Self-diffusion of poly(p-phenyleneethynylene)s in dilute solution determined by pulsed-field-gradient NMR

Self-diffusion of poly[(2,5-didodecyl-p-phenylene)ethynylene]s (didodecyl-PPE) was measured in dilute CDCl3 by pulsed-field-gradient NMR spectroscopy. The reduced versions of these didodecyl-PPEs, poly[2,5-didodecyl-p-xylylene]s (didodecyl-PPX), were also examined as random-coil analogs of equivalent chain lengths (14-120 repeat units). The coefficients for infinite-dilution self-diffusion (D0) and their concentration dependence (kF) were determined from plots of the self-diffusion coefficients versus concentration. These quantities were fit to power laws with molecular weight (M0) and their scaling constants determined from double-logarithmic plots. For didodecyl-PPX, D0 scales with M0 by −0.50±0.03 (ν) and kF scales with M0 by 0.51±0.08 (α), consistent with a random-coil conformation for this polymer in solution. For didodecyl-PPE, D0 scales with M0 by −0.71±0.06 (ν) and kF scales with M0 by 0.96±0.06 (α), indicative of a semiflexible rod-like macromolecule.