Viscoelastic Relaxation of Mono-functionally End-Associating Rouse Chains: Experimental Test

Viscoelastic test was made for end-calboxylated polyisoprene (PI-COOH) of the molecular weight M $=$ 30.5k that underwent the inter-chain association and dissociation through hydrogen bonding of the COOH groups at the chain end. As a reference, the test was made also for neat PI unimer and PI2 dimer (both having no COOH group at the chain end). These samples were diluted in oligomeric butadiene (oB) to a concentration of 10 wt{\%}. The neat unimer and dimer exhibited non-entangled Rouse behavior at this concentration, as expected from their molecular weights. At low temperatures (T $\le $ 0 C), the PI-COOH sample relaxed slower than the reference unimer but faster than the dimer, whereas the relaxation of PI-COOH approached that of the unimer with increasing T \textgreater 0 C, and this change of the relaxation time of PI-COOH was associated with changes in the relaxation mode distribution. This behavior of PI-COOH was well described by a recently proposed theory considering motional coupling between the end-associating unimer and its dimer at chemical equilibrium. On the basis of this result, an effect of the polymeric character of PI-COOH chain on the viscoelastically detected association/dissociation of the COOH groups is discussed.