Probing nonmonotonic variation of terminal relaxation in star-linear blends with a fast slip link model

Abstract

Recent experiments suggest that the terminal relaxation time τ^b of star-linear blends varies nonmonotonically with the weight fraction of the star polymer in the blend ws [Hall et al., Macromolecules 52, 7831 (2019)], when the linear polymer is more viscous than the star; i.e., the ratio of linear and star viscosities rη=η0,l/η0,s>1. We used a fast slip link model called ecoSLM to assess whether slip link models, unlike tube-based theories, can correctly capture this trend. Simulations were performed on around 150 different star-linear blends where the average number of entanglements per star arm was varied between 8 and 25, and the viscosity ratio rη was varied between 0.1 and 1000. Results agree with experiments: nonmonotonic variation of τ^b with ws is observed only when rη>1 due to partial suppression of constraint release on the star polymers in the blend. In this regime, for a given star polymer, we found that the star weight fraction ws∗ corresponding to the blend with the longest terminal relaxation time τ^b∗ decreased with increasing rη. By extrapolating plots of ws∗ versus rη, we estimated an upper-bound rηub≈0.3log⁡η0,s+1.23, where the nonmonotonic variation of τ^b with ws is expected to subside. This prediction is directly confirmed in ecoSLM simulations with rη≈100−1000. Thus, we find that rη>1 is a necessary, but not sufficient, condition for nonmonotonic variation of terminal relaxation of a star-linear blend with composition.