Linear Viscoelastic and Dielectric Properties of Strongly Hydrogen-Bonded Polymers near the Sol-Gel Transition

Linear viscoelastic and dielectric measurements were conducted for a model associating polymer system, n-butyl acrylate (PnBA)-based copolymers containing 2-ureido-4[1H]-pyrimidinone (UPy) groups as stickers. The number of stickers per chain was varied from less than one to more than two, which covered a sol-to-gel transition region. Fitting the linear viscoelasticity (LVE) to an analytical model developed in our previous study, we found necessity of distinguishing the intra- and interchain association in the model, with only the latter contributing to the gel formation. For the PnBA-Upy sample slightly above the gel point, the relaxation processes due to the Rouse motion and the sticker dissociation were commonly detected in the same range of T covering from 20 to 60 degrees C. This feature enabled us to make the time-temperature superposition separately for respective relaxation processes, and two sets of shift factors, reflecting the temperature dependence of the Rouse time To and dissociation time T were obtained accordingly. The T dependence of the ratio of these shift factors, being identical to the dependence of tau(s)/tau(0), enabled determination of activation energy of the sticker dissociation. This activation energy was found to be consistent with that determined from the model fitting of the LVE data. The dielectric measurements detected both segmental a relaxation and an ionic alpha(2) relaxation processes. The alpha(2) relaxation time was found to be shorter than tau(s), and this result was discussed in relation to a difference between fluctuation and dissociation of ionic pairs.