Concentration-superposition meets the challenge of validating very large rings

Valuable properties of linear polymers emerge when their molecular weight is several times their entanglement molecular weight, M_e,lin, providing a strong impetus to examine cyclic polymers that have M_w / M_e,lin ≡ Z_w >> 1. This regime has proven difficult to reach. Some molecular structures enable preparation of pure rings, but have a highest accessible Z_w < 15. Synthesis strategies that produce rings of Z_w >> 15 confront the difficulty of proving that there are <1% linear chain contaminants. This talk deals with such a case: putative rings produced by Reversible Radical Recombination Polymerization (R3P) up to Z_w = 300·M_e,lin. We use dilution to increase M_e(φ) without modifying topology. Using samples that have Z_w = 301 and 219 in the melt, we present results for solutions of 2.5% and 1% that have Z_w < 15 and show their accord with highly purified rings. Examining concentrations from 5% to 50% reveal unanticipated, qualitative changes in the relaxation dynamics as Z_w increases from 30 to 300—a previously unexplored regime for cyclic polymers.