Tensile and elastic properties of segmented copolyetheresteramides with uniform aramid units

Abstract

The tensile and elastic properties of segmented copolyetheresteramides with crystallisable aramid units of uniform length were investigated. The aramid concentration ranged from 3 to 30wt%. The effect of type of poly(tetramethyleneoxide) (PTMO) segment, having a different tendency to crystallise, on the tensile and elastic properties was studied. The fracture stress of the polymers was found to be mainly affected by the degree of strain-induced crystallisation of the PTMO phase and the molecular weight of the polymers and not by the aramid content. The fracture strain of the polymers is exceptionally high, ranging from 1300 to 2200%. The shear modulus increased strongly with aramid content and ranged from 1.5 to 118MPa. The uniform aramid units are very effective in forming physical crosslinks. The yield stress increased linearly with the aramid content. The yield stress and the log modulus are nearly linearly related for materials with these uniform length crystallisable units. The change of the modulus as a function of prestrain was studied. Up to 160–180% pre-strain the modulus decreases by a factor of 6–8, however, at higher strains the moduli increased again. The strong strain softening effect in the first 160% strain must be due the breaking up of the interconnecting lamellar structure. The healing of the modulus of strain-softened samples was studied with time. The healing was very slow. The materials, especially the low-modulus types, are highly elastic. The stress relaxation decreases with decreasing aramid content and the compression set was found to have a minimum at 9wt% aramid. As expected, (strain-induced) PTMO crystallisation influences the elasticity and low-temperature flexibility negatively.