6533b855fe1ef96bd12b1131

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In order to investigate the influences of different end-groups on oligomer miscibility with other substances, oligo(propylene glycol)s bearing OH groups (OPG-OH) studied in the previous paper2, were acetylated at both ends (OPG-AC). The limits of miscibility of OPG-AC with oligo(dimethylsiloxane) (ODMS) were measured turbidimetrically. A simple and reliable method for the determination of the critical points from turbidity data is presented. The observed upper critical temperatures (Tc) increase with degree of oligomerization of ODMS (ranging from 1 to 5), whereas Tc shows a minimum when the degree of oligomerization of the OPG-AC is raised (from 1 to 57,5). For lower mol. wt. oligoglycols, the Tc-values of ODMS/OPG-AC are lower by ca. 50 K as compared with those of ODMS/OPG-OH. The different end-groups play a minor role with respect to the pressure influences (up to 1 500 bar the miscibility increases in all cases). For OPG-AC the chain length of optimum miscibility with a given ODMS is found at considerably lower values than for OPG-OH. The theoretical evaluation of the experimental material on the basis of the lattice theory yields a reduction in the enthalpies of mixing up to 10% and an increase in the volumes of mixing (<0) up to 50% when OH is replaced by AC. For the treatment of end-group effects, the solubility parameter theory, when combined with the concept of molar attraction constants, presents several advantages: In particular it is possible to describe the phase separation behaviour of the different oligomer mixtures simply by means of solubility parameters and molar volumes of the different segments and end-groups and to forecast the presence or absence of a chain-length of optimum miscibility.