Network dynamics in hydrogen-bonding telechelic polymers: associate lifetime, structural relaxation and phase separation

Supra-molecular networks formed by reversible bonds between polymer chains exhibit extraordinary properties, e.g. extreme toughness and elongation at break, self-healing. We study short telechelic polymers with different H-bonding end-groups and backbone flexibility [1]. The H-bonds increase the glass transition temperature (Tg), though in flexible polydimethyl siloxanes (PDMS) it does not vary with H-bond strength whereas in much stiffer telechelic polypropylene glycol (PPG) Tg varies significantly. In contrast, viscosity strongly depends on the H-bond strength in PDMS while it remains similar in PPG. Complementary measurements of shear modulus and dielectric relaxation indicate that this can be explained by competing lifetimes of the supra-molecular associations and the chain relaxation. Only if the former exceeds the latter, viscosity increases. Furthermore, the viscoelastic properties can be enhanced tremendously by phase separating ends, controlled primarily by Tg of the aggregations. Our analysis reveals that the concept of bond lifetime renormalization [2] describes the results qualitatively but fails on a quantitative level.

[1] K. Xing, M. Tress et al., Macromolecules 10.1021/acs.macromol.8b01210
[2] E. Stukalin, M. Rubinstein et al., Macromolecules 46 (2013) 7525