The composition comprises a polyol crosslinked with a) a multifunctional epoxide; or b) an epihalohydrin; or c) a molecule or crosslinker mixture comprising multiple epihalohydrin and/or epoxide groups or molecules. The composition has a number N1 of effective ether crosslinks per crosslinker molecule as calculated by subtracting 1 from the average number of distinct polyol repeat units bound per crosslinker molecule. The remaining reactive groups on the crosslinker are ineffective in crosslinking and provide a number N2 of pendant groups per crosslinker molecule. Among the N2 groups per crosslinker there are N3 groups per crosslinker that are unreacted or otherwise retain reactivity against nucleophiles. The relationship between N1 and N2 is: N1>0.35 (N1+N2).