Network formation and molecular dynamics in hydrogen-bonding telechelic polymers: a competition between association lifetime and structural relaxation

Reversible bonds between polymer chains can form supra-molecular networks which exhibit extraordinary mechanical properties. In fact, reversible bonds represent a promising route to functional materials with e.g. self-healing properties. We study short telechelic polymers with H-bonding end-groups of different interaction strength and backbone flexibility. The glass transition temperature of flexible polydimethyl siloxanes (PDMS) does not vary with H-bond strength of the end-groups, but differs strongly from T_g in methyl-terminated PDMS. At the same time, T_g of the much stiffer telechelic polypropylene glycol (PPG) depends significantly on the H-bond interaction strength. In contrast, viscosity strongly depends on the H-bond strength in the PDMS series while it remains almost the same in PPG with different end groups. Complementary measurements of the shear modulus and dielectric relaxation indicate that these observations can be explained by competing lifetimes of the supra-molecular associations and the structural relaxations. Only if the association lifetime exceeds the characteristic segmental or chain relaxation time, the glass transition or viscosity will be affected by chain associations. Otherwise the chain end associations do not influence significantly T_g or viscosity.