Efficient Strategy to Enhance the Properties of Fully Biobased Linear Dimeric Acid Polyamides

Dimeric acid (DA) is a biobased derivative of plant oil. Its polyamides are usually weak polymers with a low melting point. In this work, an efficient strategy was developed to enhance the thermal and mechanical properties of DA polyamides (DAPAs). Two H2N-terminated DAPA prepolymers (DAPAhs and DAPAddas) were synthesized through melt polycondensation of dimeric acid with hexanediamine and decanediamine (DDA), respectively. Diethyl oxalate was used as a chain extender to increase the molecular weight, and modified DAPAs (DAPAhs-DEO and DAPAddas-DEO) were obtained. Short and easy crystallizable oxamide hard segments effectively improved the mechanical properties, crystallization, and hydrophobicity of the DAPAs. The biobased modified DAPAs exhibited good tensile strength up to 27 MPa, Tm between 105 and 151 °C, and water contact angles from 84 to 98°. Compared to DAPAhs-DEO, DAPAddas-DEO exhibited better comprehensive mechanical properties because longer DDA segments made the oxamide segments far from bulky DA segments and the oxamide segments in adjacent main chains approached easier. Different from brittle 65DAPAh, 65DAPAh-DEO, 65DAPAdda, and 65DAPAdda-DEO showed good peel strength on the aramid fabric. A simple and efficient method was established to prepare biobased DAPAs with excellent properties.