Dynamics of near-critical polymer gels

We report the viscosity and the modulus of near-critical polyester gels. Previous work has shown that these gels lie in the middle of the static crossover between mean-field and critical percolation. Above the gel point, the modulus data are well described by the scaling law attributing kT of stored elastic energy per unentangled network strand. Below the gel point, the viscosity data disagree with both the de Gennes conductivity analogy and the Rouse model, possibly due to a subtle effect of chain entanglement. PACS number(s): 61.41.+e, 82.70.Gg INTRODUCTION Polymerization involving monomers with functionality larger than 2 leads to the formation of branched poly-mers and ultimately gels. The problem of polymer gela-tion has been recognized as a phase transition in connec-tivity, and percolation ideas have been used to interpret both static [1,2] and dynamic [2] data. While the static properties of branched polymers and gels near the gel point are apparently well understood in terms of critical percolation and the crossover to the mean field [3,4], our understanding of dynamics lags far behind. Here we present a complete set of data for the dynamics of near-critical polymer gels that have been extensively charac-terized in terms of static properties [4]. The crucial ad-vantages of this study over previous ones are that (i) the reaction is stopped and samples are fully characterized in terms of molecular weight distribution both above and below the gel point (and gel fraction beyond the gel point), (ii) the gelation reaction proceeds at elevated tem-perature by an interchange reaction that maintains ergo-dicity in the gels [4], and (iii) there is a range of tempera-tures between the glass transition (Ts— =60’C) and the temperature below which the reaction is effectively stopped (-130’C) that allows for viscoelastic measure-ments to be made. BACKGROUND THEORY