Synthesis and characterization of new high-performance polymers with controlled substitution patterns

Abstract

New unsymmetrical diamine monomers, 2-trifluoromethyl-4,4′-diaminodiphenyl ether and 2-trifluoromethyl-4,4′-diaminodiphenyl sulfide, and symmetrical diamine monomer, 2,2′-bis(trifluoromethyl)-4,4′-diaminodiphenyl sulfide, were synthesized, and polymerized with PMDA, BPDA, BTDA, and ODPA using solution imidization method and conventional thermal imidization method to obtain the corresponding polyimides. While the polyimides prepared by thermal imidization method were insoluble or had poor solubility, all the polyimides prepared by solution imidization method except the polyimide from 2-trifluoromethyl-4,4′-diaminodiphenyl ether and PMDA were soluble in polar aprotic solvents regardless of the number of trifluoromethyl groups. However, in the case of the polymerization of ODPA with 4,4′-diaminodiphenyl ether that does not have trifluoromethyl group, precipitation occurred during solution imidization. The enhanced solubility of the synthesized polyimides is attributed to the unsymmetrical introduction of trifluoromethyl pendent group. All synthesized polyimides have excellent thermal stability and high $T_g$ values above 234℃. A series of para-linked aromatic polyamides having inherent viscosity of 0.53-0.91 dL/g were prepared from the above three diamine monomers and 2-cyano-4,4′-diaminodiphenyl ether. The polyamides containing trifluoromethyl or cyano pendent groups were soluble in polar aprotic solvents while the corresponding polyamides without pendent groups showed limited solubility requiring LiCl in NMP even though they have flexible ether or thioether linkages. The para-linked polyamides without flexible ether or thioether linkages prepared from 3,3′-bis(trifluoromethyl)-4,4′-diamino-1,1′-biphenyl also showed limited solubility while the meta-linked one showed good solubility in organic solvents. These solubility results indicate that the position of pendent groups is also an important factor to control the solubility of aromatic polyamides. All synth...