LATE STAGES OF PHASE-SEPARATION IN A BINARY POLYMER BLEND STUDIED BY RHEOLOGY, OPTICAL AND ELECTRON-MICROSCOPY, AND SOLID-STATE NMR

The relation between theology and the time dependent morphology: of a phase-separating binary blend of polystyrene and poly(vinyl methyl eth er) was investigated by heating a sample from the single-phase (at 90 degrees C) into the two-phase regime (at 124 degrees C, 16 K above the LCST) and maintaining its temperature there while measuring the evolu tion of the dynamic moduli G' and G'', Morphological changes occurred slowly so that there was sufficient time to cycle the dynamic mechanic al measurements repeatedly over five decades in frequency. The morphol ogy was observed on length scales From 1 mm down to 1 nm by convention al optical microscopy combined with digital image analysis, Hoffmann m odulation microscopy, TEM, and WISE NMR with spin diffusion. NMR shows that major compositional changes occur mostly in the first 20 min and then the composition remains constant at about 60:40 PS/PVME for the PS-rich matrix and 5:95 PS/PVME for the PVME-rich microdomains, The PV ME-rich microdomains are separated by thin layers of the PS-rich phase which forms the matrix. On a larger scale, shape and geometry change during the entire experiment (42 h). The linear domain growth appears to be consistent with the theories of Siggia and Doi-Ohta. The initial increase of the dynamic moduli is attributed to the formation of high ly interconnected PVME-rich and PS-rich phases during spinodal decompo sition, The subsequent decrease of the values of the dynamic moduli is considered to be the result of the loss of the interconnectivity betw een the two phases due to the breakup of the PS-rich phase network and the coalescence of the PVME-rich domains.