Mechanical Properties of Polyamide Modified by Block Copolymers: Possible Effect of Tie-Molecules

Appropriate block ABA copolymers can be suitable agents capable of increasing the number of tie-molecules in the polyamide (PA) blend. In this work, macromonomers of PA (block A), with backbone chains endcapped with —NH2 group on one end, and of polyethylenet erephthalate (PET) (block B), with —COOH groups on both ends, have been used. The —COOH and —NH2 end groups of these macromonomers were reacted via condensation reaction joining the A and В blocks to form the desired triblock copolymer PA—PET—PA. Reactions proceeded in a melt in a single screw extruder. The ABA copolymer was added in different amounts to the PA's matrix. The outside blocks of that copolymer conformed with the polymer matrix and the inner block did not co-crystallize with it. The stress-strain behaviour (yield stress, stress at break, yield strain, elongation at break, and notched impact strength) of these polymer blends was studied as a function of the mole fraction of the inner block. All mechanical characteristics were improved. It is suggested that the improvement can be attributed to the higher amount of tie-molecules in the modified polymeric material. Mechanical properties of semi-crystall ine polymers depend strongly on the presence of tie-molecules [1— 6]. These are macromolecules or their parts connecting neighbouring lamellae as shown in Scheme 1. Various ways to increase the number of tiemolecules have been attempted, e.g. annealing of the polymer in the solid state [7—10]. More signif­ icant results were obtained when mixing modified macromolecules (random copolymer poly(ethylene-cohexene) or poly(ethylene -co-octene)) with polyethy­ lene. The density of side groups (C4 or C 6) affected the thickness of lamellae and the number of tie-molecules [11, 12]. Modification of parts of the macromolecular