Phase behaviour of mixtures of polyethylene glycol and polypropylene glycol: Influence of hydrogen bond clusters on critical composition fluctuations

Abstract

The critical behaviour of a binary polymer blend of polyethylene glycol 600 (PEG600) and polypropylene glycol 1000 (PPG1000) was investigated by static and dynamic light scattering. The measurements were carried out in the homogeneous one-phase region (above the critical temperature T_c) and in the two-phase region in both coexisting phases. Additionally to the critical composition (y_PPG = 0.46, y_PPG: mass fraction of PPG), a mixture of non-critical composition (y_PPG = 0.365) was investigated in the one- and two-phase region. From the light scattering measurements, the critical exponents and the amplitudes of the correlation length ξ and the mutual diffusion coefficient D were determined. For the critical mixture the results for the critical exponents and amplitudes of ξ in the one- and two-phase regions are in accordance with the predictions of the 3d-Ising model. The measurements of D in the two-phase region in a wider temperature range gave an indication for a crossover from Ising to mean field behaviour. Since the low molar mass of the PEG and PEG oligomers would lead to Ising behaviour in the composition and temperature range studied, this crossover has to be attributed to an increase of the effective molar mass caused by clusters built by intermolecular hydrogen bonds. The assumption of these clusters is supported by static light scattering and the phase diagram. The data of the non-critical mixture could be described in the frame of the pseudospinodal concept. The results for the non-critical mixture support the interpretation of the data of the critical mixture.