Synthesis and properties of new organosoluble aromatic polyamides with cyclic bulky groups containing phosphorus

Abstract

Aromatic polyamides were prepared from various diacids with a new diamine bearing two 9,10-dihydro-9-oxa-10-oxide-10-phosphaphenanthrene-10-yl (DOPO) substituents. The polyamides possessed inherent viscosities of 0.33–0.81 dl/g and glass transition temperatures of 230–251 °C measured with DSC. All the polyamides were amorphous and readily soluble in NMP, DMAc, DMF, and DMSO. Incorporating DOPO groups into polyamides resulted in polymers with low initial decomposition temperatures (IDT, 310–362 °C), deriving from the decomposition of the DOPO groups. With thermogravimetric analysis (TGA), the DOPO-containing polyamides exhibited high integral procedure decomposition temperatures (IPDT) of 928–1086 °C, to indicate improvement on polyamides' weight loss rate, thermal stability, and heat insulating property at high temperature region. High tensile strength of 42.0–52.8 MPa and Young's modulus of 506–869 MPa measured with stress–strain tests were observed for the polyamides to indicate these polymers possessing good mechanical properties.

Introduction

Aromatic polyamides have received notable attention in research owing to their useful properties, like thermal stability, flame retardant characteristic, and excellent mechanical properties [1]. However, most of the aromatic polyamides show poor processing properties owing to their high melting points and poor solubility in organic solvents. Therefore, research effort has been devoted to improve this drawback of aromatic polyamides in the last decade. Some approaches, such as incorporation of bulky groups into the polymer chain [2], [3], [4], introducing flexible linking groups into the rigid backbones of the polymers [3], [4], [5], [6], using fluorinated monomers [4], introduction of highly polar groups into the polymers [7], and bringing non-coplanar and alicyclic monomers into the polymerization systems [8], have shown their success in preparation of organosoluble polyamides.

Phosphorus-containing groups were considered to bring improved organosolubility to high performance polyamides and polyimides [8], [9], [10], [11], [12]. Polyamides [8], [9], [10] and polyimides [12] with triphenylphosphine oxide groups in the polymer backbones have shown good organosolubility, good thermal and oxidative stability, and high glass transition temperatures. Moreover, phosphorus-containing groups were also incorporated into polymers for increasing the polymers' adhesive properties [12], [13]. On the other hand, the cyclic phosphorous group, 9,10-dihydro-9-oxa-10-oxide-10-phosphaphenanthrene-10-yl (DOPO), was widely incorporated into polymers for improving the polymer's flame retardant properties and thermal stability [15], [16]. Moreover, incorporation of DOPO groups onto polymers also brought improved organosolubility to the polymers, due to the DOPO group's bulk structure and polarity [11], [14]. In the previous work [14], [17], a novel aromatic diamine possessing two DOPO pendent groups (2DOPO-A) was synthesized through a simple preparation route. The diamine, naturally possessed bulk structures, polar groups, non-coplanarity, and cardo structure [18], [19], was demonstrated to bring enhancement on polyaspartimides' organosolubility [17]. Therefore, in this work, 2DOPO-A was polymerized with various diacids to prepare aromatic polyamides. The characterization, organosolubility, thermal properties, and mechanical properties of the resulted polyamides were discussed.