CHAPTER 5. Structural Studies on PEO-b-PPO-based Amphiphilic Polymer Co-networks

Abstract

We discuss the demands for making amphiphilic polymer co-networks based on block copolymer systems. Linear ABA triblock copolymers may very well give rise to structures of two continuous nanophases as required for co-networks; however, usually only one of the phases will be able to swell without destroying the fundamental two-phase network system. A true block copolymer-based amphiphilic polymer co-network requires more complex molecules and/or chemical crosslinks. We present an example based on both linear ABA triblock copolymers and four-armed star block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), the so-called Pluronics and Tetronics systems, respectively, both commercially available with these trade names from BASF. We show that, even in the crosslinked situation, the star molecules self-assemble and organise into a micellar structure. While the structural characteristics of the star copolymers show surprising similarities to the characteristics found in the corresponding linear PEO–PPO block copolymers, we demonstrate that crosslinking induces the important differences required for the co-network characteristics. Even the four-armed star copolymers form spherical micelles as their basic self-assembled structure, but the crosslinked architecture hinders the formation of spherical domains. The crosslinked system forms locally two-dimensional network sheets. Upon raising temperature, the increased hydrophobicity of the PPO blocks causes assembly into PPO-based cylindrical domains connecting star-centres across adjacent network-sheets.