Wholly aromatic polyamides were developed and commercialised some decades ago, and since then a great deal of effort has been expended to try to improve in particular the processability of the early materials. Work has focussed on attempting to improve polymer solubility in organic solvents, and to reduce the melting point of materials, by synthesizing a variety of macromolecular backbones with many different types of substituents. The challenge has been to modify these polymers, while not causing adverse changes in the unique and useful physical properties of this group of polymers. This has led to the development of entirely novel synthetic strategies to allow, for example, the introduction of potentially reactive groups, and also to an interest in the potential lyotropic and thermotropic liquid crystalline properties of these polymers. This review describes the various synthetic methodologies which have emerged in recent years, and then summarises and categorises progress in the understanding of the structure-property relationships of the polymers produced. Properties such as solubility, thermostability, lytropism and thermotropism are highlighted.

Recent developments in the synthesis, thermostability and liquid crystal properties of aromatic polyamides

Synthesis and characterization of polyamides and poly(amide-imide)s derived from 2,2-bis(4-aminophenoxy) benzonitrile

Characterization of Branching in Aramid Polymers Studied by MALDI−Ion Mobility/Mass Spectrometry

The processing of poly(p-phenylene terephthalamide) (PPTA) has long been a great challenge. This work reports a simple "monomers-nanofibers-macroscopic product" (MNM) hierarchical self-assembly approach to build 3D all-PPTA engineering materials. This approach mainly includes the preparation of polymerization-induced aramid nanofibers (PANFs) from monomers and the fabrication of all-PPTA materials from PANF hydrogel. Various 3D architectures, including simple solid bulks and sophisticated honeycombs (HCs), are obtained after the dehydration and shrinking of the PANF hydrogel. The tensile strength and compressive yield strength of PANF bulk are more than 62 and 90 MPa, respectively, which are comparable to typical engineering plastics. The compressive strength of PANF HC with a density of 360 kg m-3 is more than 24 MPa. The thermal stability of PANF bulk and PANF HC are as good as that of Kevlar fiber and almost no decomposition occurred before 500 °C in a nitrogen atmosphere. Furthermore, the MNM process is performed under mild conditions, without high temperature, high pressure, or corrosive solvent. The MNM process is a novel strategy for the processing of all aromatic polyamide materials with complex structures and high performances and would be another development since the breakthrough of liquid crystal spinning technology of PPTA.

Construction of Aramid Engineering Materials via Polymerization-Induced para-Aramid Nanofiber Hydrogel.

A series of polyamides and poly(amide-imide)s was prepared by direct polycondensation of ether and nitrile group containing aromatic diamines with aromatic dicarboxylic acids and bis(carboxyphthalimide)s respectively in N-methyl 2-pyrrolidone (NMP) using triphenyl phosphite and pyridine as condensing agents. New diamines, such as 2,6-bis(4-aminophenoxy)benzonitrile and 2,6-bis(3-aminophenoxy)benzonitrile, were prepared from 2,6-dichlorobenzonitrile with 4-aminophenol and 3-aminophenol, respectively, in NMP using potassium carbonate. Bis(carboxyphthalimide)s were prepared from the reaction of trimellitic anhydride with various aromatic diamines in N,N′-dimethyl formamide. The inherent viscosities of the resulting polymers were in the range of 0.27 to 0.93 dl g−1 in NMP and the glass transition temperatures were between 175 and 298 °C. All polymers were soluble in dipolar aprotic solvents such as dimethylsulfoxide, dimethylacetamide and NMP. All polymers were stable up to 350 °C with a char yield of above 40 % at 900 °C in nitrogen atmosphere. All polymers were found to be amorphous except the polyamide derived from isophthalic acid and the poly(amide-imide)s derived from diaminodiphenylether and diaminobenzophenone based bis(carboxyphthalimide)s. Copyright © 2004 Society of Chemical Industry

Synthesis and characterization of polyamides and poly (amide-imide) s derived from 2, 6-bis (3-aminophenoxy) benzonitrile or 2, 6-bis (4-aminophenoxy) benzonitrile

In order to study the influence of chemical modification on properties of polyamides, wholly aromatic polyamides have been synthesized from p-phenylenediamine, its 2,5-dimethyl and 2-methyl derivatives, m-phenylenediamine, its 2-methyl and 4-methyl derivatives, and terephthaloyl and isophthaloyl dichlorides by solution polycondensation at low temperature. The thermal stability and solubility of the methyl-substituted polyamides were compared with those of unsubstituted ones. The introduction of methyl groups in a polymeric chain led to a decrease in their thermal stability to different degrees depending on the positions of methyl groups, accompanied by an increase in their solubility. The unsymmetrical introduction of methyl groups in benzene rings had a greater effect on the increase in solubility of polyamide than did symmetrical methyl groups.

Study on wholly aromatic polyamides containing methyl‐substituted phenylene linkage

Aromatic polyimides were synthesized by polymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 2-dialkylamino-4,6-dihydrazino-1,3,5-triazines by a solution polymerization method followed by cyclodehydration. The polyimides had number-average molecular weights over 13,000–16,000 or viscosity values of 0.23–0.65 dl/g. The x-ray diffraction diagram of polyimide I showed a crystal pattern, but the pattern became less pronounced with increasing length of the polymer side chain, and thus polyimide V showed a typical amorphous pattern. Study of the thermal stability of the polyimides by thermogravimetric analysis showed that a steep weight loss occurred in the range 380–410°C. Among the polyimides, I had only poor solubility in organic solvent, while IV and V were soluble even in general organic solvents such as tetrahydrofuran and dioxane.

Thermal properties and solubility of poly‐1,4,5,8‐naphthalenetetracarboxydiimides containing 1,3,5‐triazine rings

Six methyl-substituted wholly aromatic polyamides were synthesized from the reaction of 2,5-dimethylterephthaloyl chloride with p-phenylenediamine, its 2,5-dimethyl and 2-methyl derivatives, m-phenylenediamine, or its 2-methyl and 4-methyl derivatives by solution polycondensation at low temperature. The x-ray diffraction diagrams of the polyamides obtained exhibit crystal patterns. Density values range from 1.26 to 1.37 g/cm3. NMR spectra determined in concentrated H2SO4 solution are reported. Poly(2,5-dimethylterephthalamides) have lower thermal stability than the corresponding polyterephthalamides. The increase in solubility of polyamide by the introduction of the 2,5-dimethylterephthaloyl linkage is accompanied by a decrease in thermal stability. The effect of methyl substituents on thermal properties and solubility is discussed in terms of the packing of polymeric molecules and the extent of hydrogen bonding of the amide groups.

Ryozo Takatsuka

Papers

Study on wholly aromatic polyamides containing methyl‐substituted phenylene linkage

Thermal properties and solubility of poly‐1,4,5,8‐naphthalenetetracarboxydiimides containing 1,3,5‐triazine rings

Structure and properties of poly(2,5-dimethylterephthalamides)