Dynamics of End-Associated Triblock Copolymer Networks

Abstract

In this Monte Carlo study, the relaxation dynamics of transient networks formed by triblock copolymers via end association is followed. The stress relaxation moduli, deduced from the distributions of bridge lifetimes, are examined with focus on the effects of the system variables, including the solvent quality, concentration, the middle block size, and the end block size. In general, the stress in such systems decays, upon an application of a unit shear strain, as bridges, which supported the stress, convert to dangling ends (via the end-breaking mechanism) and loops (via the fusion mechanism). The transition rates of bridges to other states depend strongly on the interaction energy between the end block segments and the solvent molecules (βe) and the end block size but weakly on the middle block size and concentration. The bridge lifetime distribution does not fit a simple exponential function, leading to a stress decay that is also nonexponential. As βe increases, the lifetime distribution becomes increasingly more nonexponential and its tail extends to longer lifetimes. The contributions from the slower modes become more significant as βe increases, leading to larger average bridging lifetimes. On the other hand, as the end block size gets longer, the bridges, on average, have shorter lifetimes. The stress relaxation appears to fit a stretched-exponential decay. As βe increases, the rubbery plateau region broadens and its height increases. The transition to flow occurs more slowly (as indicated by the larger slopes in the terminal zone) with increasing βe. As the end block size increases, both the width and height of the plateau region decrease. The transition to flow occurs at a faster rate, with longer end block size.