Competition of Mesoscales and Crossover to Tricriticality in Polymer   Solutions
  We show that the approach to asymptotic fluctuation-induced critical behavior in polymer solutions is governed by a competition between a correlation length diverging at the critical point and an additional mesoscopic length-scale, the radius of gyration. Accurate light-scattering experiments on polystyrene solutions in cyclohexane with polymer molecular weights ranging from 200,000 up to 11.4 million clearly demonstrate a crossover between two universal regimes: a regime with Ising asymptotic critical behavior, where the correlation length prevails, and a regime with tricritical theta-point behavior determined by a mesoscopic polymer-chain length. 
M. A. Anisimov, A. F. Kostko, and J. V. Sengers (Department of   Chemical Engineering and Institute for Physical Science and...