Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol)s of varying molecular weight in dilute solution

Molecular dynamics of binary mixtures of poly(propylene glycol) (PPG) and p oly(ethylene glycol)s (PEGs) of varying molecular weight due to molecular i nteractions, chain coiling and elongation in dilute solution under various conditions, ie varying number of monomer units of PEG, method of mixing of polymers and solvent environment, has been explored using microwave dielect ric relaxation times. The average relaxation time tau (o), relaxation time corresponding to segmental motion tau (1) and group rotations tau (2), of a series of binary mixtures of poly(propylene glycol) 2000 and poly(ethylene glycol) of varying molecular weight (ie PPG 2000 + PEG 200, PPG 2000 + PEG 300, PPG 2000 + PEG 400, and PPG 2000 + PEG 600 mixed by equal volume in t he pure liquid states, and PPG 2000 + PEG 1500, PPG 2000 + PEG 4000 and PPG 2000 + PEG 6000 mixed equal weights in solvent) have been determined in di lute solution in benzene and carbon tetrachloride at 10.10 GHz and 35 degre esC. A comparison of the results of these binary systems of highly associat ing molecules shows that the molecular dynamics corresponding to rotation o f a molecule as a whole and segmental motion in dilute solutions are govern ed by the solvent density when the solutes are mixed in their pure liquid s tate. Furthermore, the molecular motion is independent of solvent environme nt when the polymers are added separately in the solvent for the preparatio n of binary mixtures. It has also been observed that there is a systematic elongation of the dynamic network of the species formed during mixing of pu re liquid polymers in lighter environment of solvent with increasing PEG mo nomer units, while the elongation behaviour of the same species in the heav ier environment of carbon tetrachloride solvent is in contrast to the elong ation behaviour of the polymeric species formed in pure PEG. The role of ro tating methyl side-groups in the PPG molecular chain has been discussed in term of the breaking and reforming of hydrogen bonds in complex polymeric s pecies for the segmental motion. In all these mixtures, the relaxation time corresponding to group rotations is independent of the solvent environment and constituents of the binary mixtures. The effect of chain flexibility a nd coiling in these binary mixtures is discussed by comparing the relaxatio n times of the mixtures with their individual relaxation times in dilute so lutions measured earlier in this laboratory. (C) 2001 Society of Chemical I ndustry.