Polystyrene-MethyIcyclohexane Solutions Undergoing Phase Separation: A Study by Fluorescence Spectroscopy

The phase separation of 17 500 molecular weight polystyrene (PS)/methylcyclohexane solutions has been studied by fluorescence spectroscopy. In contrast to a previous report, the sensitivity to phase separation of the ratio of excimer to monomer fluorescence intensity, I_E/I_M was small, with a decrease observed in those regions occupied by the PS-lean phase after macroscopic phase layering and an increase observed in those regions occupied by the PS-rich phase after macroscopic phase layering. A polychromator was used to continuously monitor I_E and I_M during phase separation. The magnitude of the changes with time in I_E I_M compares favorably with those predicted by the two-phase model developed for PS/poly(vinyl methyl ether) phase separation by Gelles and Frank. For samples of critical composition, I_E I_M changed about 10-15% during phase separation for quenches of less than 0.7 K below the critical temperature, increasing steadily at long times in rich-phase regions while decreasing steadily at long times in lean-phase regions. At short times, I_E I_M variously increased and decreased, and the overall fluorescence intensity varied widely, depending upon the region of the sample cell being monitored. A sharp increase in I_E I_M with increasing PS content for PS volume fractions <0.04 was observed in miscible solutions near the critical temperature and may be due to coil contraction caused by the trend toward segregation or to increasing intermolecular interactions.