. To the best of our knowledge, this is the first example of an all-aromatic liquid crystalline poly(ether imide). Dynamic mechanical thermal analysis (DMTA) showed that the para -aryl ethers display broad β-transitions (25–160 °C), whereas the meta - and ortho -series do not show β-transitions. All PEIs with 3 or 4 aryl ether linkages become thermoplastic in nature. The purpose of this systematic study is to provided a basic set of design rules toward the design and synthesis of all-aromatic poly(ether imide) architectures.