PHASE-SEPARATION KINETICS IN A BINARY MIXTURE OF POLYETHYLENE-GLYCOL AND POLYPROPYLENE GLYCOL STUDIED BY LIGHT-SCATTERING AFTER A PRESSURE JUMP - PINNING OF DOMAIN GROWTH BY HYDROGEN-BOND STRUCTURES

The phase separation kinetics of fluid mixtures of polyethylene glycol /polypropylene glycol (a system with an upper critical mixing point) i s studied after a pressure jump from the homogeneous one-phase region into the two-phase region of the phase diagram. The growth of the emer ging domains of the coexisting phases is observed by small angle laser light scattering. In additional measurements the pressure dependence of the phase separation temperature is analyzed. In the kinetic experi ments the time-dependent structure function is detected for a mixture with near-critical as well as for a mixture with off-critical composit ion. For the near-critical mixture an increase of the maximum of the s cattering intensity with time has been found, which qualitatively is t ypical for the intermediate to late stages of spinodal decomposition. A closer analysis of the late stages reveals two maxima in the structu re factor with their own set of growth exponents for the scattering ve ctor and for the intensify. The data of the low q maximum are compatib le with a two-dimensional growth process which is interpreted as a dem ixing process in a wetting layer. The data of the high q maximum are a ccording to a three-dimensional process. It is assumed that this maxim um reflects the demixing process in the bulk phase. The values of the three-dimensional growth exponents, which are considered to be late st age values, are not compatible with observations on other fluid system s but are close to those for solid systems or, in general, for systems with suppressed hydrodynamic interactions. For the mixture with an of f-critical composition the structure function remains constant for lar ger times (pinning effect). The occurrence of a pinning effect in samp les of relatively low molecular weight M-w (M-w less than or equal to 1019 g/mol) and the apparently suppressed hydrodynamic interactions in a fluid sample are explained with specific interactions caused by hyd rogen bonding (i.e., transient entanglement or a dynamic network). (C) 1997 American Institute of Physics.