DECAY OF DENSITY-FLUCTUATIONS IN GELS
DECAY OF DENSITY-FLUCTUATIONS IN GELS
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DOI：
10.1103/physreva.43.858
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发表时间：
1991-01-15
影响因子：
2.9
通讯作者：
ODINEK, J
中科院分区：
文献类型：
--
作者：
MARTIN, JE;WILCOXON, J;ODINEK, J
在溶胶-凝胶转变附近，凝胶体系对热驱动的密度波动表现出极慢的松弛。我们用准弹性光散射方法详细地研究了硅胶在凝胶前后和凝胶点的密度起伏的衰减。在预凝胶区，支化聚合物熔体的动态结构因子S(q，t)有一个加长的指数尾部，其特征时间在凝胶点发散。由于S(q，t)的初始衰变速率在凝胶点是不发散的，因此这种临界慢化是由于平均团簇尺寸的发散引起的，并且不同于在二级热力学相变中观察到的通常的临界慢化。事实上，在凝胶点，S(q，t)变成了一个幂定律，表明了一个设定在散射场中的分维时间。这些观察结果是通过考虑渗流团簇的动力学来解释的，并在这方面描述了与粘弹性的类比。在凝胶点以外，S(q，t)仍然是一个幂函数，但强度自相关函数的弛豫部分的幅度减小。最后，研究了从反应池中稀释的团簇的动力学，观察到了S(q，t)从幂定律到伸展指数衰减的交叉。结果表明，在无限稀释度下，关联函数的长尾描述了单个渗流团簇的内模。
Near the sol-gel transition, gelling systems exhibit an extremely slow relaxation of thermally driven density fluctuations. We have made a detailed quasielastic light scattering study of the decay of density fluctuations in reacting silica sol-gels in the pre- and post-gel regimes, and at the gel point. In the pre-gel regime the dynamic structure factor S(q,t) for the branched polymer melt has a stretched exponential tail whose characteristic time diverges at the gel point. This critical slowing down is due to the divergence of the average cluster size and is distinct from the usual critical slowing down observed in second-order thermodynamic phase transitions, since the initial decay rate of S(q,t) is nondivergent at the gel point. In fact, at the gel point, S(q,t) becomes a power law, indicating a fractal time set in the scattered field. These observations are accounted for by considering the dynamics of percolation clusters, and in this connection the analogy to viscoelasticity is described. Beyond the gel point S(q,t) remains a power law, but the amplitude of the relaxing part of the intensity autocorrelation function diminishes. Finally, the dynamics of clusters diluted from the reaction bath is studied, and a crossover from power law to stretched exponential decay of S(q,t) is observed. It is shown that at infinite dilution the long-time tail of the correlation function describes the internal modes of a single percolation cluster.