Mechanical Property Enhancement of ABA Block Copolymer-Based Elastomers by Incorporating Transient Cross-Links into Soft Middle Block

We propose a new strategy to enhance mechanical properties of ABA triblock copolymer-based elastomers by incorporating transient cross-links into the soft middle block. An ABA triblock-type copolymer, poly(4-vinylpyridine)-b-[(poly(butyl acrylate)-co-polyacrylamide]-b-poly(4-vinylpyridine) (P-Ba-P), was synthesized via RAFT polymerization. In the molecular design, the poly(4-vinylpyridine) (P) end blocks with a high T-g formed pseudo-cross-link domains due to segregation against the soft Ba middle block, while acrylamide units on the middle block formed self-complementary hydrogen bonding, serving as transient cross-links. According to tensile tests, the Youngs modulus, elongation at break, maximum stress, and material toughness were 1.9 MPa, 200%, 2.6 MPa, and 2.8 MJ/m(3), respectively. Comparison between mechanical properties of P-Ba-P and those of another triblock copolymer, poly(4-vinylpyridine)-b-poly(butyl acrylate)-b-poly(4-vinylpyridine) (P-B-P), revealed that P-Ba-P showed larger Youngs modulus, longer elongation at break, and larger maximum tensile stress than P-B-P. Particularly, the material toughness of P-Ba-P (2.8 MJ/m(-3)) was more than 100 times larger than that of P-B-P (0.02 MJ/m(-3)). Rheological analysis on the basis of sticky Rouse relaxation of Ba middle block of P-Ba-P suggested that the hydrogen bonds on the middle block serve as dynamic stickers in elastic strands of elastomers under stress. Such dynamic behavior of the hydrogen bonds could prevent local concentration of applied stress for activating break/failure of the materials during elongation, leading to mechanical property enhancement of the materials. In addition, zinc chloride was blended with P-Ba-P to form metalligand coordination in the P end block domains, which also affected the mechanical properties of the elastomers.