Soluble Liquid Crystalline Poly(ester imide)s with High Glass Transition Temperatures and Improved Dielectric Properties

Abstract

At present, high-performance liquid crystalline polyesters with a good processability, low dielectric constant, and low dielectric loss at a high frequency (GHz) are in high demand ascribed to the rapid development of high-speed communication technology. Here, an imide dicarboxylic acid (IA) with a bulky phenyl side group is designed and synthesized. It is then copolymerized with 4-hydroxybenzoic acid (HBA), isophthalic acid (IPA), 4,4′-dihydroxybipheny (BP), and 4,4′-dihydroxydiphenyl ether (ODP) via solution polycondensation to obtain liquid crystalline poly­(ester imide)­s (LCPEIs). By controlling the feeding ratio of IA, the solubility, thermal stability, mechanical properties, and dielectric properties of the LCPEIs are tunable. When the molar fraction of IA reaches or exceeds 5%, the obtained LCPEIs are soluble in ordinary organic solvents, such as chloroform, N,N-dimethylformamide, N-methyl-2-pyrrolidinone, and so forth, which allows a simple solution casting to prepare isotropic LC films. The nematic LCPEIs show high glass transition temperatures (T g > 200 °C) but relatively low melting temperatures and isotropization temperatures (T m and T i, <340 °C). The LCPEI prepared with 10 mol % IA shows excellent combined properties, whose ηinh, M n (GPC), T g, T m, T i, T d 5%, tensile strength, tensile modulus, water uptake, dielectric constant, and dielectric loss (at 10 GHz) are 0.92 dL g–1, 20,900 g mol–1, 218 °C, 268 °C, 330 °C, 458 °C, 80.5 MPa, 4.0 GPa, 0.39%, 3.07, and 0.006, respectively. These LCPEIs can be promising candidates for flexible copper-clad laminate substrates.

Effect of Fluorinated Substituents on Solubility and Dielectric Properties of the Liquid Crystalline Poly(ester imides)

“…The linear coefficient of thermal expansion (CTE) is measured by thermomechanical analysis (TMA). The CTE values listed in Table are in the range of 75.2–88.9 ppm K –1 , higher than those of the LCPEIs films prepared in our previous work . It shows that the incorporation of the bulky and fluorinated substituents into polymer chains hinders their tightly alignment, thus increasing the CTE values of the films. , …”

“…The fluorinated imide diacids are activated by benzenesulfonyl chloride to produce a hybrid acid anhydride, and then they react with the hydroxyl groups of diphenols to generate the ester linkages . Here, 50 mol % HBA and 10 mol % IAs are used based on our previous work, where the corresponding PEI shows the best combined properties . The specific experiment recipes are also shown in Scheme .…”

“…24 Here, 50 mol % HBA and 10 mol % IAs are used based on our previous work, where the corresponding PEI shows the best combined properties. 14 The specific experiment recipes are also shown in Scheme 2. When BP and ODP are used as bisphenol monomers, the obtained PEIs are denoted as LCPEI-nF(D), where nF(D) is corresponding to the fluorinated IA (3FIA, 4FIA, 6FIA, or 6FDIA).…”

“…In our previous work, triphenylmethane-modified LCPEIs were prepared and characterized, which show relatively high T g s and good solubility. However, their D k and D f at high frequency have to be further researched and improved …”

“…However, their D k and D f at high frequency have to be further researched and improved. 14 For D k , the famous Clausius−Mossotti equation defines its relationship between the molar polarization and molar volume of polymer repeating unit, which is useful for preparing polymers with low D k . 15 However, there is no definite theory for designing polymers with low D f , and there are few reports on the reducing D f of the polymer films at high frequency.…”