Impact of molecular weight on miscibility and interdiffusion between poly(N-vinyl pyrrolidone) and poly(ethylene glycol)

Miscibility and mutual diffusion between poly-(N-vinylpyrrolidone) (PVP) and short-chain poly(ethylene glycol)s (PEGs) of different molecular weight were studied by optical wedge microinterferometry (WMI). The PVP M̄_n varied from 1 300 to 360 000 g · mol^-1, whereas that of PEG ranged from 200 to 6 000 g · mol^-1. Photographs of interference patterns measured at λ = 546 nm with an optical microscope were used to observe the PVP/PEG interface. PVP blends with short-chain PEG fractions, ranging in molecular weight from 200 to 1 500 g · mol^-1, were found to be fully miscible above the temperatures of PEG fusion, whereas an increase in the molecular weight of up to 3 000 g · mol^-1 renders the two polymers completely immiscible. Successive photographs of interference patterns (the interferogram of the contact interface between PEG6000 (left) and PVP is shown in the Figure) were used to determine composition-distance profiles and to calculate the composition-dependent mutual diffusion coefficient in miscible PVP/PEG blends. It was shown that the effects of PVP molecular weight are in reasonable agreement with those observed for other compatible polymer systems. In contrast to PVP behavior, the effects of PEG molecular weight indicate appreciable contribution of hydrogen bonding of terminal hydroxyl groups in short PEG chains to the miscibility and diffusivity behavior in the PVP/PEG system. The difference in the effects of PEG and PVP chain lengths is ascribed to hydrogen bonding between PEG terminal hydroxyl groups with carbonyl side groups of repeat units in PVP macromolecules and ether oxygens in the PEG chains.