AbstractBlends of water‐soluble polyethyloxazoline (PEOx) with a series of hydrophobic styreneacrylonitrile copolymers (SAN) of varying AN contents were immersed in water to determine the kinetics of water swelling and the extent of PEOx extraction. Copolymers containing 25 and 40 by weight of AN form miscible blends with PEOx; those containing higher and lower amounts of AN form immiscible mixtures with PEOx. A high degree of PEOx extraction was observed for the immiscible blends as expected, whereas surprisingly little PEOx was extracted from the miscible blends over periods up to 2 years in spite of a high degree of water swelling. Similar behavior has been noted for other blends of hydrophobic and hydrophilic polymers. In the present system, thermal analysis revealed that the sorption of large amounts of water induces a microphase separation of the two miscible polymers to form microdomains of SAN in which, evidently, segments of PEOx are entrapped to form physical cross‐links that preclude disintegration of the sample and extraction of the PEOx whose phase is highly swollen by the sorbed water. It is proposed that a similar situation probably occurs for other blend systems exhibiting such behav
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