Role of Functional Groups in Strengthening Polymer-Polymer Interfaces: Random Copolymers with Hydrogen-Bonding Functionalities

Two series of random copolymers, poly(styrene-d₈-co-4-vinylbenzamide) and poly(styrene-d₈-co-4-vinyl-N-ethylbenzamide), were prepared with varying compositions. The functionalized random copolymers were tested for their abilities to reinforce the weak interface between immiscible polymers: polystyrene and poly(2-vinylpyridine). The effect of the hydrogen-bonding groups with different interaction strengths (primary or secondary benzamide) was studied through the evaluation of interfacial fracture toughness and fracture surface characteristics. For the compositions investigated, the copolymers with the primary benzamide functionality were shown to attain higher fracture toughness values than the substituted benzamide copolymers. Additionally, the composition at which maximum interfacial strengthening was attained was much lower in the primary benzamide case (f_max = 0.06) than in the substituted benzamide case (f_max = 0.25). However, in both cases the observed strengthening was lower than our previous results using copolymers bearing phenolic groups. The effect of the copolymer functionality, including such variables as steric constraints and degree of self-association, and composition drift on the measured interfacial properties are discussed.