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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 Mn 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. Interferogram of the contact interface between PEG 6000 (left) and PVP. Magnification: 60‐fold.magnified imageInterferogram of the contact interface between PEG 6000 (left) and PVP. Magnification: 60‐fold.

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Dissolution and mutual diffusion of poly(N-vinylpyrrolidone) (PVP) in short-chain poly(ethylene glycol) PEG400 were studied by wedge microinterferometry over the temperature range of 40 -100°C. Successive photographs of interference patterns measured at ϭ 546 nm with an optical microscope at 130ϫ magnification were used to determine the PVP/PEG concentration-distance profiles. These profiles were found to be highly asymmetric, exhibiting steep concentration gradients near the surface of the glassy polymer sample. The PVP/PEG system is completely miscible, and interdiffusion kinetics are Fickian with a concentration-dependent mutual diffusion coefficient, D V . Thermal activation of diffusion was studied in terms of an Arrheniustype relation, with concentration dependent activation energy E a . Values of D V and E a are in accord with the compositional behavior of the glass transition temperature in PVP-PEG blends, indicating that PVP plasticized with PEG behaves like an elastomer.