1.5 min. Nishi et al. obtained a similar result.* As can be seen in Figure 7, a large change in concentration at very short times does not lead to a large change in the ratio of excimer-to-monomer fluorescence. As a result of this reduced sensitivity, the value of the dominant growth rate R(&) may be determined only to within a factor of 2. From the slope of the line drawn through the short-time data in Figure 9, R((3,) = 0.9 f 0.3 min-'. Nishi et al. obtained a growth rate for a 50:50 blend with higher molecular weight components annealed at 403 K which was an order of magnitude slower. The diffusion coefficient describing the decomposition process can be calculated provided the wavelength of the dominant concentration fluctuation is known. From optical microscopy, this wavelength has been determined to be of the order of 1 pm.2 From eq 9, the diffusion coefficient is then found to be of the order of -lo-" cm2/s.Finally, the fluorescence results show that phase growth is still taking place at long times where Cahn's model does not hold. From Table I, it appears that the growth rate at long times is several orders of magnitude slower than the rate during the early stages of decomposition. ConclusionsThe technique of excimer fluorescence can be used to study quantitatively the kinetics of phase separation in polymer blends. For a 10% PS/PVME blend annealed at 423 K, Cahn's model for spinodal decomposition adequately describes the process at short times, although the fluorescence results indicate that equilibrium is not reached during this period. The growth rate appears to be several orders of magnitude larger at short times than during the later stages of decomposition. ABSTRACT: A numerical method for the calculation of the binodal of liquid-liquid phase separation in a ternary system is described. The Flory-Huggins theory for three-component systems is used. Binodals are calculated for polymer/solvent/nonsolvent systems which are used in the preparation of asymmetric ultrafiltration or reverse osmosis membranes: cellulose acetate/solvent/water and polysulfone/solvent/water. The values for the binary interaction parameters are taken from literature sources. The effect of a concentration-dependent solvent/nonsolvent interaction parameter is discussed. Although knowledge of the interaction parameters for all compositions in the ternary system is rather poor, fairly good agreement has been found between calculated and experimentally found miscibility gaps when the solvent/nonsolvent parameter is taken to be concentration dependent and the other parameters, the polymer/solvent and the polymer/ nonsolvent interaction parameter, are kept constant. Acknowledgment

SynopsisThe demixing behavior on cooling of ternary systems of cellulose acetate/solvent/water has been examined for CA concentrations up to 40 wt% CA in several solvents. Cloud points have been measured as a function of cooling rate. The rapid process of liquid-liquid demixing can be discriminated from the slow process of aggregate formation by examining the dependence of the cloud point on the cooling rate and by structure analysis of quenched solutions with scanning electron microscopy. The appearance of aggregate formation depends strongly on the type of solvent. Slow cooling of ternary solutions in which acetone is the solvent leads to aggregate formation long before liquid-liquid demixing occurs.In addition, isothermal sol-gel transitions have been measured for quenched solutions at varying gelation times. It is concluded that gelation is not always preceded by aggregate formation.

No abstract

No abstract

No abstract